EFFECT OF PATTERN OF WATER-SUPPLY ON VICIA-FABA L .3. PLANT WATER RELATIONS, EXPANSIVE GROWTH AND STOMATAL REACTIONS

Leaf water potential (psi), osmotic potential (pi), turgor (P) and relative water content (RWC) of six faba bean cultivars were studied in a treatment with limited water availability from onset of flowering to the end of the growing season (d-d), and in a treatment with full irrigation during the same period (i-i), in the field in 1982 and 1984. Psi, pi and P were linearly related to leaf RWC, and there were no significant cultivar effects. The osmotic potential pi at full turgor hardly differed between the water supply treatments (-0.7 MPa in d-d and -0.6 MPa in i-i) and the slopes of pi-versus RWC showed no significant difference (P < 0.01). This indicates the absence of osmotic adjustment. Significant effects of water supply treatment on the slope of P versus RWC (the bulk modulus of elasticity, BME) were absent. It is concluded that the six faba bean cultivars showed a general lack of mechanisms to keep P on a high level at increasing water shortage. Internode expansive growth rate during flowering was in i-i 2 to 3 times higher than in d-d. Under controlled conditions (22 or 28-degrees-C; 400-mu-E m-2 s-1), expansive growth linearly declined with decreasing P. Maximum stomatal conductance in the field decreased with psi from an estimated maximum level of 1.0 s cm-1 (at full turgidity and light intensity > 800-mu-E m-2 s-1), following a negative exponential curve. It is concluded that vegetative (expansive) growth decreases faster with decreasing P or psi than the stomatal conductance (and most probably faster than the rate of photosynthesis). This result, combined with the inability to maintain P at increasing water shortage, helps to explain the positive effect of mild water shortage on reproductive growth of faba beans.