Responses by Coleoptiles of Intact Rice Seedlings to Anoxia: K+ Net Uptake from the External Solution and Translocation from the Caryopses

This study evaluated the effects of anoxia on K(+) uptake and translocation in 3-4-d-old, intact, rice seedlings (Oryza sativa L. cv. Calrose). Rates of net K(+) uptake from the medium over 24 h by coleoptiles of anoxic seedlings were inhibited by 83-91 %, when compared with rates in aerated seedlings. Similar uptake rates, and degree of inhibition due to anoxia, were found for Rb(+) when supplied over 1.5-2 h, starting 22 h after imposing anoxia. The Rb(+) uptake indicated that intact coleoptiles take up ions directly from the external solution. Monovalent cation (K(+) and Rb(+)) net uptake from the solution was inhibited by anoxia to the same degree for the coleoptiles of intact seedlings and for coleoptiles excised, 'aged', and supplied with exogenous glucose. Transport of endogenous K(+) from caryopses to coleoptiles was inhibited less by anoxia than net K(+) uptake from the solution, the inhibition being 55 % rather than 87 %. Despite these inhibitions, osmotic pressures of sap (pi(sap)) expressed from coleoptiles of seedlings exposed to 48 h of anoxia, with or without exogenous K(+), were 0.66 +/- 0.03 MPa; however, the contributions of K(+) to pi(sap) were 23 and 16 %, respectively. After 24 h of anoxia, the K(+) concentrations in the basal 10 mm of the coleoptiles of seedlings with or without exogenous K(+), were similar to those in aerated seedlings with exogenous K(+). In contrast, K(+) concentrations had decreased in aerated seedlings without exogenous K(+), presumably due to 'dilution' by growth; fresh weight gains of the coleoptile being 3.6- to 4.7-fold greater in aerated than in anoxic seedlings. Deposition rates of K(+) along the axes of the coleoptiles were calculated for the anoxic seedlings only, for which we assessed the elongation zone to be only the basal 4 mm. K(+) deposition in the basal 6 mm was similar for seedlings with or without exogenous K(+), at 0.6-0.87 micro mol g(-1) f. wt h(-1). Deposition rates in zones above 6 mm from the base were greater for seedlings with, than without, exogenous K(+); the latter were sometimes negative. We conclude that for the coleoptiles of rice seedlings, anoxia inhibits net K(+) uptake from the external solution to a much larger extent than K(+) translocation from the caryopses. Furthermore, K(+) concentrations in the elongation zone of the coleoptiles of anoxic seedlings were maintained to a remarkable degree, contributing to maintenance of pi(sap) in cells of these elongating tissues.