Exogenous paclobutrazol can relieve the low irradiance stress in Capsicum annuum seedlings

Irradiance is one of the main limiting factors affecting the production of pepper (Capsicum annuum L.) in facility production. It is therefore important to measure the growth of pepper seedlings under low irradiance and to understand how to relieve low radiation stress. In this study, pepper seedlings were cultivated under low irradiance and were treated with gibberellic acid (GA3) and paclobutrazol (PP333). Agronomic and physiological characteristics of the pepper seedlings were analyzed. Under low irradiance, the plant height, leaf area, and the chlorophyll (Chl) and malondialdehyde (MDA) content of pepper seedlings were higher than under normal irradiance, while the content of proline, and soluble protein and Chl a/b ratio of pepper seedlings were lower than those under normal irradiance; exogenous GA3 had a similar effect. When PP333 was applied to the seedlings, abnormal growth was mitigated. The effects of exogenous GA3 and PP333 on Chl fluorescence parameters were also analyzed. Under low irradiance, the maximum quantum yield reduction of photosystem (PS II) (Fv/Fm) increased significantly, and the reaction center initiated the corresponding defense mechanism for timely dissipation of excess excitation energy to reduce the damage to the plant. These results showed that spraying gibberellin under normal irradiance conditions had similar effects on pepper seedlings as under low irradiance conditions. Exogenous PP333 relieved the growth reduction by increasing the content of Chl and soluble protein and enhancing some photosynthetic parameters. These results suggest that exogenous PP333 can alleviate the abnormal growth of pepper under low radiation stress.