Effects of auxin and anoxia on the cell wall yield threshold determined by negative pressure jumps in segments of cowpea hypocotyl

The growth of any plant is based on the enlargement of its constituent cells, which is driven by the uptake of water and is supported by the irreversible yielding of the cell wall. The latter process is driven by the effective turgor (Pⁱ-Y), I.e., the excess turgor pressure beyond a critical yield threshold (Y), and it is limited by the physiological extensibility (Φ) of the cell wall, which serves as the coefficient of proportionality. It has been suggested that the plant growth hormone auxin increases Φ, with a resultant promotion of growth, while it does not affect Y (Cleland 1977). Any recorded change in Pⁱ has been very small, if any, during the promotion of growth by auxin.We have developed a novel method called the "negative pressure jump" technique which enables us directly and very quickly to determine in vivo the adjustable yield threshold (Y') with minimum perturbation of the elongation of the cell. The cell turgor is reduced to Y' not osmotically but hydraulically by application of tension to xylem vessels from the drain of the xylem perfusion system. In the case of segments of hypocotyls of cowpea seedlings, auxin was found to reduced Y' definitively (by 40-80 kPa). The contribution of this decrease is comparable in magnitude to that of the increase in Φ with respect to the promotion of growth. Once it has been decreased, Y' is not affected by anoxia.