Abstract: We assessed the correlation between photosynthetic chilling resistance and drought resistance in sugarcane (Saccharum officinarum) with 671 varieties collected from China and abroad. The predawn potential maximum photochemical efficiency (F_v/F_m) of the functionally mature leaves was measured at the predawn of extreme chilling. Furthermore, 34 important varieties were selected for the measurement of leaf F_v/F_m and photosynthetic gas exchange during the common chill season and during extended severe droughts. The results showed that, under extreme chilling condition, 97.5% of the examined varieties significantly suffered from chilling stress but only 1% of which almost completely lost their photosynthetic ability. Among the 34 important varieties, F_v/F_m under the extreme chilling condition, the F_v/F_m surveyed from the common chilling condition, the net photosynthetic rate (P_n), stomatal conductance (G_s) and transpiration rate (E) assessed under extended severe drought were positively correlated. However, the correlations between intrinsic water use efficiency (WUE_i), shortterm water use efficiency (WUE) and intercellular CO₂ concentration (C_i) were not statistically significant. Besides, the F_v/F_m measured under common chilling and under extended severe drought was not significantly correlated with photosynthetic gas exchange parameters measured during extreme chilling. Neither F_v/F_m nor P_n correlated with the sugarcane juice brix under prolonged drought. Our results suggest that sugarcane was generally sensitive to chilling stress. Compared with common chilling and severe drought, extreme chilling could better reflect the photosynthetic potentials of sugarcane. Such potential is positively correlated with photosynthetic carbon assimilation capacity under normal environment and drought condition.

Key words: chill-sensitive plant, chilling stress, drought stress, photosynthetic adaptation, sugarcane.