Within cotton plants nutrients are taken up by roots and partitioned between plant structures. At boll filling, when the demand for nutrients is greatest due to the development of seeds and lint, nutrients from vegetative plant parts are mobilised and redistributed around the cotton plant. Higher-yielding Bollgard® II varieties are suspected to have higher nutrient demands than conventional cultivars, although the mechanisms and proportions of nutrients accumulated and redistributed in different plant parts is unclear. Yield and fibre development may be limited where nutrients are not efficiently redistributed to the developing bolls in sufficient quantities. Cotton plants with a high boll retention may enhance this problem.

Fertiliser programs aim to supply nutrients to the plants at peak growth stages when nutrient uptake is greatest. Foliar and soil fertiliser applications may be needed to supply nutrients to the developing plants. Some information is available on the total plant uptake of nutrients, but little on partitioning of nutrients between vegetative and reproductive plant structures. The timing of nutrient uptake redistribution has not been studied in detail. A better understanding of this process could aid in the development of timely and effective fertiliser programs for maximising yield and fibre quality of high yielding cultivars.

Examining the nutrient partitioning and redistribution mechanisms within conventional and transgenic cultivars may aid in establishing some key nutrient limitations to yield. Further understanding of the nutrient redistribution mechanisms under nutrient stressed conditions will aid in developing best management practices for fertiliser application.

This PhD project will test several hypotheses:

• That effective translocation of nutrients (largely nitrogen, potassium, phosphorus and zinc) is essential for high cotton yields.

• That nutrient uptake is not limited by root uptake but driven by fruit load and nutrient redistribution is driven by internal physiological mechanisms.

• That supplemental nutrients applied at critical developmental stages by either soil or foliar fertilization increases nutrient uptake and promotes higher yields.

• Nutrient uptake and redistribution is more efficient in high fruit retention crops