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.

The effect of humidity on the gas exchange of leaves of the dicotyledons soybean (Glycine max (L.) Merrill), sunflower (Helianthus annuus L.), jojoba (Simmondsia chinensis (L.) Schneider), and saltbush (Atriplex halimus L.) and the monocotyledons wheat (Triticum aestivum L.), barley (Hordeum vulgare L.) sorghum (Sorghum bicolor (L.) Moench) and barnyard grass (Echinochloa crus-galli (L.) Beauv.) was examined under conditions of adequate soil moisture in a controlled environment. Photosynthesis and stomatal and internal diffusion resistances of whole, attached, single leaves were not affected by changes in humidity as the vapour pressure deficit between the leaf and atmosphere ranged from 8 to 27 mb. Transpiration increased linearly with increasing vapour pressure deficit. Whole plants of barley exhibited a different response. As humidity was increased, photosynthesis increased, transpiration expressed per unit of vapour pressure difference increased, and diffusion resistances became smaller. Reasons for the different behaviour of single leaves and whole plants are suggested. An index for water use efficiency, expressed per millibar of vapour pressure deficit, was calculated for single leaves of each species used in the experiments. This showed that water use efficiency was highest in the C4 xerophytes and lowest in the C3 mesophytes. The effect of environment on water use efficiency is examined using data from the literature.

Net photosynthesis, dark respiration and the response to photon flux density were measured on cotton leaves grown in a glasshouse. Leaves at four positions on the plant were examined from their unfolding until 70 days later. Photosynthesis and transpiration per unit of leaf area were unaffected by leaf position and, in all leaves, peak photosynthesis of about 110 ng CO2 cm-2 s-1 was attained 13-15 days after leaf unfolding, when the leaf was 75-90% of maximum area. Photosynthesis was maintained at this rate for only 12 days before declining linearly to values 20% of the maximum when leaves were 70 days old. Transpiration followed a similar pattern reaching a maximum of about 13 �g H2O cm-2 s-1 at 2 kPa vapour pressure deficit (VPD) at 13 days. Stomatal and internal conductances changed in parallel as leaves aged, with the consequence that internal CO2 concentration and water use efficiency remainedessentially constant at 220�ll-1 and 16.8 ng CO2 (�g H2O kPa VPD-1)-1 respectively. Youngest and oldest leaves saturated at lowest light levels (400-800 pE m-2 s-1) while 16-18- day-old leaves had light saturation at 1100 �E m-2 s-1. The initial slope of the light response curves increased as leaves expanded up to 10 days age then remained constant at 0.25 ng CO2 cm-2 (pE m-2)-1. Dark respiration reached a maximum of 1.5 ng CO2 mg-1 s-1 5 days after leaf unfolding, when leaf dry weight was increasing most rapidly. The relationship between the consistent pattern of gas exchange with age and the pattern of morphological development is discussed, along with internal factors associated with age-dependent photosynthesis.

SummaryRates of photosynthesis and dark respiration of the ears and flag leaves of three varieties of wheat grown at 21 DC under a constant light intensity of 3200 f.c. were measured by infrared gas analysis twice weekly throughout the period of grain development. Measurements were made on both the intact ears and the separated grains and ear structures, in air and in a mixture of nitrogen plus 320 p.p.m. C02. Dry weights of the grains, ears, and main stems were also determined.Photosynthesis by the grains was near maximal at the light intensity measured inside the glumes, and nearly balanced the loss of C02 by dark respiration, until the grains ripened. Grain photosynthesis accounted for 33-42% of gross ear photo· synthesis. Ear photosynthesis, which was much higher in awned varieties, contributed up to 76% to total grain requirements during early growth, this proportion falling to a minimum of about 26% in Sonora (awned) and 15% in Gabo (unawned) during the period of most rapid grain growth, before rising again as grain growth slowed. Over the whole period of grain development the contribution to grain requirements by ear photosynthesis was 33% in Sonora and 20% in Gabo. The rate of photosynthesis by the flag leaf blades varied apparently in response to changes in the demand for assimilates. In Sonora, requirements by the ear during the period of most rapid grain growth were equivalent to 131 and 43 mg CO 2 per ear per day for growth and respiration of the grain respectively, while net photosynthesis at that time by the ear, flag leaf blade, and stem plus sheaths was 50, 126, and 42 mg C02 per day respectively. Photosynthesis by the ear and flag leaf blade alone could meet the needs of the ear at all times, and grain growth did not appear to be limited by the supply of assimilate.