Cell Wall Yield Properties of Growing Tissue

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Abstract
Growing pea stem tissue, when isolated from an external supply of water, undergoes stress relaxation because of continued loosening of the cell wall. A theoretical analysis is presented to show that such stress relaxation should result in an exponential decrease in turgor pressure down to the yield threshold (Y), with a rate constant given by .vphi..epsilon. where .vphi. is the metabolically maintained irreversible extensibility of the cell wall and .epsilon. is the volumetric elastic modulus of the cell. This theory represents a new method to determine .vphi. in growing tissues. Stress relaxation was measured in pea (Pisum sativum L.) stem segments using the pressure microprobe technique. From the rate of stress relaxation .vphi. of segments pretreated with water was calculated to be 0.08 per megapascal h-1 while that of auxin-pretreated tissue was 0.24 per megapascal h-1. These values agreed closely with estimates of .vphi. made by a steady-state technique. The yield threshold (0.29 megapascal) was not affeced by auxin. Technical difficulties with measuring .vphi. by stress relaxation may arise due to an internal water reserve or due to changes in .vphi. subsequent to excision. These difficulties are discussed and evaluated. A theoretical analysis is also presented to show that the tissue hydraulic conductance may be estimated from the T1/2 of tissue swelling. Experimentally, peas stems had a swelling T1/2 of 2.0 min, corresponding to a relative hydraulic conductance of about 2.0 per megapascal h-1. This value is at least 8 times larger than .vphi.. From these data and from computer modeling, it appears that the radial gradient in water potential which sustains water uptake in growing pea segments is small (0.04 megapascal). This means that hydraulic conductance does not substantially restrict growth. The results also demonstrate that the stimulation of growth by auxin can be entirely accounted for by the change in .vphi.