Effect of drought stress during supplemental lighting on diurnal photosynthesis of cut rose ‘Charming Black’

To investigate the effect of supplemental lighting and irrigation during night on the diurnal photosynthetic rate, cut rose ‘Charming Black’ was subjected to three treatments: T_Control (without supplemental lighting), T_SL×NI photosynthetic photon flux density (PPFD) 90 µmol·m ^-2 ·s ^-1 without irrigation during night, and TSL×I (PPFD 90 with hourly irrigation). The diurnal photosynthetic rate showed an increasing tendency from 09:00 and the highest value occurred from 12:00 to 14:00, while it decreased from 15:00 and then remained steady in three treatment conditions. Compared with T_Control, T_SL×NI had a higher photosynthetic rate during night due to supplemental lighting. Nonetheless, from 10:00 to 15:00, the photosynthetic rate was lower than that of T_Control. Under irrigation condition TSL×I, the photosynthetic rate increased by 4.96 µmol·m ^-2 ·s ^-1 compared with T_Control at 13:00. Under T_SL×NI, the photosynthetic rate peaked earlier (at about 12:00) than under the T_SL×NI condition (13:00). Compared to T_SL×NI, T_SL×NI fluctuated dramatically in stomatal conductance and intercellular CO₂ concentration. No significant difference of water use efficiency between T_SL×NI and T_SL×NI was observed. Supplemental lighting contributed substantially to the photosynthesis during night. However, in the day time, it caused a negative effect, during which irrigation played an active role in compensating this negative effect. Thus, the irrigation and supplemental lighting compensated for each other to promote the photosynthesis of cut rose ‘Charming Black’.