Temperatures producing heat damage in leaves of Ilex ×meserveae S.Y. Hu `Blue Prince' and Ilex rugosa × cornuta Lindl. & Paxt. `Mesdob' (China Boy) were evaluated using electrolyte leakage and chlorophyll fluorescence techniques. Whole leaves were exposed to temperatures from 30 to 65C for 30 minutes to determine critical midpoint heat-killing temperatures (TJ using electrolyte leakage techniques. The Tm for `Blue Prince' and `Mesdob' was 52.4 ± 0.lC and 53.8 ± 0.lC, respectively. Dark-adapted leaves were heated for 30 minutes in darkness at temperatures between 30 and 57C before chlorophyll fluorescence was measured. Initial (F0) and peak fluorescence measurements were higher at 54 and 55C for `Mesdob' than for `Blue Prince'. Cultivar had no effect on variable fluorescence (F,). Based on the Fv: Fo ratio, `Mesdob' was estimated to have a higher optimal plant growth temperature than `Blue Prince'. The physiologic data support the hypothesis that I. cornuta as a parent conferred heat tolerance to the interspecific hybrid in this study.
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