Chill-Induced Decrease in Capacity of RuBP Carboxylation and Associated H2O2 Accumulation in Cucumber Leaves are Alleviated by Grafting onto Figleaf Gourd

Background and Aims: Chilling results in a significant decrease in Rubisco content and increased generation of reactive oxygen species (ROS) in cucumber (Cucumis sativus), a chilling-sensitive species. The role of roots in the regulation of the tolerance is unknown. Here, cucumber plants grafted onto figleaf gourd (Cucurbita ficifolia), a chilling-tolerant species were used to study the role of roots in the regulation of shoot functioning and the associated root-to-shoot communication. Methods: Gas exchange and chlorophyll fluorescence were measured using an infrared gas analyser combined with a pulse amplitude fluorimeter during chilling at 14 °C or 7 °C and subsequent recovery. At the same time, Rubisco content and activity and ROS generation were spectrophotometrically assayed. Abscisic acid and cytokinin concentrations in xylem sap were also determined by enzyme-linked immunosorbent assay. Key Results and Conclusions: Grafted plants showed a significantly higher light-saturated rate of CO2 assimilation (Asat) than ownrooted plants when roots were gradually cooled, but no differences were detected when shoots were cooled. Chill at 7 °C irreversibly reduced Asat, and significantly decreased maximum carboxylation activity, Rubisco content and initial Rubisco activity. However, grafted plants showed weaker inhibition, together with decreased electron flux in the water-water cycle. Higher activity of antioxidant enzymes with less ROS production was found in grafted plants. In addition, ABA concentration increased by 48·4-fold whilst cytokinin concentration decreased by 91·5 % in the xylem sap of own-rooted plants after exposure to a 7 °C chill. In comparison, ABA and cytokinin concentrations increased by 10·5-fold and 36·9 %, respectively, for the grafted plants. Improved plant growth was also observed in grafted plants after the chill. These results suggest that some signals coming from chilling-resistant roots (i.e. ABA and cytokinins) protect leaf photosynthesis in shoots of chilling-sensitive plants.