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a b s t r a c tA better understanding of the effects of different source-sink ratio during the grain filling period on grain growth may be relevant in order to further increase cereal grain yield. The main objective of the present work was to determine the effect of different manipulations of the source-sink ratios in wheat and barley grown at four different environmental conditions on responsiveness of sinks (grain growth and yield) and sources (spike photosynthesis and water soluble carbohydrates in the stems). Four treatments were imposed 7 days after anthesis in two contrasting locations with low-and highinputs conditions in wheat (cv. Soissons) and barley (cv. Sunrise): they were a control, a treatment removing all the spikelets from the upper half of the spikes (T S ), and shadings decreasing incident radiation by 75% on the whole canopy (S W ) or only on the leaves (having the top area of the meshes individual holes for each spike to be exposed to solar radiation, S L ). As expected grain yield was closely related to grain number per m 2 . Average grain weight was reduced by shading treatments far more markedly in S w than in S L . Interestingly, significant amounts of water soluble carbohydrates in the stems remained at maturity in S L and S w treatments and spike photosynthesis in S L was consistently higher than in the unshaded controls in both species. These results may be an indication that wheat and barley are not source-limited during grain filling and that only when subjected to an extremely severe stress, grain size would be reduced due to lack of enough assimilates available to fill them.

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Short periods (3-5 days) of high maximum temperature (>35�C), often accompanied by drought, commonly occur during grain filling of cereals. Short periods of high temperature have been shown to reduce grain weight and baking quality in wheat, but little is known about their effects on barley. Consequently, we examined the effects of high temperature and drought, alone or combined, on grain growth for two barley cultivars, Schooner and Franklin. Treatments started 15 days after anthesis and consisted of the factorial combination of three temperatures and three water regimes. The high temperature (maximum 40�C for 6 h day-1) and drought treatments were maintained for 5 or 10 days. Drought reduced individual grain weight much more (ca 20%) than high temperature (ca 5%) for both cultivars. Franklin appeared to be more sensitive to heat stress than Schooner. The reduction in individual grain weight was greatest when both stresses were combimed (ca 30%). The reduction in mature grain weight under high temperature was due to a reduction in duration of grain growth for Schooner and to a reduction in both rate and duration of grain growth for Franklin (8-12%). The reduction in duration of grain growth was the most important cause of reduced grain weight at maturity under drought alone (12-25%) or combined with high temperature (25-33%). Nitrogen content per grain was quite high and similar for all treatments, and nitrogen percentage increased when stress was severe enough to reduce starch accumulation, confirming that starch accumulation is more sensitive to post- anthesis stress than nitrogen accumulation. We conclude that drought, particularly when combined with high temperature, is more likely than heat stress to cause large reductions in grain weight of barley under field conditions.

Shading treatments of 50 % of the incident radiation were applied to the semidwarf wheat cultivar Leones INTA before and after anthesis in two field experiments in Argentina in 1987 and 1988. The treatments reduced biological (above-ground dry matter) yield, grain yield and number of grains/m 2 . Number of grains/m 2 was closely and linearly correlated with ear dry weight at anthesis and with the photothermal quotient, calculated from 20 days before to 10 days after anthesis. Grain yield was sink limited, and the shading treatments reduced sink strength. The contribution of preanthesis assimilates to grain yield was smaller in the shaded crops than in the unshaded controls; in unshaded crops, almost 40 % of grain yield was contributed by preanthesis assimilates whilst in preanthesis shaded crops this contribution was negligible. The proportion of preanthesis assimilates contributed to the grain was closely related to the decrease in stem dry weight during grain filling. The effects of shading on main stems and tillers were the same.