Nitrate reductase activity, distribution, and response to nitrate in two contrasting Phaseolus species inoculated with Rhizobium spp.

The nitrate reductase activity distribution and response of two nodulated s pecies of Phaseolus (Phaseolus vulgaris-common bean, and Phaseolus lunatus- lima bean) to different exogenous nitrate levels were studied during the ve getative period. These Phaseolus species showed to be very contrasting in r espect to the pattern of nitrate reductase (NR) activity distribution thoug ht the plant. The highest level of NR activity in P. vulgaris was clearly s hown to occur in leaves in contrast with the lowest one detected in roots a nd nodules as widely seen for other tropical species of the Phaseoleae trib e. Conversely, P. lunatus had higher NR activity in the nodules, whereas it s leaves exhibited a steadily decrease during the plant development. Indeed , at 32 days after emergence (pre-flowering stage), the nodulated P. vulgar is had approximately 95% of the total NR activity localized in its leaves, whereas in P. lunatus it was equally distributed in the nodules and in the leaves. Under long-term exposure to increasing exogenous level of nitrate, the leaf-NR activity of nodulated P. vulgaris presented a positive response , whereas the enzyme activity was very low and unresponsive in P. lunatus. In contrast, the nodule-NR activity showed a reverse response to the increa sing NO; level. The nodule-NR activity of P. lunatus significantly increase d whereas in the P. vulgaris nodules it was very low and unresponsive. This present study suggests that P. lunatus inoculated with Rhizobium tropici p resents a singular pattern of nitrate reduction distribution among leaves a nd nodules during the vegetative development. It is speculated that the nod ulated Phaseolus lunatus may have different NR isoforms in their leaves (at least a constitutive type) and an inducible form in their nodules, respons ive to long-term exposure to nitrate. (C) 2001 Elsevier Science B.V. All ri ghts reserved.