LIGHT-ASSOCIATED NITROGEN DISTRIBUTION PROFILE IN FLOWERING CANOPIES OF SUNFLOWER (HELIANTHUS-ANNUUS L) ALTERED DURING GRAIN-GROWTH

In vegetative canopies of many species, the vertical gradient of lamin a nitrogen concentration (NW) parallels the profile of light distribut ion in such a way that the actual nitrogen partitioning approaches the optimum pattern for canopy photosynthesis. This paper evaluates the h ypothesis that a strong sink for nitrogen, viz. growing grain, affects the pattern of lamina nitrogen distribution usually described for veg etative canopies. The light and NW profiles of sunflower (Helianthus a nnuus L.) crops were characterised from anthesis to physiological matu rity. The factorial combination of two plant populations (2.4 and 4.8 plants M-2) and two levels of nitrogen supply (0 and 5 g N m-2) were t he sources of variation for NW and light profiles. Before the onset of nitrogen accumulation in grain, the pattern of NW was similar to that described for other species and it was related to the distribution of light in the canopy. Important changes in the profile of NW occurred during grain filling that were unrelated to the light regime. Nitrogen was mobilised from leaves in all positions in the canopy and the rate of NW change was greater in leaves closer to the grain, which were al so the leaves where nitrogen was more concentrated. It is concluded th at the physiological mechanisms involved in determining the distributi on of leaf nitrogen in vegetative canopies do not apply to sunflower d uring grain filling.