Responses of temperate plants to drought conditions

Mitigating factors?

What about interaction of drought with other factors?

Plants often face more than one stress at a time. A combination of stresses may produce a different response than just one stress alone. In this work I am primarily interested in presenting the effect of nutrient limitations in conjunction with drought, as well as the possible interactions with changing levels of atmospheric CO2 (a possible alleviation of CO2 limitation or "stress"). If we expect that elevated CO2 will be accompanied by climate change, which could include changes in precipitation patterns, and that plant responses to elevated CO2 depend on soil fertility, then a number of questions arise: Will plants be better able to tolerate drought under future enhanced CO2 conditions? Will nutrients play a role in influencing this interaction?

Elevated CO2 and drought stress

Traditionally elevated CO2 is perceived as having a beneficial effect on carbon and water balances within a plant. Higher atmospheric CO2 levels should result in greater photosynthesis (as many plants are CO2 limited) but also lower conductances, thus allowing the plant to increase its water use efficiency, thus minimizing its exposure to drought. In terms of direct effects on growth, a higher assimilation rate should mean that more solutes are imported into growing cells, allowing them to maintain a higher turgor and thus a high expansion rate. A better supply of assimilates might also improve the expandability of the cell walls. These changes could also come about with the improved water status of the plant that is afforded to it by the greater water use efficiency. This would imply that plants should be more drought resistant when exposed to enhanced CO2 levels (Hsiao and Jackson 1999). Actual field results on this topic are extremely variable, in some cases deviating from the above assumptions (Palanisamy 1999; Schafer et al. 2002). In terms of the net interaction between CO2 and drought there is also a range of findings which alternately support the idea that CO2 can (Johnson et al. 2002) or cannot (Palanisamy 1999; Gunderson et al. 2002) compensate for drought conditions in terms of growth and photosynthesis. This question seems far from answered.

Plant nutrition and drought stress

Nutrient limitation in itself is a plant stressor. The intention here is not to go through all the effects of nutrient limitations, but highlight those that have a potential to interact with drought effects. Some things to consider:

• nutrient uptake is dependent on the availability of water in the soil
• nutrient use efficiency cannot be maximized at the same time as water use efficiency
• efficiency of nutrient transport (within the plant) is reduced with drought stress

Clearly these processes have the potential to bring on nutrient stress, which can create another set of problems for the plant. However, one adaption to nutrient stress is slow growth rate (low demand for nutrients), which leads to a low demand for water and consequently a decreased sensitivity to water stress (Nilsen and Orcutt 1996).

Predictions for the future

There seem to be many possibilities for the convergence of CO2, nutrients and drought. With the continuing human- induced environmental changes, we are likely to find out just what they are.