Effects of constant and variable nitrogen supply on sunflower (Helianthus annus L.) leaf cell number and size

The effects of nitrogen (N) availability on cell number and cell size, and the contribution of these determinants to the final area of fully expanded leaves of sunflower (Helianthus annuus L.) were investigated in glasshouse experiments. Plants were given a high (N = 315 ppm) or low (N = 21 ppm) N s upply and were transferred between N levels at different developmental stag es (5 to 60 % of final size) of target Leaves. The dynamics of cell number in unemerged (< 0.01 m in length) leaves of plants growing at high and low levels of N supply were also followed. Maximum leaf area (LA(max)) was stro ngly (up to two-fold) and significantly modified by N availability and the timing of transfer between N supplies, through effects on leaf expansion ra te. Rate of cell production was significantly (P < 0.05) reduced in unemerg ed target leaves under N stress, but there was no evidence of a change in p rimordium size or in the duration of the leaf differentiation-emergence pha se. In fully expanded leaves, number of cells per leaf (N-cell), leaf area per cell (LA(cell)) and cell area (A(cell)) were significantly reduced by N stress. While LA(cell) and A(cell) responded to changeover treatments irre spective of leaf size, significant (P < 0.05) changes in N-cell only occurr ed when the changeover occurred before the leaf reached approx. 10% of LA(m ax). There were no differential effects of N on numbers of epidermal as. me sophyll cells. The results show that the effects of N on leaf size are larg ely due to effects on cell production in the unemerged leaf and on both cel l production and expansion during the first phase of expansion of the emerg ed leaf. During the rest of the expansion period N mainly affects the expan sion of existing cells. Cell area plasticity permitted a response to change s in N supply even at advanced stages of leaf expansion. Increased cell exp ansion can compensate for low N-cell if N stress is relieved early in the e xpansion of emerged leaves, but in later phases N-cell sets a limit to this response. (C) 1999 Annals of Botany Company.