EFFECT OF NITROGEN-CONTENT ON THE PHOTOSYNTHETIC CHARACTERISTICS OF SUNFLOWER LEAVES

Photosynthesis-ir-radiance response curves and leaf nitrogen contents were measured weekly by destructive sampling over the life cycles of leaves 10, 15, 20 and 25 of sunflower plants (cv. Prosol 35) grown in large pots in the open under optimum conditions of temperature and high irradiance. Individual leaf responses were adequately described by a hyperbola of three parameters, viz. P(max), the rate of photosynthesis in saturating irradiance; R, the rate of dark respiration adjusted for temperature (30-degrees-C); and epsilon, the apparent quantum efficiency of photosynthesis at low irradiance. P(max) (range 0-40 mumol CO2 m-2 s-1) and R (0-4 mumol CO2 m-2 s-1) were non-linearly related to nitrogen content per unit leaf area (N(L)) (range 0.3-2.9 g N m-2) across all leaf positions and for all leaf ages. Epsilon (mean value 0-050 mol mol-1, s.e. 0.001) was independent of N(L). The equations for net photosynthesis derived from pot studies were shown to explain (r2=0.80) leaf photosynthesis in a crop of the same cultivar over a wide range of N(L) and irradiance.