Water stress and calcium concentration during the seed‐filling stage of soybean affect senescence

Solutions of ruthenium red (RR, 0.01 mM), ethylene glycol‐bis‐(ß‐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 at the seed‐filling stage. The volume absorptions and transpiration rates were significantly higher for NS than for 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 only. It is suggested that Ca was implicated in stomatal closure for the reduction of transpiration rates. The relative amounts of chlorophyll decreased with time but chlorophyll was least affected in Ca‐absorbing plants for both ST and NS plants. The use of RR (a Ca‐transport blocker) and EGTA (a Ca chelator) indicated the role of intracellular Ca concentrations on stomatal closure and foliar senescence at the end of the season.