Heat stress on field grown Phaseolus lunatus (lima bean) can have a significant influence on yield. Lima bean crops grown in Delaware typically yield less pounds per acre than the same cultivars grown in California. Part of this effect may be due to extreme heat conditions or fluctuations during Delaware's summers, which can affect blossom and pod set. Our purpose was to analyze the heat tolerance of various cultivars of P. lunatus using quick bioassays and to establish a relationship to yield in greenhouse temperature trials. Two assays were used. The first, a hypocotyl extension assay, consisted of a treatment of germinated seedlings at 25, 35, or 42 for 2 h and observations of hypocotyl extension at 72 and 96 h posttemperature treatments. Three cultivars [`Fordhook' 1072 (heat-sensitive), `Jackson Wonder' (heat-tolerant), and `Early Thorogreen' (heat tolerant)] were analyzed. Initial results indicated that `Jackson Wonder' and `Early Thorogreen' are capable of surviving the 42C heat shock, but `Fordhook 1072' is not. In the second assay, we measured electrical conductivity of a solution containing hypocotyl sections following incubation at various temperatures (R1). Tissue samples then were boiled and conductivity was measured again (R2). The ratios of R1/R2 × 100 were determined as percent injury. Preliminary data suggests that `Jackson Wonder' is more heat-tolerant in this assay than `Fordhook 1072'. Subsequent experiments will analyze the induction of specific heat shock proteins as a function of cultivar-specific heat tolerance.
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