CARBON PARTITIONING IN-SOURCE LEAVES OF PEACH, A SORBITOL-SYNTHESIZING SPECIES, IS MODIFIED BY PHOTOSYNTHETIC RATE

Sorbitol, together with sucrose and starch, is a major final product o f photosynthesis in most species of the Rosaceae. However, Little is k nown concerning the factors affecting the carbon fluxes into these thr ee carbohydrates. The aim of this study was to investigate the effect of net photosynthetic CO2 assimilation rate (NAR) on primary carbon pa rtitioning between sorbitol, sucrose, starch and other metabolites, an d on the kinetic pattern of labelling in mature leaves of peach (Prunu s persica [L.] Batsch). A pulse-chase labelling experiment was conduct ed on a group of peach seedlings showing a range of NAR's that were ob tained by using bye photosynthetic photon flux density treatments. Mat ure leaves undergoing one of these treatments were subjected to a 4-mi n pulse of (CO2)-C-14 followed by a 2- to 120-min chase under (CO2)-C- 12. The radioactivity in lipids, proteins and in a residual nonsoluble fraction was negligible. After a 2-min chase the organic ions were st rongly labelled, accounting for up to 40% of the total incorporated ra dioactivity at NAR's below 10.4 mu mol CO2 m(-2)s(-1). After a 30-min chase, most label was found in sorbitol, sucrose and starch with very little radioactivity in the ionic fractions. The analysis of the parti tioning of the newly-fixed carbon showed that at low NAR the C-14 flux was mostly into sorbitol rather than into sucrose (ratio = 2.6) or st arch. When the NAR was lower than 6.1 mu mol CO2 m(-2) s(-1), starch w as lightly labelled. As the NAR increased above 10.4 mu mol CO2 m(-2) s(-1) the (CO2)-C-14 diverted into starch increased dramatically. This study demonstrates that at low NAR's the newly-fixed CO2 is mainly us ed for sorbitol synthesis. However, as NAR increases, the participatio n of sucrose and starch in CO2 partitioning are favoured. These result s support the hypothesis that primary carbon partitioning depends on p hotosynthetic rate.