Objective  Tolerance to low-temperature stress of seedlings of 3 Coffea varieties was evaluated using photosynthetic as well as chlorophyll fluorescence indices for an improved prediction on cold-resistance of the plants for breeding and new cultivar introduction.

  Method  Seedlings of 3 varieties of coffee cultivars were treated under 8 ℃ for 0, 24, 48 or 72 h. Effects of the low-temp stress on chlorophyll (Chl), gas exchange indices, and Chl fluorescence measurements of the leaves were monitored. Tolerance of the seedlings to low-temp treatments was evaluated using the membership function method.

  Result  (1) The prolonged low-temp stress caused Chl to initially increase in Coffea liberica and C. arabica seedlings and declined later but decrease continuously in C. canephora. (2) The decreased Pn in seedlings was caused by non-stomatal factors. Pn, Tr, Gs, WUE, and Ls did not significantly differ among the treatments on 3 varieties under low-temp stress but did significantly between the treatments and control. (3) The total initial fluorescence (F_o) on the 3 varieties increased indicating the occurrence of significant damage to PSH. The non-photochemical quenching coefficient (qN) had a firstly-increase-then-decline pattern with the larger decreases on the maximum photochemical efficiency (F_v/F_m), photochemical quenching coefficient (qP), apparent quantum transfer rate (ETR), and actual photochemical quantum yield (Φ_PSII) of PSII in C. liberica reflecting a greater damaging effect on its leaves than in the other two varieties. According to the analysis by the membership function method, C. arabica seedlings was most cold tolerant among the three varieties.

  Conclusion  By using the photosynthetic and fluorescence indices on coffee seedlings in combination, the membership function method could accurately determine the low-temp tolerance of Coffea cultivars for breeding and/or new variety selection.