EFFECTS OF LEAF POSITION AND NITROGEN SUPPLY ON THE EXPANSION OF LEAVES OF FIELD GROWN SUNFLOWER (HELIANTHUS-ANNUUS L)

Leaf growth responses to N supply and leaf position were studied using widely-spaced sunflower plants growing under held conditions. Both N supply (range 0.25 to 11.25 g added N per plant) and leaf position sig nificantly (p = 0.001) affected maximum leaf area (LA(max)) of target leaves through variations in leaf expansion rate (LER); effects on dur ation of expansion were small. Specific leaf nitrogen (SLN, g N m(-2)) fell quite rapidly during the initial leaf expansion phase (LA < 35% LA(max)) but leveled off during the final 65% increase of leaf area. T his pattern held across leaf positions and N supply levels. Leaf nitro gen accumulation after 35% LA(max) continued up to achievement of LA(m ax); reductions in the higher SLN characteristic of the initial phase were insufficient to cover the nitrogen requirements for expansion dur ing the final phase. LER in the quasi-linear expansion phase (35 to 10 0% of LA,,) was strongly associated with SLN above a threshold that va ried with leaf position (mean 1.79 +/- 0.225 g N m(-2)). This contrast s with the response of photosynthesis at high irradiance to SLN, which has previously been shown to have a threshold of 0.3 g N m(-2); in th e present work saturation of photosynthetic rate was evident when SLN reached 1.97 g N m(-2). Thus, once the area of a leaf exceeds 35% of L A(max), expansion proceeds provided SLN values are close to the levels required for maximum photosynthesis. However, growth of leaves during the initial expansion phase ensures a minimum production of leaf area even at low N supply levels.