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Plant hydraulic conductance adapts to shoot number but limits shoot vigour in grapevines.

The rate of shoot growth (vigour) in grapevines tends to decrease as the number of shoots per plant increases. Because the underlying causes of this relationship remain unclear, they were studied by variable pruning of field-grown, deficit-irrigated Merlot grapevines (Vitis vinifera L.). Shoot number ranged from 11 to 124 per vine and was inversely correlated with shoot growth rate, leaf appearance rate, axillary bud outgrowth, internode length, leaf size, shoot leaf area, carbon partitioned to the fruit (Cfruit) per shoot, average daily maximum photosynthesis (Amax), stomatal conductance (gmax), and leaf-specific hydraulic conductance (Kl). Shoot number was positively correlated with canopy leaf area, whole-vine Cfruit, whole-plant hydraulic conductance (Kv), and canopy conductance (Kc). Higher shoot vigour was associated with higher Amax, gmax, predawn leaf water potential (Ψpd), shoot hydraulic conductance (Ks), Kl, and Kv. Vigorous shoots supported both more vegetative growth and more reproductive growth; thus fruit growth did not compete with shoot growth for photosynthates. These results indicate that the hydraulic capacity of grapevines adapts to varying shoot numbers to support leaf physiology, growth, and carbon partitioning, but adaptation may be limited, putting upper bounds on the growth of individual shoots and fruit.