Heat tolerance in leaves of tropical fruit crops as measured by chlorophyll fluorescence

Heat tolerance in 23 tropical and one temperate fruit crops was evaluated by determining chlorophyll fluorescence (the ratio of the variable fluorescence to the maximum fluorescence (F-v/F-m), and the basal fluorescence (F-0)), The ratio (R(v)) of F-v/F-m in leaves exposed to high temperature (45 degrees C for 20 min) to F-v/F-m to control temperature (25 degrees C for 20 min) was found to be highly and negatively correlated to the ratio (R(0)) of F-0 exposed to the high temperature to F-0 exposed to the control temperature (r = -0.92**), Leaves sampled in mid-July (3.5 months old) were slightly but significantly more tolerant to heat than 2.5-month-old leaves sampled in early to mid-June, The ratio of the genetic variance to the total variance in the measurements was 0.90 for R(v), and 0.89 for R(0). Pineapple, coconut palm, and Annona species in Annonaceae (cherimoya, soursop, sugar apple) were heat-tolerant, whereas Syzygium species in Myrtaceae (Java apple and rose apple), longan, and peach were sensitive.