EFFECT OF PLANT-POPULATION DENSITY ON CANOPY NET PHOTOSYNTHESIS AND THEIR RELATION TO GRAIN-YIELD IN MAIZE CULTIVARS

Field experiments were conducted to explore the relationship between canopy apparent photosynthesis (P-N,P-can) and grain yield of maize (Zea mays L.) and to determine over two years the effects of plant population density (PPD) on P-N,P-can. Yedan No. 4 hybrid (vertical leaf type) were grown at five PPD in 1989 and 1990. Measurements of P-N,P-can were made throughout the growth period in a large plastic covered chamber put over a segment of the canopy. Maximum values of P-N,P-can at ca. 1500 mu mol(photon) m(-2) s(-1) were about 2.8 mg(CO2) m(-2)(soil area) s(-1) at the anthesis stage and they decreased with age throughout grain filling. Differences in P-N,P-can between PPD could be attributed to leaf area index (LAI) or radiant energy interception. There was a quadratic regression between P-N,P-can and LAI. The critical LAI for P-N,P-can was about 4 to 6 for Shendan No. 7 and Yendan 4, respectively. Grain yield also differed between PPD, the range was 850 to 1140 g m(-2) in the two cultivars. Increasing PPD of Yedan No. 4 up to about 9 plants per m(2) and decreasing PPD of Shendan No. 7 to about 4 plants per m(2) in a non-stressed environment should increase grain yield. PN,can after anthesis was positively correlated with biological (r = 0.8750** to 0.9797**) and grain (r = 0.8932** to 0.9914**) yields.