Distribution of Nitrate Reductase Activity in Six Legumes: The Importance of the Stem

Abstract

SUMMARYNitrate reductase activity (NRA) was measured in leaves, stems and roots of six grain legumes supplied with different nitrate concentrations. The contribution of the stem relative to total plant activity was assessed.In Lupinus angustifolius L., Pisum sativum L. and Vicia faba L., legumes of temperate origin, an increased proportion of total plant NRA occurred in the shoot as nitrate concentration was increased. In Cajanus cajan (L.) Wilisp., Glycine max (L.) Merr. and Phaseolus vulgaris L., legumes of tropical origin, the proportion of total plant NRA in root and shoot was relatively constant regardless of nitrate concentration.More than 20% of total plant NRA can occur in the stem of G. max, L. angustifolius, Pisum sativum, Phaseolus vulgaris and V.faba. In C. cajan 10 to 15 % of total plant NRA was found in the stem at all nitrate concentrations. It is concluded that stem tissues can contribute significantly to overall nitrate metabolism.

Regulation of Assimilate Partitioning in Soybean

“…However, Rufty et al (24) have noted that errors in the estimate may be significant if the plant is cycling reduced N through the root. The transport model outlined in Table II The observation that the proportion of whole plant nitrate reduction which occurred in roots was not affected by the N treatment (Table II, item 9c), was in agreement with the investigations of Andrews et al (1) for legumes of tropical origin such as field bean, soybean, and pigeon pea. However, they found that the proportion of nitrate reduction occurring in roots declined with increasing nitrate supply in legumes of temperate origin (2).…”

“…The concentration of N in the xylem would decline (D) and would lead to a decrease in the shoot N pool (E), which in this study resulted in a slight decrease in shoot growth (F). Due to an apparent relationship between xylem and phloem N concentrations (Table II, items 7c and 1 1), the N content of the phloem sap leaving the shoot would decrease (G) but at a proportion similar to that of plants fed higher concentrations of nitrate. Of the N sent to the roots in the phloem a much higher proportion, but similar amount, is unloaded and used by the roots (H) thus maintaining the reduced N content of the root tissue (I), and decreasing the amount of N left to cycle through the root (J).…”

“…The plant parts of five plants from each population were finely ground, and the C and N content ofthe tissues measured with a CHN elemental analyzer (Leco Corporation, model 600). Aqueous extracts of the dried plant material were analyzed for tissue NO3-N content using a N03 specific ion electrode and a millivolt meter (models 93-07 and 81 1 (8).…”

Effect of Nodulation on the Expression of Nitrate Reductase Activity in Pea Cultivars

“…Furthermore, in soybean and cowpea plants fed with nitrate, almost all of the ^^N which is exported to the stem was as NOg" (Rufty et al, 1982;Sasakawa & LaRue, 1986). That stems of leguminous plants play an important role in nitrogen metabolism was shown by Andrews et al (1984). No NRA could be detected in leaves in the absence of added NOg" [Table l(a)], probably because the accumulated nitrate may not be available to nitrate reductase in the in vivo assay, as no treatment was applied that would destroy the permeability barriers.…”

“…Leguminous plants, like many other higher plants, can reduce NOf in leaves and roots (Shrader & Thomas, 1981). The stems of a range of legume species contain high levels of nitrate reductase activity (NRA) (Andrews et al, 1984). Root nodules and Rhizobium bacteroids also contain an active NR (Kennedy, Rigaud & Trinchant, 1975;Chen & Phillips, 1977;Manhart & Wong, 1979;Stephens & Neyra, 1983;Becana, Aparicio-Tejo & Sanchez-Diaz, 1985).…”