Physiological and Molecular Investigation of the Millerandage Problem in ‘Kalecik Karası’ (Vitis vinifera L.)

Millerandage, defined as the coexistence of both seeded and seedless berries within a grape cluster, is a common physi­ological disorder frequently observed in the ‘Kalecik Karası’ cultivar. One of the primary causes of this condition is the de­ficiency of boron, a micronutrient essential for pollen tube de­velopment and successful fertilization. Rootstocks influence boron uptake and transport, thereby affecting the severity of millerandage. In this study, the physiological and molecular basis of millerandage was investigated in ‘Kalecik Karası’ vines grafted onto 5BB (5.51), 41B (3.26) and 1103P (3.87) root­stocks. Among these, the 5BB rootstock showed the highest fruit set and Millerandage Index, indicating a strong tendency toward the formation of seedless berries despite high boron levels. Therefore, in the second phase of the study, only the 5BB rootstock was selected, and boron content in leaves as well as the expression levels of BOR-type transporter genes (VvBOR1, VvBOR3, VvBOR4) were analyzed in both seeded and seedless berries across five different phenological stages. Leaf boron levels were lowest during flowering and highest at veraison. Gene expression analysis revealed significantly higher BOR gene expression in seeded berries, especially at the fruit set stage, while expression levels remained low in seedless berries. This suggests limited boron translocation and disrupted fertilization in seedless berries. In conclusion, millerandage development is associated with both physiolog­ical effects of rootstocks and differential expression of boron transporter genes. Monitoring the expression of these genes and managing micronutrient nutrition may help improve ber­ry quality in sensitive cultivars.