Responses of leaf and tiller emergence and primordium initiation in wheat and barley to interchanged photoperiod

The influence of constant (9, 13 and 19 h) and reciprocally-interchanged ph otoperiods [at terminal spikelet (TS) or triple mound (TM)] on leaf, tiller and primordium development were examined using photoperiod-responsive cult ivars of spring wheat,'UQ 189' and spring barley,'Arapiles'. In both specie s, constant longer photoperiod reduced the: duration from sowing (S) to dou ble ridge (DR), as expected. However, photoperiod sensitivity was not restr icted to this mainly vegetative phase. There was also a marked increase or reduction in the duration of reproductive phases between TS/TM and heading (H) when plants were transferred to shorter or longer photoperiods respecti vely, compared with controls. These responses were largely independent of t he photoperiod during previous phases although minor effects of the previou s photoperiod were observed. For both species, the time course of leaf emer gence was linear, or bi-linear, depending on the final leaf number on the m ain stem. The rate of leaf emergence was faster for the first six to eight leaves than for the leaves appearing subsequently. The rate of emergence of early-formed leaves was independent of photoperiod whereas the rate of eme rgence of later leaves varied with photoperiod. Photoperiod also affected t he dynamics of tillering. The rate of leaf primordium initiation was little affected by variation in photoperiod, but the rate of spikelet initiation increased with increases in photoperiod. The rates of leaf and spikelet pri mordium initiation were both substantially higher in barley than in wheat. The fact that the reproductive phase from TS/TM to H was largely independen t of the duration of the previous phase provides evidence that this phase m ight be genetically manipulated to increase the time for floret development and hence grain number. (C) 2000 Annals of Botany Company.