Changes in sugar content and relative enzyme activity in grape berry in response to root restriction

Root restriction often depresses photosynthetic capacity and reduces shoot growth, but the photosynthate accumulation in fruit under restriction is higher than in control fruit. However, changes in metabolism responsible for these differences are unclear. To identify the metabolic mechanism by which root restriction affects sugar accumulation in fruit of ‘Kyoho’ grape (Vitis vinifera × V. labruscana), sugar metabolism and related enzyme activities in grape berries produced with and without root restriction were compared. Total sugar content of fruit produced under root restriction was higher than that of control fruit. Acid invertase (AI, EC 3.2.1.26) activity, which increases with berry development, was significantly higher in root-restricted berries than in control berries. Neutral invertase (NI, EC 3.2.1.26) activity showed a similar trend to AI, but the amount of NI activity was lower than AI in both treatments. Sucrose phosphate synthase (SPS, EC 2.4.1.14) and sucrose synthase (SS, EC 2.4.1.13) activity changed slightly with berry development, and there was no significant difference in SS and SPS activity between root restriction and control treatments. Therefore, AI appears to be the key enzyme induced by root restriction that explains the higher sugar content found in grape berry produced under root restriction.