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activities were associated with the differences in heat tolerance of turfgrasses and indicated a positive correlation between chlorophyll content and the antioxidant enzymes and a negative correlation between membrane injury index and the antioxidant enzymes

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[ Trifolium repens ( Li et al., 2019 )]. Sitosterol reinforces the stabilization of liquid-disordered membranes in plants exposed to heat stress ( Dufourc, 2008 ). Sitosterol content correlated positively to heat tolerance of hard fescue, as demonstrated by

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xanthophyll) led to decreased membrane permeability and protected the plants against heat damage ( Havaux et al., 1996 ). The association of antioxidant effects of carotenoids with improved heat tolerance in cool-season grass species is not yet well

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measuring electrolyte leakage from leaves of plants at different temperatures. Several studies have indicated that CMT is effective in detecting genetic differences with regard to heat tolerance among several crops ( Islam et al., 2014 ; Kumar et al., 2012

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Schmidt, 2000 ). Oxidative stress and differential antioxidant system activity were two key factors that contributed to differences among cultivars of creeping bentgrass in heat tolerance ( Liu and Huang, 2000 ). In addition to the antioxidant system, the

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Rady, 2014a , 2014b ). Heat tolerance can be improved by genetic selection as well as with the use of exogenous regulators, which aid the adaptation of physiological response in plants. SA and Ca 2+ are recognized as signal molecules known for their

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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.

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Abstract

Injury to turfgrass leaf segments was measured as percent electrolyte leakage as affected by the duration and level of imposed heat stress. Species differences in heat tolerance were most apparent when injury was monitored over time at 50°C, using leaf segments which were obtained from heat-hardened plants and immersed in distilled water during the stress treatment. Quantitative differences in heat tolerance in vitro were consistent with qualitative descriptions of drought resistance for most of the species tested.

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both amino acids and proteins between different cultivars of plants contrasting in heat tolerance will enable the identification of the key metabolic processes controlling genetic variations in heat tolerance. Free amino acids are constituents of

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Temperature sensitivity of net photosynthesis (PN) was evaluated among four taxa of rhododendron including Rhododendron hyperythrum Hayata, R. russatum Balf. & Forr., and plants from two populations (northern and southern provenances) of R. catawbiense Michx. Measurements were conducted on leaves at temperatures rauging from 15 to 40C. Temperature optima for PN ranged from a low of 20C for R. russatum to a high of 25C for R. hyperythrum. At 40C, PN rates for R. hyperythrum, R. catawbiense (northern provenance), R. catawbiense (southern provenance), and R. russatum were 7.8,5.7,3.5, and 0.2 μmol·m-2·s-1, respectively (LSD0.05 = 1.7). Rhododendron catawbiense from the southern provenance did not appear to have greater heat tolerance than plants from the northern provenance. Differences in dark respiration among taxa were related primarily to differences in tissue weight per unit leaf surface area. Temperature coefficients (Q5) for respiration did not vary in temperature response among taxa. Differences in heat tolerance appeared to result from a combination of stomatal and nonstomatal limitations on PN at high temperatures.

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