HETEROGENOUS STOMATAL CLOSURE IN RESPONSE TO LEAF WATER DEFICITS IS NOT A UNIVERSAL PHENOMENON

The extent and occurrence of water stress-induced "patchy" CO2 uptake across the surface of leaves was evaluated in a number of plant species. Leaves, while still attached to a plant, were illuminated and exposed to air containing [C-14]CO2 before autoradiographs were developed. Plant water deficits that caused leaf water potential depression to -1.1 megapascals during a 4-day period did result in heterogenous CO2 assimilation patterns in bean (Phaseolus vulgaris). However, when the same level of stress was imposed more gradually (during 17 days), no patchy stomatal closure was evident. The patchy CO2 assimilation pattern that occurs when bean plants are subjected to a rapidly imposed stress could induce artifacts in gas exchange studies such that an effect of stress on chloroplast metabolism is incorrectly deduced. This problem was characterized by examining the relationship between photosynthesis and internal [CO2] in stressed bean leaves. When extent of heterogenous CO2 uptake was estimated and accounted for, there appeared to be little difference in this relationship between control and stressed leaves. Subjecting spinach (Spinacea oleracea) plants to stress (leaf water potential depression to -1.5 megapascals) did not appear to cause patchy stomatal closure. Wheat (Triticum aestivum) plants also showed homogenous CO2 assimilation patterns when stressed to a leaf water potential of -2.6 megapascals. It was concluded that water stress-induced patchy stomatal closure can occur to an extent that could influence the analysis of gas exchange studies. However, this phenomenon was not found to be a general response. Not all stress regimes will induce patchiness; nor will all plant species demonstrate this response to water deficits.