Characterization of acquired thermotolerance in soybean seedlings

High surface temperatures are common to soils during periods of drought and when fields receive limited irrigation. Seedlings frequently experience these high temperatures during emergence and establishment in many regions of the world. The effects of a high temperature exposure on subsequent chlorophyll accumulation were determined in soybean (Glycine max) seedlings. Etiolated seedlings and excised cotyledons received a 30-min challenge temperature between 44 and 56°C. Following the high temperature challenge in the dark, they were returned to optimal temperatures for chlorophyll accumulation and exposed to continuous light. The results show that chlorophyll accumulation was prevented by challenge temperatures of 50°C and above in soybean cotyledons. Pre-incubation of the etiolated soybean cotyledons at 38 or 40°C before the 50°C challenge provided maximum protection against high temperature inhibition of Ch1 accumulation. The time course for inducing acquired thermotolerance and the level of acquired thermotolerance obtained was dependent upon the tissue age (number of days following germination). This study further demonstrates the effectiveness of using the Ch1-based bioassay of high temperature injury for the measurement of acquired thermotolerance.