Biofertilizers as Strategies to Improve Photosynthetic Apparatus, Growth, and Drought Stress Tolerance in the Date Palm

Abstract

Rainfall regimes are expected to shift on a regional scale as the water cycle intensifies in a warmer climate, resulting in greater extremes in dry versus wet conditions. Such changes are having a strong impact on the agro-physiological functioning of plants that scale up to influence interactions between plants and microorganisms and hence ecosystems. In (semi)-arid ecosystems, the date palm (Phoenix dactylifera L.) -an irreplaceable tree- plays important socio-economic roles. In the current study, we implemeted an adapted management program to improve date palm development and its tolerance to water deficit by using single or multiple combinations of exotic and native arbuscular mycorrhizal fungi (AMF1 and AMF2 respectively), and/or selected consortia of plant growth-promoting rhizobacteria (PGPR: B1 and B2), and/or composts from grasses and green waste (C1 and C2, respectively). We analyzed the potential for physiological functioning (photosynthesis, water status, osmolytes, mineral nutrition) to evolve in response to drought since this will be a key indicator of plant resilience in future environments. As result, under water deficit, the selected biofertilizers enhanced plant growth, leaf water potential, and electrical conductivity parameters. Further, the dual-inoculation of AMF/PGPR amended with composts alone or in combination boosted the biomass under water deficit conditions to a greater extent than in non-inoculated and/or non-amended plants. Both single and dual biofertilizers improved physiological parameters by elevating stomatal conductance, photosynthetic pigments (chlorophyll and carotenoids content), and photosynthetic efficiency. The dual inoculation and compost significantly enhanced, especially under drought stress, the concentrations of sugar and protein content, and antioxidant enzymes (polyphenoloxidase and peroxidase) activities as a defense strategy as compared with controls. Under water stress, we demonstrated that phosphorus was improved in the inoculated and amended plants alone or in combination in leaves (AMF2: 807%, AMF1+B2: 657%, AMF2+C1+B2: 500%, AMF2+C2: 478%, AMF1: 423%) and soil (AMF2: 397%, AMF1+B2: 322%, AMF2+C1+B2: 303%, AMF1: 190%, C1: 188%) in comparison with controls under severe water stress conditions. We summarize the extent to which the dual and multiple combinations of microorganisms can overcome challenges related to drought by enhancing plant physiological responses.

Keywords: PGPR; agro-physiological responses; arbuscular mycorrhizal fungi; climate change; compost; photosynthesis; plant fitness; water deficit.

FIGURE 1

Influence of different water regimes [75% field capacity (FC); open bars and 25% FC; filled bars] on (A) mycorrhization frequency and (B) intensity in control plants (non-amended, non-inoculated), and plants amended with composts (C1 or C2) and/or inoculated with arbuscular mycorrhizal fungi (AMF, exogenous AMF1 or native AMF2) or plant growth promoting rhizobacteria (PGPR) strains (B1 or B2). Data are mean ± SE of six biological replicates. Means followed by the same letters are not significantly different at P < 0.05 (Tukey’s HSD).

FIGURE 2

Influence of different water regimes (75% field capacity (FC); open bars and 25% FC; filled bars) on date palm total dry matter in control (non-amended, non-inoculated), and plants amended with composts (C1 or C2) and/or inoculated with arbuscular mycorrhizal fungi (AMF, exogenous AMF1 or native AMF2) or plant growth promoting rhizobacteria (PGPR) strains (B1 or B2) date palms. Data are mean ± SE of six biological replicates. Means followed by the same letters are not significantly different at P < 0.05 (Tukey’s HSD).

FIGURE 3

(A) Phosphorous (P) and (B) nitrogen (N) content in date palm shoots under two water regimes [75% field capacity (FC); open bars and 25% FC; filled bars] of the tested control (non-amended and non-inoculated) and biofertilizers treatments [composts C1 or C2, arbuscular mycorrhizal fungi (AMF, exogenous AMF1 and native AMF2), and/or plant growth promoting rhizobacteria (PGPR) (B1 or B2)]. Data are mean ± SE of six biological replicates. Means followed by the same letters are not significantly different at P < 0.05 (Tukey’s HSD).

FIGURE 4

(A) Leaf water potential, (B) stomatal conductance, and (C) chlorophyll fluorescence of date palm plants under two water regimes [75% field capacity (FC); open bars and 25% FC; filled bars] and grown under control (non-amended and non-inoculated) or biofertilizer applications [composts C1 or C2, arbuscular mycorrhizal fungi (exogenous AMF1 and native AMF2), and/or plant growth promoting rhizobacteria (PGPR) (B1 or B2)]. Data are mean ± SD of six biological replicates. Means followed by the same letters are not significantly different at P < 0.05 (Tukey’s HSD).

FIGURE 5

(A) Chlorophyll a, (B) chlorophyll b, (C) total chlorophyll, and (D) carotenoid content in leaves of date palm plants under two water regimes [75% field capacity (FC); open bars and 25% FC; filled bars] and further grown without (control; non-amended and non-inoculated) or with biofertilizers [composts C1 or C2, arbuscular mycorrhizal fungi (AMF, exogenous AMF1 and native AMF2), and/or PGPR (B1 or B2)]. Data are mean ± SE of six independent biological replicates. Means followed by the same letters are not significantly different at P < 0.05 (Tukey’s HSD).

FIGURE 6

(A) Total soluble sugar content, (B) protein content, (C) peroxidase (POX) activity, and (D) polyphenol oxidase (PPO) activity in date palm shoots under two water regimes [75% field capacity (FC); open bars and 25% FC; filled bars] of the tested control treatments (non-amended and non-inoculated) and biofertilizers treatments [composts C1 or C2, arbuscular mycorrhizal fungi (exogenous AMF1 and native AMF2), and/or plant growth promoting rhizobacteria (PGPR) (B1 or B2)]. Data are mean ± SE of six biological replicates. Means followed by the same letters are not significantly different at P < 0.05 (Tukey’s HSD).

FIGURE 7

(A) Malondialdehyde (MDA) and (B) hydrogen peroxide (H₂O₂) content in date palm shoots under two water regimes (75% field capacity (FC); open bars and 25% FC; filled bars) of the tested control treatments (non-amended and non-inoculated) and biofertilizers treatments [composts C1 or C2, arbuscular mycorrhizal fungi (AMF, exogenous AMF1 and native AMF2), and/or plant growth promoting rhizobacteria (PGPR) (B1 or B2)]. Data are mean ± SE of six independent biological replicates. Means followed by the same letters are not significantly different at P < 0.05 (Tukey’s HSD).

FIGURE 8

Principal component analysis (PCA) of the different studied (A) traits and (B) treatments under drought stress conditions (25% FC). Chl a, chlorophyll a; Chl b, chlorophyll b; EC, electrical conductivity; Fv/Fm, chlorophyll fluorescence; g_s, stomatal conductance; H₂O₂, hydrogen peroxide; LA, leaf area; LWP, leaf water potential; MDA, malondialdehyde; MI, mycorrhizal intensity; MF, mycorrhizal frequency; N (soil), nitrogen content in soil; N (plant); nitrogen content in plant; NL, leaf numbers; OM, organic matter; P (soil), Phosphorous content in soil; P (plant), Phosphorous content in plant shoot; POX, peroxidase; PPO, polyphenol oxidase; RL, root length; SH, shoot height; TOC, total organic carbon; T Chl, total chlorophylls.

FIGURE 9

Suggested model for the regulatory network involved in date palm growth and tolerance to drought in response to compost, arbuscular mycorrhizal fungi (AMF) and plant growth promoting rhizobacteria (PGPR). According to this model, AMF colonization of a plant root permits the extension of hyphae extending into the surrounding soil, providing availability and storage of nutrients such as phosphorus and nitrogen for the plant. Also, AMF help to promote the synthesis of aquaporins which by changing the root hydraulic conductivity can enhance water uptake and water homeostasis maintenance under drought conditions. PGPR function as plant enhancer and facilitate the drought-exposed plants by improving nutrient uptake (N), water balance and osmoregulation through hormones (CKs and ABA)-mediating stomatal pores and regulating plant biochemical mechanisms (reducing the degradation of chlorophyll content and lipid peroxidation, increasing production of protein that reduces the damaging effect of ROS and can help maintain photosystem functionality under drought stress). Further, PGPR affect the EPS, allowing the increase of the water holding capacity. The compost functions as a soil conditioner in the process of decomposition and nutrient cycling (capture and delivery), which are driven by the activity of soil microorganisms affecting the soil microorganism activity (e.g., AMF and PGPR). The resulting changes in soil characteristics permit soil aggregation and enhance water holding capacity. Additionally, the plant–AMF/PGPR-compost associations act on physiological (increases in the photosynthetic pigments, and ABA-mediating higher stomatal conductance, permitting the increase of internal CO₂ and photosynthetic capacity) and biochemical (accumulation of osmolytes and activation of antioxidant metabolites/activities allowing leaf osmotic adjustment, ROS scavenging, and alleviation of oxidative stresses) parameters. Solid lines represent the analyses carried out in this study. Dashed lines indicate mechanisms found in the literature.