RADIATION EFFECTS ON RATE OF DEVELOPMENT IN WHEAT GROWN UNDER DIFFERENT PHOTOPERIODS AND HIGH AND LOW-TEMPERATURES

This study was designed to examine whether development rate of wheat m ight be changed by radiation at high temperature and more generally wh ether development might be varied by changed source:sink ratio. In the genotype studied, when temperature was high (27/22 degrees C) and esp ecially when the photoperiod was short (9 h), leaf primordia were prod uced more slowly under low than high natural radiation, and this slowi ng was reflected in lower final spikelet numbers and in fewer grains p er ear. Under the higher source:sink condition of low temperature (17/ 7 degrees C), and under either short or long photoperiod, rates of pri mordia production were not measurably changed by radiation, but more l ight did result in more spikelets. The lengthening of plastochron inte rvals (thermal time between the appearance of leaf primordial) associa ted with low radiation did not translate into significantly increased phyllochron intervals (thermal time between emergence of leaves). Howe ver, phyllochron intervals were responsive to both photoperiod and tem perature. Under short photoperiod, source limitation associated with h igh temperature extended the thermal time to ear emergence. Thus, an i ncrease in temperature from 17/7 degrees C to 27/22 degrees C doubled the thermal time to heading from 1241 to 2571 degrees Cd. Source limit ation associated with low radiation also increased thermal time to hea ding by up to 400 degrees Cd and calendar time by up to 17 days. Under long photoperiod, where development was rapid, radiation did not chan ge the timing of developmental stages. It is concluded that although d evelopment in wheat is generally not source-limited in the traditional environments for cultivation, under high temperatures and shorter pho toperiods development may be slowed by limiting source:sink ratios.