Environmental Influences on Panicle Differentiation and Spikelet Number of Pennisetum americanum

Abstract

Coaldrake, P. D. and Pearson, C. J. 1986. Environmental influences on panicle differentiation and spikelet number of Pennisetum americanum.—J. exp. Bot. 37: 865-875. Pearl millet (Pennisetum americanum (L.) Leeke) has a juvenile phase after which the time to panicle initiation is reduced by short daylengths. To understand more fully the mechanism underlying temperature x daylength interactions on panicle initiation and differentiation, plants were grown (a) at a range of constant temperatures under a short daylength from sowing until after panicle differentiation and (b) at one temperature until 20 d after emergence and then at a range of temperatures during a 10 d exposure to short daylength. Temperature prior to panicle initiation determined the number of leaves initiated on the main stem and the size of the apical dome at the start of panicle initiation. The number of leaves, in turn, influenced the duration of the phase from panicle initiation to anthesis: this phase required a constant thermal time when expressed as day degrees per leaf. At anthesis, panicle length was positively correlated with the number of leaves on the main stem (and temperature) prior to panicle initiation. Changing the temperature only during exposure to inductive daylengths affected the rate of growth of the apical dome so that panicle differentiation began within 10 d at high temperature (30 °C) whereas differentiation did not commence in 10 d at 21 °C. Panicles developed normally if differentiation had commenced under inductive daylengths whereas panicles were abnormal when plants were returned to long daylengths after panicle initiation but before visible differentiation. Relative extension rates of the panicle during differentiation were correlated positively with temperature. The results are consistent with the hypothesis that panicle initiation depends on the apex attaining a critical size and that temperature, by determining the number of leaves initiated on the main stem, affects the size of the apical dome and thus the onset of panicle initiation, the duration of