Foliar heat tolerance of three holly species (Ilex spp.): responses of chlorophyll fluorescence and leaf gas exchange to supraoptimal leaf temperatures

Temperature sensitivity of CO2 assimilation (A(CO2)), dark respiration, and chlorophyll fluorescence was evaluated among three taxa of hollies including I. aquifolium L., I. cornuta Lindl. and Paxt., and I. rugosa Friedr. Schmidt. Variations in foliar heat tolerance among these species were manifested in temperature responses for A(CO2). Temperature optima of A(CO2) for I. rugosa, I. cornuta, and I. aquifolium were 22.0, 26.3, and 27.9 degrees C, respectively (LSD(0.05) = 2.9). Temperature responses of respiration were similar among taxa and did not appear to be contributing factors to variations in A(CO2). At 40 degrees C, potential photosynthetic capacity, measured under saturating CO2, was 4.1, 9.4, and 14.8 micromoles.m-2.s-1 for I. rugosa, I. aquifolium, and I. cornuta, respectively (LSD(0.05) = 5.1). Variations in the relative dark-acclimated fluorescence temperature curves were used to assess thresholds for irreversible heat injury. The critical fluorescence temperature threshold (T(C)) was similar (48.0 degrees C) for all taxa. The fluorescence temperature peaks (T(P)) were 52.0, 52.8, and 53.5 degrees C for I. rugosa, I. cornuta, and I. aquifolium, respectively (LSD(0.05) = 0.9). Based on these results, I. rugosa was the most heat-sensitive species, followed by I. aquifolium and I. cornuta. Ilex cornuta also had substantially greater potential photosynthetic capacity than the other species at 40 degrees C, indicating superior metabolic tolerance to high temperatures