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

Abstract

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 supply and were transferred between N levels at different developmental stages (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 strongly (up to two-fold) and significantly modified by N availability and the timing of transfer between N supplies, through effects on leaf expansion rate. Rate of cell production was significantly (P < 0.05) reduced in unemerged target leaves under N stress, but there was no evidence of a change in primordium size or in the duration of the leaf differentiation-emergence phase. 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 irrespective of leaf size, significant (P < 0.05) changes in N(cell) only occurred when the changeover occurred before the leaf reached approx. 10% of LA(max). There were no differential effects of N on numbers of epidermal vs. mesophyll cells. The results show that the effects of N on leaf size are largely due to effects on cell production in the unemerged leaf and on both cell production and expansion during the first phase of expansion of the emerged leaf. During the rest of the expansion period N mainly affects the expansion of existing cells. Cell area plasticity permitted a response to changes in N supply even at advanced stages of leaf expansion. Increased cell expansion can compensate for low N(cell) if N stress is relieved early in the expansion of emerged leaves, but in later phases N(cell) sets a limit to this response.