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Succulent shock: old and young compete for water
It is a well-established idea that during water shortage, plants export water from old leaves to young leaves. The old leaves then senesce, die and drop off. But, suppose the sequence of events is the other way round: perhaps water shortage induces programmed cell death in older leaves (they are, in developmental terms, nearer to senescence than are young leaves). In programmed cell death the plant makes use of the resources (including water) from the dying cells, transporting them to the younger regions, and thus the older leaves wither and fall off. What then would happen if water stress does not induce programmed cell death: do the older leaves export water, but nevertheless survive, or do they retain it? It is this that has exercised Rabas and Martin (University of Kansas, pp. 529-536). They used three succulents, Kalanche tubiflora (a CAM plant), Carpobrotus edulis (regarded as an inducible CAM plant but behaved as a C3 species in these experiments) and Sedum spectabile (inducible CAM plant) in which they examined the effects of water deficit on water relations, CO2 fixation and nocturnal acid accumulation. They obtained no evidence for transfer of water from old to young leaves in any species even under water-stress conditions. Removal of old leaves led to increases in young leaf water content, especially in C. edulis and K. tubiflora, suggesting that there is normally competition for water between young and old leaves, rather than a preferential movement from old leaves. This is supported further by the finding that rates of CO2 fixation (whether C3 or CAM) increased in young leaves after removal of old leaves, especially in conditions of water deficit. These results thus contradict the view that plants move water from old to young leaves during water deficit. It will be interesting to do a similarly rigorous analysis with non-succulent plants.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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