Department of Plant Sciences

The efficiency with which cotton plants redistributed N from the vegetative to the reproductive organs in three cotton crops in north-west NSW, Australia that differed in yield potential was examined. It is generally assumed that differences in boll numbers result in corresponding changes in the demands placed on leaves and stems for the supply of the N required for boll production. Cotton plants were partitioned into leaves, stems and fruit every ten days from flowering to maturity. The N content of these fractions differed between the crops: the lowest-yielding crop (2130 kg lint/ha) had the highest proportion of N in the fruit (74%), whereas the highest-yielding crop (3270 kg lint/ha) had the lowest (64%). The lowest-yielding crop redistributed 55% of its leaf-N and 7% of its stem-N to the fruit, while the highest-yielding crop redistributed 24% of leaf-N and 8% of the stem-N. These findings indicated that higher boll loads (increasing the demand for N) in situations where nitrogen is a non-limiting factor will not necessarily equate to a higher N-use efficiency. Differences in redistribution of N are more likely caused by complexities of water, other nutrients and environmental conditions. This information can be used to better understand the way high yielding crops use N, and to aid in optimising the N use efficiency of high yielding cotton crops.