INTERACTION BETWEEN MILD NACL SALINITY AND SUBLETHAL SO2 POLLUTION ONWHEAT TRITICUM-AESTIVUM CULTIVAR WILGOYNE (CIANO-GALLO) .1. RESPONSESOF STOMATAL CONDUCTANCE, PHOTOSYNTHESIS, GROWTH AND ASSIMILATE PARTITIONING

Wheat (Triticum aestivum cultivar 'Wilgoyne' (Ciano/Gallo)) plants wer e exposed to a factorial combination of two levels of salinity (contro l and 50 mM NaCl), and three levels of SO2 (< 10 (ambient), 231, and 4 41 nl l-1) in fumigation chambers with rain-exclusion tops for 4 h per day for up to 51 days. The objective was to investigate whether mild salinity stress could decrease the sensitivity of plant growth to subl ethal SO2 stress. Exposure to 441 nl l-1 SO2 significantly decreased C O2 stomatal conductance, net photosynthesis rate, and at the end of ex periment, chlorophyll concentrations in the youngest fully expanded (Y FE) leaves. Inhibition of photosynthesis by 441 nl l-1 SO2 fumigation resulted in a change in assimilate allocation in favour of shoot growt h, leading to an increase in shoot to root ratio. Exposure to 441, but not 231 nl l-1 SO2 significantly decreased plant dry weight and relat ive growth rate (RGR) of both root and shoot. 50 mM NaCl treatment fir st decreased shoot growth, resulting in a decrease in shoot to root ra tio, but later a significant decrease in root growth developed, probab ly owing to limited assimilate supply from the shoot, and resulted in an increase in shoot to root ratio. There was generally no significant interaction between SO2 fumigation and NaCl salinity on net photosynt hesis rate and leaf chlorophyll concentration. However, CO2 stomatal c onductance in the YFE leaves of 50 mM NaCl-treated and SO2-fumigated p lants recovered more quickly after the completion of fumigation than t he non-saline and SO2-fumigated plants. 50 mM NaCl salinity and 441 nl l-1 SO2 fumigation had an antagonistic interaction on both shoot and root dry weight. Exposure to 231 nl l-1 SO2 increased the shoot to roo t ratio in plants with 50 mM NaCl. No significant interaction between SO2 fumigation and 50 mM NaCl was observed on the RGR of both shoot an d root. The results showed that 50 mM NaCl salinity did not provide an effective protection against SO2 uptake in the leaves by increasing s tomatal resistance during SO2 fumigation. Other physiological mechanis ms could be involved in the antagonistic interaction between 50 mM NaC l salinity and 441 nl l-1 SO2 fumigation on plant growth.