Potassium as a Key Limiting Factor: Foliar Application Improves Cold Tolerance in Augustinegrass via CAT Activation

Identifying the factors limiting cold resistance in augustinegrass is essential for expanding this shade-tolerant tropical turfgrass into temperate regions. We hypothesized that leaf potassium content is closely associated with its cold tolerance. To test this, we first analyzed the correlation between leaf potassium content and cold resistance across 30 germplasms, which confirmed a positive relationship and suggested that low potassium may limit cold tolerance. We then applied foliar potassium at 0, 15, 30, and 60 mM to increase leaf potassium content and evaluate its effect on cold resistance. The 15 mM treatment was most effective, increasing whole-plant fresh weight by 91.5% under cold stress compared to the control. To understand the underlying physiological mechanism, we measured the impact of foliar potassium on four key processes: photosynthesis (chlorophyll content, fluorescence, enzyme activity, stomatal aperture, gas exchange, and carbon assimilation products), osmotic adjustment (proline), membrane stability (relative conductivity and MDA), and ROS scavenging (SOD and CAT activity). Foliar potassium significantly enhanced photosynthetic performance; increased soluble sugars, starch, and proline; reduced MDA; and boosted both SOD and CAT activities. Pearson correlation analysis linked most physiological indicators to improved fresh weight. Critically, multiple linear regression identified leaf CAT activity as the primary factor, explaining 80% of the variation in cold resistance. qPCR analysis confirmed that CAT gene expression matched the increased enzyme activity. Field trials validated that a 15 mM potassium foliar spray effectively enhances cold tolerance. These findings demonstrate that leaf potassium is a major limiting factor for cold resistance in augustinegrass, and that foliar application of 15 mM potassium represents an effective management strategy, primarily by enhancing leaf CAT activity to improve ROS scavenging and overall stress tolerance.