Adaptations of Photosynthetic Electron Transport, Carbon Assimilation, and Carbon Partitioning in Transgenic Nicotiana plumbaginifolia Plants to Changes in Nitrate Reductase Activity

Abstract

Transgenic Nicotiana plumbaginifolia plants that express either a 5-fold increase or a 20-fold decrease in nitrate reductase (NR) activity were used to study the relationships between carbon and nitrogen metabolism in leaves. Under saturating irradiance the maximum rate of photosynthesis, per unit surface area, was decreased in the low NR expressors but was relatively unchanged in the high NR expressors compared with the wild-type controls.However, when photosynthesis was expressed on a chlorophyll (Chl) basis the low NR plants had comparable or even higher values than the wild-type plants. Surprisingly, the high NR expressors showed very similar rates of photosynthesis and respiration to the wild-type plants and contained identical amounts of leaf Chl, carbohydrate, and protein. These plants were provided with a saturating supply of nitrate plus a basal level of ammonium during all phases of growth. Under these conditions overexpression of NR had little impact on leaf metabolism and did not stimulate growth or biomass production. Large differences in photochemical quenching and nonphotochemical quenching components of Chl a fluorescence, as well as the ratio of variable to maximum fluorescence, (F,/FM), were apparent in the low NR expressors in comparison with the wild-type controls. Light intensity-dependent increases in nonphotochemical quenching and decreases in F,/FM were greatest in the low NR expressors, whereas photochemical quenching decreased uniformly with increasing irradiance in all plant types. Nonphotochemical quenching was increased at all except the lowest irradiances in the low NR expressors, allowing photosystem II to remain oxidized on its acceptor side. l h e relative contributions of photochemical and nonphotochemical quenching of Chl a fluorescence with changing irradiance were virtually identical in the high NR expressors and the wild-type controls. Zeaxanthin was present in all leaves at high irradiances; however, at high irradiance leaves from the low NR expressors contained considerably more zeaxanthin and less violaxanthin than wild-type controls or high NR expressors. l h e leaves of the low NR expressors contained less Chl, protein, and amino acids than controls but retained 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 Chl or protein basis in the low NR expressors compared with the wild-type controls or high NR expressors. We conclude that large decreases in NR have profound repercussions for photosynthesis * Corresponding author; fax 33-1-30-83-30-96. 171and carbon partitioning within the leaf but that increases in NR have negligible effects.The hypothesis that the pathways of carbon and N assimilation in leaves compete for energy and carbon skeletons in higher plants is widely accepted, but very little evidence is available to support this view (Robinson, 1988)....

Effects of Iron Excess on Nicotiana plumbaginifolia Plants (Implications to Oxidative Stress)

“…2), qQ and qNP (Fig. 31, soluble protein/Chl ratio (Table I), and carbohydrate content (Table 11) were nearly identical with the values obtained by Foyer et al (1994) for N. plumbaginifolia wild-type plants.…”

“…All measured Chl fluorescence parameters of the control plants were identical with those previously reported for unstressed N. plumbaginifolia wild-type plants (Foyer et al, 1994).…”

A Mechanism of Nonphotochemical Energy Dissipation, Independent from PsbS, Revealed by a Conformational Change in the Antenna Protein CP26

“…In this way, a plant that has stored Zea upon previous exposure to high light is able to trigger qE more promptly (Demmig-Adams et al, 1990) by avoiding the temporal lag necessary to release Viola from the V1 site (Caffarri et al, 2001) and its deepoxidation when no Zea is present. Published results suggest a more general role for xanthophyll cycle: it was shown that downregulation of nitrate reductase activity, and thus lower NADPH use, increases zeaxanthin levels (Foyer et al, 1994). Consistently, inhibition of transketolase activity increases starch accumulation and zeaxanthin levels in dark adapted leaves, allowing for faster and stronger excess energy dissipation in the event of excess light (Henkes et al, 2001).…”

Partitioning of nitrate reductase activity in Coffea arabica L. and its relation to carbon assimilation under different irradiance regimes

“…Carbon and N assimilation in leaves apparently compete for energy and carbon skeletons in higher plants, but very little evidence is available to support this view (Robinson, 1988;Foyer et al, 1994). In addition, it has been suggested that when photosynthesis is light-limited, the increased nitrate assimilation in roots may allow greater control over the use of limited energy between nitrogen and carbon assimilation (Smirnoff and Stewart, 1985).…”

“…In contrast, under limiting conditions for the photosynthetic process and consequently for the leaf nitrate assimilation, higher NRA would occur in roots, compensating the leaf NRA decreases. In fact, it has been documented that nitrate reduction is regulated by concurrent photosynthesis (Pace et al, 1990;Foyer et al, 1994).…”