Rate of Leaf Appearance and Final Number of Leaves in Wheat: Effects of Duration and Rate of Change of Photoperiod

Abstract

This study was conducted to test the hypothesis that photoperiod or its rate of change significantly affects the rate of leaf appearance (RLA) and final number of leaves (FNL) in wheat, as suggested from several time-of-sowing experiments. Two wheat cultivars (Condor and Thatcher) were sown in the field on 2 Sep. 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 min d-1), two faster rates of change (8·5 and 13·3 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 re-randomised, i.e. some plots received different photoperiod regimes before and after TS.The rate of leaf appearance maintained strong linear relationships with thermal time. It was greater for Condor [0·012-0·013 (°C d)-1] than for Thatcher [0·011-0·012 (°C d)-1] and did not alter during plant development or in response to the change in photoperiod at TS. Rate of leaf appearance on the main culm was not influenced by the rate of change of photoperiod nor by the average photoperiod.Cultivar and photoperiod significantly affected FNL on the main culm. Condor produced more leaves than Thatcher under long but not under short photoperiods. The rate of change of photoperiod did not affect FNL independently of the effect of average photoperiod. Most of the variation in FNL due to photoperiod resulted from differences in duration of leaf initiation.The lack of effects of the photoperiod treatments on RLA contrast with previous reports of its effects on the rate of phasic development from seedling emergence to double ridge. Therefore, the number of visible leaves on the main culm (NL) at double ridge and at TS were not constant. However, NL on the main culm at double ridge was closely correlated with FNL.