Photosynthetic attributes and seed yield of sunflower (<i>Helianthus annuus</i> L.) as influenced by different levels and ratios of nitrogen and phosphorus fertilizers

A field experiment was conducted at the Main Research Station, University of Agricultural Sciences, Dharwad, India, on medium black soils during the<i> kharif</i> (wet) season of 1999. The experiment was laid out in a randomized complete block design with varying N/P ratios (0.67 to 2.00) along with a control with a constant level of potassium (60 kg ha^-1). The results revealed that the number of green leaves plant^-1, the dry matter accumulation in the leaves, leaf area (dm² plant^-1) and leaf area index (LAI) increased up to the flowering stage (65 DAS) and thereafter declined. In the early stages (seedling and button stages) there was no significant variation with respect to the number of green leaves plant^-1among the treatments except in the control. Similarly, leaf area and LAI did not vary at the seedling stage. Treatments receiving N/P ratios of ?1.0 or 1.0 with higher doses of nitrogen (120 kg N ha^-1) gave a significantly higher number of green leaves plant^-1, leaf area and LAI as compared to N/P ratios of <1.0 and the control in later stages. The dry matter accumulation in the leaves (g plant^-1) differed in all the stages, but higher values were recorded in these same treatments. Thus, due to the higher number of green leaves, higher LAI and greater dry matter accumulation in the leaves, the treatments with an N/P ratio of ?1.0 or 1.0 with 120 kg N ha^-1produced higher seed yields (3188 to 3554 kg ha^-1) than other N/P ratios (2761 to 3009 kg ha^-1). The highest yield (3554 kg ha^-1) was recorded with an N/P ratio of 1.0 in the treatment receiving 120 kg N and 120 kg P₂O₅ ha^-1. The correlation coefficients between these photosynthetic attributes and seed yield were also positive and significant.