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 initiatio n to maximum area, over two markedly different periods, During the fir st one (2/3 of total leaf development time), cell production occurred while cell and leaf expansions were slow, Rapid expansion (95% of tota l) 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 w ater potential and xylem [ABA]. Plants which experienced water deficit during the slow expansion period had markedly reduced expansion durin g the second period (i.e. 1 or 2 weeks after cessation of deficit), wh ile all variables characterizing water status were returned to the lev el 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 immediat ely reduced leaf expansion via cell area, without affecting cell numbe r per leaf, These experiments indicate a role, in the response to wate r deficits, for events occurring very early in the development of pea leaves, while leaf expansion is too slow to be measured with macroscop ic 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 r esults suggest that long-term temporal analysis may be essential in th e study of dicot leaf expansion compared to monocot leaves where tempo ral analysis can be inferred from spatial analysis.