Effect of specific leaf nitrogen content on photosynthesis of sugarcane

Summary In the laboratory, rate of photosynthesis (P) of the youngest fully emerged leaf lamina on a shoot, whether taken from a 4 or 8 month old sugarcane plant, increased approximately linearly with specific leaf nitrogen content (SLN) from < 1.0 to c. 1.7 g m‐2; the rate of increase was greater the higher the incident photosynthetically active radiation (PAR) flux density, over the range 80 to 1600 μmol m‐2s‐1. In a field crop receiving standard fertiliser, including 150 kg N ha‐1, SLN of the green leaf canopy decreased more or less linearly with depth, from > 1.7 g m‐2at the top to c. 1.0 g m‐2at the bottom at 15 wk of age, and progressively with time to 1.2 to 0.85 g m‐2from top to bottom of the canopy by week 45. Where an extra 300 N ha‐1were applied in top‐dressings, the decrease in SLN with time was somewhat smaller, to 1.3 to 0.9 g m‐2from top to bottom of the canopy by week 45. Calculations with a model, using the data from laboratory and field, suggest a decrease of about 25% in canopy gross P and about 30% in canopy net P from wk 15 to wk 45 in the crop receiving standard fertiliser only; the calculated decreases where the crop received additional nitrogen were about 17% and 20% respectively. There would seem to be a need for a change in type of cultivar to make profitable use of augmented nitrogen fertiliser.