Continuous light may induce photosynthetic downregulation in onion – consequences for growth and biomass partitioning

Onions were grown in environmentally controlled growth chambers for 85 days to investigate the effect of relatively low light intensity (350 µmol m⁻² s⁻¹) 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 bulb-forming 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 CO₂-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 CO₂ exchange and plant mass at the end of the experiment.