4.3.5  How rapidly do cell walls respond to changes in P?

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Precise changes in P can be achieved by varying the concentration of slowly penetrating solutes (e.g. mannitol) in the bathing medium. Imposing an increase in Πoutside (= Ψoutside) induces an abrupt, almost equivalent decrease in P. Changes in wall rheology often follow rapidly, in the order of minutes and may be faster. In maize roots, Frensch and Hsiao (1994) used this approach to show that loss of P was compensated for by resetting of Pth to a lower level, helping to hold P – Pth constant and maintain growth rate. Speculation on how cell wall properties are rapidly modified appears in Section 4.3.6. The lower P in roots subjected to lower Ψoutside was temporary; recovery of P by up to 0.3 MPa followed over the next 30 min. Cells deep within the root recovered first, suggesting that sugars unloading from the stele to re-establish P act in concert with changes in cell walls to maintain root growth under drying conditions.


Figure 4.23    Effect of a perturbation in P on elongation of giant algal cells (Nitella). A 50 kPa increase in P caused a transient (within minutes) increase in growth followed by a return to initial elongation rate. Sustained high P did not further affect elongation of cells (From Green et al. 1971; reproduced with permission of the American Society of Plant Physiologists)

Cells of the giant alga Nitella have provided the most convincing evidence for wall properties of single cells responding to P (Green et al. 1971). Again, P was manipulated by making rapid changes in Πoutside. When a 50 kPa increase in P was induced by placing the cell in hypotonic solution (Figure 4.23), the elongation rate rose rapidly (within minutes) but quickly settled to the same rate as the cells in the original solution. That is, cell elongation was independent of P within 10–15 min of the initial perturbation. Clearly cell walls were responding to the altered water status of the cells, becoming either less extensible (lower φ) or yielding less readily to internal pressure (higher Pth).

Later experiments have been performed on barley seedlings in which roots were pressurised in order to elevate artificially shoot P. In this case, a sustained rise in shoot P induced by pressurising the roots and xylem sap caused a temporary jump in leaf elongation rates but a return to existing rates was, as with Nitella, rapid. Again, the cell wall properties (φ and Pth) enunciated in the Lockhart equation responded to the water status of the growing cell.

Growth can also be manipulated by photoinhibiting pea seedlings with blue and red light (Kigel and Cosgrove 1991). No change in water supply was imposed and yet reduced cell wall relaxation was apparent through either lowering of φ (blue light) or increases in Pth (red light). It appears that cell wall properties induce changes in growth rate, regardless of whether growth is perturbed by light treatments or hydraulic effects. Substantial effort is now directed towards defining molecular events which start and stop growth.