The 2012 ASHS Annual Conference

Sales of garden roses in the United States have been declining over the last 20 years partly due to the impression that garden roses are difficult to grow. The lack of heat tolerant cultivars is one of the limiting factors for growing garden roses in sub-tropical conditions such as the southern U.S. Field evaluation of garden performance is crucial to a breeding program for landscape roses, but challenging due to yearly fluctuations in weather conditions. Development of a rapid screening technique to screen large amounts of rose germplasm which would enable accurate selection against the most heat susceptible material prior to field trials would be of great value to a breeding program. Both chlorophyll fluorescence and cell membrane thermostability (CMT) have been used as indicators of heat tolerance in other agricultural and horticultural crops. Detached leaf protocols, using chlorophyll fluorescence and CMT, were developed for screening heat tolerance in garden roses. To test whether chlorophyll fluorescence or CMT would provide the best resolution an experiment using four accessions, two identified as heat tolerant (J06-30-3-3 and J06-30-3-6) and two as heat susceptible (‘Sweet Chariot’ and ‘Vineyard Song’); and  two stress methods, water bath (45 ºC for 45 min.) and an oven (45 ºC for 45 min.) were used. Leaves from field grown plants were harvested within the first hour of sunrise and kept in the dark at high humidity until heat stress was applied to intact leaves. Chlorophyll fluorescence on dark adapted leaves was measured 30 minutes after the stress period, followed by CMT measurement on the same leaves. In both cases, CMT successfully separated the heat tolerant accessions from the heat susceptible accessions. CMT measurements resulted in a lower coefficient of variance when leaves were stressed in a water bath compared to chlorophyll fluorescence measurements on the same leaves. CMT was a more reliable indicator of heat tolerance in garden roses than was chlorophyll fluorescence.