Water relations of salt stressed wheat
The present study was conducted to investigate the water relations of individual plant cells and the biophysical parameters controlling plant growth in the context of salt stress. Growth and water relations were studied in growing as well as in mature zones of the first emerged leaf of wheat seedlings (cv. Flanders, a British variety) in the context of NaCl stress. Various levels of NaCl (0,25,50,75,100,125 and 150 mol m) -3 were used to salinize the media. I In the case of leaf elongation rate a two phase response was found i. e. an immediate decrease and then, a recovery in the elongation rate. Leaf elongation rate decreased within 1-2 minutes of the onset of stress and, later, a recovery started 1-2 h after the salt addition. The time taken for the recovery was proportional to the levels of external salinity. After 24 h the elongation rate was almost fully recovered for all the NaCl concentrations. A similar response was observed when equi-osmolar concentrations (with NaCl) of mannitol were added to the media. In control plants turgor pressure of the expanding cells was about 0.45 MPa while tissue osmotic pressure was equal to 1.1 MPa showing that the cell had a low water potential (-0.6 MPa). The transpiration tension was equivalent to 0.1 MPa. Turgor pressure in th e growing cells did not change after the salt addition (0- 150 mol m-3 NaCl), however, the tissue osmotic pressure continuously increased with time. Turgor pressure dropped when more -3 than 150 mol m NaCl were applied to the media i. e. 200 and 250 mol m. -3 This is presented as evidence that growing leaf cells - maintained their turgor pressure In response. to . the salt stress by taking up osmotically -active solutes present in the cell wall. The salt stress had not any effect on Instron tensiometric measurements of elastic and plastic extension of the cell wall. A different turgor pressure response was found in the mature cells. Turgor pressure was about 1.0 MPa, almost twice that in the growing cells, while tissue osmotic pressure was similar to that found in the growing cells i. e. 1.1 MPa. After the application of the stress the turgor pressure dropped within 15- 20 min of the application of all the concentrations of NaCl. The osmotic pressure of osmotically active solutes present in the cell wall, nwr was almost negligible i. e. 4 0.1 MPa, in mature cells and so could not contribute to turgor maiýtenance. The extent of decrease was proportional to the external stress of 25, 50 and 75 mol m-3 NaCl only. Turgor pressure recovery, due to osmotic adjustment, started after about 10-12 h of the stress initiation. Complete turgor recovery was achieved after 24-48 h of the onset of stress depending on the applied NaCl concentration. Tissue osmotic pressure increased continuously with time. An increase in the nw was inferred during the whole experimental period and after 6d of the stress application that appeared to correspond to the magnitude of external stress. The concentrations of major ions and sugars were determined to measure their contribution towards the osmotic adjustment. Under control conditions Na +, ci-, PO 4 3- ' so 4 2- , glucose, fructose and sucrose were present in small amounts, while, K+ and No 3- were the-major osmotica. Their concentrations were about 200. mol _m-3. After the stress a large increase in the concentrations of Na + and Cl was observed, the sucrose concentration increased to a small extent. However, other osmotica remained Uniform for whole of the experimental time. A small decrease was observed in k+ concentration in response to higher salt levels. volumetric elastic modulus, -c, of mature cells was remained unchanged by the salt stress. However, the apparent resistance of the root cortex to osmotically driven water flow increased with the increase in stress level. No conclusion could be drawn about the contribution of these parameters to the control of growth and to leaf water relations in the context of salt stress. The possible use of turgor pressure recovery in the mature cells was investigated for assessing the extent of salt tolerance of various Pakistani wheat varieties. These varieties were previously rated according to their performance in absolute grain yield in response to NaCl stress. No simple correlation was found.