Annals of Botany

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Scientific progress involves both expanding the boundaries of what we know and, from time to time, revising our ideas about what we thought we already knew. The paper by Lecher et al. from Balcarce, Argentina and Montpellier, France (pp. nn–nn) provides a clear example of the second category. They grew Arabidopsis thaliana and Helianthus annuus plants under water deficit and under well-watered conditions. In both species, water deficit markedly reduced leaf growth rate although the period of leaf expansion was similar in the two treatments. Final leaf area in the droughted plants was therefore much smaller than that of control plants. When droughted plants were re-watered, leaves still in the expansion phase showed a much increased expansion rate. However, the really surprising result was that leaves of droughted plants which had stopped expanding (and were assumed therefore to have reached their ‘final’ size) started expanding again. This was especially dramatic in A. thaliana, with some leaves increasing their area by up to 186 %. Increases in H. annuus were smaller (up to 27 %) but nevertheless significant. The increases in area did not involve cell division but were solely caused by cell expansion. Analysis of the response in relation to leaf age showed that the longer the gap between the cessation of expansion in droughted conditions and its re-initiation induced by re-watering, the smaller was the response, eventually declining to zero in the oldest leaves. This indicates that there is a ‘developmental window’ during which it is possible for leaf cells to resume expansion growth. Under the conditions used in these experiments, the window was 4 days in H. annuus and 11 days in A. thaliana. This window is likely to represent a period in which the biophysical/biochemical changes in the cell wall are making the wall more rigid, but until full rigidity is achieved, the wall is able to respond to increased turgor pressure.