Photosynthesis plays an essential role in plant growth and crop yield, and the mechanisms of the effects of cadmium (Cd) on photosynthetic performance require more attention. The acute toxicity of Cd in soil to the photosynthetic capacity of Hybrid Pennisetum was evaluated using gas exchange parameters, A/C_i curves, light response curves, and chlorophyll a fluorescence transients after exposure to elevated Cd concentrations (0, 10, 20, 50, 70, and 100 mg kg⁻¹) for a 3-month period. The results indicated that leaf Cd concentration in Hybrid Pennisetum increased with the strength of soil Cd stress and ranged from 4.9 to 15.8 μg g⁻¹ DW. The accumulation of leaf Cd severely restricted photosynthesis and its non-stomatal limitation in regulating the photosynthetic performance of Hybrid Pennisetum. The leaf chloroplasts at 10 and 20 mg kg⁻¹ Cd concentrations showed no noticeable change, but the chlorophyll content significantly decreased by 9.0–20.4% at 50–100 mg kg⁻¹ Cd concentrations. The Cd treatments also decreased plant ribulose-1,5-bisphosphate (RuBP) activity (V_cmax) and regeneration capacity (J_max), triose phosphate utilization (TPU), light-saturated photosynthesis (A_max), apparent quantum yield (AQY), light saturation point (LSP), and dark respiration (R_day), but Cd treatment increased the light compensation point (LCP). The shape of chlorophyll a fluorescence transients in leaves was altered under different Cd treatments. The increased OJ phase and the decreased IP phase in fluorescence induction curves suggested that Cd toxicity inhibited both light use efficiency and photodamage avoidance ability. These results suggested that the decrease in photosynthesis through exposure to Cd may be a result of the decrease in leaf chlorophyll content, Rubisco activity, and RuBP regeneration, inhibition of triose phosphate utilization, reduction of the ability to use light and provide energy, and restrictions on electron transport in PSII.

Soils co-contaminated with polycyclic aromatic hydrocarbons (PAHs) and cadmium (Cd) have serious environmental impacts and are highly toxic to humans and ecosystems. Phytoremediation is an effective biotechnology for the remediation and restoration of PAH- and Cd-polluted soils. Pot experiments were conducted to investigate the individual and combined effects of PAHs (1238.62 mg kg^-1) and Cd (23.1 mg kg^-1) on the phytoremediation potential of Bermuda grass grown in contaminated soils. Bermuda grass exhibited a significant decrease in plant growth rate, leaf pigment content, root activity, plant height and biomass and a remarkable increase in malondialdehyde content and electrolyte leakage when grown in PAH- and Cd-contaminated soils compared with grass grown in uncontaminated soils. The activity of soil enzymes, including urease, alkaline phosphatase, sucrose, and fluorescein diacetate hydrolysis, were reduced in soil with PAH and Cd stress. Furthermore, the toxicity of combined PAHs and Cd on Bermuda grass growth and soil enzyme activity was much higher than that of PAH or Cd stress alone, suggesting a synergistic effect of PAHs and Cd on cytotoxicity. To scavenge redundant reactive oxygen species and avoid oxidative damage, Bermuda grass increased plant catalase, superoxide dismutase, and peroxidase activity and soluble sugar and proline content. The bioconcentration factor of Cd in Bermuda grass grown under Cd alone and combined PAH and Cd exposure was greater than 1 for both, suggesting that Bermuda grass has a high Cd accumulation ability. Under PAH alone and combined PAH and Cd exposure conditions, a higher PAH removal rate (41.5-56.8%) was observed in soils planted with Bermuda grass than in unplanted soils (24.8-29.8%), indicating that Bermuda grass has a great ability to degrade PAHs. Bermuda grass showed great phytoremediation potential for the degradation of PAHs and phytoextraction of Cd in co-contaminated soils.

To examine the impacts of various straw mulching techniques, this study used the indoor soil column test as the primary research method and the field test as the validation test on the salinity dynamics of saline and alkaline soils. The experiment in this study was designed with five treatments: SC means for straw covered on the soil surface; DB means for straw buried 40 cm below the soil surface; S1D1, S2D1, and S1D2 represent the ratio of soil surface cover to the amount of straw buried 40 cm below the soil surface as 1:1, 2:1, and 1:2, respectively. The results of the indoor soil column test showed that all kinds of straw mulching techniques could effectively reduce soil moisture evaporation, and the straw mulching and deep burial mode was more effective: after 45 days of evaporation, compared with that of CK, the cumulative evaporation of soil moisture were reduced by 29.61%, 27.49%, 37.87%, 65.85%, and 54.58% for SC, DB, S1D1, S2D1, and S1D2, respectively; the straw mulching and deep burial mode could reduce the soil evaporation intensity more effectively than the single-layer straw mulching mode: the mean soil evaporation rates of CK, SC, DB, S1D1, S2D1 and S1D2 after 45 days of evaporation were 1.27 mm/day, 0.90 mm/day, 0.92 mm/day, 0.80 mm/day, 0.43 mm/day, and 0.58 mm/day; various straw mulching techniques could inhibit the accumulation of salts in the surface soil and effectively regulate the distribution of salts in the soil profile, among which the straw mulching and deep burial mode had the best effect of salinity suppression: after 30 days of evaporation, the re-salinization levels of the 0–40 cm soil layer of SC, DB, S1D1, S2D1, and S1D2 were reduced by 66.78%, 43.08%, 33.95%, 92.04% and 45.94% compared with that in the CK, respectively; there was a significant positive correlation between cumulative evaporation of soil moisture and cumulative soil salinity, which implied that cumulative soil salinity increased with the increase in cumulative evaporation of soil moisture. The results of the field experiment justified the results of the indoor soil column test: after four months of evaporation, the field moisture contents of CK, SC, DB, S1D1, S2D1, and S1D2 in the 0–20 cm soil layer were 14.77%, 3.51%, 15.10%, 15.26%, 18.73%, and 2.94%, respectively; during the experimental period, the salt inhibition rate of SC, DB, S1D1, S2D1 and S1D2 in 0–20 cm soil layer were 35.46%, 44.76%, 50.98%, 54.80% and 37.30%, respectively. Therefore, in a comprehensive view, S2D1 treatment had the best effect of salt and vapor suppression on saline soil. This study is of great significance for the resource utilization of straw waste, the improvement of water utilization and efficiency, and the management of soil salinization.

Field experiments were conducted to evaluate the effects of wheat straw return methods, which included the use of surface straw mulch and a buried straw layer, on soil water content, electrical conductivity (EC), and sodium adsorption ratio (SAR) of saline sodic soils in an effort to identify useful ways for reducing soil salt accumulation and enhancing soil water content. The results showed that the straw return treatments were effective for inhibiting salt accumulation and soil water loss, resulting in a reduction of EC and SAR but an enhancement of soil water content. After a year-long experiment, compared with the treatment with no straw return, the straw burial and straw mulching treatments decreased the EC by 10.5% and 3.5%, reduced the SAR by 7.4% and 21.5%, and increased the soil water by 0.9% and 4.4%, respectively. Furthermore, the combined application of straw layer burial and surface straw return had a more significant effect than the individual treatments; the positive effect of straw return occurred mainly focused in the topsoil (0–40 cm) and decreased with increasing soil depth. Our results allowed us to conclude that burial of the straw layer was necessary to enhance the effects of surface mulch, and the combination of surface mulch (3.0 t ha⁻¹ of wheat straw) and straw layer burial (6.0 t ha⁻¹ of wheat straw) proved to be a better straw return method than the others.