ADAPTATIONS OF PHOTOSYNTHETIC ELECTRON-TRANSPORT, CARBON ASSIMILATION, AND CARBON PARTITIONING IN TRANSGENIC NICOTIANA-PLUMBAGINIFOLIA PLANTS TO CHANGES IN NITRATE REDUCTASE-ACTIVITY

Transgenic Nicotiana plumbaginifolia plants that express either a 5-fo ld increase or a 20-fold decrease in nitrate reductase (NR) activity w ere used to study the relationships between carbon and nitrogen metabo lism in leaves. Under saturating irradiance the maximum rate of photos ynthesis, per unit surface area, was decreased in the low NR expressor s but was relatively unchanged in the high NR expressors compared with the wild-type controls. However, when photosynthesis was expressed on a chlorophyll (Chi) basis the low NR plants had comparable or even hi gher values than the wild-type plants. surprisingly, the high NR expre ssors showed very similar rates of photosynthesis and respiration to t he wild-type plants and contained identical amounts of leaf Chi, carbo hydrate, and protein. These plants were provided with a saturating sup ply of nitrate plus a basal level of ammonium during all phases of gro wth. Under these conditions overexpression of NR had little impact on leaf metabolism and did not stimulate growth or biomass production. La rge differences in photochemical quenching and nonphotochemical quench ing components of Chi a fluorescence, as well as the ratio of variable to maximum fluorescence, (F-V/F-M), were apparent in the low NR expre ssors in comparison with the wild-type controls. Light intensity-depen dent increases in nonphotochemical quenching and decreases in F-V/F-M were greatest in the low Nh expressors, whereas photochemical quenchin g decreased uniformly with increasing irradiance in all plant types. N onphotochemical quenching was increased at all except the lowest irrad iances in the low NR expressors, allowing photosystem II to remain oxi dized on its acceptor side. The relative contributions of photochemica l and nonphotochemical quenching of Chi a flourescence with changing i rradiance were virtually identical in the high NR expressors and the w ild-type controls. Zeaxanthin was present in all leaves at high irradi ances; however, at high irradiance leaves from the low NR expressors c ontained considerably more zeaxanthin and less violaxanthin than wild- type controls or high NR expressors. The leaves of the low NR expresso rs contained less Chi, protein, and amino acids than controls but reta ined more carbohydrate (starch and sucrose) than the wild type or high NR expressors. Sucrose phosphate synthase activities were remarkably similar in all plant types regardless of the NR activity. In contrast phosphoenolpyruvate carboxylase activities were increased on a Chi or protein basis in the low NR expressors compared with the wild-type con trols or high NR expressors. We conclude and carbon partitioning withi n the leaf but that increases in NR have negligible effects.that large decreases in NR have profound repercussions for photosynthesis and ca rbon partitioning within the leaf but that increases in NR have neglig ible effects.