Comparative Effects of Soil Moisture Stress and Restricted Root Zone Volume on Morphogenetic and Physiological Responses of Soybean [Glycine max(L.) Merr.]

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Abstract
Significant differences in response to soil moisture stress (SMS) and restricted root zone volume (RRZV) were found in two cultivars of soybean [Glycine max (L.) Merr.] (‘Forrest’ and ‘Williams’) plants grown under controlled-environment conditions. Leaf water potentials of SMS-treated plants were 0·4-0·6 MPa lower than those of controls and stomata1 conductances 23-56% lower. In the case of RRZV treatment, however, there were no differences in either parameter. Initiation of new leaves as reflected in the plastochron index was strongly reduced by SMS but was unaffected by RRZV. Photosynthetic rates (CO2 fixation dm2 of leaf) of plants given SMS were reduced by 11-21% while those of RRZV-treated plants were unaffected. SMS caused a strong preferential allocation of dry matter to the root at the expense of the shoot in both cultivars. RRZV, however, had no effect on assimilate distribution in ‘Forrest’ and only slightly favoured root growth in ‘Williams.’ Carbohydrate concentrations of both alcohol-soluble and insoluble fractions were increased significantly by SMS, especially in the leaves, but were little affected by RRZV. Nitrogen concentration in the root fraction was reduced by 22-24% and that in the leaf and stem fractions by 7-14% under SMS but was not affected appreciably by RRZV. Phosphorus concentration in the leaf, stem, and root fractions was reduced by 45-65% under SMS but was relatively unaffected by RRZV. These findings suggest that SMS and RRZV are basically different in their mechanism of action and that the impairment of growth resulting from these two stresses may involve different physiological processes. Our results also indicate that the suppressive effects of small containers on plant growth do not necessarily result from inadvertent SMS.