Different mechanisms to obtain higher fruit growth rate in two cold-tolerant cucumber (Cucumis sativus L.) lines under low night temperature

One cold-sensitive cultivar (Jinyan 4) and two cold-tolerant inbred lines (NY-1 and XC-1) of cucumber (Cucumis sativus L.) were subjected to temperatures of 28 °C/22 °C (day/night, control) or 28 °C/12 °C (day/night, cold treatment) in a 10 h photoperiod (7:00–17:00). Under control conditions, cucumber fruits grew fast during afternoon and early night, and slow during late night and morning. Under 28 °C/12 °C conditions, the two cold-tolerant inbred lines maintained relatively higher fruit growth rates than the cold-sensitive cultivar by different mechanisms. Compared to Jinyan 4, NY-1 fruits had higher growth rates during cold nights while XC-1 fruits grew faster during the next day. Under the 28 °C/12 °C temperature regime, the assimilate accumulation in the fruits of all tested genotypes followed a similar trend with the corresponding fruit growth rates. After a cold night treatment, the net CO2 assimilation rates of one- and two-fruit plants, which had increased sink demand, were higher than that of plants without fruits in all tested genotypes. This response indicates that feedback inhibition might be an important reason for the reduction of photosynthesis on the next day. In addition, after a cold night treatment, the levels of exportable sugars (sucrose and stachyose) in mature leaves of XC-1 were higher than those measured in Jinyan 4 and NY-1, which might explain why XC-1 fruits had faster assimilate accumulation rates in the next morning. Higher activity of sucrose-phosphate synthase, a key enzyme of sucrose and stachyose biosynthesis, constituted an additional evidence that faster sucrose and stachyose biosynthesis in mature leaves may occur in XC-1 than in Jinyan 4 and NY-1 at that time. In conclusion, our results showed that cucumber genotypes may use different mechanisms to enhance their cold tolerance.