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<p>See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/343617599</p><p>Solar radiation effects on growth, anatomy, and physiology of apple trees in a</p><p>temperate climate of Brazil</p><p>Article  in  International Journal of Biometeorology · August 2020</p><p>DOI: 10.1007/s00484-020-01987-w</p><p>CITATIONS</p><p>8</p><p>READS</p><p>551</p><p>3 authors, including:</p><p>Homero Bergamaschi</p><p>Universidade Federal do Rio Grande do Sul</p><p>200 PUBLICATIONS   2,761 CITATIONS</p><p>SEE PROFILE</p><p>Gilmar Arduino Bettio Marodin</p><p>Universidade Federal do Rio Grande do Sul</p><p>130 PUBLICATIONS   1,110 CITATIONS</p><p>SEE PROFILE</p><p>All content following this page was uploaded by Homero Bergamaschi on 10 September 2020.</p><p>The user has requested enhancement of the downloaded file.</p><p>https://www.researchgate.net/publication/343617599_Solar_radiation_effects_on_growth_anatomy_and_physiology_of_apple_trees_in_a_temperate_climate_of_Brazil?enrichId=rgreq-ea801bbf3fe45a7eac9a09067c2bbfbc-XXX&enrichSource=Y292ZXJQYWdlOzM0MzYxNzU5OTtBUzo5MzQyMDYwODM0NDQ3MzZAMTU5OTc0MzQ5NTA1Ng%3D%3D&el=1_x_2&_esc=publicationCoverPdf</p><p>https://www.researchgate.net/publication/343617599_Solar_radiation_effects_on_growth_anatomy_and_physiology_of_apple_trees_in_a_temperate_climate_of_Brazil?enrichId=rgreq-ea801bbf3fe45a7eac9a09067c2bbfbc-XXX&enrichSource=Y292ZXJQYWdlOzM0MzYxNzU5OTtBUzo5MzQyMDYwODM0NDQ3MzZAMTU5OTc0MzQ5NTA1Ng%3D%3D&el=1_x_3&_esc=publicationCoverPdf</p><p>https://www.researchgate.net/?enrichId=rgreq-ea801bbf3fe45a7eac9a09067c2bbfbc-XXX&enrichSource=Y292ZXJQYWdlOzM0MzYxNzU5OTtBUzo5MzQyMDYwODM0NDQ3MzZAMTU5OTc0MzQ5NTA1Ng%3D%3D&el=1_x_1&_esc=publicationCoverPdf</p><p>https://www.researchgate.net/profile/Homero-Bergamaschi?enrichId=rgreq-ea801bbf3fe45a7eac9a09067c2bbfbc-XXX&enrichSource=Y292ZXJQYWdlOzM0MzYxNzU5OTtBUzo5MzQyMDYwODM0NDQ3MzZAMTU5OTc0MzQ5NTA1Ng%3D%3D&el=1_x_4&_esc=publicationCoverPdf</p><p>https://www.researchgate.net/profile/Homero-Bergamaschi?enrichId=rgreq-ea801bbf3fe45a7eac9a09067c2bbfbc-XXX&enrichSource=Y292ZXJQYWdlOzM0MzYxNzU5OTtBUzo5MzQyMDYwODM0NDQ3MzZAMTU5OTc0MzQ5NTA1Ng%3D%3D&el=1_x_5&_esc=publicationCoverPdf</p><p>https://www.researchgate.net/institution/Universidade_Federal_do_Rio_Grande_do_Sul?enrichId=rgreq-ea801bbf3fe45a7eac9a09067c2bbfbc-XXX&enrichSource=Y292ZXJQYWdlOzM0MzYxNzU5OTtBUzo5MzQyMDYwODM0NDQ3MzZAMTU5OTc0MzQ5NTA1Ng%3D%3D&el=1_x_6&_esc=publicationCoverPdf</p><p>https://www.researchgate.net/profile/Homero-Bergamaschi?enrichId=rgreq-ea801bbf3fe45a7eac9a09067c2bbfbc-XXX&enrichSource=Y292ZXJQYWdlOzM0MzYxNzU5OTtBUzo5MzQyMDYwODM0NDQ3MzZAMTU5OTc0MzQ5NTA1Ng%3D%3D&el=1_x_7&_esc=publicationCoverPdf</p><p>https://www.researchgate.net/profile/Gilmar-Marodin?enrichId=rgreq-ea801bbf3fe45a7eac9a09067c2bbfbc-XXX&enrichSource=Y292ZXJQYWdlOzM0MzYxNzU5OTtBUzo5MzQyMDYwODM0NDQ3MzZAMTU5OTc0MzQ5NTA1Ng%3D%3D&el=1_x_4&_esc=publicationCoverPdf</p><p>https://www.researchgate.net/profile/Gilmar-Marodin?enrichId=rgreq-ea801bbf3fe45a7eac9a09067c2bbfbc-XXX&enrichSource=Y292ZXJQYWdlOzM0MzYxNzU5OTtBUzo5MzQyMDYwODM0NDQ3MzZAMTU5OTc0MzQ5NTA1Ng%3D%3D&el=1_x_5&_esc=publicationCoverPdf</p><p>https://www.researchgate.net/institution/Universidade_Federal_do_Rio_Grande_do_Sul?enrichId=rgreq-ea801bbf3fe45a7eac9a09067c2bbfbc-XXX&enrichSource=Y292ZXJQYWdlOzM0MzYxNzU5OTtBUzo5MzQyMDYwODM0NDQ3MzZAMTU5OTc0MzQ5NTA1Ng%3D%3D&el=1_x_6&_esc=publicationCoverPdf</p><p>https://www.researchgate.net/profile/Gilmar-Marodin?enrichId=rgreq-ea801bbf3fe45a7eac9a09067c2bbfbc-XXX&enrichSource=Y292ZXJQYWdlOzM0MzYxNzU5OTtBUzo5MzQyMDYwODM0NDQ3MzZAMTU5OTc0MzQ5NTA1Ng%3D%3D&el=1_x_7&_esc=publicationCoverPdf</p><p>https://www.researchgate.net/profile/Homero-Bergamaschi?enrichId=rgreq-ea801bbf3fe45a7eac9a09067c2bbfbc-XXX&enrichSource=Y292ZXJQYWdlOzM0MzYxNzU5OTtBUzo5MzQyMDYwODM0NDQ3MzZAMTU5OTc0MzQ5NTA1Ng%3D%3D&el=1_x_10&_esc=publicationCoverPdf</p><p>1 23</p><p>International Journal of</p><p>Biometeorology</p><p>ISSN 0020-7128</p><p>Int J Biometeorol</p><p>DOI 10.1007/s00484-020-01987-w</p><p>Solar radiation effects on growth, anatomy,</p><p>and physiology of apple trees in a temperate</p><p>climate of Brazil</p><p>L. C. Bosco, H. Bergamaschi &</p><p>G. A. B. Marodin</p><p>1 23</p><p>Your article is protected by copyright and</p><p>all rights are held exclusively by ISB. This e-</p><p>offprint is for personal use only and shall not</p><p>be self-archived in electronic repositories. If</p><p>you wish to self-archive your article, please</p><p>use the accepted manuscript version for</p><p>posting on your own website. You may</p><p>further deposit the accepted manuscript</p><p>version in any repository, provided it is only</p><p>made publicly available 12 months after</p><p>official publication or later and provided</p><p>acknowledgement is given to the original</p><p>source of publication and a link is inserted</p><p>to the published article on Springer's</p><p>website. The link must be accompanied by</p><p>the following text: "The final publication is</p><p>available at link.springer.com”.</p><p>ORIGINAL PAPER</p><p>Solar radiation effects on growth, anatomy, and physiology of apple</p><p>trees in a temperate climate of Brazil</p><p>L. C. Bosco1</p><p>& H. Bergamaschi2 & G. A. B. Marodin3</p><p>Received: 12 February 2020 /Revised: 15 June 2020 /Accepted: 3 August 2020</p><p># ISB 2020</p><p>Abstract</p><p>The aim of study was to characterize patterns of interception and distribution of photosynthetically active radiation (PAR) in an</p><p>apple orchard and to examine its relationship with morphophysiological characteristics of “Royal Gala” and “Fuji Suprema”</p><p>apple trees. The experiments were conducted during three production cycles in two distinct orchard areas, one covered by black</p><p>anti-hail netting and another uncovered (control). We analyzed PAR characteristics with data from meteorological sensors</p><p>installed on the canopy, as well as growth, anatomical, and physiological variables of apple trees. The reduction of PAR by</p><p>netting influenced the components of radiation balance. PAR intercepted, absorbed, transmitted, and reflected by the canopy</p><p>under netting decreased by 33%, 31%, 32%, and 46%, respectively, in comparison to uncovered canopy. When leaf area index</p><p>(LAI) was 1.5 (under netting) and 2.5 (uncovered), maximum PAR interception efficiency was reached. During the three</p><p>production cycles, a light extinction coefficient of 1.09 and 0.76 was found under netting and in the control, respectively.</p><p>Plant height was greater under netting in all three cycles for both cultivars. Number of leaves, LAI, and shape index did not</p><p>differ between treatments. At stage 85, leaves of “Royal Gala” under netting showed lower chlorophyll content and thinner</p><p>parenchymas in comparison to the control. However, physiological and anatomical characteristics of Fuji “Suprema” did not</p><p>change under anti-hail netting.</p><p>Keywords Malus domesticaBorkh . Light extinction coefficient . Photosynthetic rate . Anti-hail netting</p><p>Introduction</p><p>Solar radiation is an abiotic factor that triggers several eco-</p><p>physiological processes in nature. The spectrum of solar radi-</p><p>ation used by plants includes incident photosynthetically ac-</p><p>tive radiation, which is intercepted, absorbed, and used in</p><p>physiological events such as photosynthesis, transpiration,</p><p>and respiration. Photosynthetic rate is influenced by leaf</p><p>morphological and anatomical characteristics, chlorophyll</p><p>and N content in the plant, genetic makeup of the cultivars</p><p>(Solomakhin and Blanke 2008; Hunche et al. 2010; Bhusal</p><p>et al. 2018), plant architecture (Da Silva et al. 2014), and</p><p>microclimate (Amarante et al. 2007; Amarante et al. 2009).</p><p>The climate of the southern region of Brazil is character-</p><p>ized by strong convective storms that cause a large number of</p><p>damaging hail events followed by annual losses of tens of</p><p>millions of dollars (Martins et al. 2017). Therefore, the instal-</p><p>lation of protective netting over orchards is recommended to</p><p>avoid losses in production and fruit quality (Bosco et al.</p><p>2015). Anti-hail netting reduces the amount of direct solar</p><p>radiation and increases the amount of diffused light in the</p><p>apple orchard (Bosco et al. 2018; Tanny 2013).</p><p>Understanding the basic relationships between apple trees</p><p>grown under netting and intercepted solar radiation is essential</p><p>to generate interception and</p><p>distribution patterns of photosyn-</p><p>thetically active radiation on the canopy. Furthermore, it is pos-</p><p>sible to determine parameters that can be used in plant growth</p><p>simulation models, such as interception efficiency and the use</p><p>of solar radiation and light extinction coefficient (k). Few</p><p>* L. C. Bosco</p><p>leosane.bosco@ufsc.br</p><p>1 Department of Agriculture, Biodiversity and Forest, Federal</p><p>University of Santa Catarina, Rodovia Ulysses Gaboardi, Km 3,</p><p>Curitibanos, Santa Catarina 89520-000, Brazil</p><p>2 Department of Forage Plants and Agrometeorology, Federal</p><p>University of Rio Grande do Sul, Av. Bento Gonçalves, 7712, Porto</p><p>Alegre, Rio Grande do Sul, Brazil</p><p>3 Department of Horticulture and Silviculture, Federal University of</p><p>Rio Grande do Sul, Av. Bento Gonçalves, 7712, Porto Alegre, Rio</p><p>Grande do Sul, Brazil</p><p>International Journal of Biometeorology</p><p>https://doi.org/10.1007/s00484-020-01987-w</p><p>Author's personal copy</p><p>http://crossmark.crossref.org/dialog/?doi=10.1007/s00484-020-01987-w&domain=pdf</p><p>https://orcid.org/0000-0003-2623-2590</p><p>https://orcid.org/0000-0002-7932-2074</p><p>mailto:leosane.bosco@ufsc.br</p><p>studies on apple trees have shown the relationship between</p><p>intercepted solar radiation and leaf area (Palmer 1977; Jackson</p><p>1978; Jackson and Palmer 1979; Wünsche and Lakso 2000; Da</p><p>Silva et al. 2014). Leaf area index (LAI) is an ecological char-</p><p>acteristic that represents the structure of the canopy and controls</p><p>several ecosystem functions and processes such as water inter-</p><p>ception, gas exchange, carbon fixation, and radiation attenua-</p><p>tion (Saitoh et al. 2012). Through this variable, canopy leaf</p><p>density and interception efficiency of solar radiation can be</p><p>determined. The measurement of LAI is difficult to obtain, es-</p><p>pecially in species such as the apple tree. Thus, there are indirect</p><p>methods for estimating LAI, such as using models based on</p><p>accumulated degree days and PAR attenuation (Cardoso et al.</p><p>2010; Forchesatto et al. 2016).</p><p>The method for estimating the LAI based on the Beer–</p><p>Lambert law adapted by Monsi and Saeki (2005) requires the</p><p>determination of k. For plant communities, the amount of radi-</p><p>ation transmitted by the canopy is reduced logarithmically with</p><p>an increase in LAI. The conduction system and the arrangement</p><p>of the plants determine the attenuation pattern of solar radiation</p><p>that passes through the canopy, which is expressed through k.</p><p>For the same LAI, the lower k, the greater is the fraction of solar</p><p>radiation that goes through the canopy and reaches the ground.</p><p>This relationship between canopy radiation extinction and re-</p><p>spective LAI is important to estimate biomass and fruit produc-</p><p>tion, aiming to develop better apple cultivars and management</p><p>practices (Grappadelli 2003). A detailed analysis of the LAI</p><p>seasonal pattern is essential for the characterization of agricul-</p><p>tural ecosystems, such as those of apple trees. However, peri-</p><p>odic assessment is difficult to be carried in the field due to</p><p>limitations in cost, labor, or access to sites.</p><p>Plants can acclimate to low radiation availability through</p><p>morphophysiological changes or may not need all the radia-</p><p>tion available in some climatic regions, as reduced radiation</p><p>caused by coverage is not limiting to plant development. In</p><p>terms of acclimatization or phenotypic plasticity, plants can</p><p>show changes in growth and at an anatomical and a physio-</p><p>logical level. However, if the amount of solar radiation avail-</p><p>able under the net is potentially enough, the plant will not</p><p>change its phenotypic characteristics. Microclimate changes</p><p>can alter morphological and anatomical characteristics of</p><p>leaves and consequently interfere with complex plant eco-</p><p>physiological functions (Solomakhin and Blanke 2010). A</p><p>few studies on apple trees have shown that the microclimate</p><p>in orchards under netting has no influence on the micromor-</p><p>phology and cuticular properties of leaves (Hunche et al.</p><p>2010), while others have identified changes in leaf anatomy</p><p>of plants grown under netting (Solomakhin and Blanke 2010).</p><p>Leaf anatomy can influence photosynthetic rate due to chang-</p><p>es in CO2 diffusion rate from the environment to the carbox-</p><p>ylation sites in chloroplasts (Vemmos et al. 2013; Bhusal et al.</p><p>2018). Also, the thickness of the palisade parenchyma influ-</p><p>ences the penetration of sunlight into the leaves, while the</p><p>thickness of the spongy parenchyma can cause scattering of</p><p>radiation within the leaf, thus altering uptake.</p><p>Studies on the anatomy of apple tree leaves grown under</p><p>anti-hail netting may help to understand the effect of micro-</p><p>climate (more specifically of solar radiation) on plant</p><p>morphophysiology. As far as we know, only one study in</p><p>Europe has investigated these issues (Solomakhin and</p><p>Blanke 2010). Thus, the motivation for this study was based</p><p>on the hypothesis that plants grown under less light availabil-</p><p>ity (netting) have morphological, anatomical, and physiolog-</p><p>ical characteristics that allow them to have greater radiation</p><p>interception efficiency and a lower light extinction coefficient.</p><p>This study aimed to characterize patterns of interception</p><p>and distribution of photosynthetically active radiation (PAR)</p><p>in an apple orchard and to examine its relationship with</p><p>morphophysiological characteristics of “Royal Gala” and</p><p>“Fuji Suprema” apple trees.</p><p>Material and methods</p><p>Location and characterization of the experiments</p><p>The experiments were carried out in a commercial apple or-</p><p>chard in southern Brazil (28° 24′ 52.5′′ S, 50° 50′ 53.8′′ W,</p><p>altitude of 930 m) from September to April during three veg-</p><p>etation periods (2008/2009, 2009/2010, and 2010/2011).</p><p>According to the Köppen classification, the climate is sub-</p><p>tropical (Cfb) with temperate summers (Alvares et al. 2013).</p><p>The soils of the region are classified as brown Oxisol of flat</p><p>to wavy relief, with retractable character and humic A horizon</p><p>with organic carbon content greater than 10 g kg−1 until a</p><p>depth of 70 cm (SiBCS 2018).</p><p>“Royal Gala” and “Fuji Suprema” apple cultivars planted</p><p>in 1999 with M9 rootstocks were evaluated. A high-density</p><p>orchard was installed with plant spacing of 1.0 m and row</p><p>spacing of 3.5 m. Plant rows were set up in a North-South</p><p>direction, using a central leader system with support.</p><p>An area of the orchard was covered with a black anti-hail</p><p>net (4 × 7 mm mesh). The net was installed 1 year after the</p><p>seedlings were planted. The net was placed on the support</p><p>structure as to form a double pitch roof, with an opening of</p><p>20 cm between the rows for eventual hail runoff. Another</p><p>uncovered (control) area of the orchard was set up 15 m from</p><p>the first. This area was installed and conducted similarly to the</p><p>one under netting following specific technical standards for</p><p>integrated apple production (NTEPI 2006).</p><p>Growth variables</p><p>The growth of 10 labeled plants of “Royal Gala” and “Fuji</p><p>Suprema” was evaluated through trunk diameter, measured at</p><p>the fruit development (Stage 72) . The diameter was measured</p><p>Int J Biometeorol</p><p>Author's personal copy</p><p>with a caliper 10 cm above the transition between the root-</p><p>stock and the canopy. In addition, plant height was measured,</p><p>and the number of branches was counted. In the first two</p><p>cycles, four branches were randomly selected, two in the up-</p><p>per layer and two in the lower layer of the canopy to count the</p><p>number of leaves. The average number of leaves in each can-</p><p>opy layer was multiplied by the number of branches to esti-</p><p>mate the total number of leaves in each plant.</p><p>Leaf area index is a dimensionless measure of vegetation</p><p>cover and represents the ratio between the sum of the leaf</p><p>areas and the soil area occupied by the plant. We determined</p><p>leaf area index (LAI) using leaf area measurements of apple</p><p>trees found in Bosco et al. (2012) and considering a soil area</p><p>of 3.5 m2. Through this variable, canopy leaf density and solar</p><p>radiation interception efficiency can be determined.</p><p>We used shape index (SI), which is determined through</p><p>leaf length-to-width ratio, to characterize leaf shape. Based</p><p>on this index, we considered leaves with</p><p>SI lower than 1 to</p><p>be proportionally wider, while leaves with SI greater than 1 to</p><p>be proportionally longer.</p><p>Climate variables</p><p>After pruning, sensors were installed to monitor the microcli-</p><p>mate of the canopy in both treatments. More details on mea-</p><p>surements and changes of the microclimate can be found in</p><p>Bosco et al. (2018). Photosynthetically active radiation (PAR,</p><p>400–700 nm) was measured using photovoltaic cell sensors</p><p>facing upwards and downwards to measure incident and</p><p>reflected PAR, respectively. The incident PAR sensors were</p><p>installed 2.7 m above the ground (top of the canopy). In the</p><p>treatment with anti-hail netting, radiation that passed through</p><p>the net was considered incident PAR. Thus, radiation was</p><p>measured between the net and the canopy. The reflected</p><p>PAR sensors were installed facing downwards at ground level</p><p>and at 2.7 m. For incident and reflected PAR, measurements</p><p>were made by sensor bars at each position, which were con-</p><p>sidered two replications. In both treatments, sensors were con-</p><p>nected to automatic data acquisition systems, consisting of a</p><p>Campbell 40-channel AM 416 multiplexer, CR21X</p><p>datalogger, and a storage unit.</p><p>A fraction of the PAR absorbed by the crop was calculated</p><p>by the amount of received radiation (incident and reflected by</p><p>the soil) and “lost” radiation (transmitted and reflected by the</p><p>crop) (Varlet-Grancher et al. 1989).</p><p>The intercepted PAR (PARint) was calculated by</p><p>subtracting PAR incident (PARinc) on the canopy from</p><p>PAR transmitted at ground level (Varlet-Grancher et al. 1989).</p><p>The intercepted PAR is the amount of PAR that reaches</p><p>different layers of the canopy. This happens as the incident</p><p>PAR propagates down through canopy layers to the ground.</p><p>However, the absorbed PAR is the fraction absorbed by can-</p><p>opy layers and includes PAR reflected off the top of the</p><p>canopy back into the atmosphere and PAR reflected off the</p><p>ground back into the canopy (Varlet-Grancher et al. 1989).</p><p>The difference between intercepted and absorbed PAR de-</p><p>pends on the closure, density, and composition of the canopy</p><p>(Lordan et al. 2018), in addition to canopy cover and reflec-</p><p>tance. If a canopy has healthy and dense leaves (as is the case</p><p>of an apple orchard), the intercepted PARwill be similar to the</p><p>absorbed PAR.</p><p>PAR interception efficiency (Eint) by the crop was calcu-</p><p>lated based on the PARint:PARinc ratio.</p><p>The extinction coefficient (k) was estimated by methodol-</p><p>ogy described in Monsi and Saeki (2005).</p><p>Anatomical and physiological variables</p><p>Leaf anatomy, chlorophyll content, and photosynthetic rate</p><p>were measured for “Royal Gala” and “Fuji Suprema” in the</p><p>vegetation period during the phases of fruit development (fruit</p><p>size up to 20 mm—stage 72) and fruit maturation (advanced</p><p>ripening—stage 85), periods in which the leaves were fully</p><p>developed (Meier 2003).</p><p>To characterize leaf anatomy, leaves exposed to solar radi-</p><p>ation were collected in the middle layer of the canopy of four</p><p>plants of each cultivar, treatment, and evaluation period. The</p><p>anatomical study of apple tree leaves was developed at the</p><p>plant anatomy laboratory, where some fixed leaf fragments</p><p>were sectioned into smaller parts and dehydrated in ethyl al-</p><p>cohol series. These were embedded inmethacrylate glycol and</p><p>cross-sectioned in 5 μm thickness, in a Leica 1400Microtome</p><p>(Karl Zeiss), distended over slides and stained with an aque-</p><p>ous solution of 1.0% Astra blue and 0.0125% basic fuchsin</p><p>(Alves de Brito and Alquini 1996). Photomicrographs of the</p><p>sections were taken under a DMR microscope, using bright-</p><p>field microscopy. Based on the photomicrographs of the cross</p><p>sections, five measurements of thickness of the abaxial and</p><p>adaxial epidermis as well as the palisade and spongy paren-</p><p>chyma were made for each repetition.</p><p>Fixed fragments of the leaves of each treatment and</p><p>repetition were sectioned and prepared following the method</p><p>of Arnott (1959) for stomatal characterization. Leaf fragments</p><p>were arranged on slides with the abaxial surface facing up-</p><p>wards and photomicrographs of the epidermal cells and sto-</p><p>mata were taken through bright-field microscopy in a DMR</p><p>microscope. Stomata (S) and epidermal cells (E) were count-</p><p>ed. The stomatal index (I) was calculated by the methodology</p><p>described in Sack and Buckley (2016).</p><p>Twenty (20) leaves exposed to solar radiation were collect-</p><p>ed in the middle layer of the canopy for analysis of leaf chlo-</p><p>rophyll content. From these leaves, four repetitions with five</p><p>discs were collected for each treatment. The discs were imme-</p><p>diately placed on dark glass wrapped in aluminum foil to</p><p>prevent light from entering. We added 20 ml of 96% PA</p><p>alcohol to each glass. The samples were stored for 1 week in</p><p>Int J Biometeorol</p><p>Author's personal copy</p><p>a cool and dark place. After this period, the optical density of</p><p>the liquid material was read on a spectrophotometer at 649 and</p><p>665 nm. The concentration of chlorophyll a, b, and total in the</p><p>reading solutions was determined by the methodology de-</p><p>scribed in Wintermans and Demots (1965).</p><p>Eight plants (four per treatment) were selected for the de-</p><p>termination of photosynthetic rate. A healthy and fully ex-</p><p>panded leaf was marked in the middle layer of the canopy of</p><p>each plant to determine net photosynthetic rate (P) using a LI-</p><p>COR 6400 infrared gas analyzer. The measurements were</p><p>performed with an artificial light source (LI-6400-02B</p><p>LED). The leaf was subjected to fluxes of incident PAR of</p><p>0, 100, 200, 400, 600, 800, and 1500 μmol m−2 s−1. To ex-</p><p>press the P in response to PAR, a hyperbolic polynomial</p><p>function was adjusted by: P ¼ aþ Pmax�PAR</p><p>bþPAR</p><p>� �</p><p>, in which a</p><p>is dark respiration, Pmax is maximum net photosynthetic rate</p><p>and b is the adjustment coefficient of the equation. The light</p><p>compensation point (Г) corresponded to the PAR value in</p><p>which P is equal to zero. The apparent quantum efficiency</p><p>(Фa) was estimated by adjusting a linear equation in a range</p><p>in which the variation of P in function PAR was linear.</p><p>Statistical analysis</p><p>The means of all the variables evaluated in each treatment</p><p>were subjected to Bartlett, Kolmogorov-Smirnov and</p><p>Durbin-Watson tests to verify whether they showed homoge-</p><p>neity of variance, normal distribution, and data independence,</p><p>respectively. These conditions were confirmed, and regression</p><p>analysis was carried out for solar radiation, radiation intercep-</p><p>tion efficiency, extinction coefficient, LAI, and photosynthet-</p><p>ic rate. Analysis of variance was performed for growth and</p><p>anatomical and physiological variables to quantify the effect</p><p>of the growth environment (treatment). When analysis of var-</p><p>iance was significant (F test), it means separation was per-</p><p>formed by the Tukey test. Statistical significance was tested</p><p>at 5%.</p><p>Results</p><p>Growth</p><p>Plant height under netting was greater than in the uncovered</p><p>control in all three production cycles, both for “Royal Gala”</p><p>and for “Fuji Suprema” (Table 1). Number of branches dif-</p><p>fered only in one cycle for both cultivars—there were fewer</p><p>branches under netting than in the control. Number of</p><p>branches in each plant or treatment may have varied according</p><p>to pruning intensity because the pruning of the orchard in the</p><p>last year was carried out by a different team. Trunk diameter</p><p>did not differ between treatments for “Royal Gala.” However,</p><p>trunk diameter was larger under netting than in the control in</p><p>two cycles for “Fuji Suprema.”</p><p>Number of leaves, leaf area index (LAI), and shape index</p><p>(SI) did not differ between treatments (Table 2). Leaf area</p><p>index per plant varied from 2.6 in the first cycle to 3.3 in the</p><p>second cycle. Leaf area index data reported in the literature for</p><p>apple trees indicate values varying between 1.3 at stages close</p><p>to flowering and 3.0 during fruit maturation (Wünsche and</p><p>Lakso 2000).</p><p>Solar radiation</p><p>Incident PAR on the orchard under anti-hail netting was on</p><p>average 32% lower than in the uncovered control (Bosco et al.</p><p>2018). Thus, the black anti-hail net used in these evaluations</p><p>has an average transmissivity of 68% for PAR. The reduction</p><p>of PAR by the net influenced the components of radiation</p><p>balance. Through linear regression analysis, we found that</p><p>PAR intercepted, absorbed, transmitted, and reflected by the</p><p>canopy under netting decreased by 33%, 31%, 32%, and 46%,</p><p>respectively, in comparison to uncovered treatment (Fig. 1).</p><p>The reflected fraction was the smallest component of the bal-</p><p>ance of PAR in the apple orchard under netting (Fig. 1d).</p><p>The average reflectance of the apple tree was 3% in both</p><p>treatments (Fig. 2).</p><p>The ratio between absorbed and intercepted PAR was 0.98</p><p>by the uncovered crop and 0.99 under netting. In other words,</p><p>virtually all intercepted PAR was absorbed by the canopies in</p><p>both treatments.</p><p>PAR interception efficiency</p><p>PAR interception efficiency increased considerably from the</p><p>beginning of sprouting to the beginning of fruit formation, a</p><p>period in which there is intense increase in leaf size and num-</p><p>ber. The pattern of evolution of interception efficiency was</p><p>similar between the treatments.</p><p>The evolution of PAR interception efficiency as a function</p><p>of crop LAI was fit to a sigmoidal model in both treatments</p><p>(Fig. 3). This model describes the evolution of interception</p><p>efficiency, with the increase of LAI to a point of stabilization.</p><p>Maximum PAR interception efficiency occurred when LAI</p><p>was 1.5 under netting and 2.5 in the control. In the production</p><p>three cycles, we found that when plants showed low LAI, the</p><p>slope of the interception efficiency curve was greater under</p><p>netting than in the control. This indicates a faster increase in</p><p>radiation interception in the treatment with a lower incidence</p><p>of PAR, but with a greater amount of diffused light (Fig. 3).</p><p>Extinction coefficient (k)</p><p>For the same LAI, there was higher radiation extinction in the</p><p>orchard under netting than in the control. In the three</p><p>Int J Biometeorol</p><p>Author's personal copy</p><p>production cycles, we found a k of 1.09 under netting and 0.76</p><p>in the control (Fig. 4).</p><p>Photosynthetic rate, chlorophyll content, and leaf</p><p>anatomy</p><p>The net photosynthetic rate did not differ between treatments</p><p>for both “Royal Gala” and “Fuji Suprema,” in spite of the</p><p>different radiation fluxes (Fig. 5).</p><p>Maximum net photosynthetic rate, light compensation</p><p>point, dark respiration, and apparent quantum efficiency did</p><p>not differ between treatments at both stages (Table 3).</p><p>Therefore, even though netting reduced global solar radiation,</p><p>it was not limiting for photosynthesis. In general, when the</p><p>saturation point is not reached, the anti-hail net reduces pho-</p><p>tosynthesis. However, when the plant reaches the light satura-</p><p>tion point, the net can reduce photoinhibition problems, espe-</p><p>cially in periods with high air temperature (Lebese et al. 2011;</p><p>Mupambi et al. 2018).</p><p>Chlorophyll a and b contents in “Royal Gala” leaves did</p><p>not differ between treatments at stage 72. However, leaf chlo-</p><p>rophyll a and b contents in the control were higher than under</p><p>netting at stage 85 (Table 4). There was no difference in chlo-</p><p>rophyll content between the treatments for “Fuji Suprema”</p><p>(Table 4), which corroborates the results of Amarante et al.</p><p>(2009). These authors reported that the increase in chlorophyll</p><p>content in response to shading depends on the cultivar.</p><p>At stages 72 and 85 together, the overall average</p><p>thickness of the adaxial and abaxial epidermis was</p><p>18.1 μm and 14.2 μm for “Royal Gala,” respectively,</p><p>while these measurements were 16.9 μm and 13.4 μm</p><p>for “Fuji Suprema,” respectively (Table 5). Therefore,</p><p>the thickness of the adaxial epidermis is greater than that</p><p>of the abaxial epidermis.</p><p>There were no significant differences in adaxial and abaxial</p><p>epidermis thickness in “Fuji Suprema” and “Royal Gala” be-</p><p>tween the treatments (Table 5; Fig. 6).</p><p>At both stages (72 and 85), the overall average thickness of</p><p>the palisade and spongy parenchyma was 107.5 μm and</p><p>133 μm for “Royal Gala,” respectively. The thickness of the</p><p>parenchyma did not differ between treatments at stage 72. At</p><p>stage 85, leaves under netting showed thinner palisade</p><p>(23.7%) and spongy (21.6%) parenchyma compared to those</p><p>in the control, influencing total thickness. The thickness of the</p><p>palisade and spongy parenchyma of “Fuji Suprema” did not</p><p>differ between treatments, showing overall averages of</p><p>105 μm (palisade) and 133 μm (spongy) (Table 5).</p><p>The thickness of the palisade parenchyma in relation to the</p><p>spongy parenchyma (PP/SP) varied between 0.7 and 0.9, in-</p><p>dicating that PP is generally 10 to 30% thinner than SP.</p><p>However, this ratio did not differ between treatments</p><p>(Table 5).</p><p>The existence of two palisade parenchyma cell layers was</p><p>found in both cultivars (Fig. 6). Studies on apple tree leaves</p><p>have reported up to three layers (Talwara et al. 2013; Bhusal</p><p>et al. 2018).</p><p>Table 1 Plant height, number of</p><p>branches and trunk diameter of</p><p>‘Royal Gala’ and ‘Fuji Suprema’</p><p>under anti-hail netting (AHN) and</p><p>in the uncovered control (C)</p><p>Cycle Plant height (m) Number of branches Trunk diameter (cm)</p><p>AHN C Means AHN C Means AHN C Means</p><p>“Royal Gala”</p><p>1 3.0 a 2.5 b 2.8 20 19 20 4.5 4.5 4.5</p><p>2 3.0 a 2.7 b 2.9 17 18 18 4.6 4.6 4.6</p><p>3 3.0 a 2.7 b 2.9 15 b 18 a 17 5.0 5.1 5.1</p><p>“Fuji Suprema”</p><p>1 3.2 a 2.6 b 2.9 25 25 25 5.8 5.6 5.7</p><p>2 3.2 a 2.4 b 2.8 20 21 21 6.7 a 5.7 b 6.2</p><p>3 3.1 a 2.4 b 2.8 19 b 23 a 21 7.0 a 5.9 b 6.5</p><p>Different lowercase letters on the row indicate a significant difference between treatments by the Tukey test</p><p>(p</p><p>are part of the plant adaptive strategy. In order to reduce water</p><p>losses, they retain air moisture around the leaf boundary layer</p><p>(Jackson 2003), in addition to repelling pests and pathogens.</p><p>Stomata were present only in the leaf abaxial epidermis,</p><p>corroborating with the description made by Jackson (2003)</p><p>and Hunsche et al. (2010). The stomatal index did not differ</p><p>between treatments, ranging from 11.7 to 14.6 for “Royal</p><p>Gala” and 12.1 to 13.9 for “Fuji Suprema” (Table 5). The</p><p>stomatal index is considered constant within the same species,</p><p>but variable when plants are submitted to different growth</p><p>conditions (Pompelli et al. 2010). This is another indication</p><p>that the photosynthetic apparatus of the plants under netting</p><p>did not differ from those in the control.</p><p>Discussion</p><p>The use of anti-hail nets in apple orchards in southern Brazil is</p><p>part of a management practice that aims to reduce or eliminate</p><p>the risk of damage to the orchard caused by hailstorms.</p><p>Consequently, our results showed that the use of anti-hail</p><p>netting can change PAR availability, growth, as well as certain</p><p>anatomical and physiological characteristics of apple trees.</p><p>Anti-hail netting enhances plant height and trunk diameter</p><p>but does not affect the final number of leaves, LAI, and SI.</p><p>There are several factors that can impact vegetative growth</p><p>under netting, but the results are not always consistent among</p><p>scientific studies (Mupambi et al. 2018). Genetic (Lordan</p><p>et al. 2018) and shade avoidance (De Wit et al. 2016) are</p><p>characteristics that may change vegetative growth, vigor,</p><p>and apical dominance in trees. In our study, trunk diameter</p><p>of “Fuji Suprema” was larger under netting. This is most like-</p><p>ly because the solar radiation in the uncovered control treat-</p><p>ment did not promote shade avoidance or changes in photo-</p><p>receptors as shown in Solomakhin and Blanke (2008).</p><p>Therefore, photosynthesis occurred in all parts of the plant</p><p>containing leaves, not just in the outer shoots directly exposed</p><p>to solar radiation (Zervoudakis et al. 2012).</p><p>The use of anti-hail netting did not change leaf shape, rep-</p><p>resented by SI, although there were differences between the</p><p>cultivars. Leaf shape and size can influence the amount of</p><p>radiation available to the plant, which is crucial for flowering.</p><p>The thinning of apple trees to obtain the ideal leaf-fruit ratio,</p><p>for example, is a common practice in the pursuit of better fruit</p><p>color and size. There is clearly an important relationship be-</p><p>tween leaves and fruits for apple tree, and we understand that</p><p>quantifying SI and LAI was more important than plant height</p><p>and trunk diameter, because they play an essential role in</p><p>flowering, fruit size and can be an early indicator of other</p><p>important characteristics in terms of plant management</p><p>(Khan et al. 2014; Migicovsky et al. 2018).</p><p>The average reflectance of the apple trees was 3% in both</p><p>treatments and less than that shown in the literature. According</p><p>to Larcher (2003), leaves generally reflect 6 to 10% of the radi-</p><p>ation within the visible radiation spectrum, although this can</p><p>reach 24% in apple tree leaves (Landsberg et al. 1973). The</p><p>mechanisms that influence the amount of energy reflected by</p><p>the leaves are more related to leaf maturation or age, leaf position</p><p>and size, solar declination, pigment content, cell structure, and</p><p>water within the cells (Monsi and Saeki 2005; Larcher 2003)</p><p>than to the reduction of solar radiation. The results showed that</p><p>anti-hail netting can reduce intercepted, absorbed, transmitted,</p><p>and reflected PAR. Moreover, anti-hail nets can improve</p><p>Fig. 4 Extinction coefficient of photosynthetically active solar radiation</p><p>(coefficient b of linear equation) as a function of leaf area index (LAI) and</p><p>radiation interception efficiency (Ln(1-Eint)) in “Royal Gala” orchard</p><p>under anti-hail netting (AHN; black circle) and in the control (C; white</p><p>circle). F test of the analysis of variance of the regression and coefficients</p><p>was significant (p</p><p>al. 2010;</p><p>Fochesatto et al. 2016). The results of the treatment with anti-</p><p>hail netting were, on average, greater than those of the control.</p><p>This can be attributed to the smaller or equivalent amount of</p><p>diffuse radiation during most of the experimental period.</p><p>Even though netting reduced global solar radiation, it was</p><p>not limiting for photosynthesis because PAR interception ef-</p><p>ficiency was similar between treatments. Our results are com-</p><p>parable to those obtained by Stampar et al. (2002) and</p><p>Solomakhin and Blanke (2008), who did not find significant</p><p>differences in the photosynthetic rate of apple plants grown</p><p>under black netting and without cover. However, they differ</p><p>from those of Amarante et al. (2007) and Amarante et al.</p><p>(2009), who found a high photosynthetic rate in plants grown</p><p>without cover. In general, when the light saturation point is</p><p>not reached, the anti-hail net can reduce photosynthesis, and</p><p>when the plant reaches this point, the net can reduce</p><p>photoinhibition, especially in periods with high air tempera-</p><p>ture (Lebese et al. 2011; Mupambi et al. 2018). Our results</p><p>demonstrated that light saturation point was reached, on aver-</p><p>age, with Pmax around 21 μmol m−2 s−1, without significant</p><p>difference between treatments.</p><p>Table 4 Chlorophyll content (mg cm−2) in “Royal Gala” and “Fuji</p><p>Suprema” leaves under anti-hail netting and in the control at the different</p><p>stages of fruit development (stage 72) and fruit maturation (stage 85)</p><p>Variable Stage 72 Stage 85</p><p>AHN C Means AHN C Means</p><p>“Royal Gala”</p><p>Chlorophyll a 0.14 0.16 0.15 0.19 b 0.24 a 0.22</p><p>Chlorophyll b 0.05 0.05 0.05 0.07 b 0.09 a 0.08</p><p>Chlorophyll a + b 0.19 0.21 0.20 0.26 b 0.33 a 0.30</p><p>Chlorophyll a:b 2.80 3.20 3.00 2.71 2.67 2.69</p><p>“Fuji Suprema”</p><p>Chlorophyll a 0.17 0.17 0.17 0.21 0.22 0.22</p><p>Chlorophyll b 0.06 0.06 0.06 0.07 0.07 0.07</p><p>Chlorophyll a + b 0.23 0.23 0.23 0.28 0.29 0.29</p><p>Chlorophyll a:b 2.83 2.83 2.83 3.00 3.10 3.05</p><p>Different lowercase letters in the same row indicate a significant differ-</p><p>ence between treatments by the Tukey test (p</p><p>Pesq Agropec Bras</p><p>42:925–931. https://doi.org/10.1590/S0100-204X2007000700003</p><p>Amarante CVT, Steffens CA, Miqueloto A et al (2009) Disponibilidade</p><p>de luz em macieiras ‘Fuji’ cobertas com telas antigranizo e seus</p><p>efeitos sobre a fotossíntese, o rendimento e a qualidade dos frutos.</p><p>Rev Bras Frutic 31:664–670. https://doi.org/10.1590/S0100-</p><p>29452009000300007</p><p>Arnott HJ (1959) Leaf clearings. 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Braz Arch Biol</p><p>Techno l 55 :89–95 . h t t p s : / / do i . o rg /10 .1590 /S1516 -</p><p>89132012000100011</p><p>Int J Biometeorol</p><p>Author's personal copy</p><p>View publication stats</p><p>https://doi.org/10.2307/2402570</p><p>https://doi.org/10.2307/2402570</p><p>https://doi.org/10.1590/S1519-69842010000500025</p><p>https://doi.org/10.1590/S1519-69842010000500025</p><p>https://doi.org/10.1590/S0100-29452011000500006</p><p>https://doi.org/10.1104/pp.16.00476</p><p>https://doi.org/10.1104/pp.16.00476</p><p>https://doi.org/10.1080/21580103.2012.673744</p><p>https://doi.org/10.1080/21580103.2012.673744</p><p>https://www.embrapa.br/buscae-ublicacoes/-publicacao/1094003/sistemarasileiroelassificacaoe-olos</p><p>https://www.embrapa.br/buscae-ublicacoes/-publicacao/1094003/sistemarasileiroelassificacaoe-olos</p><p>https://www.embrapa.br/buscae-ublicacoes/-publicacao/1094003/sistemarasileiroelassificacaoe-olos</p><p>https://doi.org/10.1007/s10725-008-9302-7</p><p>https://doi.org/10.1007/s10725-008-9302-7</p><p>https://doi.org/10.1111/j.1744-7348.2009.00372.x</p><p>https://doi.org/10.1111/j.1744-7348.2009.00372.x</p><p>https://doi.org/10.1016/j.scienta.2013.08.025</p><p>https://doi.org/10.1016/j.biosystemseng.2012.10.008</p><p>https://doi.org/10.1016/j.biosystemseng.2012.10.008</p><p>https://doi.org/10.1051/agro:19890501</p><p>https://doi.org/10.1016/j.scienta.2013.05.036</p><p>https://doi.org/10.1016/0926-6585(65)90170-6</p><p>https://doi.org/10.1016/0926-6585(65)90170-6</p><p>https://doi.org/10.21273/HORTSCI.35.7.1202</p><p>https://doi.org/10.21273/HORTSCI.35.7.1202</p><p>https://doi.org/10.1016/j.pnsc.2009.10.001</p><p>https://doi.org/10.1016/j.pnsc.2009.10.001</p><p>https://doi.org/10.1590/S1516-89132012000100011</p><p>https://doi.org/10.1590/S1516-89132012000100011</p><p>https://www.researchgate.net/publication/343617599</p><p>Solar radiation effects on growth, anatomy, and physiology of apple trees in a temperate climate of Brazil</p><p>Abstract</p><p>Introduction</p><p>Material and methods</p><p>Location and characterization of the experiments</p><p>Growth variables</p><p>Climate variables</p><p>Anatomical and physiological variables</p><p>Statistical analysis</p><p>Results</p><p>Growth</p><p>Solar radiation</p><p>PAR interception efficiency</p><p>Extinction coefficient (k)</p><p>Photosynthetic rate, chlorophyll content, and leaf anatomy</p><p>Discussion</p><p>References</p>