The impact of dark chilling followed by exposure to high light intensities on ultrastructure and selected biochemical reactions of photosynthesis in Glycine Max (L.) Merrill

Abstract

The impact of dark chilling on ultrastructure and key reactions of photosynthesis was assessed in two soybean genotypes, 'Maple Arrow' and 'Fiskeby V'. Both these genotypes are regarded as chilling tolerant (albeit differentially tolerant). These genotypes were subjected to three nights of dark chilling followed by exposure to high light intensities. Half of the plants were subjected to whole plant chilling, while the other half were only shoot-chilled. The shoot-chilled plants were grown in the presence/absence of nitrate supplementation. The aim of this study was to increase the current understanding of the physiological, biochemical and ultrastructural basis for the limitation of photosynthesis by dark chilling in soybean genotypes. The photosynthetic response to dark chilling in the presence or absence of root chilling was also compared and the ameliorating effect of nitrate treatment on photosynthetic limitation during dark chilling was evaluated. Non-intrusive analysis conducted to assess the physiological and biochemical impact of dark chilling included photosynthetic gas exchange measurements. Intrusive analysis included ultrastructural and anatomical studies, measurement of the activities of key enzymes involved in photosynthesis and Western Blot analysis. The results demonstrated the existence of genotypic differences in the response of photosynthesis in two chilling tolerant soybean genotypes to dark chilling. Photosynthesis was inhibited to a lesser extent in 'Maple Arrow' than in ' Fiskeby V'. It appeared that not only mesophyll limitation, but also stomatal limitation, was much more evident in 'Fiskeby V'. Novel evidence is provided for the existence of an inverse relationship between loss of FBPase activity and loss of photosynthetic capacity in the two chilling tolerant soybean

genotypes. The results presented suggest that 'Maple Arrow' is capable of "sensing" dark chilling stress much better than 'Fiskeby V' . ~o severe ultrastructural disruption was visible in the plants investigated. The changes observed in ultrastructure corroborate the changes in photosynthetic capacity. No genotypic differences in leaf anatomy could be seen in untreated plants. The comparative investigation regarding the effects of dark chilling, in the presence or absence of root chilling, on the photosynthetic response in 'Maple Arrow' and 'Fiskeby V' confirmed that whole-plant dark chilling caused a greater decrease in photosynthetic capacity than shoot chilling alone. In the absence of root chilling it was not possible to evaluate the ameliorating effect of nitrogen on the inhibition of photosynthesis by dark chilling because photosynthesis was not inhibited in the two genotypes under these conditions. Although dark chilling of only the shoots did not induce a significant decrease in photosynthesis, the trend did suggest that 'Maple Arrow' was more responsive to the change in temperature. In conclusion, the results presented strongly suggest a more dark chilling tolerant physiological and biochemical make-up in 'Maple Arrow' compared to 'Fiskeby V'. However, this effect was only visible during whole plant chilling. Genotypic differences in the effectiveness of the host - Bradyrhizobium interaction could have contributed towards the chilling sensitivity of 'Fiskeby V' . When roots remained at higher temperatures during chilling, the differential chilling sensitivity between the genotypes were not revealed, suggesting that 'Fiskeby V' is more sensitive to root chilling than 'Maple Arrow'.