The response of 4 crops of autumn-and spring-sown field beans cv. Maris Bead to irrigation applied during the vegetative, flowering and pod-filling phases was examined during 1981-2 and 1982-3. The crops, grown on a Templeton silt-loam, were irrigated weekly in amounts equal to the difference between the estimated evapotranspiration and rainfall of the previous week using trickle irrigation.Seed yield of fully irrigated autumn-and spring-sown crops (averaged over the two seasons), was 5-2 and 3-3 t/ha respectively, about 45 % greater than the yield of unirrigated crops. The increase in yield due to irrigation was mainly associated with an increase in total dry matter (TDM) production as harvest index varied little within each sowing. The yield response to each mm of applied irrigation water ranged from 0 to 9 kg/ha and did not appear to be related in any way to the developmental phase of the crop when irrigation was applied.Seed yield within each sowing was closely correlated with the number of beans per unit area and the number of pods per plant. Autumn sowings yielded about 55 % more than spring sowings mainly due to a higher harvest index which was associated with a much heavier mean weight per bean.

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S U M M A R YThe growth and yield of four crops of field bean cv. Maris Bead in response to irrigation and sowing date were analysed in relation to leaf area expansion and senescence and their absorption and utilization of photosynthetically active radiation (PAR). Total dry matter (D.M.) production and seed yield were strongly correlated with total green area duration (GAD) and post-flowering GAD respectively.Total D.M. production was also strongly related to radiation absorbed by the green surfaces of the crop although autumn sowing and drought both decreased the constant of proportionality, i.e. the growth efficiency (£ g ). Autumn sowings yielded more than spring sowings because they grew for longer and received 22 % more radiation. Their harvest index was also about 40 % higher than in spring sowings. These more than compensated for their smaller E g . Drought decreased yield mainly by decreasing radiation received and E % . Growth duration was shorter and harvest index was smaller.The rate of phenological development was strongly dependent upon temperature and to a lesser extent on photoperiod. The average thermal duration from emergence to flowering was 790 °Cd above a base of 0 °C. The time from sowing to the end of the pod growth was well represented by a simple multiplicative model in which development rate was a linear function of temperature above a base of 0 °C and photoperiod above a base of 6 h. The average photothermal duration required for 10 crops was 980 °Cd.An attempt was also made to determine the crop physiological and environmental factors which govern the change in size of the yield components of field bean crops caused by irrigation and sowing date. The final number of pods per plant was closely correlated with the rate of supply of assimilates during pod filling. Irrigation increased assimilate flux by increasing leaf area, growth rate and total dry matter during pod growth. Both the rate and duration of pod growth were little affected by irrigation. Autumn sowings produced heavier pods and beans due to both a faster rate and a longer duration of growth which were associated with a greater production of assimilate during the seed growth period. Seed growth depends on both the current assimilate and stored reserves, the latter especially when plants were subjected to environmental stress.