| Summary: | An infusion technique was used with an aqueous radiocalcium ( 45CaCl2) solution during the seed-filling stage of well-watered and moisture-stressed soybean in a greenhouse. The kinetics of infusion volume showed a quadratic reduction in absorption which approached zero on the sixth day for non-irrigated plants. The concentration of 45Ca increased quadratically from the point of injection towards the apex independent of both water status and plant parts. The difference in concentration of 45Ca between irrigated and non-irrigated plants was significant (P < 0.05) and concentrations attained the maximum values at the sixth node from the plant base. Seeds contained considerably less 45Ca than either stem or leaves. === The effects of a long (LD, 16h) and a short (SD, 12h) photoperiod with two water stress levels (SL) of stress (ST) and no stress (NS) on the distribution of 45Ca in plant organs (PO) of leaves, petioles, and stem at different node number (NN) of soybean were studied during the seed-filling stage. The univariate and Manova analyses showed the main effects of photoperiod (PP), SL, and PO to be highly significant (P < 0.001) on Ca distribution. The long PP increased Ca concentration in top leaves compared with the short PP regardless of SL. Water stress significantly (P < 0.001) modified the Ca distribution and reduced its concentration in PO within NN irrespective of the photoperiod (a measure of light stress). A possible mechanism for the regulation of Ca distribution is discussed in terms of nitrate reduction. === Ca uptake was also studied by immersing the central tip of a trifoliate leaf in various concentrations of 45CaCl2 solutions and drought conditions during the seed-filling period of soybean. The beta-ray gauging and the diurnal leaf temperature variation showed similar characteristics for leaf water status. The activities of 45Ca were significantly higher (P < 0.0001) at 5, 10, 20, and 30 mM concentrations for water-stressed and non-stressed leaves compared with the control. 45Ca activities at 5, 10, and 20 mM Ca concentrations between stressed and non-stressed leaves were not significant, but the difference in their mean values at 30 mM Ca concentration was significant (P = 0.0159). The relationship between 45Ca uptake and Ca concentration was parabolic for both stressed (R 2 = 0.77) and non-stressed (R2 = 0.81) leaves. Autoradiograms indicated Ca movement through the mid-rib and veins of the tip-immersed trifoliate leaf but showed no activity in other plant parts. An activity gradient developed between seeds when a pod-tip was immersed in the radioactive solution. Solutions of ruthenium red (RR, 0.01 mM), Ethylene Glycol-bis-(beta-aminoethyl ether)- N,N,N,N-Tetraacetic Acid (EGTA, 0.1 mM), calcium (Ca, 1 mM), and double distilled water (control) were fed through a bottom branch of soybean with (ST) and without (NS) water stress. The volume absorptions and transpiration rates were significantly higher for NS than ST plants and decreased almost linearly with time for all treatments. The transpiration rates of Ca-feeding ST plants and the control overlapped while the NS plants approached the same rate of transpiration by the third week. Ca was implicated in stomatal closure for the reduction in the transpiration rates. The relative amounts of chlorophyll decreased with time but chlorophyll was least affected for Ca-absorbing plants for both ST and NS plants. The use of RR (Ca transport blocker), and EGTA (Ca chelator) indicated the role of intracellular Ca conce
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