Exploring induced mutagenesis as a tool for grapevine intra-varietal improvement: increased diversity in ripening periods and bunch traits with climate resilience potential

Abstract

The wine industry currently relies on a limited number of grapevine cultivars, comprised of numerous clones with slight differences in their viticultural, oenological, or stress-tolerance traits. To adapt to and mitigate the impacts of rising global temperatures and associated stress pressures, characterising and conserving intra-varietal diversity is crucial for cultivar improvement. Mutagenesis can enhance genetic diversity, offering a means to identify and conserve potentially useful traits. Induced mutagenesis using chemical or physical mutagens is widely employed in plants to introduce genetic variation.

As a relatively young cultivar, Pinotage (a South African-bred red-wine cultivar) has very few registered clones compared to ancient cultivars. To increase the diversity of currently available Pinotage planting materials, an intra-varietal improvement scheme was implemented using gamma irradiation. The treatment was applied to in vitro plantlets of three widely planted Pinotage clones (Pi7B, Pi45R and Pi48I), targeting the meristems of the latent buds, resulting in a collection of ~1400 putatively mutated plants. A subset of 500 individuals (irradiated treatment vines and unirradiated somaclonal controls) was pre-selected and field-planted in 2021. Fruiting began in 2023, enabling detailed analysis of viticultural and oenological characteristics towards selection.

Phenotyping techniques (using OIV descriptors and agronomic measurements) and phenological scoring (using E-L scale), including ripeness monitoring, were employed to perform the initial characterisation of the fruiting putative mutant grapevines. Diverse phenotypes were observed amongst the population, including variation in berry skin colour, flesh colour, seed characteristics and bunch compactness. The bunch architecture traits and compactness were a particular focus given the links with improved ripening and reduced fungal infections of less compact bunches. This population also displayed a wide range of phenological progressions and ripening periods. Variation in the véraison-harvest period has implications for climate resilience, as rising global temperatures advance phenology and ripening, resulting in unbalanced phenolic accumulation and sugar-to-acid ratios, restraining grape and wine quality. Our results on this population will be shared to highlight the potential of gamma irradiation and induced mutagenesis for increasing intra-varietal diversity in grapevine cultivars and clones.