Abstract
Water movement across plant tissues occurs along two paths: from cell-to-cell and in the apoplasm. We examined the contribution of these two paths to the kinetics of water transport across the parenchymatous midrib tissue of the maize (Zea mays L.) leaf. Water relations parameters (hydraulic conductivity, Lp; cell elastic coefficient, ε; half-time of water exchange for individual cells, T½) 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)·10−5 centimeters per second per bar, ε = 103 ± 72 bar, and T½ = 7.9 ± 4.8 seconds (n = seven cells); whereas, in the abaxial region, Lp = (2.5 ± 0.9)·10−5 centimeters per second per bar, ε = 41 ± 9 bar, and T½ = 1.3 ± 0.5 seconds (n = 7). This zonal variation in Lp, ε, and T½ 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)·10−6 square centimeters per second (n = 6). The diffusivity of the cell-to-cell path (Dc) calculated from pressure probe data ranged from Dc = 0.4·10−6 square centimeters per second in the adaxial region to Dc = 6.1·10−6 square centimeters per second in the abaxial region of the tissue. Dt ≃ Dc suggests substantial cell-to-cell transport of water occurred during rehydration. However, the tissue diffusivity calculated from the kinetics of pressure-propagation across the tissue (Dt′) was Dt′ = (33.1 ± 8.0)·10−6 square centimeters per second (n = 8) and more than 1 order of magnitude larger than Dt. Also, the hydraulic conductance of the midrib tissue (Lpm per square centimeter of surface) estimated from pressure-induced flows across several parenchyma cell layers was Lpm = (8.9 ± 5.6)·10−5 centimeters per second per bar (n = 5) and much larger than Lp.
These results indicate that 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.
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