[Effects of light quality on growth and photosynthesis of broccoli at different reproductive periods]

To understand the light requirements of broccoli, we investigated the effects of different light qualities on the growth and photosynthesis in the 'Naihanyouxiu' cultivar at various growth stages in an experiment. There were four light treatments, white-to-red light ratio 4:1 (WR), white-to-blue light ratio 4:1 (WB), white-purple light ratio of 4:1 (WP), and white-green light ratio of 4:1 (WG), with the white light as control (CK). Results showed that all light quality treatments reduced broccoli germination rate but did not affect the growth during the germination stage. During the seedling, rosette and heading stages, WR treatment increased plant height, leaf length, leaf width, leaf spread, relative chlorophyll content, and net photosynthetic rate (P_n), and promoted the accumulation of dry matter. Moreover, the single head weight of WR treatment increased by 15.5%, 16.2%, 66.6%, and 42.7% compared to CK, WB, WP, and WG, respectively. Plants treated with WB showed similar growth and P_n to CK, while that of WP and WG both exhibited significantly lower growth and P_n. Compared to CK, WR treatment increased total sugar, starch, and nitrogen content, followed by WB treatment though the difference was not significant, and WP and WG treatments showed no difference or a significant decrease compared to CK during the rosette stage. In summary, supplementing 20% red light to white light could enhance photosynthetic capacity, increase the accumulation of dry matter, and then promote the growth and head formation of broccoli. Supplementing blue light showed no impact, while supplementing purple and green light significantly inhibited growth. These findings could provide theoretical support for light quality regulation during different growth stages of broccoli.