Nitrate Accumulation and Reduction in Papaw Fruits

Abstract

Nitrate reductase activity was present in roots, stems, leaves, and fruits of the papaw. In fruits, the location and activity of nitrate reductase varied with stage of maturation. Nitrate reductase activity was induced in the exocarp of green and mature green fruits by light and light plus exogenous nitrate. The endogenous nitrate in the exocarp of ripe fruit was not available for reduction and hence no induction occurred in the presence of light only. As fruit tissues comll1enced to show colour changes, there was a marked loss in their nitrate reductase activity. At the mature green stage of maturity, phloem transport appeared to predominate over xylem transport and this reduced the net influx of nitrate to fruit. The levels of nitrate observed in fruits at various stages of maturity may be explained in terms of the following: changes in nitrate reductase activity associated with fruit maturation; the availability of endogenous nitrate for reduction; factors controlling the net influx of nitrate into fruits and distribution of nitrate within fruits.