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Increasing duration of stem elongation by exposure to short photoperiod would result in higher spike dry weight at anthesis, which is positively associated with the number of fertile florets and grains in wheat. However, it is not easy to determine whether photoperiod effects on fertile florets and grains are only mediated by assimilate supply to the growing spike when spike weight variation is attained only with photoperiod treatments. The aim of this study was to determine whether photoperiod effects on number of fertile florets and grains may be direct, that is, not mediated by assimilate supply, by comparing the magnitude of photoperiod effects with those of shading the canopy. Spike dry weight at anthesis was changed through the factorial combination of different photoperiod (natural and 6 h extended photoperiod) and shading (un-shaded and 67 ± 3% shaded) treatments during stem elongation of Buck Manantial, a cultivar known for its photoperiod sensitivity in this phase. Both treatments modified spike dry weight at anthesis and the number of fertile florets and grains, independently. When duration of stem elongation was lengthened by exposure to natural photoperiod and when incident radiation was high, spike dry weight at anthesis increased by 33% (NP+0 v. NP+6) and 27% (un-shaded v. shaded), respectively. The number of fertile florets increased similarly to spike dry weight (34% NP+0 v. NP+6 and 28% un-shaded v. shaded) resulting in higher number of grains. Most photoperiod effects on the number of fertile florets and, consequently, on the number of grains, were mediated by assimilate supply to the growing spike as the same relationship between the number of fertile florets and spike dry weight at anthesis was observed for photoperiod and shading treatments (R2 = 0.99, P

Yield increases in temperate cereals have been associated with increases in grain number. An experiment was conducted to investigate whether altering the duration of the stem elongation period in wheat and barley increases floret fertility and thereby grain number. A photoperiod-responsive spring wheat (UQ189) and spring barley (Arapiles) were grown in a naturally lit phytotron. Plants were grown at constant (9, 13 and 19 h) and reciprocally interchanged photoperiod [at terminal spikelet (TS, wheat) or triple mound (TM, barley)] to alter the duration of the stem elongation period. An increased duration of the late reproductive phase from TS to heading in wheat resulted in more fertile florets per spike. A similar relationship was observed in barley but only for a limited range of duration of the stem elongation period. Shorter photoperiods reduced the rate of floret development and extended the time to reach the fertile floret stage. The fact that the duration of the late reproductive phase during which the spike and stem competing for assimilates was associated with the number of fertile florets per spike suggests that extending the stem elongation period in cereals could be a way to reduce assimilate competition and thereby increase the number of fertile florets and grain yield.

In this paper we describe the effects of photoperiod (9, 12, 15, 17, 19 and 21 h) and temperature (21/17 and 16/12°C) on rate of leaf 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 leaves irrespective of their level of insertion. Increased temperature significantly decreased phyllochron, but the degree of this effect differed between cultivars. As photoperiod was shortened below the optimum, phyllochron lengthened marginally and similarly in all varieties (by approximately 0.1 days per hour change in photoperiod). For very short photoperiods this was true only for the first six leaves, whilst for leaves at higher insertions there was a major effect of reducing photoperiod on lengthening phyllochron. The actual daylength required for producing this major effect on phyllochron was cultivar-dependent. These results suggest that, when making predictions of heading date using phyllochron, it may not be acceptable to assume that leaf number and time are always linearly related at shorter photoperiods, particularly when considering leaves at higher insertions.

This paper addresses three issues relating to development in wheat. Whether thermal amplitude changes rates of leaf appearance and phenological development, whether the responses of leaf appearance to a wide range of temperature can be acceptably described in terms of rates and cardinal (base and optimum) temperatures, and whether similar rates and cardinal temperatures can be applied to parallel processes of development. The two studies used a total of 22 naturally-lit growth chambers and 44 treatments, and the disparate wheat cultivars Sunset, Condor, Rosella and Cappelle Desprez. All plants were vemalised and then grown under photoperiods extended to 18 h. In the first study, thermal amplitudes varying between 0 and 14�C around a common mean of 19�C did not change rates of leaf appearance and had no consistent effect on phenological development at any stage to anthesis. Consequently, conclusions from data collected under one amplitude at this mean temperature can be extrapolated with confidence to another. In the second study, with six mean temperature regimes of 10-25�C, rate of leaf appearance was significantly increased progressively by increasing temperature up to 19�C. All cultivars had an optimum temperature for this process which approximated 22�C but they differed in their base temperatures which ranged from -5.7 to -1.9�C. Optimum and base values for the concurrent phase of phenological development ranged between similar extremes, but the rankings of base values amongst genotypes differed markedly for leaf appearance and phenology. For the process of culm extension, temperature significantly, but not linearly, affected final length but not internode number. Length was unchanged by temperatures between 10 and 16�C but was significantly reduced above 19�C. Cultivars differed in their respective optimum and base temperatures for rate of elongation, ranging from ca 19 to 21�C and from 2 to 5�C. The concurrent phenological process had a higher optimum of 22-25�C but again a similar range in base temperature with differing cultivar rankings. We conclude from the second study that cardinal temperatures for leaf appearance, culm elongation and phenology were different, and for each process there were cultivar differences.

In this review we assess the universality of several assumptions that are commonly made about development in wheat. The assumptions tested are that: (1) wheat is most sensitive to the environmental variables of temperature and photoperiod during the vegetative period; (2) any responses 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 any 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 that, in terms of development, all wheats are responsive to temperature throughout their life cycles though to differing degrees, most are responsive to photoperiod at least until heading and, contrary to expectations, with potentially increasing sensitivity once the flowering process is triggered, and many are responsive to a memory of vernalising temperatures to well beyond the double ridge stage. In all these responses we show that there is considerable genotypic variation and that it is usually difficult to guess the responses of one genotype to the main environmental variables from the responses of another. This is partially because overall sensitivity to each of the main variables can differ, and the responses can be interactive, but also because the primary responses and interactions can differ between developmental phases. This can result in a high level of complexity of response whenever any variable is changed. The level of complexity is a negative feature when it comes to modelling and forecasting responses across widely differing environments but a highly positive feature when considering the enormous genotypic variation available for selection.