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The effects of low light intensity on sucrose synthase, the key enzyme in sucrose metabolism were studied in nectarine (Prunus persica L. var. nectarina) grown in a greenhouse. Experiments were conducted under two light regimes: a photosynthetic photon flux (PPF) of 588 ± 0.32 μmol m⁻² s⁻¹ (full sun) and a PPF of 150 ± 1 μmol m⁻² s⁻¹ (low light). Concentrations of sucrose were lower under low light [1.47 ± 0.2 mg g⁻¹ fresh weight (FW)] than in full sun (1.56 ± 0.4 mg g⁻¹ FW), whereas there was no effect on the concentrations of glucose and fructose. The activity of sucrose synthase (SS) was also lower under shade (0.15 ± 0.05 nmol min⁻¹ g⁻¹ FW) than in full sun (0.26 ± 0.05 nmol min⁻¹ g⁻¹ FW). An 87 kDa polypeptide was detected by western blotting using antiserum specific for SS, and was present at similar levels in low light and in full sun. Electron microscopy and immunogold-labelling showed that SS was localised mainly in the vacuole rather than in the cytoplasm of phloem parenchyma (PP) cells and in nacreous cell walls (NCW). However, SS activity decreased in low light, suggesting that low light reduced the specific activity but not the amount of SS in nectarine leaves. As sucrose production in nectarine leaves is affected primarily by the activity of SS, to improve the quality of greenhouse-grown nectarines, additional light should be provided. [ABSTRACT FROM AUTHOR]