Interactive Effects of Nitrogen and Phosphorus Deficiency on Photosynthesis and Leaf Growth in Maize.

Nitrogen (N) and phosphorus (P) are often limiting in agricultural systems and must be augmented through fertilization. Simulation models that can account for N and P limitations are important tools for assessing the impact of nutrient limitations on crop yield. A study was conducted to evaluate the interactive effects of Nitrogen (N) and Phosphorus (P) on corn growth, development, chlorophyll content and photosynthesis and incorporate the results into MAIZSIM, a maize simulation model. Com plants were grown in outdoor, sunlit Soil Plant Atmosphere Research (SPAR) chambers with four levels of phosphorus i.e., 0, 0.01, 0.05 and 0.2 m mol l^-1 and three levels of nitrogen i.e., 2.0, 5.0 and 12.0 m mol l^-1 Plants were fertigated three times a day for three minutes each @ 4 1iters per minute through an automated  fertigation  system. The maize plants were grown at constant 400 mmol C02 mole^-1 and 28/18 °C day/night temperatures.   Canopy photosynthesis, dark respiration, and transpiration were measured at every 5 minutes throughout crop growth period automatically using computer controlled instrumentation. Light response and A/ci curves were measured with Li-Cor 6400 photosynthesis system. Chlorophyll content was measured in the lab and estimated using a chlorophyll meter (SPAD). Destructive measurements were taken four times by harvesting the six to nine plants while maintaining the uniform spacing at the end of each harvest. Leaf area was measured on sampled leaves. Non-destructive measurements were taken on tagged plants. The results indicated that when plant growth was P-limited, response to increased light intensity and C02 with increase in nitrogen levels was limited. At higher levels of phosphorus, there is a continuous increase in carbon assimilation with increase in nitrogen levels. A good correlation was observed between chlorophyll content and SPAD chlorophyll reading with little response in P limited plants to increased nitrogen levels. Algorithms to quantify the effects of N and P deficiencies were tested in the maize model, MAIZSIM