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After the flood - photosynthesis
Ranunculus repens is a terrestrial plant that thrives in damp habitats. It can survive total immersion for several days during which it develops aerenchyma and exhibits the petiole elongation response (coming up for air). However, in western Ireland there is a habitat that provides a more extreme challenge to the flexibility of this species. Turloughs are temporary limestone lakes (lough is the Gaelic word for lake) that exist for several months of the year, and in these lakes there is a distinct population of R. repens. Morphologically, the population is distinguished by having much more dissected leaves than the normal form. The morphological differences are genetically determined and it is thus possible that the turlough population is a distinct ecotype adapted to this unusual habitat. Certainly the coping mechanisms exhibited by normal populations during temporary submergence are not adequate for long-term survival in several metres of water. This has led Lynn and Waldren (Dublin; pp. 707-714) to compare the turlough population with a normal ruderal population in respect of several physiological characters, of which I focus on photosynthesis. Plants were grown in flooded or non-flooded soil for 8 weeks before photosynthetic rates were measured. In both populations, flooded plants showed slightly higher rates than non-flooded plants, but rates in the turlough population were two- to three-times greater in both flooded and non-flooded conditions than those of the ruderal population. When photosynthesis was measured in submerged plants, both populations exhibited rates of only about 5 % of those in non-submerged plants, maintaining the difference between the two populations. Intriguingly, given the alkalinity of the lakes, neither form could use bicarbonate as a carbon source. Taking these and their other data, the authors suggest that the leaf morphology of the turlough population may enhance gas exchange, permitting the accumulation of storage carbohydrates for times when photosynthetic rates are severely depressed by submergence.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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