Ascorbate content of wheat leaves is not determined by maximal l-galactono-1,4-lactone dehydrogenase (GalLDH) activity under drought stress

Although ascorbic acid (AA) is a high-abundance metabolite, relatively little is known about the factors controlling its accumulation in leaves. To address this issue, we examined the role of l -galactono-1,4-lactone dehydrogenase (GalLDH), the enzyme which catalyses the last step of this pathway, in the control of AA content under optimal and stress conditions. In a range of species, no clear relationship between AA content and leaf GalLDH protein and activity was found under optimal growth conditions. To explore the effect of drought stress on GalLDH activity and protein content, wheat (Triticum aestivum L.) was selected for detailed analysis, using two cultivars that differ in their constitutive AA level. In well-watered plants, the AA content of cv Buck Chambergo (BCH) was over twice that of cv Cooperativa Maipún (CM) but dehydroascorbic acid content was similar in both cv. In agreement with this, dehydroascorbate reductase and glutathione reductase activities were higher in cv BCH than in cv CM, indicating a higher capacity for AA regeneration. Neither leaf DHA content nor activities of AA regenerating enzymes were modified by drought. Although drought caused a substantial increase in GalLDH protein and activity in the low AA cv CM, this treatment had no effect on these parameters in cv BCH. Notably, leaf AA content was unaffected by drought in either cv. These results suggest that GalLDH protein and activity cannot be used as an indicator for changes in the capacity for ascorbate biosynthesis and that AA biosynthesis is constrained by other factors under stress. This can be explained by the importance of regeneration in maintaining AA levels and possibly also by redox regulation of GalLDH.     

Nitrogen Assimilation and Leaf Development in Indeterminate Soybeans as Influenced by Post-Flowering Photoperiod

Guiamét, J. J., Balatti, P. A. and Montaldi, E. R. 1986.Nitrogen assimilation and leaf development in indeterminatesoybeans as influenced by post-flowering photoperiod.—J.exp. Bot. 37: 1611–1618. The effects of photoperiod on nitrogen fixation and leaf developmentin indeterminate soybeans were studied during early reproductivegrowth. Soybean plants cv. Williams were grown under short days(SD: 8 h-natural daylight (N.D.)+16 h-darkness) or long days(LD: 8 h-N.D. + 8 h-low intensity artificial light+ 8 h-darkness)from full bloom until mid pod filling. Long days greatly increased plant growth, both on the basisof leaf area or weight, mainly due to higher net assimilationrate. Average daily rates of N₂-fixation increased under LD;however, average N₂-fixation rates on a nodule weight or N basisdid not vary, suggesting that changes were not in nodule efficiencybut in nodule biomass. As compared to SD, LD reduced N contentin vegetative parts (pooled roots, stems and leaves), individualleaf blades and fruits. This seemed to be due to greater drymatter accumulation relative to N₂-fixation. The 2nd and 5th trifoliolate leaves showed larger specific leafweight (SLW) under LD. Soluble protein content on a dry weightbasis was higher in the 5th (younger) leaf than in the 2nd,but did not vary due to photoperiod. On the other hand, chlorophylland Fraction I protein content decreased in terms of dry weightunder LD. A larger proportion of leaf N was allocated to solubleproteins under LD, thus compensating for the lower N content.On the whole, growth enhancement by LD seemed unrelated to increasedavailability of N or to greater leaf soluble protein or FractionI content. Key words: Photoperiod, leaf development, soybean, nitrogen fixation