Natural genetic variation for fruit set rate within Malbec grapevine (Vitis vinifera L.) clones

Fruit set is the transformation of flower ovaries into berries. Fruit set rate determines the number of berries produced per bunch, which is a major component of yield in grapevines. Malbec is a black-berried grapevine cultivar recognized for producing high-quality wines, and particularly relevant for Argentina's winemaking industry. Clonal variation has been reported for Malbec at the phenotypic and molecular levels. However, less is known about clonal variation for agronomically relevant features affecting yield. In this work we evaluated 25 Malbec clones for the fruit set rate and other related features over multiple seasons.

The mean fruit set rate was 38.6% (ranged between 13.1% and 65.8%) in 2021/22, and 32.1% (ranged between 9.8% and 50%) in 2022/23 season, evidencing a wide range of within-season variation. Besides the expectable interannual differences, significant and positive correlations were found over seasons for the fruit set rate, number of flowers per inflorescence and number of berries per bunch. Moreover, multivariate clustering analyses consistently grouped the evaluated clones into three distinct groups. Two of these groups showed similar number of flowers per inflorescence, but significantly different fruit set rates and number of berries per bunch. Representative clones of the latter two groups were in-depth analyzed in 2023/24, supporting previous seasons results and revealing differences in their floral phenotypes.

We found a wide range of clone-dependent variation for the evaluated traits, which generated great differences in the reproductive performance and yield within Malbec. We hypothesized that the observed differences were associated to somatic mutations, affecting the correct development and functionality of flower organs in a clone-specific way.