Two approaches to in vitro screening of raspberry cultivars for cold hardiness

Improved cold hardiness of new raspberry (Rubus idaeus L.) cultivars is one major objective in many raspberry breeding programs. In vitro evaluation o f cold hardiness would accelerate breeding progress. The aim of this work w as to study the possibility of screening raspberry genotypes for cold hardi ness in vitro. Two different approaches were examined; determining cold har diness after low-temperature acclimation treatment, and using accumulation of sugars as a biochemical indicator of hardiness. Raspberry cultivars 'Festival', 'Titan' and 'Willamette' grown on three med ia were exposed to three acclimation treatments (0, 2 and 4 weeks cold temp erature). During acclimation, plants were analysed for soluble carbohydrate content. After acclimation, cold hardiness (LT50) was determined by contro lled freezing of the plants. Total soluble carbohydrates, sucrose, glucose, fructose and raffinose accum ulated during cold hardening. The hardiest cultivar 'Festival' contained mo re soluble carbohydrates, sucrose and raffinose, but less glucose and fruct ose than the other cultivars. The results indicate the importance of sucros e on cold hardening in raspberry. The amounts of total soluble carbohydrate s, sucrose and raffinose correlated with cold hardiness in these cultivars and might be used as an indicator of hardiness when screening raspberry cul tivars for cold hardiness in vitro. No cultivar differences in cold hardiness were detected in non-acclimated p lants. Four weeks acclimation (including 2 weeks at +15 degrees C, 2 weeks at +2 degrees C, 24 h at -2 degrees C and 3 days at +2 degrees C) on MS bas al culture medium without growth regulators discriminated best between the cold hardy cultivar 'Festival' and less hardy 'Titan' and 'Willamette. It w as apparent from this study that in vitro screening of red raspberry germpl asm cold hardiness is promising as a preliminary indicator of hardiness.