Effects of multiple stresses on radish growth and resource allocation : I. Responses of wild radish plants to a combination of SO2 exposure and decreasing nitrate availability.

Acclimation of wild radish plants to a simultaneous combination of SO2 fumigation and decreasing nitrate availability was investigated. Plants were grown for 24 d under continuous daytime (10h) exposure to 0 or 0.4 ppm SO2 and were grown in a nutrient solution with stable nitrate concentrations of 100 μM for the first 15 d, 50 μM from day 15 to day 19, and 25 μM from day 19 to day 24. Analysis of relative growth rates (RGR) showed that radish plants responded rapidly to changes in nitrate availability and that SO2 treatment affected those responses. Shoot RGR of plants from both treatments and root RGR of control plants showed rapid declines and subsequent recoveries in response to decreasing nitrate availability. Root RGR of SO2-treated plants declined rapidly in response to decreased nitrate availability, but did not recover as quickly or completely as root RGR of control plants. Analysis of specific leaf weights and tissue nitrogen concentrations showed that control plants had significantly higher amounts of nitrogen in tissues after nitrate availability was lowered, and had higher rates of nitrate uptake in comparison to SO2-treated plants; especially when nitrate availability was highest. Furthermore, control plants had temporarily higher rates of root respiration in comparison to SO2-treated plants, suggesting that control plants temporarily allocated more resources to physiological processes occurring in roots, such as nutrient uptake. Although SO2-induced changes in growth and resource allocation of plants were relatively small, it was probable that SO2 treatment of radish plants affected plant nitrogen balance, and subsequently affected the ability of plants to respond to decreased nitrate availibility, by affecting resource partitioning to nitrate uptake and root growth.