Characterization Of Fruit Growth And Assimilate Translocation Of Cucumber Under Low Night Temperature

Energy-Saving Sunlight Greenhouse is very popular in China. However, the patterns of fruit growth and carbohydrate translocation of cucumber, an important stachyose-transporting vegetable under a normal–day–low–night–temperature thermoperiod (similar to the condition in Energy-Saving Sunlight Greenhouse) still remain poorly understood. In this study, One cold-sensitive cultivar (Jinyan 4) and two cold-tolerant lines (NY-1 and XC-1) of cucumber (Cucumis sativus L.) were treated with temperatures of 28°C/22°C (day/night, control) or 28°C/12°C in a 10 h photoperiod (7:00-17:00) to investigate how chilling night effect cucumber fruit growth and carbohydrate translocation. Some information helpful for cold-tolerant cucumber breeding was obtained .Under control conditions, cucumber fruits grew fast during the afternoon and early night, and slow during the late night and morning. Under 28°C/12°C, two cold-tolerant lines adopted distinctively different mechanisms to keep relatively higher fruit growth rate. Compared to Jinyan 4, NY-1 had a higher fruit growth rate during cold nights while XC-1 fruit grew faster during the next day. Assimilate accumulation of fruits was in conformity with the growth rate under 28°C/12°C. The increase of net CO₂ assimilation rate by fruit set after a cold night indicated that feedback inhibition was partially responsible for the reduction of photosynthesis. To investigate why NY-1 fruit grew faster than that of Jinyan 4 during cold night, carbohydrates and related enzymes were assayed from 0 h to 4 h after the start of the dark period from source leaf to fruit sink. Compared to the normal night temperature (22°C, control), sucrose, stachyose and galactinol increased in mature leaves under cold-night treatment (12°C) while sucrose, glucose and fructose in fruits remained unchanged. In peduncles, where stachyose is catabolized to sucrose after long distance transport, cold nights simultaneously induced a significant increase of stachyose (substrate) and a decrease of sucrose (product), indicating that the metabolic step from stachyose to sucrose in peduncles is crucial to translocation inhibition in cold nights. This decrease was more pronounced in the cold-sensitive cultivar. Similar growth rates of fruits on one-fruit and two-fruit plants under cold-night treatment further confirmed that it is sink activity rather than source supply limiting the source-sink translocation. No significant genotypic differences in enzyme activities involved in the stachyose-sucrose conversion, including alkalineα-galactosidase, acidα-galactosidase, galactokinase, uridine diphosphate (UDP)-galactose pyrophosphorylase, UDP–glucose-4`epimerase and sucrose synthase, were observed when assayed in an adenosine triphosphate (ATP)-rich in vitro environment. However, the ATP concentration was much higher in peduncles of the cold-tolerant line, indicating that a limiting ATP supply may be partially responsible for the stronger inhibition of the stachyose-sucrose pathway observed in the cold-sensitive cultivar (Jinyan 4).To investigate why the fruit of XC-1 accumulate assimilate faster than that of Jinyan 4 and NY-1 in the next morning after a cold night, several carbohydrates and enzymes were assayed in mature leaves of fruit carrying nodes. Distinctly different partitioning of fixed carbon between starch and sugars (sucrose and stachyose) was observed among Jinyan 4, NY-1 and XC-1, indicating that the faster assimilate accumulation rate of XC-1 fruit in the morning after a cold night may be caused by the higher exportable carbohydrate level in the mature leaves. Higher sucrose-phosphate synthase (EC.2.4.1.14) activity constituted additional evidence that faster sucrose and stachyose biosynthesis in mature leaves was occurred in XC-1 than in Jinyan 4 and NY-1 at that time.