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Heat stress may limit the growth of turfgrasses through the induction of oxidative stress, causing cellular and physiological damage. The objective of the study was to examine the association of heat and oxidative stresses between warm-season (C₄) and cool-season (C₃) turfgrasses. Plants of zoysiagrass (Zoysia matrella L. Merr. cv. Manila) (C₄) and tall fescue (Festuca arundinacea Shreber cv. Barlexus) (C₃) were exposed to optimal temperature conditions (24 °C for tall fescue and 34 °C for zoysiagrass) or heat stress (10 °C above the respective optimal temperature for each species) in growth chambers. Zoysiagrass exhibited less severe decline in turf quality and photochemical efficiency and less severe oxidative damage in cellular membranes as demonstrated by lower membrane electrolyte leakage and lipid peroxidation compared with tall fescue when both were exposed to heat stress. The activities of superoxide dismutase (SOD) and peroxidase (POD) declined with heat stress for both species, but to a lesser extent in zoysiagrass than in tall fescue, whereas catalase activity did not change significantly under heat stress and did not exhibit species variation. Our results demonstrate that the superior heat tolerance in zoysiagrass in comparison with tall fescue was associated with greater oxidative scavenging capacity as a result of the maintenance of higher SOD and POD activities.

Fatty acid metabolism may be involved in plant adaptation to drought stress. The objective of this study was to identify saturated and unsaturated fatty acids associated with leaf dehydration tolerance by comparing fatty acid composition and unsaturation levels at equivalent leaf water status of two bermudagrass genotypes contrasting in drought resistance. A drought-resistant hybrid bermudagrass (Cynodon dactylon × C. transvaalensis) genotype (‘Tifway’) and a drought-sensitive bermudagrass (C. dactylon) genotype (‘C299’) were maintained under well-watered (control) or water-withheld (drought) conditions. Drought treatment was imposed until soil water content decreased to 5% or leaf relative water content (RWC) dropped to 28% to 29%. ‘Tifway’ maintained higher RWC and lower electrolyte leakage (EL) at 5 and 10 days of drought stress. Leaves of ‘Tifway’ maintained lower EL when RWC of both genotypes declined to the same level of water deficit (28% to 29%) by the end of drought periods. The degree of fatty acid unsaturation, expressed as the double bond index, decreased in both genotypes during drought stress, which was mainly associated with the decline in linoleic (C18:2) and linolenic acids (C18:3) and an increase in palmitic (C16:0) and stearic acids (C18:0). A lipid composition characterized by a greater amount of unsaturated fatty acids was detected in ‘Tifway’ relative to ‘C299’ exposed to the same level of water deficit, mainly as a result of a greater content of C18:2 and a lower content of C16:0 and C18:0. Our results suggest that the ability to maintain a greater composition of unsaturated fatty acids in membrane lipids may contribute to superior leaf dehydration tolerance in bermudagrass.

The accumulation of different types of metabolites may reflect variations in plant adaptation to different severities or durations of drought stress. The objectives of this project are to examine changes in metabolomic profiles and determine predominant metabolites in response to short-term (6 days) and long-term (18 days) drought stress with gas chromatography–mass spectrometry analysis in a C₄ perennial grass species. Plants of hybrid bermudagrass (Cynodon dactylon × C. transvaalensis cv. Tifdwarf) were unirrigated for 18 days to induce drought stress in growth chambers. Physiological responses to drought stress were evaluated by visual rating of grass quality, relative water content, photochemical efficiency, and electrolyte leakage (EL). All parameters decreased significantly at 6 and 18 days of drought stress, except EL, which increased with the duration of drought stress. Under short-term drought stress (6 days), the content did not change significantly for most metabolites, except methionine, serine, γ-aminobutyric acid (GABA), isoleucine, and mannose. Most metabolites showed higher accumulation under long-term drought stress compared with that under the well-watered conditions, including three organic acids (malic acid, galacturonic acid, and succinic acid), 10 amino acids (proline, asparagine, phenylalanine, methionine, serine, 5-hydroxynorvaline, GABA, glycine, theorine, valine), seven sugars (sucrose, glucose, galactose, fructose, mannose, maltose, xylose), one nitrogen compound (ethanolamine), and two-sugar alcohol (myo-inositol). The accumulation of those metabolites, especially malic acid, proline, and sucrose, could be associated with drought adaptation of C₄ hybrid bermudagrass to long-term or severe drought stress.

Heat is a major factor limiting growth of C₃ grass species. Elevated CO₂ may mitigate the adverse effects of heat stress or enhance heat tolerance. The objective of this study was to determine metabolic changes associated with improvement of heat tolerance by elevated atmospheric CO₂ concentration in tall fescue (Festuca arundinacea). Plants (cv. Rembrandt) were exposed to ambient day/night temperature (25/20 °C) or heat stress (35/30 °C) and ambient CO₂ concentration (400 ± 10 μmol·mol⁻¹) or double ambient CO₂ concentration (800 ± 10 μmol·mol⁻¹) in growth chambers. Turf quality (TQ), shoot growth rate, and leaf electrolyte leakage results demonstrated that heat stress at ambient CO₂ concentration inhibits turf growth and reduces cell membrane stability, whereas heat-stressed plants under elevated CO₂ concentration exhibit improved TQ, shoot growth rate, and membrane stability. Plants exposed to heat stress under elevated CO₂ exhibited a significantly greater amount of several organic acids (shikimic acid, malonic acid, threonic acid, glyceric acid, galactaric acid, and citric acid), amino acids (serine, valine, and 5-oxoproline), and carbohydrates (sucrose and maltose) compared with heat-stressed plants at ambient CO₂. The increased production or maintenance of metabolites with important biological functions such as those involved in photosynthesis, respiration, and protein metabolism could play a role in elevated CO₂ mitigation of heat stress damage. Therefore, elevated CO₂ conditions may contribute to improved heat stress tolerance as exhibited by better TQ and shoot growth of heat-stressed plants. Practices to harness the power of CO₂ may be incorporated into turfgrass management for plant adaptation to increasing temperatures, particularly during summer months.

The objective of this study was to compare differential nutrient responses to heat stress in relation to heat tolerance for warm-season (C₄) common bermudagrass [Cynodon dactylon (L.) Pers.] and cool-season (C₃) kentucky bluegrass (Poa pratensis L.). Both species were exposed to two temperature regimes in growth chambers: optimal day/night temperature conditions (24/20 °C for kentucky bluegrass and 34/30 °C for bermudagrass) or heat stress (10 °C above the respective optimal temperature for each species). Heat injury in leaves was evaluated and the concentrations of several major macronutrients [nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg)] in both grass species were measured at 0, 7, 14, 21, and 28 days of treatment. Heat stress reduced leaf photochemical efficiency and cellular membrane stability in both species, but bermudagrass leaves exhibited less damage in these parameters than kentucky bluegrass. Heat stress caused a significant decline in N, P, and K concentration, beginning at 7 days in kentucky bluegrass, but had no significant effects on N, P, and K concentration in bermudagrass during the 28-day treatment period. The concentration of Ca and Mg increased under heat stress in both kentucky bluegrass and bermudagrass, but there were no significant differences between the species under optimal or high-temperature conditions, suggesting they were not involved in heat responses in either species. The differential responses of N, P, and K to heat stress could at least partially account for the differences in heat tolerance between the two species and demonstrate the importance of sufficient N, P, and K in turfgrass adaptation to heat stress.

Salinity is a detrimental abiotic stress for plant growth in salt-affected soils. The objective of this study was to examine photosynthetic responses to salinity stress in two warm-season turfgrasses differing in salinity tolerance. Salt-tolerant species seashore paspalum (Paspalum vaginatum) and salt-sensitive species centipedegrass (Eremochloa ophiuroides) were exposed to salinity at three NaCl concentrations (0, 300, and 500 mm) in a growth chamber. Turf quality, relative water content (RWC), and leaf photochemical efficiency (Fv/Fm) declined, whereas electrolyte leakage (EL) increased under the two NaCl regimes for both grass species, and the changes were more dramatic in centipedegrass than that in seashore paspalum as well as in the higher salinity concentration. Two grass species showed different phytosynthetic responses to salinity stress. The earlier inhibition of photosynthesis in seashore paspalum was mainly associated with stomatal closure. As salinity increased and salinity stress prolonged, the inhibition of photosynthesis in seashore paspalum was mainly associated with non-stomatal factors. The inhibition of photosynthesis in centipedegrass was associated with both stomatal closure and non-stomatal factors at both salinity levels. The sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) analysis demonstrated the Rubisco large subunit had no obvious decrease during the whole stress period under the 300-mm and 500-mm treatments in seashore paspalum, whereas it significantly decreased in centipedegrass under both the 300-mm and 500-mm treatments. The results indicated that the superior salinity tolerance in seashore paspalum, compared with centipedegrass, could be attributed to its maintenance of Rubisco stability, chlorophyll content, photochemical efficiency as well as photosynthetic rate (Pn) capacity under salinity stress.

Low-temperature storage in darkness is usually used for preserving seedlings for a short period. To investigate whether grafted watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] seedlings are superior to non-grafted ones under low-temperature storage in darkness and to study their physiological differences during storage, watermelon (‘Zaojia 84-24’) scions were grafted to pumpkin (Cucurbita moschata Duch. ‘Zhuangshi’) rootstocks. Carbohydrate levels; chlorophyll and malondialdehyde contents; the activities of superoxide dismutase, catalase, and peroxidase; and photochemical efficiency were assayed during 6 days of storage at 15 °C in darkness. After that, seedlings were transplanted into an artificial climate chamber. The net photosynthetic rate and stomatal conductance (g _S) were measured on the first and third days after transplanting. The results showed that the grafted watermelon seedlings had more soluble sugar and chlorophyll contents, higher activities of antioxidant enzymes, and less malondialdehyde content than the non-grafted ones after 6 days of storage. In addition, low-temperature storage in darkness damaged the photosystem II of non-grafted watermelon seedlings more than that of grafted ones. After transplanting, grafted seedlings had a higher net photosynthetic rate. The results suggest that grafted watermelon seedlings were more suitable for the low-temperature storage in darkness than the non-grafted ones.