CHILLING SENSITIVITY OF PHOTOSYNTHETIC OIL-SEEDLINGS .1. COTTON AND SUNFLOWER

Despite the high probability that sensitive post-emergent seedlings will be exposed to suboptimal temperatures which significantly affect subsequent development and yield, growers of long-season, indeterminate crops must rely upon empirically determined 'earliest planting dates'. Most 'chilling-stress' studies have dealt, not with photosynthetic seedlings, but rather with the effect of low, non-freezing temperatures ( < 10-degrees-C) upon germinating seeds and pre-emergent seedlings. A photosynthetic seedling growth system was used to monitor the effects of temperatures from 10 to 35-degrees-C upon the roots and aerial portions of chilling-sensitive cotton and relatively chilling-resistant sunflower oil-seedlings. Length measurements of cotton seedlings were too variable to be reliable indicators of temperature treatment effects, but root and shoot fresh weights could be used as non-destructive measurements of differences in root and shoot temperature responses in both cotton and sunflower seedlings. The fresh weights were indicative of the relative water status of seedling roots and shoots, including that of seedlings returned to 30-degrees-C after exposure to other temperatures. Growth of both cotton and sunflower seedlings was inhibited to some degree at non-optimal temperatures and the capacities of the roots and shoots to return to normal water status, as indicated by fresh weights and relative water contents, was correlated with sensitivity to suboptimal temperatures.