PRIMARY PARTITIONING AND STORAGE OF PHOTOSYNTHATE IN SUCROSE AND STARCH IN LEAVES OF C-4 PLANTS

A procedure involving pulse labelling of leaves with (CO2)-C-14 was de veloped to measure the primary (initial) partitioning of photosynthate between sucrose and starch. Partitioning of photosynthate into sucros e and starch was determined in leaves of C-4 plants and compared with the patterns of storage of carbon in these products during the light p eriod. The ratio of primary partitioning into sucrose and starch varie d from about 0.5 in those species that accumulated mostly starch in th e leaves (Amaranathus edulis L., Atriplex spongiosa F. Muell. and Flav eria trinervia (Spreng.) C. Mohr) to about 8 in Eleusine indica (L.) G aertn., which accumulated mostly sucrose. No label was detected in fre e glucose or fructose. Generally there was a reasonable link between t he primary partitioning of photosynthate and the type of carbohydrate stored in the leaf during the day. However, the ratio of carbon initia lly partitioned into sucrose versus starch was about 3 to 4 times high er in leaves of NADP-malic enzyme-type monocotyledonous species compar ed with phosphoenolpyruvate carboxykinase-type species, although the r atio of sucrose to starch accumulated in leaves during the day was ver y similar in the two groups. Sucrose and starch were the principal car bohydrates accumulated in leaves during the day. None of the species e xamined contained significant amounts of fructan and only one species, Atriplex spongiosa, contained substantial amounts of hexose sugars. I n most of the species studied, the proportion of photosynthate partiti oned into starch was greater at the end of the day than at the beginni ng. With the exception of Flaveria trinervia, the rate of CO2 assimila tion did not decline during the day, showing that, under our condition s, accumulation of carbohydrate in the leaves did not lead to feedback inhibition of photosynthesis in these C-4 species.