GROWTH AND WATER-USE OF FABA BEANS (VICIA-FABA) IN A SUBHUMID CLIMATE .1. ROOT AND SHOOT ADAPTATIONS TO DROUGHT STRESS

Root growth, shoot growth and water-use of faba beans (Vicia faba L.) were studied to ascertain the nature and timing of morphological adaptations to drought stress. Spring-sown crops were grown in an alluvial silt loam in Canterbury, New Zealand, during 1982/83. Extractable water capacity for faba beans in this soil is about 120 mm. The crops were either irrigated frequently to avoid drought stress (If treatment) or not irrigated at all (Io treatment). Grain-yields were 3.8 and 2.9 t ha-1 respectively for the If and Io crops. Actual soil water-deficit, potential soil water-deficit, soil matric potential at 0.2 m depth, periodic evapotranspiration, and crop height increase were used as indices of drought stress. From around day 70. all the drought-stress indices were greatest in the Io crop. The Io crop responded to stress by reducing its rate of height increase (from about day 70), decreasing its rate of leaf-area expansion slightly (between days 69 and 81), greatly increasing root growth (from about day 78), producing leaves of smaller specific area (from around day 80), and shedding leaves (from around day 94). Radiation conversion efficiency was about 9% greater in the Io treatment. Drought-stress had no detectable effects on shoot dry-matter until after day 120. Thus the observed adaptations were reasonably successful in reducing the drought stress experienced by the Io crop. Furthermore, all these adaptations occurred during flowering and pod-fill, periods suggested by some authors to be when faba beans are especially sensitive to drought. Our results offer some explanation for previous results which suggest faba bean yields are equally sensitive to drought stress during all developmental phases. © 1990.