Crop loss from heat damage is becoming a prevalent problem for many blueberry growers in the northwestern United States. The region, which includes Oregon and Washington, is the leading producer of blueberries in the country [U.S. Department of Agriculture (USDA), 2019]. In 2018, these two states produced a combined total of 150,700 t of blueberries (49% of the total U.S. production) (USDA, 2019). In 2015, the Washington blueberry industry lost an estimated $10 million (about 10% of farm gate value) due to heat damage and inadequate water for cooling and irrigation (Schreiber, 2016). Similar losses were reported in Oregon (Ore. Blueberry Commission, personal communication). High temperature events such as this have become more common in the region over the last two decades and are resulting in more frequent reports of heat damage in many crops, including blueberry (Abatzoglou et al., 2014; Houston et al., 2018).
Northern highbush is the primary type of blueberry grown in cooler regions such as Oregon and Washington. Unlike southern highbush (a complex hybrid based largely on V. corymbosum and V. darrowii Camp.) and rabbiteye blueberry (V. virgatum Ait.), both of which are typically grown in warmer climates, northern highbush cultivars tend to be poorly adapted to high temperatures. During hot weather, net photosynthesis declines considerably in northern highbush blueberry, and high leaf temperatures result in large increases in plant water use (Bryla, 2011; Hancock et al., 1992). When high temperatures coincide with fruiting, water and carbohydrates are diverted from the fruit to supply leaves and other vegetative components of the plant, resulting in small or shriveled berries, hastened fruit ripening, and a reduction in fruit quality and storage (Lobos and Hancock, 2015). Berries exposed to direct sunlight tend to be the most susceptible to heat damage. Unlike leaves that cool via transpiration, blueberries have very few stomata on their surface and, therefore, do not have an effective means of cooling (Konarska, 2015).
At the cellular level, high temperatures disrupt the thermal stability of membranes and proteins, causing ion leakage and inhibition of physiological processes associated with fruit development (Inaba and Crandall, 1988; Schrader et al., 2011; Yu et al., 2016). Like many fruit, blueberries possess inherent qualities such as a waxy cuticle that provides natural protection against heat damage. The cuticle consists of a polyester matrix or cutin layer and an epicuticular wax layer. The latter, often referred to as the “bloom,” is deposited on and in the cutin matrix and contains long-chain alkanes, acids, alcohols, and esters (Gülz, 1994). Without the wax, blueberries are prone to infections by bacteria and fungi, physical damage, and water loss (Jenks and Ashworth, 1999; Riederer and Schreiber, 2001). Knowledge of the ultrastructure and thickness of the cuticle may help us to understand how to select and manage cultivars for increased resistance to heat damage. Apart from transpiration, the cuticle protects the berries against sunburn resulting from exposure to ultraviolet radiation and excess heat absorption (Samuels et al., 2008; Shepherd and Griffiths, 2006).
Overhead sprinklers are sometimes used to cool blueberry fields during hot temperatures. However, most new blueberry fields in northwestern United States are irrigated by drip (Strik and Yarborough, 2005). Options for reducing heat damage with drip are currently limited. To contend with this problem, some growers install dual irrigation systems and use microsprinklers to cool the berries and use drip tubing to irrigate the plants. These dual systems are similar to those used in apple [Malus ×sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.], where cooling not only prevents heat damage but also improves fruit size and color (Gindaba and Wand, 2005; Iglesias et al., 2002). The microsprinklers are located above the canopy and produce fine droplets of water that evaporate quickly.
Currently, there are many questions regarding the use of microsprinklers to reduce heat damage in northern highbush blueberry, including the temperature at which cooling is needed. Most growers focus their efforts on the later stages of berry development and initiate cooling whenever air temperature is expected to exceed 30 to 32 °C (F.-H. Yang, personal observation). However, there is no scientific basis for this decision, nor is there any information on how frequently the system should be run for cooling. Some growers run their microsprinklers continuously in hot weather, while others cycle them on for 15 to 30 min every hour.
The objective of the present study was to characterize and determine the critical temperatures and heating times for fruit damage in northern highbush blueberry. We also examined the ultrastructure of the berry cuticle to consider whether resistance to heat damage could be a heritable function of the amount of wax on the fruit. Damage was evaluated in late-season cultivars during green and blue stages of berry development. Late-season cultivars ripen in late summer when temperatures are warmer and, therefore, are often vulnerable to heat damage.
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