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.