INFLUENCE OF AMMONIUM AND NITRATE ON THE GROWTH AND PHOTOSYNTHESIS OF SUGARCANE

Five-month-old sugarcane (Saccharum officinarum L. cv. Ja 605) plants were grown for 45 days on mixed peat and vermiculite in a growth chamber in which the plants were irrigated frequently with complete nutrient solution containing NO3- or NH4+ (2 mM) as sole nitrogen source. Measurement of the ionic composition of the draining solution from the plants supplied with NH4+-N failed to reveal NO3-, strongly suggesting only a minor role for nitrification in the nutrient medium. In NH4+-fed plants, shoot growth and stalk diameter were increased as compared with those fed with NO3-. In the youngest, fully expanded leaves of NH4+-fed plants, the concentrations of free amino compounds, aspartate, asparagine and glutamine, were higher; total N, soluble protein, malate and chlorophyll levels were similar and the concentrations of the principal carbohydrates were slightly lower than in the corresponding leaves of the NO3--fed plants. Net photosynthesis (whole plant), room temperature chlorophyll a fluorescence, quantum yield, in vitro carboxylase and decarboxylase activities involved in CO2 photoassimilation were similar regardless of the nitrogen source. It may be concluded, therefore, that although nitrogen assimilation as NO3- or NH4+ theoretically requires different amounts of energy and reducing equivalents supplied by the chloroplast, we were unable to detect any marked biochemical differences associated with photosynthesis between the two N forms.