Uptake of Dinitrophenol & its Effect on Transpiration & Calcium Accumulation in Barley Seedlings

Abstract

There is still no general agreement as to the significance of transpiration in the overall process of ion accumulation in the shoots of intact plants. In a recent review, Russell and Barber (19) concluded that ions are first actively transported into the stele of the root and that thereafter transpiration may or may not affect their rate of movement to the shoot depending on the salt status of the root, the external ionic concentration, and the rate of transpiration. In contrast, Hylmo and his co-workers (12, 13, 14, 16), while acknowledging the existence of an active transport component, stress the importance of transpiration in the movement of ions to the shoot. Since the present work was completed, Falk (15) has shown, for K and NO3, that active transport predominates over the concentration range generally found in soil solution and nutrient cultures, while at high concentrations (> 10 mm) mass flow in the transpiration stream becomes progressively dominant. Moore et al. (17), concluded most, if not all, Ca was taken up passively. They showed with excised barley roots that 2,4-dinitrophenol (DNP) did not inhibit Ca uptake although it inhibited K and Mg uptake. If their conclusion is correct, Ca must accumulate in the shoot independent of metabolism in the root either by movement into the xylem vessels through the symplast (this assumes that at least part of symplast is free space for Ca) or along cell walls. The route along the walls would quite probably be blocked by the suberized thickening of the endodermal cell walls. An alternative route would be directly up the root along the cortical cell walls, but this route would seem to be quite slow. If Ca is accumulated in the shoot passively, then the accumulation might be directly correlated with transpiration. However, in view of the work of Epstein (7), Crafts (6), and others, it seems more plausible that Ca moves across the plasmalemma by the expenditure of metabolic energy (like other ions), into the symplast and finally into the xylem. In the present work, the Ca level, DNP level, O.0 level, root viability, and transpiration rates were varied to determine if Ca was actively accumulated by the barley plant.