007 Evaluation of the Spring Frost Susceptibility of Strawberry Genotypes Using Chlorophyll Fluorescence Measurements
Frost tolerance of flower buds is one of the most important characteristics of strawberry cultivars that produce fruit early or very early in the season. The objective of this study was to evaluate chlorophyll fluorescence (CF) as a suitable rapid method to assess spring frost injury of strawberry flowers. More specifically, to determine if there was a relationship between a decrease in CF and the appearance of visual symptoms (visual expression of necrosis (VEN) based on the amount of dark, damaged, and/or water soaked tissue of the pistil) due to frost. Sixty-six strawberry genotypes with varying levels of chilling susceptibility were used. The plants were grown in a greenhouse under a 16-h light period at 20-22 °C during the daytime and 16-18 °C at night. For the CF and VEN measurements, the plants were stored at -3 °C for 24 h followed by 24 h in the greenhouse. The CF measurements were made on dark-adapted tissue, using the Fv/Fm test of an OS-500 modulated fluorometer. For the VEN method, the flowers which had dark, damaged, and/or water soaked pistils were counted. The results showed that variable fluorescence (Fv) decreased as the temperature was lowered. The spring frost resistant cultivars maintained Fv at a stable level and had a smaller regression slope (ß1), whereas the susceptible cultivars showed a very dramatic decrease in Fv. The CF method gave results that correlated with the VEN results. The strong relationship between chilling tolerance determined via visual and florescence techniques supports the use of CF in selecting resistant spring frost selections in a breeding program. The use of CF will allow the breeder not only to select for spring frost-resistant selections independently of environmental changes, but also to select frost resistant seedlings prior to planting in the field.