DEVELOPMENT IN WHEAT AS AFFECTED BY TIMING AND LENGTH OF EXPOSURE TO LONG PHOTOPERIOD

Seeds of a spring wheat (Triticum aestivum L. cv. 'Condor') were verna lized and then grown at 19 degrees C in two naturally-lit environments , one with a moderate (12 h) and the other with long (18 h) photoperio d. Treatments consisted of transfers of plants from the moderate to th e long photoperiod chamber on different occasions, or for periods of d ifferent durations, The main objectives were to determine whether whea t development responds to current and previous photoperiodic environme nts and whether there is a juvenile phase when the plants are insensit ive to photoperiod. Plants under constant 18 h photoperiod had fewer l eaves which appeared faster than those under constant 12 h photoperiod (i.e. phyllochron was increased from 4.4 to 5.1 d leaf(-1)). Plants t ransferred from 12 h to 18 h photoperiod at terminal spikelet appearan ce (ISA) reached anthesis 4 d earlier than plants retained at 12 h, wh ile plants under continuous long photoperiod (18 h) completed this pha se most rapidly, Thus, there was some evidence for a historic effect o f photoperiod on development. Exposure to long photoperiod during the first 5 d after plant emergence accelerated the rate of development to wards anthesis, suggesting that there was no juvenile period of photop eriodic insensitivity. There were, however, changes during ontogeny in the degree of sensitivity to long photoperiod, increasing from seedli ng emergence to a maximum c. 15 d later, and then decreasing again. Al though all treatments were imposed before TSA, the response was not li mited to the pre-TSA phase, suggesting that well before the terminal s pikelet appeared, the plant was already committed to the initiation of this spikelet. Spikelet number decreased with delayed transfer to lon g photoperiod with a minimum for plants transferred to long days from 16-20 d after seedling emergence. Additionally, there was a trend for an increase in the rate of leaf appearance (decrease in phyllochron) w hen the plants were exposed to long days between 10 and 35 d after see dling emergence. Although the differences were small, when considered in conjunction with the effects on final leaf number they become impor tant in explaining differences in time to anthesis.