Growth and physiology of spring wheat under saline conditions
A series of experiments were carried out in solution culture in growt~ ~ooms and a glasshouse, to study the effects of sallnlty on leaf extension rate ion concentrations, sap osmotic pressure, net photosynthesis and related parameters, stom~tal frequency, specific leaf weight and a number of agronomlc parameters of spring wheat. Rate of net photosynthesis, transpiration rate, stomatal conductance and sub-stomatal carbon dioxide concentration per unit area of leaf were determined using an Infra-red Gas Analyser. Experiments 1 and 3 were conducted in growth rooms set at a temperature cycle of 24°Cj16°C day and night and photoperiod of 16 hours. The seedlings received light from a bank of 125W fluorescent 'warm white' lights which provided between 200-300 ~mol m- 2 s-l photosynthetically active radiation at initial plant level. Experiments 2, 4 and 5 were carried out in a glasshouse with no control of light and temperature. In Experiment 1 the salinity treatments tested were control (0 mol m- 3 NaCI), 'constant' and 'variable' salinity. In the constant salinity treatment plants were grown at 100 mol m- 3 NaCI all the time after initial salt stress. In the variable salinity treatment a 12 day cycle was repeated with daily increments of 10 mol m- 3 NaCI after initial salt stress of 50 mol m- 3 NaCI till it reached to 150 mol m- 3 NaCI. During the final two days of the cycle salinity was stepped down from 150 to 100 to 50 mol m- 3 NaCl. In Experiment 2 the salinity levels tested were 0, 50, 100 and 150 mol m- 3 NaCI. CaCI was added in this and later experiments at 20:1 (mol Na~:mol ca2+) ratio. The results of the both Experiments 1 and 2 suggested that salinity had no effect on leaf appearance stage but tiller production was decreased. Salinity decreased leaf extension rate and final leaf length but leaf extension duration was not affected. Although leaf extension rate was the main factor influencing final leaf length, there were no consistent quantitative relationships between these parameters in different leaves and at different salinity levels. Plants in variable salinity performed better than those in constant salinity but these treatments were not significantly different and gave similar results. The results of Experiment 2 showed that a gradient of Na+ and Cl- concentrations was found in different leaves. Higher Na+ and Cl- concentrations were found in lower leaves than in expanding leaves. Calculated Na+ and CI- contents (ion concentrations x dry weight) suggested that these ions were mainly located in roots, stem and tillers irrespective of salinity levels. The effect of salinity was to increase concentrations of leaf Na+, Cl- and sap osmotic pressure in the youngest fully expanded leaves whereas K+ concentration was inconsistently affected. When gas exchange measurements were made in situ on leaves, light intensity showed wide i variation due to movement of clouds. Variations in light intensity and absence of any equilibration prior to measurements made it difficult to detect any effects of salinity on gas exchange. Therefore to determine the effects of salinity on gas exchange in expanding and senescing leaves, in Experiments 3, 4 and 5, a strong light source capable of providing photon flux densities at or near light saturation for gas exchange was used. In Experiments 3 and 4 light response curves were produced using neutral density filters. Using an exponential model, maximum net photosynthesis photosynthetic efficiency, photon flux compensation point and dark respiration for salinities and leaf insertions were calculated. In Exper~~ent 3 the .s~linity levels tested were 0, 100 and 200 mol m NaCI. Sa11n1ty decreased green lamina area, maximum and net photosynthesis, stomatal conductance, transpiration rate, leaf productivity but increased dark respiration and photon flux compensation point. Photosynthetic efficiency and transpiration efficiency were inconsistently affected. In Experiment 3 at 200 mol m- 3 NaCI leaf 6 senesced rapidly. Therefore in Experiment 4 the salinity levels tested were 0, 75 and 150 mol m- 3 NaCI. In Experiment 4 the parameters studied were identical to those in Experiment 3 except that the measurements were performed on leaf 5 and the flag leaf. In Experiment 4 a similar trend for gas exchange parameters was noted at 0 and 150 mol m- 3 NaCl but at 75 mol m- 3 NaCI Pn was higher than in the control due to delayed senescence. In both Experiments 3 and 4 leaf sap Na+, CI- and osmotic pressure increased and Pn decreased during senescence but there were no consistent relationships between these parameters for different leaves and salinity treatments. Experiments 2, 3 and 4 suggested that salinity increased stomatal frequency per unit leaf area but stomatal frequency per leaf and specific leaf weight were inconsistently affected. Experiment 5 was conducted to examine the effects of salinity on changes in gas exchange in the flag leaf and two penultimate leaves simultaneously. The salinity levels tested were 0, 75 and 150 mol m- 3 NaCI. The leaf x salinity interaction showed that salinity had larger effects on the flag leaf than leaves 2 and 3. The leaf x salinity interaction was significant for leaf temperature, net photosynthesis, stomatal conductance, transpiration rate and transpiration efficiency but not for sUb-stomatal carbon dioxide concentration. Salinity significantly decreased all the yield components and grain yield. The results of these experiments suggest that salinity had large effects on photosynthesis, dry matter production and grain yield and that ion concentrations do not determine the observed changes in net photosynthesis with leaf age in salt stressed plants.