Oecologia (1989) 81 : 124-131 Oecologia 9 Springer-Verlag1989 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 James S. Coleman*, Harold A. Mooney, and James N. Gorham Department of Biological Sciences, Stanford University, Stanford, CA 94305 USA Summary. Acclimation of wild radish plants to a simulta- neous combination of SOz fumigation and decreasing ni- trate availability was investigated. Plants were grown for 24 d under continuous daytime (10 h) exposure to 0 or 0.4 ppm SO2 and were grown in a nutrient solution with stable nitrate concentrations of 100 gM for the first 15 d, 50 gM from day 15 to day 19, and 25 gM 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 re- sponses. Shoot RGR of plants from both treatments and root RGR of control plants showed rapid declines and sub- sequent recoveries in response to decreasing nitrate avail- ability. Root RGR of SO/-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 availabili- ty 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 tempo- rarily allocated more resources to physiological processes occurring in roots, such as nutrient uptake. Although SOz- 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 subse- quently affected the ability of plants to respond to de- creased nitrate availability, by affecting resource partition- ing to nitrate uptake and root growth. Key words: Plant stress - Raphanus sativus X raphanistrum - Air pollution effects - Plant nutrition - Physiological responses Plants show extensive plasticity in their acquisition and par- titioning of resources in response to environmental condi- tions that limit resource availability (Bloom et al. 1985; Ro- binson 1986; Szaniawski 1987). Factors that limit carbon * Current address and addressfor offprint requests: Biological Lab- oratories, 16 Divinity Avenue, Harvard University, Cambridge, MA 02138, USA gain, such as air pollution or low light, shift the resource partitioning of plants to favor shoot growth and leaf pro- duction (Davidson 1969; Lechowicz 1987; Szaniawski 1987; Mooney and Winner 1989). For example, fumigation of radish plants (Raphanus sativus cv. "Cherry Belle") with SO/reduced their photosynthetic performance by reducing carboxylating capacity, and subsequently reduced their growth rates (Mooney et al. 1988). However, these plants compensated for SO/-induced reductions in carbon gain by increasing partitioning of resources to the production of new leaves, thus reducing the loss of growth potential (Mooney et al. 1988). Plants may also compensate for nu- trient or water deficiency by partitioning proportionally more resources to root production (Chapin 1980; Bloom et al. 1985 ; Robinson 1986; Szaniawski 1987; Ingestad and Agren 1988; Mooney and Winner 1989), or by increasing the efficacy of resource uptake by roots (Clarkson 1985; Robinson 1986; Chapin et al. 1987). For example, wild rad- ish plants (R. sativus X raphanistrum) responded to nitrate depletion in the root environment with increased root: shoot biomass, nitrate uptake and nitrogen-use efficiency (Koch 1988; Koch et al. 1989; Koch and Mooney 1989). These responses enabled radish plants to grow at high rela- tive growth rates in environments with relatively low nitrate availability (Koch et al. 1989; Koch and Mooney 1989). Although these types of whole-plant responses to indi- vidual shoot and root stresses have been extensively studied in radish and other plants, little is known regarding the responses of plants to simultaneous limitations of carbon gain and nutrient availability. Yet, it is the response of plants to combinations of resource limitations that may ultimately determine plant growth, survival and reproduc- tive output in natural environments (Chapin et al. 1987). Given that radish plants responded to SO2-induced carbon limitation by partitioning resources to shoots at the expense of roots, we hypothesized that SO2 fumigated plants would have a reduced capacity to exhibit compensatory root re- sponses to step-decreases in nitrate availability than control plants. Here we report a partial test of this hypothesis. Material and methods Plant materials and growth conditions Seeds of wild radish (Raphanus sativus X raphanistrum) were obtained from wild populations growing near Stanford,