Growth of epicotyls, turgor maintenance and osmotic adjustment in pea plants ( Pisum sativum L.) subjected to water stress

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The growth of pea epicotyls was dramatically reduced when subjected to water stress induced by PEG 6000. The degree of inhibition was proportional to the concentration of PEG, although variability among cultivars was observed. Intraspecific variability in growth under water stress could be due to differences in the osmotic adjustment or turgor maintenance capability of each variety. To test this hypothesis, osmotic adjustment (the difference in Ψ s at saturation in watered epicotyls and Ψ s at saturation when epicotyls were at 70% relative water content (RWC), as measured from log Ψ s against log RWC plots) and turgor maintenance (measured from Ψ s versus Ψ w plots as Ψ w at the point of turgor loss) were calculated in epicotyls. All cultivars were capable of osmotic adjustment from 0.30 to 0.65 MPa, while turgor maintenance varied between −2.436 and −3.906 MPa. A significant correlation between growth and osmotic adjustment, and turgor maintenance was observed, but only at the highest concentrations of PEG assayed. The coefficient of correlation was at 30 mM PEG, r=0.70 ( P<0.01) and r=−0.79 ( P<0.01), and at 46 mM PEG, r=0.64 ( P<0.05) and r=−0.89 ( P<0.01) for osmotic adjustment and turgor maintenance, respectively. Water stress induced the accumulation of soluble sugars in epicotyls between 2.8- and 5.1-fold. Their contribution to osmotic adjustment was very important, varying from 34 to 46% depending on cultivar. Free proline in the epicotyls increased between 5- and 50-fold. Its contribution to osmotic adjustment varied from 3 to 5% depending on cultivar. To determine whether osmotic adjustment and turgor maintenance were related in epicotyl and adult stages, a comparison was made between them, and a significant correlation found for turgor maintenance ( r=0.78; P<0.01). The results obtained indicate that measurements made at early stages of development could be used to identify drought-tolerant genotypes.