A protocol to assess heat tolerance in a segregating population of raspberry using chlorophyll fluorescence

A detached leaf protocol was developed to assess heat tolerance in raspberry (Rubus L. subgenus Idaeobatus) by measuring chlorophyll fluorescence. In order to establish the heat screen protocol for field measurements in a segregating population, several baseline experiments were performed on genotypes with known heat tolerance based on visual observations in multiple climates. 'Mandarin' and 'Qualicum' were used as heat tolerant and heat susceptible controls, respectively. We determined that: optimum collection time was in the morning before ambient temperatures reached 28 degrees C, critical temperature (Tc) was 45 degrees C, minimum duration of dark adaptation was 15 min, and optimum heat shock duration was 30 min. Parameters established in baseline experiments were subsequently used for screening a large population (196 genotypes) planted in the field, where warm conditions commonly occur (maximum temperatures ranging between 30 and 35 degrees C). A significant difference in chlorophyll fluorescence was found between the two controls; 'Qualicum', the heat sensitive cultivar, had the lowest Fv/Fm (variable fluorescence/maximum fluorescence) value, while 18.8% of the segregating population had a higher Fv/Fm value than 'Mandarin', the heat tolerant cultivar. We have identified 106 individuals with an Fv/Fm above 0.600 (representing 52% of the population), and 7 individuals above 0.680 (4% of the population). The distribution of heat tolerance within the population was essentially normal, and thus quantitative trait analysis is a plausible approach for studying heat tolerance. This protocol has potential application to other Rosaceae species, and is not dependent on visual assessment. (C) 2011 Elsevier B.V. All rights reserved.