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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.

Citrus (Citrus sp.) germplasm collections are a valuable resource for citrus genetic breeding studies, and further utilization of the resource requires knowledge of their genotypic and phylogenetic relationships. Diverse citrus accessions, including citron (Citrus medica), mandarin (Citrus reticulata), pummelo (Citrus maxima), papeda (Papeda sp.), trifoliate orange (Poncirus trifoliata), kumquat (Fortunella sp.), and related species, have been housed at the Florida Citrus Arboretum, Winter Haven, FL, but the accessions in the collection have not been genotyped. In this study, a collection of 80 citrus accessions were genotyped using 1536 sweet orange–derived single nucleotide polymorphism (SNP) markers, to determine their SNP fingerprints and to assess genetic diversity, population structure, and phylogenetic relationships, and thereby to test the efficiency of using the single genotype-derived SNP chip with relatively low cost for these analyses. Phylogenetic relationships among the 80 accessions were determined by multivariate analysis. A model-based clustering program detected five basic groups and revealed that C. maxima introgressions varied among mandarin cultivars and segregated in mandarin F₁ progeny. In addition, reciprocal differences in C. maxima contributions were observed among citranges (Citrus sinensis × P. trifoliata vs. P. trifoliata × C. sinensis) and may be caused by the influence of cytoplasmic DNA and its effect on selection of cultivars. Inferred admixture structures of many secondary citrus species and important cultivars were confirmed or revealed, including ‘Bergamot’ sour orange (Citrus aurantium), ‘Kinkoji’ (C. reticulata × Citrus paradisi), ‘Hyuganatsu’ orange (Citrus tamurana), and palestine sweet lime (Citrus aurantifolia). The relatively inexpensive SNP array used in this study generated informative genotyping data and led to good consensus and correlations with previously published observations based on whole genome sequencing (WGS) data. The genotyping data and the phylogenetic results may facilitate further exploitation of interesting genotypes in the collection and additional understanding of phylogenetic relationships in citrus.