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We examined the effect of water stress on leaf demography in four grass species known to differ in degree of drought resistance. Stress was imposed by growing plants in solution culture and lowering water potential of the solution either slowly, or rapidly, by the addition of polyethylene glycol. Rapid stress reduced the number of leaves by increasing leaf death rate and decreasing birth rate. Gradual stress reduced the number of leaves by temporarily decreasing leaf birth rate; death rate also decreased, partially compensating for the reduction in birth rate. Death rates were age specific; rapid stress had little effect on young leaves, and gradual stress increased leaf longevity to the greatest extent in those leaves that emerged immediately prior to the imposition of stress. As a result, plants subjected to rapid stress had a young leaf age structure, while plants subjected to gradual stress had an older age structure. Differences among species in tolerance to water stress were related in part to differences in leaf age structure. Species with a young leaf age structure exhibited less of an increase in leaf mortality in response to rapid stress, and more of an increase in leaf longevity in response to gradual stress than other species. Key words: drought tolerance, leaf turnover, leaf age structure, polyethylene glycol, leaf senescence. |
