Responses to photoperiod change with phenophase and temperature during wheat development

This paper describes responses of four wheat genotypes to photoperiod in terms of several numerical parameters and examines how they are changed by temperature and stage of development. The cultivars are an awned selection of Sunset, Condor, Rosella and Cappelle Desprez. The three phenological phases were to double ridge (Phase I), from then to terminal spikelet (Phase II) and from then to heading (Phase III). Photoperiod regimes included a 9-h sunlit treatment and five longer photoperiods achieved by extension with low-intensity incandescent lamps to 12, 15, 17, 19 and 21 h. Temperature regimes were sinusoidal with daily maxima/minima of 16/12 and 21/17 degrees C. The parameters were adapted from those used in models by Major and Roberts et al. and partitioned responses into one or two photoperiodic sensitivities (Ps and Ps(2)), an actual maximum length of the phase (Lm(a)), which occurs at the critical photoperiod (Pc), a potential maximum length (Lm(p)) and a basic length (Lb) that occurs at and above the optimum (Po) photoperiod. Development in all phases was modified by photoperiod, temperature, cultivar and their interactions. Regardless of phase, Po was always longer at the higher temperature though it differed among cultivars and tended to be longer in late development. Photoperiod sensitivity could be described by a single parameter (Ps) during Phase I when all responses were quantitative. However, as development progressed, sensitivity also increased, particularly at short photoperiods and in the most sensitive cultivars, for which the model required a second sensitivity parameter (Ps(2)) 3-5 fold larger than Ps. During the last phase, some photoperiodic responses became qualitative. It is concluded that (i) differences among cultivars in response to photoperiod can be conveniently partitioned into parameters to describe photoperiodic sensitivity, (ii) the parameters appear to be unrelated, suggesting that plant breeders could manipulate them independently to tailor cultivars for particular environments, (iii) the parameters are sensitive to temperature, thus precluding extrapolations from one thermal environment to another and (iv) the lengths of all three phenological phases examined were affected by photoperiod, with that of the final phase being more sensitive than the early phase to double ridge.