H2S and rhizobia jointly regulate the growth of plant, photosynthesis, and chlorophyll fluorescence under water deficiency
Previous studies showed that rhizobia could promote plant growth and increase biomass accumulation under metal contaminated environment (Shen et al., 2019). Rhizobia also improve alfalfa productivity and increase biomass in different alfalfa cultivars under salt condition (Bertrand et al., 2015). In the present study, water deficiency reduced the shoot and root biomass, but rhizobia can significantly alleviate the decrease of water deficiency-induced biomass in soybean seedlings (Fig. 1A). Moreover, our data indicated that 100 μM NaHS and the inoculation of rhizobia substantially increased the shoot and root biomass of soybean plants under water deficiency condition (Fig. 1B, C). similarly, the study of Zhang et al. (2020) showed that H2S and rhizobia can jointly regulate the biomass and growth in soybean under N deficiency. These results suggested that H2S and rhizobia jointly alleviated stressful environments in plants, likely by increasing biomass yield and maintaining a higher level of nitrogen fixation. Therefore, we concluded that the interaction of H2S and rhizobia more effectively enhanced plant growth under water deficiency condition.
Plants could avoid leaf water loss by adjusting stomatal conductance and transpiration (Jin & Pei, 2015; Stanton & Mickelbart, 2014). In the present study, we found that the leaf RWC was increased by Q8+NaHS than that of the control plants under SW condition (Fig. 1E). In response to drought, higher leaf RWC in leaves may be due to reduce water loss in most plants (Yin et al., 2013), as exemplify by phenomenons showing high biomass under water deficiency in Q8 and Q8+NaHS treatments, which tended to have an increased leaf RWC when water availability decreased. Besides, the leaf RWC was increased by NaHS in S. oleraceaseedlings under drought condition (Chen et al., 2016). These results clearly indicated that H2S and rhizobia markedly alleviated water loss of plant leaves under water deficiency condition.
H2S promoted chlorophyll synthesis and reduced chlorophyll loss under environmental stress (Zhang et al., 2010; Zhang et al., 2009). For instance, under aluminum stress the increase of chlorophyll content caused by exogenous H2S substantially promoted the growth of rape (Qian et al., 2014). Moreover, exogenous H2S remarkably mitigated the deterioration of chlorophyll content under cadmium stress (Tian et al., 2016). Similarly, Mostofa et al. (2015) reported the restoration of chlorophyll content with exogenous H2S in rice under cadmium stress. Further, Ding et al.(2019) also claimed that the higher plant growth was due to the increase of chlorophyll content by H2S-mediated under salinity stress. Our results also indicated that H2S and rhizobia jointly mediated the increase in chlorophyll content improved the growth of soybean plants under water deficiency conditions, suggesting that H2S and rhizobia enhance the synthesis complexes and protein molecules of chloroplasts and mitochondria. However, it is noteworthy that the chlorophyll content was increased gradually with increasing water deficiency (Fig. 1D). Similar results had reported that drought stress could induce the increase in photosynthetic pigments inArabidopsis thaliana (Gamar et al., 2019). However, many papers reported a decrease in chlorophyll content due to drought stress in other legumes (Basal et al., 2020; Buezo et al., 2019; Hao et al., 2013). Munawar et al. (2019) reported a notable decline in photosynthetic pigments in drought-stressed broccoli plants. Naz et al. (2016) found the decline in chlorophyll molecules in cucumber under drought. Drought stress significantly diminished chlorophyll molecules in radish (Akram et al. 2015). Some possible explanations for this discrepancy included the differences in plant species, the drought stress intensity and duration of treatment. In this study, under SW condition, the Pn, Tr, and Gs were higher in H2S and rhizobia treatment than in the control treatment (Fig. 3A, B, D). These results showed that Q8+NaHS-treated plants exhibited a high photosynthetic rate compared with the control treatment under water deficiency condition. The major reason of low photosynthesis rate is caused by inadequate ribulose-1, 5-bisphosphate (RUBP) synthesis, as a result of decreased ATP synthesis (Lawlor, 2002). Hence, the higher photosynthesis by Q8+NaHS-treated plants under water deficiency condition could be related to the positive role of H2S and rhizobia on cellular ATP production in soybean leaves. Moreover, H2S acted as modulator of PSII activity in leaves. Specifically, ETR, PSII, and Fv/Fm parameters were increased by Q8+NaHS treatment (Fig. 4A, B, and D), indicating that the photochemical efficiency of PSII was increased by H2S and rhizobia. These were consistent with the increase in photosynthetic rate by Q8+NaHS treatment under water deficiency. In general, above results suggested that the interaction of H2S and rhizobia jointly enhanced photosynthesis and alleviated the inhibition of soybean biomass due to lack of water.