Effects of nitrogen supply and population density on plant development and yield components of irrigated sunflower (Helianthus annuus L.)

Abstract

Abstract During two seasons the growth and development of an F1 hybrid of sunflower ( Helianthus annuus L.) cv. Hysun 30 was tested at nitrogen supply rates between 0 and 500 kg N ha −1 and at three population densities: 5, 10 or 15 plants m −2 . Shoot dry matter and N content per plant increased with increases in applied N and decreased with increases in population density. The relationship between relative growth rates and relative N uptake rates could be described by a single quadratic function over all N and populations treatments in both experiments. Early in crop development in dense populations there was a partitioning of dry matter into the stem at the expense of the production of leaf primordia, decreasing the number of leaves. Throughout crop development stem dry weight % shoot dry weight remained greater in plants at high than at low population densities. Floret numbers per capitulum were also less but this was as a result of limited receptacle expansion during floret production and not due to a smaller generative area on the floral meristem. After floret initiation, dry matter and nitrogen accumulation rates decreased in all organs in dense populations. Population density had no effect on the duration of expansion of individual leaves but rates of expansion were lower in dense populations. Low N supplies increased the duration but decreased the rates of leaf expansion. Increased N supply could compensate for the effects of dense populations except where direct population effects occurred, e.g. on floret numbers and rates of leaf expansion. There was no response of N concentration (nitrogen % organ dry weight) in separate organs to population but there was a positive response to increase in N application rates. Nitrogen distribution was independent of dry matter distribution between organs. The percentage of initiated florets that produced filled seeds was less (38%) on 15 plants m −2 than on 5 plants m −2 (68%). Seed number, single seed weight, oil yield per plant and harvest index were increased by increasing nitrogen application and decreased by increasing population density. Oil % seed dry weight was not affected by population but was decreased by increasing N application rates. Fitted response surfaces describing seed number per plant, single seed weight and oil yield per plant in terms of plant N status and population density are given and may be useful for predicting crop yield.