High daily light integral positively regulate photosynthetic capacity through mediating nitrogen partitioning and leaf anatomical characteristic in flowering Chinese cabbage

In controlled environment agriculture, optimization of lighting regimes, i.e. daily light integral (DLI, photosynthetic photon flux density (PPFD) x photoperiod duration), plays vital roles to maximize harvestable yield and reduce lighting costs. Regulating PPFD or photoperiod have been widely used for energy-efficient cultivation, while the mechanisms of multiple DLI contributing to photosynthesis capacity are poorly understood. Hence, the treatment of 16/8 h light/dark compared with 24 h continuous lighting (CL), at 200 and 300 mu mol m- 2 s- 1 PPFD, with the DLI from 11.52 to 25.92 mol m- 2 d-1, were conducted in flowering Chinese cabbage respectively. The results showed that higher DLI significantly promoted relative growth rate and dry matter accumulation, irrespective of the combination of PPFD and photoperiod, consequently facilitated faster harvest compared to the lower DLI of 11.52 mol m- 2 d-1 treatment. Under higher DLI, the increase in Rubisco content was the primary factor accelerating photosynthetic capacity, followed by the increases in gm. Additionally, the enhancement of Rubisco was ascribed to the increased allocation of nitrogen to the carboxy system. Furthermore, the increase of gm under higher DLI was largely resulted from the increase in Sm/S and Sc/S caused by increased accumulation of soluble sugar and starch. Unexpectedly, at the same DLI of 17.28 mol m- 2 d-1, extending photoperiod from 16 to 24 h d-1 had no influence on photosynthetic capacity, although 24 h 200 mu mol m- 2 s- 1 treatment reduced the abaxial stomatal density and light-saturated stomatal conductance, and increased the thickness of cell walls and the contents of chlorophyll and Rubisco, compared with 16 h 300 mu mol m- 2 s- 1 treatment. To summarize, plant growth and ripening were promoted as the DLI increased. And optimized nitrogen allocation accounted for the promotion of photosynthetic capacity. Increasing photoperiod at the same DLI had no promotion on photosynthetic capacity.