Water relations of growing pea epicotyl segments

The water relations of growing epicotyl segments of pea (Pisum sativum L.) were studied using the miniaturized pressure probe. For epidermal cells stationary turgor pressures of P=5 to 9 bar and half-times of water exchange of individual cells T_1/2=1 to 27 s were found. The volumetric clastic modulus (e{open}) of epidermal cells varied from 12 to 200 bar and the hydraulic conductivity, Lp=0.2 to 2·10^-6 cm s^-1 bar^-1. For cortical cells P=5 to 11 bar, T_1/2=0.3 to 1 s, Lp=0.4 to 9·10^-5 cm s^-1 bar^-1 and e{open}=6 to 215 bar. The T_1/2 of cortical cells was extremely low and the Lp rather high as compared to other higher plant cells. The T_1/2-values of cortical cells were sometimes observed to change from short to substantially longer values (T_1/2=3 to 20 s). Both short and long pressure relaxations showed all the characteristics of non-artifactual curves. The change is apparently due to an increase in Lp and not e{open}, but the reason for the change in cell permeability to water is not known. In osmotic exchange experiments on peeled segments using solutions of different solutes, the half-time of osmotic water exchange for the whole segment was approximately 60 s. Water exchange occurred too quickly to be rate controlled by solute diffusion in the wall space. The data suggest that the short T_1/2-values in the cortical cells are the physiologically relevant ones for the intact tissue and that a considerable component of water transport occurs in the cell-to-cell pathway, although unstirred layer effects at the boundary between the segment and solution may influence the measured half-time. Using the theory of Molz and Boyer (1978, Plant Physiol. 62, 423-429), the gradient in water potential necessary to maintain the uptake of water for cell enlargement can be calculated from the measured diffusivities to be approximately 0.2 and 1 bar for growth rates of 1% h^-1 and 5% h^-1, respectively. Thus, although the T_1/2-values are short and Lp rather high, there may be a significant osmotic disequilibrium in the most rapidly growing tissue and as a consequence the influence of water transport on the growth rate cannot be excluded.