Water Transport in the Midrib Tissue of Maize Leaves

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Abstract
Water movement across plant tissues occurs along 2 paths: from cell-to-cell and in the apoplasm. The contribution of these 2 paths to the kinetics of water transport across the parenchymatous midrib tissue of the maize (Z. mays L.) leaf was examined. Water relations parameters (hydraulic conductivity, Lp; cell elastic coefficient, .epsilon.; half-time of water exchange for individual cells, T1/2) of individual parenchyma cells determined with the pressure probe varied in different regions of the midrib. In the adaxial region, Lp = (0.3 .+-. 0.3) .cntdot. 10-5 cm s-1 per bar, .epsilon. = 103 .+-. 72 bar, and T1/2 = 7.9 .+-. 4.8 s (n = seven cells), whereas, in the abaxial region, Lp = (2.5 .+-. 0.9) .cntdot. 10-5 cm s-1 per bar, .epsilon. = 41 .+-. 9 bar, and T1/2 = 1.3 .+-. 0.5 s (n = 7). This zonal variation in Lp, .epsilon., and T1/2 indicates that tissue inhomogeneities exist for these parameters and could have an effect on the kinetics of water transport across the tissue. The diffusivity of the tissue to water (Dt) obtained from the sorption kinetics of rehydrating tissue was Dt = (1.1 .+-. 0.4) .cntdot. 10-6 cm2 s-1 (n = 6). The diffusivity of the cell-to-cell path (Dc) calculated from pressure probe data ranged from Dc = 0.4 .cntdot. 10-6 cm2 s-1 in the adaxial region to Dc = 6.1 .cntdot. 10-6 cm2 s-1 in the abaxial region of the tissue. Dt .simeq. Dc suggests substantial cell-to-cell transport of water occurred during rehydration. The tissue diffusivity calculated from the kinetics of pressure-propagation across the tissue (Dt'') was Dt'' = (33.1 .+-. 8.0) .cntdot. 10-6 cm2 s-1 (n = 8) and > 1 order of magnitude larger than Dt. Also, the hydraulic conductance of the midrib tissue (Lpm cm-2 of surface) estimated from pressure-induced flows across several parenchyma cell layers was Lpm = (8.9 .+-. 5.6) .cntdot. 10-5 cm s-1 per bar (n = 5) and much larger than Lp. The preferential path for water transport across the midrib tissue depends on the nature of the driving forces present within the tissue. Under osmotic conditions, the cell-to-cell path dominates, whereas under hydrostatic conditions water moves primarily in the apoplasm.