DEVELOPMENT RATE IN WHEAT AS AFFECTED BY DURATION AND RATE OF CHANGE OF PHOTOPERIOD

A field study was conducted to test the hypothesis that wheat development rate responds to the rate of change of photoperiod. Two wheat cultivars (Condor and Thatcher) were sown on 18 Aug. 1992 at Melbourne (38° S). Photoperiod was extended artificially to give five treatments up to terminal spikelet initiation (TS) viz.: natural photoperiod (rate of change of photoperiod, 2·3 mind d-1), two faster rates of change (9·8 and 13·1 min d-1) and two constant photoperiods of 14·0 and 15·5 h. After TS, the two constant photoperiods were extended to 15·0 and 16·5 h, respectively and treatments were randomly re-allocated, i.e. some plots received different photoperiod regimes before and after TS. There were no significant differences among treatments in the length of the period from sowing (S) to seedling emergence (E) phase, ranging from 15 to 16·3 d. The rate of development from E to TS responded to increases in photoperiod in both cultivars, increasing with average photoperiod across all treatments but there was no effect of rate of change of photoperiod independent of its average photoperiod. The rate of development from TS to anthesis (A) did not show any trend with average photoperiod. This lack of effect of photoperiod on the period from TS to A contrasts with other results from the literature and possible reasons for this conflicting result are discussed. Rate of change of photoperiod did not affect the duration of the phase from TS to A either. Therefore, the effect of photoperiod on the duration of the S-A period was strongly and positively correlated to that of the length of the E-TS phase.