PHYLLOCHRON IN WHEAT AS AFFECTED BY PHOTOPERIOD UNDER 2 TEMPERATURE REGIMES

In this paper we describe the effects of photoperiod (9, 12, 15, 17, 1 9 and 21 h) and temperature (21/17 and 16/12 degrees C) on rate of lea f appearance and phyllochron in two spring wheats, a semi-winter wheat , and a winter wheat. Under long photoperiods only, all leaves on the main culm of a cultivar emerged at a common rate within a temperature regime, so it was acceptable to assign a specific phyllochron to leave s irrespective of their level of insertion. Increased temperature sign ificantly decreased phyllochron, but the degree of this effect differe d between cultivars. As photoperiod was shortened below the optimum, p hyllochron lengthened marginally and similarly in all varieties (by ap proximately 0.1 days per hour change in photoperiod). For very short p hotoperiods this was true only for the first six leaves, whilst for le aves at higher insertions there was a major effect of reducing photope riod on lengthening phyllochron. The actual daylength required for pro ducing this major effect on phyllochron was cultivar-dependent. These results suggest that, when making predictions of heading date using ph yllochron, it may not be acceptable to assume that leaf number and tim e are always linearly related at shorter photoperiods, particularly wh en considering leaves at higher insertions.