Expansion of pea leaves subjected to short water deficit: cell number and cell size are sensitive to stress at different periods of leaf development

We have followed the expansion of individual pea leaves from initiation to maximum area, over two markedly different periods. During the first one (2/3 of total leaf development time), cell production occurred while cell and leaf expansions were slow. Rapid expansion (95% of total) occurred for a second period lasting 1/3 of total development time, whereas cell division was virtually completed. Water deficits of 15 d were applied during either slow or rapid expansion, and characterized by measurements of soil water potential, stomatal conductance, leaf water potential and xylem [ABA]. Plants which experienced water deficit during the slow expansion period had markedly reduced expansion during the second period (i.e. 1 or 2 weeks after cessation of deficit), while all variables characterizing water status were returned to the level of the control. This ‘after effect’ was accounted for by a reduced cell number per leaf, while individual cell area was not affected. In contrast, water deficit occurring during rapid leaf expansion immediately reduced leaf expansion via cell area, without affecting cell number per leaf. These experiments indicate a role, in the response to water deficits, for events occurring very early in the development of pea leaves, while leaf expansion is too slow to be measured with macroscopic methods. This role would be accounted for by cell production during the first 2/3 of leaf development while cell expansion would account for changes in the area of leaves experiencing a later stress. These results suggest that long-term temporal analysis may be essential in the study of dicot leaf expansion compared to monocot leaves where temporal analysis can be inferred from spatial analysis.