An investigation of sugarcane nitrogen physiology: sources, uptake, and metabolism

Abstract

Sugarcane has one of the highest biomass per unit of assimilated nitrogen (N) ratios of any commercial crop plant. By contrast efficiency of uptake of applied fertiliser N is very low. Current N fertilising practices apply approximately double the N required for cane production. Average N application in Australian sugarcane is ~2.8 kg N t-1 cane, compared with the optimum calculated by Keating et al. (1997) of 1.4 kg N t-1 cane. Low world sugar prices, increasing costs of production and community pressures to make the industry sustainable and more environmentally friendly are increasing the need for a better understanding of sugarcane N metabolism. Such improved knowledge can then flow into improved agronomic practice in farm N management. This thesis describes investigations of the uptake, assimilation and metabolism of nitrogen in sugarcane, using Queensland commercial varieties, especially the southern region cultivar Q141. Nitrate (NO3-) and ammonium (NH4+) uptake preferences were investigated using aerated hydroponic culture systems. Plant biomass measures showed no clear preference for either N form although higher NH4+ concentrations resulted in reduced plant growth and eventually plant death. Q141 sugarcane reduced most of the NO3- in its roots and transported the assimilated N as amino acids in the xylem sfream. Asparagine was the main amino acid present in the xylem sap. In fieldgrown sugarcane, asparagine also was the major amino acid in the xylem sap in high N fertilised compared with low N fertilised plants. The activities of the rate-limiting N assimilation enzymes, nitrate reductase (NR) and glutamine synthetase (GS), were measured in leaves and roots of two Saccharum species and six Saccharum hybrid varieties. Both enzymes had higher activities in leaves compared with roots. There were significant differences in the leaf activity of NR between the different varieties and species but not in the root activity. No differences in GS activity were found between different varieties and species. The tracing of NH4+ uptake and assimilation in aerated hydroponic Q141 sugarcane, using pulse-chase 15N experiments, supported the findings of the enzymatic profiling and amino acid distribution studies. 15N label was incorporated rapidly in the amino acid pools of the roots and more slowly in the leaves. The pools of asparagine and glutamine incorporated the highest amounts of 15N label. The synthesis of g-aminobutyric acid in the roots was probably a stress response. The potential for N input into sugarcane from atinospheric N2 via endogenous symbiotic N2-fixing bacteria was investigated using the 15N natural abundance technique. Reports from overseas claim high potential N incorporation by N2 fixation and past studies confirmed the presence of N2-fixing bacteria in Queensland commercial sugarcane plants and sugarcane fields. Overall, 6 d15N values did not show biological N2 fixation to be a major N source for Queensland commercial sugarcane. The results did however show the influence of high N fertilisation applications on plant d15N. The drop in d15N, due either to 15N discrimination on uptake of N via the roots and/or the foliar uptake of volatilised d15N depleted-NH3 can potentially result in an incorrect interpretation of the N2 fixation potential. Sugarcane can use either NO3- or NH4+ inorganic N sources but does not rely on N2 fixation. The potentially detrimental effect at high NH4+ concentrations on plant growth has implications for the current fertilising strategy of applying a single high dose of urea fertiliser to fields at a time when the crop is establishing itself This practice may actually hinder root and plant growth. Levels of organic and inorganic nitrogen pools in various plant parts indicated a capacity of roots to assimilate the majority of nitrogen taken up by the plant, followed by transport of assimilated nitrogen mostly as asparagine. Stem storage of assimilated organic nitrogen is also predominantly as asparagine. These results showed that the roots of sugarcane plants have a central place in the assimilation of inorganic nitrogen. Further research into sugarcane root physiology is required and strongly recommended.n