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Summary: • C assimilation (A) has been shown to limit the growth of young Lactuca sativa (lettuce) plants following an interruption in their external N supply. Further data from these plants were used to test two hypotheses: that N‐limited growth of lettuce is associated with lower stomatal conductance (gs); and that reductions in gs result from adjustments to stomatal frequency or distribution.• The photosynthetic characteristics, nitrate and organic N‐concentrations, as well as epidermal and stomatal distributions, were determined in leaves of hydroponically grown lettuce plants supplied continuously with N or with N removed for up to 14 d.• Although N‐limited plants had lower maximum rates of A, comparisons at equivalent values of gs showed that A was not directly limited by organic‐N but by gs. Reductions in gs under N‐limiting conditions did not associate with adjustments to stomatal frequency or distribution.• Associations between plant N and A could arise either through stomata responding directly to signals induced by N deprivation or to increased CO2 partial pressure at the sites of carboxylation. [ABSTRACT FROM AUTHOR]

Sorbitol, together with sucrose and starch, is a major final product of photosynthesis in most species of the Rosaceae. However, little is known concerning the factors affecting the carbon fluxes into these three carbohydrates. The aim of this study was to investigate the effect of net photosynthetic CO2 assimilation rate (NAR) on primary carbon partitioning between sorbitol, sucrose, starch and other metabolites, and on the kinetic pattern of labelling in mature leaves of peach (Primus persica [L.] Batsch). A pulse-chase labelling experiment was conducted on a group of peach seedlings showing a range of NAR's that were obtained by using five photosynthetic photon flux density treatments. Mature leaves undergoing one of these treatments were subjected to a 4-min pulse of 14CO2 followed by a 2- to 120-min chase under 12CO2. The radioactivity in lipids, proteins and in a residual nonsoluble fraction was negligible. After a 2-min chase the organic ions were strongly labelled, accounting for up to 40% of the total incorporated radioactivity at NAR's below 10.4 µmol CO2 m-2s-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 partitioning of the newly-fixed carbon showed that at low NAR the 14C flux was mostly into sorbitol rather than into sucrose (ratio = 2.6) or starch. When the NAR was lower than 6.1 µmol CO2 m-2s-1, starch was lightly labelled. As the NAR increased above 10.4 µmol CO2 m-2s-1 the 14CO2 diverted into starch increased dramatically. This study demonstrates that at low NAR's the newly-fixed CO2 is mainly used for sorbitol synthesis. However, as NAR increases, the participation of sucrose and starch in CO2 partitioning are favoured. These results support the hypothesis that primary carbon partitioning depends on photosynthetic rate. [ABSTRACT FROM AUTHOR]