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Creeping bentgrass (Agrostis stolonifera) is an excellent cool-season turfgrass that is widely used in urban gardening, landscaping, and golf turf. Triennial field experiments from 2017 to 2019 were conducted to investigate effects of the foliar application of chitosan (CTS), γ-aminobutyric acid (GABA), or sodium chloride (NaCl) on mitigating summer bentgrass decline (SBD) and exploring the CTS, GABA, or NaCl regulatory mechanism of tolerance to summer heat stress associated with changes in chlorophyll (Chl) loss and photosynthetic capacity, osmotic adjustment (OA), oxidative damage, and cell membrane stability. The findings demonstrated that persistent ambient high temperatures above 30 °C during the summer months of 2017, 2018, and 2019 significantly reduced the turf quality (TQ), Chl content, photochemical efficiency of PSII (Fv/Fm and PIABS), leaf relative water content, and osmotic potential (OP) but significantly increased electrolyte leakage (EL) and the accumulations of free proline, water-soluble carbohydrate (WSC), hydrogen peroxide (H2O2), and malondialdehyde (MDA). The foliar application of CTS, GABA, or NaCl could significantly alleviate SBD, as reflected by improved TQ and delayed Chl loss during hot summer months. Heat-induced declines in Fv/Fm, PIABS, the net photosynthetic rate (Pn), the transpiration rate (Tr), and water use efficiency (WUE) could be significantly mitigated by the exogenous application of CTS, GABA, or NaCl. In addition, the foliar application of CTS, GABA, or NaCl also significantly improved the accumulations of free proline and WSC but reduced the EL, OP, and H2O2 content and the MDA content in leaves of creeping bentgrass in favor of water and redox homeostasis in summer. Based on the comprehensive evaluation of the subordinate function value analysis (SFVA), the CTS had the best effect on the mitigation of SBD, followed by GABA and NaCl in 2017, 2018, and 2019. The current study indicates that the foliar application of an appropriate dose of GABA, CTS, or NaCl provides a cost-effective strategy for mitigating SBD.

Drought is a serious outcome of climate change reducing the productivity of forage species under arid and semi-arid conditions worldwide. Diethyl aminoethyl hexanoate (DA-6), a novel plant growth regulator, has proven to be involved in the amelioration of critical physiological functions in many agricultural crops under various abiotic stresses, but the role of the DA-6 in improving seed germination has never been investigated under drought stress. The present study was carried out to elucidate the impact of the DA-6 priming on seeds germination of white clover under drought stress. Results showed that seed priming with the DA-6 significantly mitigated the drought-induced reduction in germination percentage, germination vigor, germination index, seed vigor index, root length, shoot length, and fresh weight after 7 days of seed germination. The DA-6 significantly increased the endogenous indole-3-acetic acid, gibberellin, and cytokinin content with marked reduction in abscisic acid content in seedlings under drought stress. In addition, the DA-6 significantly accelerated starch catabolism by enhancing the activities of hydrolases contributing toward enhanced soluble sugars, proline content and ameliorated the antioxidant defense system to enhance the ability of reactive oxygen species scavenging under drought stress. Furthermore, exogenous DA-6 application significantly increased dehydrins accumulation and upregulated transcript levels of genes encoding dehydrins (SK2, Y2SK, or DHNb) during seeds germination under water deficient condition. These findings suggested that the DA-6 mediated seeds germination and drought tolerance associated with changes in endogenous phytohormones resulting in increased starch degradation, osmotic adjustment, antioxidants activity, and dehydrins accumulation during seed germination under water deficient condition.

Cool-season creeping bentgrass (Agrostis stolonifera) has the ability to form fine sports turf, but high temperatures result in summer bentgrass decline (SBD), especially in transitional and subtropical zones. Physiological responses in combination with the alteration in turf quality (TQ) will contribute to a better understanding of SBD in a subtropical zone. Field experiments were conducted from 2017 to 2019 to test the adaptability to summer stress among four cultivars (13M, Penncross, Seaside II, and PA-1). A constant ambient high temperature above 30 °C significantly decreased the TQ of the four cultivars during the summer months in 2017, 2018, and 2019. Significant declines in the chlorophyll content, photochemical efficiency of photosystem II (Fv/Fm and PIABS), leaf relative water content (RWC), and osmotic potential (OP) were induced by summer stress, whereas gradual increases in water-soluble carbohydrates, proline, hydrogen peroxide (H2O2), malondialdehyde (MDA), and electrolyte leakage (EL) were observed in the four cultivars during the summer months. The 13M and Penncross cultivars exhibited better performance than Seaside II and PA-1 in response to summer stress from 2017 to 2019, which is associated with better maintenance of photosynthesis, water status, WSC and proline accumulation, and cell membrane stability. The 13M and Penncross cultivars could be used as potential candidates for turf establishment in a subtropical zone. Physiological responses together with alterations in TQ also provided critical information for the breeding and development of germplasm with heat tolerance in creeping bentgrass species.

Persistent high temperature decreases the yield and quality of crops, including many important herbs. White clover (Trifolium repens) is a perennial herb with high feeding and medicinal value, but is sensitive to temperatures above 30 °C. The present study was conducted to elucidate the impact of changes in endogenous γ-aminobutyric acid (GABA) level by exogenous GABA pretreatment on heat tolerance of white clover, associated with alterations in endogenous hormones, antioxidant metabolism, and aquaporin-related gene expression in root and leaf of white clover plants under high-temperature stress. Our results reveal that improvement in endogenous GABA level in leaf and root by GABA pretreatment could significantly alleviate the damage to white clover during high-temperature stress, as demonstrated by enhancements in cell membrane stability, photosynthetic capacity, and osmotic adjustment ability, as well as lower oxidative damage and chlorophyll loss. The GABA significantly enhanced gene expression and enzyme activities involved in antioxidant defense, including superoxide dismutase, catalase, peroxidase, and key enzymes of the ascorbic acid–glutathione cycle, thus reducing the accumulation of reactive oxygen species and the oxidative injury to membrane lipids and proteins. The GABA also increased endogenous indole-3-acetic acid content in roots and leaves and cytokinin content in leaves, associated with growth maintenance and reduced leaf senescence under heat stress. The GABA significantly upregulated the expression of PIP1-1 and PIP2-7 in leaves and the TIP2-1 expression in leaves and roots under high temperature, and also alleviated the heat-induced inhibition of PIP1-1, PIP2-2, TIP2-2, and NIP1-2 expression in roots, which could help to improve the water transportation and homeostasis from roots to leaves. In addition, the GABA-induced aquaporins expression and decline in endogenous abscisic acid level could improve the heat dissipation capacity through maintaining higher stomatal opening and transpiration in white clovers under high-temperature stress.

γ-Aminobutyric acid (GABA) participates in the regulation of adaptability to abiotic stress in plants. The objectives of this study were to investigate the effects of GABA priming on improving thermotolerance in creeping bentgrass (Agrostis stolonifera) based on analyses of physiology and proteome using iTRAQ technology. GABA-treated plants maintained significantly higher endogenous GABA content, photochemical efficiency, performance index on absorption basis, membrane stability, and osmotic adjustment (OA) than untreated plants during a prolonged period of heat stress (18 days), which indicated beneficial effects of GABA on alleviating heat damage. Protein profiles showed that plants were able to regulate some common metabolic processes including porphyrin and chlorophyll metabolism, glutathione metabolism, pyruvate metabolism, carbon fixation, and amino acid metabolism for heat acclimation. It is noteworthy that the GABA application particularly regulated arachidonic acid metabolism and phenylpropanoid biosynthesis related to better thermotolerance. In response to heat stress, the GABA priming significantly increased the abundances of Cu/ZnSOD and APX4 that were consistent with superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities. The GABA-upregulated proteins in relation to antioxidant defense (Cu/ZnSOD and APX4) for the reactive oxygen species scavenging, heat shock response (HSP90, HSP70, and HSP16.9) for preventing denatured proteins aggregation, stabilizing abnormal proteins, promoting protein maturation and assembly, sugars, and amino acids metabolism (PFK5, ATP-dependent 6-phosphofructokinase 5; FK2, fructokinase 2; BFRUCT, β-fructofuranosidase; RFS2, galactinol-sucrose galactosyltransferase 2; ASN2, asparagine synthetase 2) for OA and energy metabolism, and transcription factor (C2H2 ZNF, C2H2 zinc-finger protein) for the activation of stress-defensive genes could play vital roles in establishing thermotolerance. Current findings provide an illuminating insight into the new function of GABA on enhancing adaptability to heat stress in plants.

As a state-of-the-art method, the digital image correlation (DIC) technique is used to capture the fracture properties of wood along the longitudinal direction, such as the crack propagation, the strain field, and the fracture process zone (FPZ). Single-edge notched (SEN) specimens made of Douglas fir (Pseudotsuga menziesii) from Canada with different notch-to-depth ratios are tested by three-point-bending (3-p-b) experiment. The crack mouth opening displacements (CMOD) measured by the clip gauge and DIC technique agree well with each other, verifying the applicability of the DIC technique. Then, the quasi-brittle fracture process of wood is analyzed by combing the load-CMOD curve and the strain field in front of the preformed crack. Additionally, the equivalent elastic crack length is calculated using the linear superposition hypothesis. The comparison between the FPZ evolution and the equivalent elastic crack shows that specimens with higher notch-to-depth ratios have better cohesive effect and higher cracking resistance.