Seed pre-soaking with brassinolide enhances photosynthetic resistance of foxtail millet seedlings under mesosulfuron-methyl + iodosulfuron-methyl-sodium stress

Abstract

The mixture of mesosulfuron-methyl + iodosulfuron-methyl-sodium (ME+IMS) as post-emergence dual-purpose herbicide exhibits phytotoxicity in foxtail millet (Setaria italica (L.) P. Beauv.) Reducing ME+IMS stress in foxtail millet is therefore of practical significance. This study assessed the effects of seed pre-soaking with different concentrations of brassinolide (0.01, 0.1, 1.0 and 2.0 mg L-1) on foxtail millet grown under ME+IMS (2.81+0.56 g ai ha-1) stress. The agronomic characteristics, photosynthetic pigment, gas exchange parameters, chlorophyll fluorescence and P700 parameters of two foxtail millet cultivars (‘Jingu 21’ and ‘Zhangza 5’) were analyzed 7 d after ME+IMS treatment at the 5th leaf stage. Compared with ME+IMS treatment alone, brassinolide treatment within a concentration range of 0.1-1.0 mg L-1 had a positive effect on all plant parameters to varying degrees. Pre-soaking of seeds with 0.1 mg L-1 brassinolide increased the leaf area (41.8%), fresh mass (179.2%), net photosynthetic rate (Pn, 38.4%), transpiration rate (Tr, 61.5%), stomatal conductance (gs, 103.1%), photosystem II maximum quantum yield (Fv/Fm, 8.6%), effective quantum yield (Y(II), 16.0%), electron transport rate (ETR(II), 17.8%), photosystem I photochemical quantum yield (Y(I), 16.7%) and electron transport rate (ETR(I), 38.7%) in ‘Zhangza 5’ leaves under ME+IMS stress. An increase in plant height (53.0%), total chlorophyll (38.3%), carotenoid contents (40.0%), and maximal P700 change (Pm, 34.3%) were also observed following 1.0 mg L-1 brassinolide treatment compared with ME+IMS in ‘Jingu 21’. Comprehensive analysis showed that pre-sowing seed treatment with 0.1-1.0 mg L-1 brassinolide could enhance the growth and photosynthetic resistance of foxtail millet seedlings to ME+IMS stress.