and the environment ( Jenni and Yan, 2009 ). Tipburn is one of the physiological disorders associated with heat stress and the symptom appears as brown necrosis around the margin of the affected leaves. This disorder is also believed to be associated
Heat stress imposed on roots of container-grown plants is an important problem in the nursery industry. In a number of nursery-grown species, substrate temperatures over 30 °C may cause root growth to slow considerably ( Johnson and Ingram, 1984
When plants experience high temperature stress, they respond by synthesizing a discrete set of proteins called heat shock proteins (HSPs). This response is not unique to plants, but is observed in all other eukaryotes. It is now known that the HSPs are evolutionarily conserved proteins, and furthermore, that HSPs function not only during stress, but also during normal growth and development. My laboratory has characterized several of the major groups of HSPs in higher plants. We have cloned genes encoding plant HSP70 proteins and low molecular weight (LMW) HSPs (17-23 kDa). Using this information we have investigated the expression of HSPs both in the field, and under laboratory conditions which mimic field situations. We have determined the temperature limits for expression of HSPs in vegetative tissues, and have also found that HSPs are frequently produced in plant reproductive structures, even in the absence of stress. As a first step toward understanding HSP function, we have characterized the intracellular localization of HSPs. Results show that there are unique HSPs in the cytoplasm, chloroplast and endomembrane system. These ubiquitous proteins appear to play essential roles in many cellular processes.
Mature green `Sunbeam' tomato fruit (Lycopersicon esculentum Mill.) were treated in water for 1 hr at 27 (ambient), 39, 42, 45, or 48°C, and then either ripened at 20°C (nonchilled) or stored at 2°C (chilled) for 14 days before ripening at 20°C. The most-effective heat treatment was 42°C, which reduced decay 67% in chilled fruit and 53% in nonchilled fruit. Heat treatment had no effect on time required to ripen the fruit. Red-ripe tomatoes had higher respiration rates and evolved more ethylene following nonchilling storage, but heat treatment had no effect on respiration or ethylene evolution. Red color development was enhanced by heat treatment, and inhibited by chilling. At red ripe, fruit were firmer as a result of storage at the chilling temperature, while heat treatment had no effect on firmness. Heat-treated fruit were preferred in terms of taste and texture over nontreated fruit in informal taste tests, with the exception of the 45°C treatment. With increasing temperature of heat treatment, there was increased electrolyte leakage following chilling storage. Of the 15 flavor volatiles analyzed, the levels of five were decreased with increasing temperature of heat treatment. Storage at the chilling temperature reduced the levels of six flavor volatiles. Prestorage heat treatments can reduce decay with only minimal adverse effects on tomato fruit quality.
fruit set in other crops. For instance, the delivery of abundant, viable pollen was not sufficient to ensure adequate fruit set in heat-stressed tomatoes ( Peet et al., 1997 ). When Brassica napus (L.) plants were exposed to short periods of high
microclimate in the summer by decreasing leaf temperature and leaf transpiration rate, thus alleviating heat stress ( Aberkani et al., 2008 ). The cultivation area under shade is constantly increasing in Mediterranean countries such as Israel, Morocco, and
( Fig. 2 ) suggest that bell pepper plants were under heat stress, particularly those that were unshaded. Root zone temperature under plastic mulch affects plant growth and yield in several vegetable crops ( Díaz-Pérez et al., 2008 ). Root zone
reduced root zone temperature to a value closer to optimal root zone temperature (≈27 °C) for bell pepper compared with unshaded conditions ( Díaz-Pérez, 2013 ). Decreased disease associated with shading may be related to amelioration of heat stress of
Temperature and chlorophyll fluorescence characteristics were determined on leaves of various horticultural species following a dark adaptation period where dark adaptation cuvettes were shielded from or exposed to solar radiation. In one study, temperature of Swietenia mahagoni (L.) Jacq. leaflets within cuvettes increased from ≈36C to ≈50C during a 30-minute exposure to solar radiation. Alternatively, when the leaflets and cuvettes were shielded from solar radiation, leaflet temperature declined to 33C in 10 to 15 minutes. In a second study, 16 horticultural species exhibited a lower variable: maximum fluorescence (Fv: Fm) when cuvettes were exposed to solar radiation during the 30-minute dark adaptation than when cuvettes were shielded. In a third study with S. mahagoni, the influence of self-shielding the cuvettes by wrapping them with white tape, white paper, or aluminum foil on temperature and fluorescence was compared to exposing or shielding the entire leaflet and cuvette. All of the shielding methods reduced leaflet temperature and increased the Fv: Fm ratio compared to leaving cuvettes exposed. These results indicate that heat stress from direct exposure to solar radiation is a potential source of error when interpreting chlorophyll fluorescence measurements on intact leaves. Methods for moderating or minimizing radiation interception during dark adaptation are recommended.
Forty hybrid broccoli [Brassica oleracea L. (Italica Group)] accessions were screened for heat tolerance and holding ability over three planting dates in 1988 at the Long Island Horticultural Research Laboratory in Riverhead, N.Y. Holding periods were quantified using the number of consecutive days between the time individual heads reached 10 cm diameter and cutting, which occurred when the sepals had fully expanded and had just begun to separate. In 1989 and 1991, heat stress was applied at various weeks during maturation to determine the most sensitive stage or stages of plant development in terms of reduction in holding period and head weight. Field studies and heat stress experiments indicate that heat stress may be most critical during the time the immature inflorescence measures 5 to 10 mm in diameter. This stage corresponds to ≈ 3 weeks before harvest for summer plantings in the northeastern United States.