Seasonal changes of leaf area distribution and light response curves, and incorporation of vapor pressure deficit into a canopy photosynthesis model in corn

Amount and vertical distribution of leaf area determine light attenuation and interception by a crop, and thus, productivity. Light response curves (photosynthetic responses to light, LRC) serve, among other things, to obtain interpolated values. Carbon balance of corn can be incorporated into models that predict large scale balances of greenhouse gases. This study was conducted on corn to quantify seasonal changes in vertical distribution of leaf area and LRC and to estimate post-silking CO{dollar}{bsol}sb2{dollar} balance.; Experiments were conducted in 1994 and 1995 near Mead, Nebraska. Plants were grown under irrigated or unirrigated conditions at N rates of 0, 68 or 135 kg ha{dollar}{bsol}sp{lcub}-1{rcub}.{dollar} Vertical distribution of leaf area was determined as leaf area indices (LAI) at 0.10 m height intervals. Light response curves and leaf N contents (N{dollar}{bsol}rm{bsol}sb{lcub}lf{rcub}){dollar} were evaluated in the upper, middle and lower canopy. Diurnal variation of vapor pressure deficit (VPD) was used to adjust LRC measured only once a day. Canopy photosynthesis (P{dollar}{bsol}rm{bsol}sb{lcub}can{rcub}){dollar} was modeled using three canopy layers and three LRC. Respiration components of the CO{dollar}{bsol}sb2{dollar} balance were based on dry weight.; A bell-shaped curve fitted the vertical distribution of leaf area reasonably well. Light response curves differed significantly between canopy layers. All LRC flattened (approached light saturation at less light intensities) with time. Leaf nitrogen content of plants at 0 N-rate remained fairly constant, while N{dollar}{bsol}rm{bsol}sb{lcub}lf{rcub}{dollar} of plants at 135 N-rate decreased continuously. A linear, but distinct relationship between N{dollar}{bsol}rm{bsol}sb{lcub}lf{rcub}{dollar} and photosynthetic capacity existed in each N treatment. Peak predicted P{dollar}{bsol}rm{bsol}sb{lcub}can{rcub}{dollar} shifted from solar noon without to mid-morning with VPD-adjusted LRC. Daily CO{dollar}{bsol}sb2{dollar} balance was comparable between N treatments.; Leaf area in corn is distributed symmetrically (bell-shaped) in the vertical. One LRC seemed inadequate to represent an entire canopy, day or season. The relatively high photosynthetic N-use efficiency or daily CO{dollar}{bsol}sb2{dollar} balance of plants with 0 N rate was insufficient to explain their lower grain yield.