PHOTOSYNTHESIS, PHOTOSYNTHETIC METABOLISM, AND TRANSLOCATION OF ASSIMILATES IN C4 PLANTS AT NORMAL AND LOW-TEMPERATURES

Photosynthesis, photosynthetic metabolism, and carbohydrate content we re investigated in the C4 plants Amaranthus retroflexus L. and Atriple x tatarica L. as related to the inhibiting effect of low temperature o n the translocation of assimilates. When the intact plants of A. tatar ica were cooled, the export of C-14-compounds from chlorenchyma into t he veins was inhibited to a greater extent than the total (CO2)-C-14 a ssimilation. The local cooling of leaf petioles of both plant species also inhibited both translocation of assimilates and photosynthesis, a nd, in addition, changed the ratio between C-14-Starch and C-14-sucros e synthesized de novo in the course of photosynthesis. The increase in monosaccharide content and in the ratio of starch to sucrose was the most evident response to cooling. Only one species, A. retroflexus, re stored its photosynthetic rate under continuous petiole cooling. The d ifferent responses to cold stress in two plant species could probably result from the structural peculiarities of their assimilating and tra nsporting systems, the different sizes of their carbohydrate pools (in particular, in vacuoles), and the different controls over their redis tribution.