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

Abstract

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 (A_sat) than own-rooted plants when roots were gradually cooled, but no differences were detected when shoots were cooled. Chill at 7 °C irreversibly reduced A_sat, 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.

Chilling, chlorophyll fluorescence, Cucumis sativus, electron transport flux, grafting, photosynthesis, reactive oxygen species, water–water cycle