FOLIAR HEAT TOLERANCE OF 3 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 holli es including I. aquifolium L., I. cornuta Lindl. & Paxt, and I. rugosa Friedr. Schmidt. Variations in foliar heat tolerance among these spec ies were manifested in temperature responses for A(CO2). Temperature o ptima of A(CO2) for I. rugosa, I. cornuta, and I. aquifolium were 22.0 , 26.3, and 27.9 degrees C, respectively (LSD0.05 = 2.9). Temperature responses of respiration mere similar among taxa and did not appear to be contributing factors to variations in A(CO2). At 40 degrees C, pot ential photosynthetic capacity, measured under saturating CO2, was 4.1 , 9.4, and 14.8 mu mol . m(-2). s(-1) for I. rugosa, I. aquifolium, an d I. cornuta, respectively (LSD0.05 = 5.1). Variations in the relative dark-acclimated fluorescence temperature curves mere used to assess t hresholds for irreversible heat injury. The critical fluorescence temp erature 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 degree s C for I. rugosa, I. cornuta, and I. aquifolium, respectively (LSD0.0 5 = 0.9). Based on these results, I. rugosa was the most heat-sensitiv e 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.