Many of the most productive apple-growing regions in the world are located in semiarid climates such as Washington State, South Africa, Israel, Chile, and Australia. Washington State is the primary apple-production area in the United States, with a climate characterized by high ambient temperatures and solar radiation during the growing season. Plant growth and development is dependent on temperature, with an optimum range that is species dependent (Bita and Gerats, 2013; Hatfield and Prueger, 2015). In apple, high ambient temperatures and high solar radiation cause the fruit surface to overheat, leading to the development of fruit sunburn (Racsko and Schrader, 2012; Wünsche et al., 2004). The use of PN for reducing incoming solar radiation and protection against sunburn is gaining in popularity in commercial apple orchards (Mupambi et al., 2018). PN (also called antihail nets or shade nets) initially was developed as a physical barrier against hail damage. PN can protect trees against bird damage, fruit bats, insects, strong winds, and sand storms (Arthurs et al., 2013; Shahak et al., 2004). The colors of early PN were primarily black and white. Since then, PN was modified by adding chromatic elements, giving rise to photoselective PN that can modify the light spectrum within the orchard canopy (Shahak et al., 2008). For this study, the term PN will be used in the general sense and photoselective PN where it was specified as such in previous studies.
The use of PN can change the orchard environment. Depending on the porosity and material used, PN can reduce the amount of photosynthetically active radiation (PAR) reaching the underlying plants and modify the light quality in the ultraviolet, PAR, and near-infrared wavelengths (Castellano et al., 2008; Shahak et al., 2016). The effects of PN on air temperature have been inconsistent. Middleton and McWaters (2002) reported that perceived cooler ambient temperatures under PN are due to reduced radiant heat (from lower sunlight levels) rather than a change in ambient temperature. PN has been reported to reduce ambient temperature (Middleton and McWaters, 2002; Solomakhin and Blanke, 2010), but Arthurs et al. (2013) found that 50% red, blue, and pearl PN increased ambient temperature. Kalcsits et al. (2017) found no significant differences in ambient temperature under photoselective PN compared with an uncovered control. The inconsistent results obtained may be due to radiation shielding not being used with the temperature sensors in some studies. For example, ambient temperature readings from a sensor that is exposed to solar radiation have been reported to be 4 to 6 °C higher than ambient temperature readings from inside a Stevenson Screen (Middleton and McWaters, 2002). Photoselective PN reduced soil temperature under Washington State conditions at depths of 20 and 40 cm (Kalcsits et al., 2017). PN also has been reported to lower the wind speed (Arthurs et al., 2013; Kalcsits et al., 2017; Middleton and McWaters 2002) and increase relative humidity (Middleton and McWaters, 2002; Solomakhin and Blanke, 2010).
Heat damage from excessively high leaf temperatures affects photosynthetic reactions, posing limitations for growth and survival of plants (Berry and Bjorkman, 1980; Seemann et al., 1984). Heat stress can cause morphological, physiological, and biochemical changes that reduce photosynthetic efficiency, plant growth, and productivity (Ashraf and Harris, 2013). Leaves exposed to full sunlight can heat up substantially above ambient temperature (Sharkey, 2005). The leaf temperature of ‘Honeycrisp’ apple was reported to be 36.2 °C when ambient temperature was 28.8 °C (Kalcsits et al., 2017). Enzymes involved in chlorophyll biosynthesis are affected by heat stress (Tewari and Tripathy, 1998). Heat stress deactivates RuBisCo, an enzyme involved in the first major step of carbon dioxide fixation (Salvucci and Crafts-Brandner, 2004). With increasing heat stress, the ratio of photorespiration to photosynthesis increases (Sharkey, 2005). Inactivation of PSII was observed above 38 °C in Solanum tuberosum L., together with damage to the thylakoid membrane (Havaux, 1996). Yan et al. (2011) reported that PSII performance in leaves of Sorghum bicolor L. was negatively affected by high temperature, with the acceptor side of PSII being more sensitive than the donor side and reaction center.
Saturation of the leaf photosynthetic apparatus occurs at high levels of solar radiation and results in a decrease in photosynthetic light use efficiency (Li and Yang, 2015). This process is referred to as photoinhibition and occurs when the rate of photodamage to PSII is greater than the rate of repair of photodamaged PSII (Murata et al., 2007). The measurement of photosynthetic light use efficiency can be used to estimate plant photosynthesis and net primary production (Flanagan et al., 2015; Liu et al., 2013). Leaf-level photosynthetic light use efficiency can be measured using leaf spectral reflectance and chlorophyll fluorescence. Leaf spectral reflectance can be used to obtain the photochemical reflectance index, which provides a measure of photosynthetic light use efficiency (Gamon et al., 1992). The most common chlorophyll fluorescence protocols used to measure the photosynthetic efficiency of PSII are Fv/Fm and ΦPSII (Maxwell and Johnson, 2000). Photosynthetic light use efficiency as measured by Fv/Fm began to decline at temperatures above 37 °C in Vitis californica (Gamon and Pearcy, 1989).
Modification of the orchard environment under PN results in changes in apple tree physiology. PN has been reported to improve water use efficiency in ‘Cripps Pink’ apple (Gindaba and Wand, 2007a) and reduce stem water potential in ‘Smoothee Golden Delicious’ (Shahak et al., 2004). The effect of PN on leaf photosynthesis has been variable. Leaf photosynthesis was increased in ‘Cripps Pink’, ‘Royal Gala’, and ‘Fuji’ (Bastías et al., 2012; Gindaba and Wand, 2007a, 2007b; Solomakhin and Blanke, 2008), reduced in ‘Fuji’ and ‘Golden Delicious’ (Ebert and Casierra, 2000; Solomakhin and Blanke, 2008), and unaffected in ‘Starkrimson’ and ‘Fuji’ (Romo-Chacon et al., 2007; Solomakhin and Blanke, 2008). Photosynthesis in field-grown apples has been shown to decline above the ambient temperature of 30 °C (Gindaba and Wand, 2007b; Pretorius and Wand 2003). In growing regions like Washington State, ambient temperatures regularly exceed this threshold during the growing season. In addition, under climate change in the future, heat stress events are more likely to occur during the growing season (Teixeira et al., 2013). Here, the primary objective was to determine whether the reduction in excessive solar radiation under blue photoselective PN at different ambient temperature conditions impacts leaf-level photosynthetic light use efficiency, leaf gas exchange, leaf spectral reflectance, and plant water status in a semiarid climate.
AgWeatherNet2017AgWeatherNet Current Conditions Map. 24 Sept. 2017. <http://weather.wsu.edu/>.
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