Impaginato 431 Adv. Hort. Sci., 2020 34(4): 431­440 DOI: 10.13128/ahsc­9764 Climatic and physiological parameters related to the progress and prediction of apple sunburn damage in a neotropical climate V. Severino 1 (*), M. Arias­Sibillotte 1, S. Dogliotti 1, E. Frins 2, J. Gonzalez­ Talice 1, J.A. Yuri 3 1 Facultad de Agronomía, Universidad de la República (UDELAR), Garzón 780, CP 12900 Montevideo, Uruguay. 2 Facultad de Ingeniería, UDELAR, Julio Herrera y Reissig 565, CP 11300 Montevideo, Uruguay. 3 Centro de Pomáceas, Universidad de Talca, 1 Poniente 1141, Talca, Chile. Key words: hydric potential, Malus domestica, proline, spectroradiometry, reflectance indices. Abstract: Apple production in neotropical climate is affected by sunburn and the high interannual variability in meteorological conditions makes prediction and management of damage difficult. Non­destructive methods associated with physiological variables are keys to monitoring but their development is still incipient. In our study occurrence of sunburn, meteorological conditions and physiological parameters was monitored throughout four crop cycles. Fruit visu­ al assessment and reflectance measures in field, as well as, pigments, proline and hydric potential in laboratory, were accomplished. The results show that the availability of water in the soil was more related to the evolution of sun­ burn than air temperature. Plant Senescence Reflectance Index (non­destruc­ tive predictor) discriminated between healthy and damaged fruits and fruit hydric potential and proline content were good indicators of sunburn, although such variables are determined when damage has already occurred. Our results suggest focusing future research on the water balance of the system and on the physiological indicators of osmotic stress as a way to predict damage. 1. Introduction Fruit sunburn has been reported since 1870, and although several studies have touched upon the matter since the early 20th century (Racsko and Schrader, 2012), it is still a cause of significant economic loss in apple production (Reig et al., 2019). Although some expressions of damage may be easily perceived in the field, on occasions the symptoms of sunscald are imperceptible and only appear after months of cold stor­ age (usually after three months), which makes the damage difficult to control (Yuri et al., 2000). Symptoms appear as brown stains on the fruits’ (*) Corresponding author: vseverin@fagro.edu.uy Citation: SEVERINO V., ARIAS­SIBILLOTTE M., DOGLIOTTI S., FRINS E., GONZALEZ­TALICE J., YURI J.A., 2020 ­ Climatic and physiological parameters related to the progress and prediction of apple sunburn damage in a neotropical climate. ­ Adv. Hort. Sci., 34(4): 431­440 Copyright: © 2020 Severino V., Arias­Sibillotte M., Dogliotti S., Frins E., Gonzalez­Talice J., Yuri J.A. This is an open access, peer reviewed article published by Firenze University Press (http://www.fupress.net/index.php/ahs/) and distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Competing Interests: The authors declare no competing interests. Received for publication 18 September 2020 Accepted for publication 30 November 2020 AHS Advances in Horticultural Science http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ Adv. Hort. Sci., 2020 34(4): 431­440 432 exterior, being Granny Smith the most susceptible reported cultivar (Felicetti and Schrader 2008; Hernandez et al., 2014). There is consensus that sunburn is related to the combination of high temperature and irradiance (UV­ B range is thought to be essential) during the fruit growth period (Yuri et al., 2000, 2010; Racsko and Schrader, 2012; Torres et al., 2013; Darbyshire et al., 2015; Torres et al., 2016 a, b), however, there is a limited understanding of the physiological aspects of the changes in the fruit’s internal quality (Racsko and Schrader, 2012) and generation of sun­related physi­ ological disorders in fruit. This problem requires fur­ ther research into the environmental and physiologi­ cal processes that occur prior to and during sun injury development and more importantly, biochemi­ cal changes that may contribute to resistance to envi­ ronmental conditions that cause sun­related disor­ ders in fruit (Morales­Quintana et al., 2020). Stress adaptation mechanisms of plants, such as chlorophyll reduction (Ballester et al., 2017), dissipa­ tion of excitation energy, increase of solutes of low molecular weight (Wen and Moriguchi, 2015) and changes in pigmentation (Merzlyak et al., 2003) have been previously studied in relation to sunburn in apple fruit. Based on these results, in the last decade, work has been done on the development of non­ destructive methods to predict and detect sun dam­ age based on the composition and location of skin pigments as well as the optical properties of the underlying fruit tissue (Solovchenko et al., 2010; Torres et al., 2016 a, b). In this direction, sunburn has been related to: fruit reflectance values in the visible and near­infrared (NIR) spectra (Solovchenko et al., 2010; Torres et al., 2016 a), crop water stress index and chlorophyll fluorescence (Torres et al., 2013, 2016 b). About temperature effect, it has been reported that increases in fruit temperature above a certain limit may cause enzymes denaturation and protein coagulation, leading to tissue damage (Yuri et al., 2010). Studies performed in cv. Fuji fruits showed a highly susceptible caused by excessive heat and did not sustain damage when exposed to UV radiation only (Yuri et al., 2000). Air temperatures of 38­42°C increases the heat­shock proteins induction (Woolf and Ferguson, 2000) and sunburn symptomatology appear with fruit temperature of 46°C and higher (Racsko and Schrader, 2012). The water status of the plant and fruit has also been related to sunburn, although fewer studies have focused attention on this aspect. Recent work in Chile analyses the association of acclimation events with fruit water relations and osmoregulation occur­ ring in sun­exposed fruit tissue (Torres et al., 2013). Studies in Japan and South Africa discuss the effect of foliar ABA on antioxidant levels and the incidence of sunburn with variable results (Mupambi et al., 2018). Antioxidant system plays a crucial part in the elimina­ tion of free radicals under stress conditions (Chen and Murata, 2002). Compatible solutes such as pro­ line, betaine and polyols are accumulated in response to abiotic stress (Suzuki, 2015). These solutes affect the osmotic balance and the mem­ brane stability and have been proven to maintain tur­ gor pressure, cellular volume and electrolyte concen­ tration (Roberts, 2005). Proline is known to be a sta­ bilizer of sub­cellular structures (Kautz et al., 2015) and although many studies have established a con­ nection between proline and antioxidant activity in apple plant leaves and xylem under abiotic stress (Šircelj et al., 2005; Nemeskéri et al., 2015; Afonso et al., 2017) no relationship between proline and sun­ burn has yet been reported. Most of the existing research has been carried out in latitudes similar to that of the present study but in more arid climates such as, Chile, Australia, and South Africa (southern hemisphere) or Spain, Turkey, and Washington State (north hemisphere), however, few studies have addressed sunburn in humid growth­season conditions like in Eastern New York State (Reig et al., 2019). The region where the study was conducted, defined as neo­tropical (Bernardi et al., 2016), has been considered restrictive for apple quality in relation to sunburn aspects (FAO­MGAP, 2013) due to the occurrence of high temperatures during the fruit growth period. Changes in El Niño evolution after 1976 may have played a role in alter­ ing the relationship between temperature extreme events in Uruguay and the atmospheric circulation (Renom et al., 2011). The average maximum temper­ atures of the summer period show a high iner­annual variability (71­86%) and lower variability in the medi­ um (10 years) or long­term (>30 years) components, 23% and 6% respectively (Tiscornia et al., 2016), so it is expected that the climate in the region will contin­ ue to be favorable to the occurrence of burning. The aims of this work were to study apple sunburn progress and its relation to meteorological variables in a neo­tropical climate, and to establish correla­ tions between fruit physiological parameters and reflectance index. Severino et al. ‐ Apple sunburn in a neotropical climate 433 2. Materials and Methods Plant material The experiment was conducted during the 2012/2013 to 2015/2016 crop cycles (hereinafter, cycles 1 to 4), on a Granny Smith/M7 plantation established in 2003. The crop is located in Uruguay (southeastern of South America) with the coordi­ nates of 34°38’18’’ S and 56°40’06’’ W and 45 meters above sea level.The climate of this regional ecotone is classified by Bernardi et al. (2016) as neo­tropical. Crop had planting distance of 4x1.5 m, rows arranged from N to S and trained in central leader system. The soil types are mainly Argiudolls and Hapluderts and a drip irrigation system with a maximum daily watering capacity of 4.5 mm is installed. Three fruits per tree from ten trees per row, in a total of five rows were selected between 40 and 50 days after full bloom (DAFB) in the four evaluated cycles. Trees and rows were randomly marked, and 150 fruits exposed to radiation were classified by visu­ al assessment of different external initial conditions: A) 50 fruits with no visible sunburn (HF=healthy fruits); B) 50 fruits with red color (RF=red fruits); C) 50 fruits with an early degree of sunburn (SBF=sunburn fruits) [sunburn browning, according to the classifica­ tion of Racsko and Schrader (2012)] as indicated in fig­ ure 1. The flowering dates for cycles 1 to 4 were, September 27 (cycle 1), October 28, 3 and 14, to cycles 2, 3 and 4 respectively. The exposed side of each fruit was defined as the one directly exposed to sunlight, facing the space between rows, and the internal side as the one facing the trunk, with no direct exposition to solar radiation. Field tests The sunburn progress in each marked fruit was assessed by observation. Its frequency varied between 1 week and 1 month, with weekly observa­ tions predominating. In each observation, fruits were reclassified according to the categories mentioned above (HF, RF, SBF). Reflectance measurements of the exposed side of ten fruits of each condition were recorded with an ILT 950 spectroradiometer (International Light Technologies, USA) at 91, 99 and 154 DAFB in cycle 4. Percent reflectance was calculat­ ed based on a dark spectrum and a reference spec­ trum from a white reference standard. The content of Chlorophyll (CHL), Anthocyanins (ANT), Carotenoids (CAR), Flavonoids (FLA) and Senescence indexes were calculated based on the reflectance measurements. Chlorophyll was calculated according to the following indexes: CHL1, CHL2 (Mullan, 2013) RARSa, RARSb, PSSRa, MSR, CL1, CL2 (Solovchenko et al., 2010) and Anthocyanins, according to the Anthocyanin Reflectance Index (ARI) (Solovchenko et al., 2010). Carotenoids were calculated according to the following indexes: RARSc (Mullan, 2013), CRI1 and CRI2 (Solovchenko et al., 2010) and Flavonoids according to the Flavonoid Reflectance Index (FRI) (Solovchenko et al. , 2010). The Normalized Phaeophytization Index (NPQI), the Pigment Simple Ratio (PSR), the Normalized Difference Pigment Index (NDPI), the Structural Independent Pigment Index (SIPI) (Solovchenko et al., 2010) and the Plant Senescence Reflectance Indexes (PSRI480 PSRI500) (Mullan, 2013) were also calculated (Table 1). An automated meteorology station located 1900 m from the crop recorded the maximum temperature (Tmax) (°C) and rainfall (RF) (mm) variables in the fruit’s growth period. The soil water balance (SWB) for each growth cycle was calculated. Plot characteristics and local and regional meteorology stations were used. Local variables used were irrigation (mm), daily rain­ fall (mm), root deep (m), phenological stages (days) and soil texture. The ETo (reference evapotranspira­ tion) (mm) was recorded at the meteorology station of INIA Las Brujas using Penman­Monteith (Allen et al., 1998). Crop coefficient (Kc) was adjusted to reflect the wetting frequency of soil surface and local climatic conditions according to Allen et al. (2006): Kcini = Kcini (*) + (I­10) [kcini (**) ­ Kcini (*)] (40­10) Kcmid = Kcmid (Tab) + [0.04 (u2 ­ 2) ­ 0.004 (RHmin ­ 45)] ( h ) 0.3 3 Where: Kcini(*): value for Kc ini from figure 29 in Allen et al. (2006). Kcini(**): value for Kc ini from figure 30 in Allen et al. Fig. 1 ­ Examples of fruit categories. RF=red fruits, HF=healthy fruits, SBF=sunburn fruits. Adv. Hort. Sci., 2020 34(4): 431­440 434 (2006). I: average infiltration depth (mm). Kc mid (Tab): value for Kc mid taken from Table 12 in Allen et al. (2006) apples, cherries and pears crops with active ground cover without frosts. u2: mean value for daily wind speed at 2 m height over grass during the mid­season growth stage (m s­ 1), for 1 m s­1