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Dry and salty: a recipe for reduced photosynthesis
There are several situations in which plants suffer from reduced availability of water. One of these is low soil water content and another is salinity where water uptake is impeded by highly negative osmotic potentials. Plants that grow in habitats in which they are exposed to both salt and drought are therefore exposed to double jeopardy. One such species is the shrub Lycium nodosum, a member of the Solanaceae, which grows in the coastal scrub communities in Venezuela. Tezara and colleagues at Caracas (pp. 757-765) report that plants may be exposed to salt both from the soil and from sea spray. They are also exposed to very low soil water content during the dry season. The authors investigated plants growing in natural habitats varying in salt exposure in both the wet and dry seasons and greenhouse-grown plants exposed to individual ‘stress factors’: water deficit or salt spray or salt irrigation. Plants growing nearer the sea have more succulent leaves than those growing further inland and succulence was also induced in the greenhouse-grown plants that were exposed to salt spray or salt irrigation, suggesting that this feature is an adaptation to salinity. A further adaptation was osmotic adjustment, especially in salt-irrigated plants. In general, effects on photosynthesis were more severe for water deficit than for salinity but both factors eventually led to marked reductions in photosynthetic CO2 fixation. Reduction in the rate of photosynthesis was, as may be expected, correlated with stomatal closure, but it was also clear that carboxylation efficiency was lowered. However, in contrast to some other plants, this was not caused by photo-inhibition (chlorophyll fluorescence ratios were unaffected, indicating tolerance to high irradiance), suggesting that there are metabolic factors that affect photosynthetic efficiency. Overall, the authors conclude that these responses ‘allow the plants to maintain a positive carbon balance and growth in a stressful environment’.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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