Interrelationships of Nitrate and 6-Azauracil in the Growth, Enzymology, and Sucrose Production of Immature Sugarcane

Abstract

Sugarcane grown in sand culture was subjected to extreme levels of nitrate (NO₃) to simulate conditions of restricted growth and ripening, moderate growth, and succulent growth. The pyrimidine analog 6-azauracil was supplied concurrently as a foliar spray. There were three objectives: 1, To measure the perseverance of the growth regimes in the presence of a powerful growth inhibitor; 2, to evaluate 6-azauracil as a growth-retardant in high-nitrogen (N) stimulated plants, and as a ripening agent for slowgrowing cane in a simulated ripening condition; and 3, to investigate the effectiveness of 6-azauracil as an enzyme inhibitor under extreme conditions of N stress. 6-azauracil (0.05 percent) succeeded in suppressing growth regardless of NO₃ supply. High NO₃ (50 meq./liter) alleviated but did not overcome the inhibitor effect. It was suggested that N fertilization might be prolonged to increase cane tonnage, with subsequent growth termination being based upon a chemical agent rather than the withholding of N. Low NO₃ (0.5 meq./liter) reduced growth, increased sucrose content of leaf and immature storage tissues, and raised Brix and polarization values for mature storage tissue. 6-azauracil also produced a ripening effect, as evidenced by increasing Brix and polarization values in the low NO₃ plants, but failed to do so when NO₃ supply was raised. Nitrate generally increased the activity of invertase, amylase, phosphatase, ATP-ase, and oxidases. 6-azauracil retarded enzyme action, but was less effective when the NO₃ supply was high. Failure to severely retard invertase and amylase is believed to account for Brix and polarization decline. Although 6-azauracil was applied as a spray to the cane leaves, the leaf tissues and leaf enzymes did not appear to be damaged by the inhibitor. The chemical is believed to be translocated to meristem and immature storage tissues while leaves remain relatively undamaged, and presumably photosynthetically active.