Onions were grown in environmentally controlled growth chambers for 85 days to investigate the effect of relatively low light intensity (350 mmol m À2 s À1 ) at two different total irradiance periods (12-h and 24-h photoperiods) on growth and photosynthetic performance. To test whether photosynthetic downregulation occurred due to carbohydrate feedback, we used onions that differed in bulbforming capacity. Allium fistulosum (L. cv. 'Kinka') is a non-bulbing onion, with potentially limited carbohydrate storage capacity, while Allium cepa (L. cv. 'Cal 296') is a bulb-forming onion with possibly greater carbohydrate storage capacity. In A. fistulosum, photosynthetic downregulation was observed in 24-h plants as indicated by reductions in the light-and CO 2 -saturated photosynthetic capacity (A sat and A max , respectively) by 26%, reduced maximum rate of carboxylation (V cmax ) by ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) by 33%, reduced maximum rate of electron transport (J max ) by 27% and 3-fold higher foliar sugar concentration. In contrast, the photosynthetic and biochemical capacity of A. cepa was not affected by exposure to 24-h photoperiod, presumably because substantial amounts of foliar carbohydrates were re-allocated to bulbs. In 24-h A. cepa, up to 84% of total plant mass was allocated to bulbs, while in 12-h plants, more mass was allocated to leaves. Production of greater leaf area in 12-h plants compared with 24-h plants compensated for lower total daily irradiance such that 12-h and 24-h plants of both species exhibited similar daily total leaf net CO 2 exchange and plant mass at the end of the experiment.Abbreviations -A max , photosynthetic capacity at saturating light level (1500 mmol m À2 s À1 PPFD) and saturating atmospheric CO 2 (C a of 80 Pa CO 2 ) (mmol CO 2 m À2 s À1 ); A sat , net photosynthesis at saturating irradiance (1500 mmol m À2 s À1 PPFD) and C a of 37 Pa CO 2 (mmol CO 2 m À2 s À1 ); C a , atmospheric CO 2 partial pressure (Pa); C i , intercellular CO 2 partial pressure (Pa); J max , maximum rate of electron transport (mmol m À2 s À1 ); L s , relative stomatal limitation to photosynthesis (%); Q comp , photosynthetic light compensation point (mmol m À2 s À1 ); Q sat , photosynthetic light saturation point (mmol m À2 s À1 ); V cmax , maximum rate of Rubisco carboxylation (mmol m À2 s À1 ).

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