Environmental Stresses that Cause Sunburn of Apple

Transcription

1 Environmental Stresses that Cause Sunburn of Apple Larry Schrader, Jianguang Zhang 1 and Jianshe Sun Washington State University Tree Fruit Research & Extension Center 1100 N. Western Avenue, Wenatchee, WA, USA, Former Visiting Professor; Current Address: College of Horticulture, Agricultural University of Hebei, Baoding City, , Hebei Province, China Keywords: [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.], Malus x domestica, thermal death, fruit surface temperature, UV-B radiation, air temperature, solar irradiation, wind velocity, relative humidity Abstract Sunburn of apple costs growers millions of dollars in economic losses annually. We have identified two types of sunburn in apple and their causes. One type (sunburn necrosis) is caused by thermal death of epidermal and subepidermal cells when the peel reaches 52 ± 1 C. A necrotic spot appears on the sun-exposed side of the fruit. The second type (sunburn browning) is sublethal and results in a yellow, bronze, or brown spot on the sun-exposed side of the fruit. When we experimentally induced sunburn in attached fruit of several cultivars, the fruit surface temperatures (FST) at which sunburn browning occurred varied between 46 C and 49 C depending on cultivar. Maximum FST exceeded C on 22 days in 00 and on 23 days in 01 in our orchard. Sunlight was required for sunburn browning. Protecting fruit from UV-B radiation usually prevented sunburn browning. The FST reached a maximum between 14 and 16 hours between June 15 and September, and was affected by several meteorological parameters. Maximum FST was highly correlated with maximum daily air temperature (r = 0.90**). Maximum FST also was highly correlated (r = 0.88**) with the mean of maximum hourly air temperatures between 1100 and 1700 hours. Mean solar radiation between 1100 and 1700 hours also was highly correlated (r = 0.65**) with maximum FST. Mean wind velocity and mean relative humidity (RH) between 1100 and 1700 hours were inversely correlated (r = _ 0.24** and _ 0.66**, respectively) with maximum FST. During 00 and 01, we found that FST never exceeded C on days when maximum air temperature was below C. When maximum air temperature exceeded C, maximum FST almost always exceeded C. When maximum air temperature was between and C, solar radiation, wind velocity and RH also were important determinants of maximum FST. INTRODUCTION Sunburn causes important economic losses in a large number of fruits and vegetables (Barber and Sharpe, 1971). Sunburn and sunscald have been used interchangeably in the literature, and this has caused confusion. Jones and Aldwinckle (1990) defined sunburn as damage to fruit by exposure to solar radiation, and defined sunscald as injury to the bark and underlying tissues caused by freezing. We therefore use the term sunburn to describe this fruit disorder. In Washington State and several other regions of the world, sunburn causes serious economic losses in apples [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.] (Bergh et al., 1980; Simpson et al., 1988; Warner, 1997; Schrader et al., 01; Wünsche et al., 01). Brooks and Fisher (1926) reported fruit surface temperatures on the sun-exposed side of fully exposed apples were up to 14 C above air temperatures and concluded injury to apples resulted from heat, and not from other radiation effects. In contrast, Rabinowitch et al. (1974) reported that in addition to heat, visible light was necessary for development of typical sunburn symptoms in tomatoes. They concluded that sunburn is caused by the concurrent action of two external factors, heat and light. Rabinowitch et al. (1986) later reported air temperature thresholds of 38 to C and.5 to 42.5 C were essential for development of sunburn injury in cucumbers and peppers, respectively. Light in the visible spectrum was reported to be essential, as no bleaching Proc. XXVI IHC Environmental Stress Eds. K.K. Tanino et al. Acta Hort. 618, ISHS 03 Publication supported by Can. Int. Dev. Agency (CIDA) 397

2 occurred in the dark. They suggested that sunburn occurs when photosynthesis is disturbed by excessive heat, so that light energy is redirected into damaging photodynamic processes. To better understand sunburn of apple, we examined the effects of sunlight and fruit surface temperature (FST) on incidence of sunburn damage (Schrader et al., 01). We described two types of sunburn in apple. We called the first type sunburn necrosis and the second type sunburn browning. We established that sunburn necrosis was induced when the FST reached 52 ± 1 C. Electrolyte leakage increased significantly with necrosis, indicating that membrane integrity is lost during thermal death. Sunburn browning is sublethal and results in a yellow, bronze, or brown spot on the sun-exposed side of the fruit. It occurred when the FST reached 46 to 49 C in the presence of sunlight. We concluded that FST is critical to development of both types of sunburn in apples, and that solar irradiation also is required for sunburn browning (Schrader et al., 01). In order to predict the incidence of fruit sunburn, it is essential to document the comprehensive conditions that cause this disorder. The present study was conducted to elucidate the correlations between maximum FST and several meteorological factors so as to establish a solid basis for understanding the underlying causes of apple fruit sunburn and to develop a predictive computer model. In this paper, we report the effects of air temperature, solar radiation, wind velocity and relative humidity on FST and the incidence of sunburn in apple. MATERIALS AND METHODS Fuji BC-2 apple trees on M.26 rootstocks (6 years old in 00) were used for fruit surface temperature determinations. Trees were planted with a 2 m within-row and 4.5 m between-row spacing with north to south row orientation in a randomized complete block with four replications. Trees were trained to a central leader with support, and irrigated with microsprinklers. Maximum FST was determined by attaching copper-constantan thermocouples (0.4 mm diameter) with a small piece of Band-Aid tape to a fruit. Thermocouples were placed on the shoulders of the southwest side of well-exposed fruits, and were moved as fruits matured to obtain maximum FST for those fruits. In other studies (data not shown), we established that FST determinations were similar when thermocouples were taped to the fruit surface or inserted under the peel. The tape method was preferred for long-term measurements, as the thermocouples could be moved as fruit changed position and peel was not damaged to allow entry of pathogens. We also compared infrared thermometry and thermocouples and obtained similar FST. Thermocouples were connected to a CR-10X datalogger (Campbell Scientific, Inc., Logan, UT). Dataloggers were programmed to scan the FST every 5 seconds and then record the average FST of each 5-min period. Meteorological data was collected from a Campbell ET-106 weather station that monitored solar radiation, air temperature, relative humidity, precipitation, wind velocity and wind direction continuously. The weather station was positioned within meters of the apples whose FST were monitored. RESULTS We observed that sunburn browning was induced only when the fruit surface temperature (FST) attained a threshold temperature. This threshold temperature was reached only on clear, sunny days, and varied in different cultivars (data not shown). We have established this by experimentally inducing sunburn in attached apples in full sunlight with controlled-temperature devices, and also by observing natural sunburn on apples to which thermocouples were attached for several weeks. Light was required for induction of sunburn browning. When the experimental induction chambers were covered to exclude sunlight, apples heated to their threshold temperatures did not sunburn (Schrader et al, 01). We have several lines of evidence to support the hypothesis that UV-B radiation is required for induction of sunburn browning, but additional experiments are in progress. Based on many observations and experiments with various cultivars, we concluded that sunburn of apple did not occur unless FST exceeded C. We therefore designed experiments to 398

3 identify the meteorological factors that interacted to cause maximum FST of over C. Maximum Air Temperatures during 00 and 01 Growing Seasons, and Dates on Which Fruit Surface Temperature (FST) Reached at Least C Between June 15 and September, 00, the maximum air temperature varied from 15 to 38 C (Fig. 1). There were 22 days during this season on which the maximum FST on the sun-exposed side of an apple reached C (denoted by open circles in the figure). The maximum air temperature on those 22 days was at least C, but not more than 38 C. During 01, the maximum air temperature between June 15 and September ranged from 18 C to slightly over 38 C (Fig. 2). There were 23 days during this season on which the maximum FST on the sun-exposed side of an apple reached at least C. The maximum air temperature on those 23 days was at least 31.5 C, but not more than 38.5 C. Maximum FST was frequently 10 to 15 C higher than the maximum air temperature, but was not increased by a constant amount each day. Therefore, factors other than maximum air temperature had to be considered in predicting the maximum FST on a given day. At What Time of Day Does Sunburn Occur? Thermocouples were attached to fruit and the FST was recorded throughout the day and night for the period between June 1 and October 1, 00, and between June 15 and October 7, 01. We concentrated on those days when FST was at least C, and plotted a frequency distribution for those 23 days in 01. These 23 events occurred between 14 and 16 hours, but the highest frequency for maximum FST occurred between 15 and 1615 hours (Fig. 3). During 00, the maximum FST was attained between 1515 and 16 hours, but the highest frequency occurred between 15 and 16 hours (Fig. 3). Effect of Air Temperature on Fruit Surface Temperature (FST) Air temperature between 0800 and 10 hours was highly correlated (r = 0.92**) with FST between those hours (Fig.4). After noon, FST usually surpassed air temperature by several degrees. Maximum FST was highly correlated (r = 0.90**) with maximum daily air temperature during 01 (Fig. 5), and can be predicted from the equation y = 1.17x where y = FST and x = maximum air temperature. For example, when the maximum air temperature was C, FST was predicted to reach 47.2 C. When the maximum air temperatures attained each hour between 1100 and 1700 hours were averaged, this average temperature was also highly correlated (r = 0.88**) with maximum FST (Fig. 6). Maximum FST was predicted to be 1.21x For example, with an average maximum air temperature of 32 C between 1100 and 1700 hours, FST was predicted to reach 46.7 C. Effect of Solar Radiation on Maximum Fruit Surface Temperature Solar radiation also influenced maximum FST. Solar radiation and FST were highly correlated (r = 0.95**) between 0800 and 10 hours (Fig. 7). Maximum FST was highly correlated (r = 0.65**) with the mean solar radiation between 1100 and 1700 hours (Fig. 8). Maximum FST could be estimated from y = 0.03 x where y = maximum FST and x = mean solar radiation. For example, if solar radiation averaged 7 W m -2 between 1100 and 1700 hours, FST was predicted to reach.3 C. Effect of Wind Velocity on Maximum Fruit Surface Temperature Wind velocity was less important than air temperature and solar radiation in affecting FST. Greater wind velocity lowered FST by disturbing the boundary layer around the fruit. Wind velocity was negatively correlated (r = _ 0.24**) with FST (Fig. 9). According to the regression equation, maximum FST was decreased about 5 C as wind velocity increased from 0.5 to 3.5 m s -1. Therefore even moderate wind can reduce FST enough to prevent the incidence of sunburn on many days. 399

4 Effect of Relative Humidity on Maximum Fruit Surface Temperature Relative humidity also was negatively correlated (r = _ 0.66**) with FST (Fig. 10). Relative humidity was negatively correlated with both average solar radiation between 1100 and 1700 hours (r = **) and average maximum air temperature between 1100 and 1700 hours (r = **). DISCUSSION Sunburn of apple occurred only when the FST exceeded C, and several factors influenced FST. Maximum FST cannot be predicted from air temperature alone. We observed that factors such as clouds, wind and precipitation caused rapid fluctuations of FST. For example, appearance of a few clouds markedly decreased solar radiation, and quickly decreased FST below the threshold temperature required to induce sunburn browning. Likewise, wind quickly reduced FST. We have not firmly established how long that threshold temperature must be maintained to induce sunburn browning, but we expect that to 60 minutes are required. It may be that a shorter period at a higher FST will have the same effect as a longer period at lower FST. We established earlier (unpublished) that, under full sunlight, a sun-exposed spot on an apple remains at maximum temperature for about minutes on a clear day. We therefore chose to use a 1-hour period of induction when we experimentally induced sunburn browning in different cultivars and determined their threshold temperatures for sunburn browning. Further studies are needed to elucidate these complex interactions. Maximum FST was attained prior to 5 PM in both 00 and 01. Therefore sunburn does not occur after 5 PM as the FST is decreasing below the threshold temperature required for incidence of sunburn. Rabinowitch et al. (1986) reported air temperature thresholds of 38 to C and.5 to 42.5 C were essential for development of sunburn injury in cucumbers and peppers, respectively. However, they did not report FST. In apples, we found that at air temperatures between and 38 C, FST above C could be attained. The threshold temperature for sunburn was cultivar-dependent and in the range of 46 to 49 C. This is for fruit that were previously fully exposed to sunlight and therefore acclimated to heat and sunlight. Studies are in progress to determine if threshold temperatures are different for fruit that are not acclimated to heat and sunlight. The observation that relative humidity is negatively correlated with FST is consistent with our observations that water-stressed apple trees are more susceptible to sunburn (unpublished). Fruit on trees lacking vigor and on trees with inadequate water are more likely to sunburn (unpublished). We agree with Rabinowitch et al. (1974, 1986) that solar irradiation is essential for induction of sunburn, but we believe UV-B radiation is more important than visible light for induction of sunburn browning. Sunburn necrosis is normally induced by high FST in sunlight, but can be induced experimentally without direct sunlight (Schrader et al., 01), as thermal death is a heat-induced phenomenon. ACKNOWLEDGMENTS The financial support of the Washington State Tree Fruit Research Commission is gratefully acknowledged. Literature Cited Barber, H.N. and Sharpe, P.J.H Genetics and physiology of sunscald of fruits. Agric. Meteorol. 8: Bergh, O., Franken, J., Van Zyl, E.J., Kloppers, F. and Dempers, A Sunburn on apples preliminary results of an investigation conducted during the 1978/79 season. The Deciduous Fruit Grower :8-22. Brooks, C. and Fisher, D.F Some high-temperature effects in apples: Contrasts in the two sides of an apple. J. Agric. Res. 32:1-16. Compendium of Apple and Pear Diseases Jones, A.L. and Aldwinckle, H.S., eds. 0

5 Amer. Phytopathological Soc., St. Paul, MN. Rabinowitch, H.D., Ben-David, B. and Friedmann, M Light is essential for sunscald induction in cucumber and pepper fruits, whereas heat conditioning provides protection. Scientia Hort. 29: Rabinowitch, H.D., Kedar, N. and Budowski, P Induction of sunscald damage in tomatoes under natural and controlled conditions. Scientia Hort. 2: Schrader, L.E., Zhang, J. and Duplaga, W.K. 01. Two types of sunburn in apple caused by high fruit surface (peel) temperature. Online. Plant Health Prog. doi: /php RS. Simpson, J., Rom, C.R. and Patterson, M Causes and possible controls of sunburn on apples. Good Fruit Grower 39: Warner, G Sunburn is a hot topic in orchards of Washington. Good Fruit Grower 48: Wünsche, J.N., Greer, D.H., Palmer, J.W., Lang, A. and McGhie, T. 01. Sunburn The cost of a high light environment. Acta Hort. 557: Figurese Maximum Air Temperature ( C) 15 6/1 6/15 6/29 7/13 7/27 8/10 8/24 9/7 9/21 Date Fig. 1. Maximum daily air temperature for 00. Open circles denote those days on which the fruit surface temperature was > C. 1

6 Maximum Air Temperature ( C) 15 6/15 6/29 7/13 7/27 8/10 8/24 9/7 9/21 Date Fig. 2. Maximum daily air temperature for 01. Open circles denote those days on which the fruit surface temperature was > C Frequency : 16: 16:15 16:00 15: 15: 15:15 15:00 14: 14: Time Fig. 3. Time frequency for fruit surface temperatures to exceed C in 00 and 01. 2

7 Temperature (ºC) Fruit Temperature 7:12 8:24 9:36 10:48 12:00 13:12 Time of Day y = 1.95x r = 0.92** Air Temperature Maximum Fruit Temperatures ( 0 C) 55 y = 1.17x r = 0.90** Maximum Air Temperatures ( 0 C) Fig. 4. Air temperature versus fruit surface Fig. 5. Regression between maximum fruit temperature on a clear day between surface temperatures and daily maxi- 8 AM and noon. mum air temperatures during 01. Maximum FruitTemperatures ( 0 C) 55 y = 1.21x r = 0.88** 15 Average Daily Maximum AirTemperatures ( 0 C) Fig. 6. Regression between maximum fruit surface temperature and mean of hourly maximum air temperatures attained between 1100 and 1700 hours. Solar Radiation (W/m 2 ) Solar Radiation :00 11: 11:00 10: 10:00 9: 9:00 8: 8:00 y = 0.03x r = 0.95** Time Fruit Temperature Fig. 7. Correlation between solar radiation and fruit surface temperature between 0800 and 10 hours Fruit Surface Temp. (ºC) 3

8 Maximum Fruit Temperature ( 0 C) 55 y = 0.0x r = 0.65** Solar Radiation (W/m 2 ) Fig. 8. Regression between solar radiation (1100 to 1700 hr) and maximum fruit surface temperature. 55 Maximum Fruit Temperature ( 0 C) y = x r = ** Wind Velocity (m/s) Fig. 9. Regression between mean wind velocity (1100 to1700 hours) and maximum fruit surface temperature. 4

9 55 Maximum Fruit Temperature ( 0 C) y = -0.12x r = ** Relative Humidity (%) Fig. 10. Regression between mean relative humidity (1100 to 1700 hours) and maximum fruit surface temperature. 5