Construction of a Nocturnal Low-Temperature Tolerance Index for Strawberry and Its Correlation with Yield

Strawberry is widely cultivated due to its short growth cycle, high yield, and significant profits. In high-latitude cold regions, the planting area of overwintering strawberry has expanded rapidly in recent years. However, although daytime temperatures inside solar greenhouses rise quickly with solar radiation, plants are frequently subjected to persistent nocturnal low-temperature stress (nocturnal temperature below 10 °C). This stress restricts photosynthesis, delays growth, and markedly reduces yield. Therefore, accurately evaluating the tolerance of strawberry varieties to low nocturnal temperatures is crucial for unheated overwintering production in cold regions. This study selected Snow White, Benihoppe, and Kaorino as experimental materials for overwintering cultivation trials in a typical cold-region solar greenhouse. We measured and analyzed growth and development, photosynthetic characteristics, phenological traits, and fruit yield. Based on photosynthetic physiology and phenotypic traits, we constructed the Photosynthesis–Fluorescence Index (PFI), the Production–Phenotype Index (PPI), and the Nocturnal Cold Tolerance Index (NCTI). The results showed that Kaorino exhibited significantly higher values in all three indices compared with Benihoppe and Snow White. After exposure to low night temperatures, Kaorino exhibited rapid photosynthetic induction, strong maintenance of PSII activity, vigorous growth, early maturation, and high yield. Moreover, all three composite indices were strongly and positively correlated with total yield (R² > 0.97), demonstrating their effectiveness in distinguishing the nocturnal low-temperature tolerance of strawberry cultivars. These composite indices provide a scientifically robust method for selecting suitable cultivars for unheated overwinter strawberry production in high-latitude cold regions.