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 l eaves 10, 15, 20 and 25 of sunflower plants (cv. Prosol 35) grown in l arge 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 fo r temperature (30-degrees-C); and epsilon, the apparent quantum effici ency 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 n itrogen content per unit leaf area (N(L)) (range 0.3-2.9 g N m-2) acro ss 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=0.8 0) leaf photosynthesis in a crop of the same cultivar over a wide rang e of N(L) and irradiance.