Differential shrinkage of mesophyll cells in transpiring cotton leaves: implications for static and dynamic pools of water, and for water transport pathways
Martin Canny A D , Suan Chin Wong A , Cheng Huang B and Celia Miller CA Plant Science Division, Research School of Biology, RN Robertson Building, The Australian National University, Canberra, ACT 0200, Australia.
B Centre for Advanced Microscopy, The Australian National University, Canberra, ACT 0200, Australia.
C Division of Plant Industry, CSIRO, Canberra, ACT 2601, Australia.
D Corresponding author. Email: martin.canny@anu.edu.au
Functional Plant Biology 39(2) 91-102 https://doi.org/10.1071/FP11172
Submitted: 3 August 2011 Accepted: 15 November 2011 Published: 13 December 2011
Abstract
Shrinkage of palisade cells during transpiration, previously measured for sclerophyllous leaves of Eucalyptus where cells shrank equally, was compared with shrinkage in thin mesophytic leaves of cotton (Gossypium hirsutum L.). Selected vapour pressure differences (Δe) from 0.6 to 2.7 kPa were imposed during steady-state photosynthesis and transpiration. Leaves were then cryo-fixed and cryo-planed paradermally, and images obtained with a cryo-scanning electron microscope (CSEM). Diameters of palisade ‘cavity cells’ within sub-stomatal cavities, and surrounding palisade ‘matrix cells’ were measured on CSEM images. Cavity and spongy mesophyll cells shrank progressively down to Δe = 2.7 kPa, while matrix cells remained at the same diameter at all Δe. Diameters were also measured of cavity and matrix cells quasi-equilibrated with relative humidities (RHs) from 100% to 86%. In leaves quasi-equilibrated with 95% RH, the cavity cells shrank so much as to be almost unmeasurable, while matrix cells shrank by only 6%. These data suggest that there are two distinct pools of water in cotton leaves: cavity plus spongy mesophyll cells (two-thirds of leaf volume) which easily lose water; and matrix cells (one-third of leaf volume), which retain turgor down to relative water loss = 0.4, providing structural rigidity to prevent wilting. This phenomenon is probably widespread among mesophytic leaves.
Additional keywords: cell interconnections, relative water loss, sites of evaporation, stomatal cavities, transpiration pathway, wilting resistance.
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