GROWTH AND WATER-USE OF FABA BEANS (VICIA-FABA) IN A SUBHUMID CLIMATE .2. SIMULATION ANALYSIS OF CROP RESPONSES TO DROUGHT

Although faba bean (Vicia faba L.) crop yields are sensitive to drought stress, this species shows a number of potentially advantageous adaptations to drought stress in the field. These adaptations have implications for the breeding and management of faba beans. In this study, a computer model was used to analyse the water relations of the frequently irrigated (If) and unirrigated (Io) faba bean crops described in paper I of this series. The objective was to assess the effectiveness of observed root and shoot adaptations to drought, paying particular attention to increased root growth and decreased leaf-area index under drought conditions. The model describes water movement throught the soil/plant/atmosphere continuum using resistance-analogue concepts. Measured crop characteristics (root distribution, leaf-area index, etc.) and weather conditions were input frequently over 92 simulated days. The model performed well at simulating water use by the crops. Simulations were also carried out in which the input data for crop characteristics were altered, to assess the likely performance of the Io crop had it not adapted as observed. The results indicated that the root and shoot adaptations were quite successful at reducing drought stress in the Io crop; without them the crop would probably have died prematurely. At least up to about 21 days from maturity the increased root growth of the Io crop was the most important of the observed adaptations. Both simulations and field observations indicated the Io crop to be under substantially more drought stress than the If crop from about 65 days after sowing, although shoot dry-matter did not differ between these crops for a further 55 days. Even though shoot dry-matter was relatively insensitive to the drought, composition of shoot material was probably much affected, since other studies indicate the energy-demanding process of N2-fixation is much reduced by drought stress. © 1990.