Photosynthetic acclimation to elevated CO2 in relation to Rubisco gene expression in three C3 species.

Wheat (Triticum aestivum L. var. DL 1266-5), sunflower (Helianthus annuus L. var. MSFH 17) and mungbean [Vigna radiata (L.) Wilczek var. P 9072] were grown in field under atmospheric (360 +/- 10 cm3 m(-3), AC) and elevated (650 +/- 50 cm3 m(-3), EC) CO2 concentrations in open top chambers for entire period of growth and development. Photosynthetic acclimation to elevated CO2 was examined by comparing photosynthesis rate (Pn), Pn/Ci curves, leaf contents of RuBP carboxylase/oxygenase (Rubisco), change in the transcripts of Rubisco small subunit (SSU) gene and leaf carbohydrate constituents in AC and EC grown plants. The study indicated that photosynthetic acclimation to elevated CO2 concentration in wheat occurred because of down regulation of Rubisco, through limitation imposed on Rubisco SSU gene expression, as a consequence of sugar accumulation in the leaves. Leaf starch accumulators, sunflower and mungbean, showed no down regulation of Pn under EC. The Rubisco contents (%) in leaf soluble protein and rbcS transcript levels were not significantly affected in EC plants compared to AC plants of sunflower and mungbean. The study indicated that accumulation of excess assimilates in the leaves as starch was less inhibitory to Pn and would, therefore, be an important trait for sustenance of Pn not only under EC, but also under AC, where Pn inhibited by end products.

Effect of different temperature on starch synthase activity in excised grains of wheat cultivars.

Excised grains of wheat (Triticum aestivum) varieties HD 2285 (relatively tolerant) and HD 2329 (susceptible type) were incubated for 1 hr at 15 degrees, 25 degrees, 35 degrees and 45 degrees C. In an another treatment, excised grains were incubated for 1 hr at increasing temperature (15 degrees, 25 degrees, 35 degrees and 45 degrees C) continuously, thus exposing the grains to gradual rise in temperature. The above treated grains were then analysed for the activity of soluble starch synthase (SSS) and granule bound starch synthase (GBSS) assayed at 20 degrees C. SSS activity decreased as the pre-exposure temperature was higher, though the tolerant variety showed lesser decrease. Decrease in SSS activity was lesser when excised grains were exposed to gradual rise in temperature from 15 degrees to 45 degrees C as compared to direct exposure to 45 degrees C. Pre-exposure of excised grains to different temperatures however, had no significant effect on GBSS activity.

Changes in rubisco content in relation to saccharides accumulation in wheat leaves during day.

Two wheat varieties, T. durum (HD 4502) and T. aestivum (Kalyansona) were examined for photosynthesis rate and contents of sugars and rubisco protein in the flag leaf, at forenoon and afternoon at anthesis stage. A decrease in photosynthesis rate was observed in the afternoon compared to forenoon in both the varieties and was associated with an increase in non-reducing sugars and a decrease in rubisco content in the leaves.

Photosynthetic acclimation to rising atmospheric carbon dioxide concentration.

With rising level of CO2 in the atmosphere plants are expected to be exposed to higher concentration of CO2. Since, CO2 is a substrate limiting photosynthesis particularly in C3 plants in the present atmosphere, the impact of elevated CO2 would depend mainly on how photosynthesis acclimates or adjusts to the long term elevated level of CO2. Photosynthetic acclimation is a change in photosynthetic efficiency of leaves due to long term exposure to elevated CO2. This change in photosynthetic efficiency could be a biochemical adjustment that may improve the overall performance of a plant in a high CO2 environment or it could be due to metabolic compulsions as a result of physiological dysfunction. Acclimation has generally become synonymous with the word response, if long term exposure to elevated CO2 decreases the photosynthesis rate (Pn) at a given CO2 level, it is called negative acclimation, if it stimulates Pn at a given CO2 level, it is called positive acclimation. Photosynthetic acclimation is clearly revealed by comparing Pn of ambient and elevated CO2 grown plants at same level of CO2. Species level differences in acclimation to elevated CO2 have been reported. The physiological basis of differential photosynthetic acclimation to elevated CO2 is discussed in relation to the regulation of photosynthesis and photosynthetic carbon partitioning at cellular level.