4.3.4 Cell wall properties: determinants of growth rate

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Figure 4.21  Turgor pressure measured in soybean leaves elongating at different rates imposed by water deficits. Plants were grown in chambers (circles) and outdoors (squares). Note that the linear relationship between growth and P holds in both environments in spite of shifts in slope and intercept, denoting changes in φ and Pth respectively


Figure 4.22 Spatial distribution of (a) elongation rates and (b) turgor pressures along apical zones of maize roots grown either in well-watered (Ψ = -0.02 MPa; filled circles) or rather dry (Ψ = -1.6 MPa; open circles) vermiculite. Note that drought only depressed growth at positions more than 2 mm from the apex but P was lower at all positions on droughted roots. (From Spollen and Sharp 1991; reproduced with permission ofthe American Society of Plant Physiologists)

Investigating the relationship of cell and tissue growth to P gives some indication of the importance of φ and Pth, in growth regulation. Led by the idea that the primary determinant of growth is P, many experimenters have set out to show that growth and P are correlated in multicellular tissues. Occasionally strong correlations are found but more often not. First, the Lockhart analysis is not designed for multi-cellular tissues which have specialised growth zones, poorly defined cell geometries or strong gradients in water activity. Moreover, during the long periods taken to assess growth rates, water-stressed tissues might develop higher Π, leading to recovery in P. Simultaneous changes in cell wall rheology can be induced at low P, raising growth rates above the low values predicted if φ and Pth, were constant. Overall, short-term perturbations to growth of single cells and simple tissues are most likely to reveal the subtle role played by wall rheology in growth.

Bunce (1977) showed that drought—induced variations in P in soybean leaves correlated with variations in growth rate (Figure 4.21). Furthermore, a comparison between plants grown in chambers and outdoors reveals a shift in the relationship between leaf elongation and P. Specifically, leaf growth of plants grown outdoors was halved by a 0.1 MPa drop in P while chamber—grown plants experienced only a 30% drop over the same range. Putting aside the heterogeneity within a growing soybean leaf, Equation 4.11 suggests that both φ and Pth have changed in response to plant growth conditions. Cell walls of plants growing outside yielded more readily to P (lower Pth) and then extended faster than the cell walls of plants in chambers (greater φ). In bean leaves exposed to light to initiate rapid growth (van Volkenburg and Cleland 1986), φ and Pth changed over time but long—term growth effects were more closely related to φ than P - Pth. Short-term fluctuations in growth were ascribed to changes in P - Pth, implying subtle roles for the two variables of the Lockhart equation in whole plants.

In roots, too, cell wall properties participate in growth regulation (Figure 4.22a and b). Drought—induced water deficits in maize roots lowered P in apices to about 0.3 MPa, almost 0.4 MPa below that of well—watered roots (Figure 4.22b) (Spollen and Sharp 1991). Overall growth rates of the two sets of roots in wet and dry conditions were 2.8 and 1.0 mm h-1, respectively. In broad terms, this might be thought to represent a case of lowered P inhibiting root growth but local rates of elongation along the root axis (Figure 4.22a) show that the distribution of growth is also affected by drought. Figures 4.22(a) and (b) together demonstrate that even in roots with a restricted water supply and low P, tissues less than 2 mm from the apex of the roots always elongated at the same rate. Reduced water supply to the roots therefore induced a change in either φ or Pth of the most apical cells, allowing these distal cells to continue elongating unabated in dry conditions.