Which factor determines the leaf structure of bearing units in grapevines?

The structure of the leaves on bearing units in grapevines is predominantly influenced by the size of the clusters. As grapes develop and the clusters grow, the vine allocates resources to support the size and weight of the fruit. This demand can lead to changes in leaf structure to optimize light capture and photosynthesis, which are crucial for providing energy to the growing grapes. Larger clusters may necessitate larger, more efficient leaves to sustain the increased fruit load, ensuring that the vine can adequately nourish each cluster and maintain overall health.

Other factors like soil quality, aging of the vine, and water availability can influence grapevine growth and health, but they are not the primary determinants of leaf structure in relation to bearing units. While they can indirectly impact how a vine produces leaves (influencing vigor, stress conditions, or nutrient availability), the direct relationship between cluster size and leaf structure emphasizes the vine's adaptive responses to support successful fruit development.