Responses by Coleoptiles of Intact Rice Seedlings to Anoxia: K+ Net Uptake from the External Solution and Translocation from the Caryopses |
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Authors: | HUANG S; GREENWAY H; COLMER T D |
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Institution: | 1 School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, 6009 WA, Australia |
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Abstract: | This study evaluated the effects of anoxia on K+ uptake andtranslocation in 34-d-old, intact, rice seedlings (Oryzasativa L. cv. Calrose). Rates of net K+ uptake from the mediumover 24 h by coleoptiles of anoxic seedlings were inhibitedby 8391 %, when compared with rates in aerated seedlings.Similar uptake rates, and degree of inhibition due to anoxia,were found for Rb+ when supplied over 1·52 h,starting 22 h after imposing anoxia. The Rb+ uptake indicatedthat intact coleoptiles take up ions directly from the externalsolution. Monovalent cation (K+ and Rb+) net uptake from thesolution was inhibited by anoxia to the same degree for thecoleoptiles of intact seedlings and for coleoptiles excised,aged, and supplied with exogenous glucose. Transportof endogenous K+ from caryopses to coleoptiles was inhibitedless by anoxia than net K+ uptake from the solution, the inhibitionbeing 55 % rather than 87 %. Despite these inhibitions,osmotic pressures of sap (sap) expressed from coleoptiles ofseedlings exposed to 48 h of anoxia, with or without exogenousK+, were 0·66 ± 0·03 MPa; however,the contributions of K+ to sap were 23 and 16 %, respectively.After 24 h of anoxia, the K+ concentrations in the basal10 mm of the coleoptiles of seedlings with or without exogenousK+, were similar to those in aerated seedlings with exogenousK+. In contrast, K+ concentrations had decreased in aeratedseedlings without exogenous K+, presumably due to dilutionby 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 werecalculated for the anoxic seedlings only, for which we assessedthe elongation zone to be only the basal 4 mm. K+ depositionin the basal 6 mm was similar for seedlings with or withoutexogenous K+, at 0·60·87 µmolg1 f. wt h1. Deposition rates in zones above6 mm from the base were greater for seedlings with, thanwithout, exogenous K+; the latter were sometimes negative. Weconclude that for the coleoptiles of rice seedlings, anoxiainhibits net K+ uptake from the external solution to a muchlarger extent than K+ translocation from the caryopses. Furthermore,K+ concentrations in the elongation zone of the coleoptilesof anoxic seedlings were maintained to a remarkable degree,contributing to maintenance of sap in cells of these elongatingtissues. |
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Keywords: | Anoxia caryopsis coleoptile Oryza sativa osmotic pressure potassium uptake translocation seedlings spatial distribution |
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