SENSITIVITY OF WHEAT PHASIC DEVELOPMENT TO MAJOR ENVIRONMENTAL-FACTORS - A REEXAMINATION OF SOME ASSUMPTIONS MADE BY PHYSIOLOGISTS AND MODELERS

In this review we assess the universality of several assumptions that are commonly made about development in wheat. The assumptions tested a re that: (1) wheat is most sensitive to the environmental variables of temperature and photoperiod during the vegetative period; (2) any res ponses to vernalisation and photoperiod are complete by the time that the apex has become reproductive and the stems begin to elongate; (3) cultivars differ little in their responses to temperature aside from a ny responses to vernalisation; (4) cultivar differences in 'intrinsic earliness' or 'basic development rate' are unaffected by temperature; (5) photoperiod and vernalisation responses are quantitative and that these responses are well understood and can be generalised. We show th at, in terms of development, all wheats are responsive to temperature throughout their life cycles though to differing degrees, most are res ponsive to photoperiod at least until heading and, contrary to expecta tions, with potentially increasing sensitivity once the flowering proc ess is triggered, and many are responsive to a memory of vernalising t emperatures to well beyond the double ridge stage. In all these respon ses we show that there is considerable genotypic variation and that it is usually difficult to guess the responses of one genotype to the ma in environmental variables from the responses of another. This is part ially because overall sensitivity to each of the main variables can di ffer, and the responses can be interactive, but also because the prima ry responses and interactions can differ between developmental phases. This can result in a high level of complexity of response whenever an y variable is changed. The level of complexity is a negative feature w hen it comes to modelling and forecasting responses across widely diff ering environments but a highly positive feature when considering the enormous genotypic variation available for selection.