ABSTRACT
The long-term effect of limiting soil nitrogen (N) availability on foliar antioxidants, thermal energy dissipation, photosynthetic and respiratory electron transport, and carbohydrates was investigated in Spinacia oleracea L. Starch, sucrose, and glucose accumulated in leaves of N-limited spinach at predawn, consistent with a downregulation of chloroplast processes by whole-plant sink limitation in response to a limited supply of N-based macromolecules throughout the plant. On a leaf-area or dry-weight basis, levels of chlorophyll, carotenoid pools, photosynthetic electron transport capacity, as well as activities for the predominantly chloroplast-localized antioxidant enzymes ascorbate peroxidase (EC 1.11.1.11) and glutathione reductase (EC 1.6.4.2) were much lower in N-limited versus N-replete plants. When expressed on a chlorophyll basis, foliar levels of all of these parameters were similar in N-replete versus N-limited plants. However, on a total-protein basis, antioxidant enzyme activities were higher in N-limited plants. Nitrogen-limited spinach showed higher levels of thermal energy dissipation and of zeaxanthin and antheraxanthin at midday, as well as slightly higher ascorbate contents relative to chlorophyll. These results indicate that strong, long-term N limitation led not only to alterations in the balance between different processes but also to an overall downregulation of light collection, photosynthetic electron transport capacity, and chloroplast-based antioxidant enzymes. This is further supported by the finding that glucose-feeding of excised leaves led to strong concomitant decreases in photosynthetic electron transport capacity and ascorbate peroxidase activity. On a leaf-area basis, neither superoxide dismutase (EC 1.15.1.1) activity nor dark repiration rates showed a treatment effect. This indicates that overall mitochondrial electron transport activity does not decrease under long-term N limitation and is consistent with localization of an important fraction of foliar superoxide dismutase in mitochondria.
ABSTRACT
The effects of exogenously supplied isoprene on chlorophyll fluorescence characteristics were examined in leaf discs of four isoprene-emitting plant species, kudzu (Pueraria lobata [Willd.] Ohwi.), velvet bean (Mucuna sp.), quaking aspen (Populus tremuloides Michx.), and pussy willow (Salix discolor Muhl). Isoprene, supplied to the leaves at either 18 &mgr;L L-1 in compressed air or 21 &mgr;L L-1 in N2, had no effect on the temperature at which minimal fluorescence exhibited an upward inflection during controlled increases in leaf-disc temperature. During exposure to 1008 &mgr;mol photons m-2 s-1 in an N2 atmosphere, 21 &mgr;L L-1 isoprene had no effect on the thermally induced inflection of steady-state fluorescence. The maximum quantum efficiency of photosystem II photochemistry decreased sharply as leaf-disc temperature was increased; however, this decrease was unaffected by exposure of leaf discs to 21 &mgr;L L-1 isoprene. Therefore, there were no discernible effects of isoprene on the occurrence of symptoms of high-temperature damage to thylakoid membranes. Our data do not support the hypothesis that isoprene enhances leaf thermotolerance.
