Allocation of nitrogen and dry matter for two soybean genotypes in response to water stress during reproductive growth

Drought stress significantly limits soybean [Glycine mau (L.) Merr.] yield in the Southeastern United States. The Plant Introduction 416937 (PI), whic h has lower yields than adapted cultivars under favorable conditions but a relatively lesser yield reduction under water-stress conditions, has been i dentified as a potential source of drought avoidance germplasm. It is uncle ar whether the mechanism of drought avoidance is associated with shoot or r oot. Also unclear is the effect of the pi's restricted yield potential on t he extent of its yield reduction in response to a water stress. To determin e the differences in response between the PI and an adapted cultivar, Delta pine 105, to reproductive sink size and water stress, inoculated Pi and Del tapine plants were grown in sand-filled pots in controlled-environment cham bers. The fixed rooting volume of the pot culture restricts the influence t hat genotypic differences in rooting patterns may have in accessing soil wa ter. During the 24-day period of pod development between R-3 and R-6 growth stages, plants were subjected to one of two water regimes, either well-wat ered or water-stressed to a leaf water potential of about -0.95 MPa. Within each water treatment, plants of both genotypes were depodded at the R3 sta ge to remove all pods (full depodding), one-half of the pods (partial depod ding), or no pods (no depodding). Tissues of plants harvested at the Rb sta ge were separated dried to a constant mass, weighed, and analyzed for nitro gen. Photosynthate production was calculated from dry matter and nitrogen c ontent. Photosynthate production and nitrogen fixation by Deltapine plants were unaffected within a pod load by the mild water stress, but both photos ynthate production and nitrogen fixation by the PI plants were diminished b y the mild water stress except when a reproductive sink was absent. It thus appears that a sizeable component of the drought tolerance observed in fie ld experiments for the PI plants may he attribute to root characteristics. Leaf nitrogen concentration decreased during water stress in Deltapine plan ts but not in the PI plants. Also, the decrease in nitrogen concentration i n stems was greater in response to increased reproductive load fur Deltapin e plants than for the PI plants. These data suggest that the PI does not re mobilize leaf nitrogen as readily as Deltapine.