Manipulating Grapevine Bud Fruitfulness

Abstract

Bud fruitfulness is a key component of reproductive performance of grapevine. It plays a significant role in annual yield variation of vineyards as it is a prerequisite of crop production in the following season. Bud fruitfulness is defined as the formation of inflorescence primordia (IP) in mature latent buds and can be expressed as the average number of IP per bud. Various exogenous and endogenous factors influencing the development of IP and subsequent yield have been extensively studied. However, the research on molecular genetic control of bud fruitfulness, especially how it interacts with environmental factors is still lacking. Temperature and light have been shown to be two of the most critical factors affecting the formation of IP in latent buds. A better understanding of bud fruitfulness in relation to the two environmental factors as well as cultural practices is imperative for producing a consistent crop. The purpose of this thesis is to investigate the molecular mechanism of effects of temperature and light on grapevine bud fruitfulness during initiation and differentiation of IP. The project also aims to manipulate bud fruitfulness in field using canopy management practices and explore the influence on reproductive performance in the following seasons. Semillon cuttings were propagated and exposed to six regimes of combined light (90, 200 and 600 PAR) and temperature (day/night 30/25°C and 20/15°C) in growth rooms for six months from budburst to leaf fall. Bud samples were collected at three stages (E-L Stage 17, 35 and 38) for bud transcription analysis and fruitfulness was assessed at E-L Stage 35, 38 and 43. Results showed that both number and size of IP were positively correlated to temperature and light within the given range. Shoot vigour was negatively associated with bud fruitfulness, indicating that there may be competition for resources between shoot growth and bud development. RNA-seq analysis revealed that temperature had a greater influence at early development (pre-flowering, E-L Stage 17) with 8530 differentially expressed genes (DEGs), while light was most important later (veraison, E-L Stage 35) with 5716 DEGs. Gene ontology enrichment analysis showed that the DEGs were mainly involved in biological functions of stress management under the temperature treatment and active cellular development under the light treatment. Canopy management practices are widely adopted in commercial vineyards and are important non-environmental factors that can influence bud fruitfulness. Intensive shoot thinning was applied on Semillon vines in field at E-L Stage 17 to investigate the effect of this practice on canopy architecture and reproductive performance of grapevine over time. Plant area index and light interception by the canopy were captured at different growing stages and bud fruitfulness was assessed at dormancy by recording number and size of IP. Inflorescence and bunch architecture, and yield components were measured in the following seasons. It was found that shoot thinning created a more open canopy and improved bud fruitfulness with more and larger IP. Inflorescence architecture was increased in the next season, suggesting a carry-over effect of the treatment on the enlarged IP. A compensation in bunch development was shown by increased berry number and berry weight and moreover, the extent of compensation was accumulative when the practice was imposed in consecutive seasons. In this trial, the influence of bunch position generated by spur pruning on IP development and bunch architecture was investigated on control vines. A consistent pattern of bunch architecture variation was found at different positions and can be traced back to the formation of IP in compound buds in the preceding growing season. The size of IP was positively related to the final bunch weight, and large variation between the IP sizes led to a larger difference in bunch volume and weight. To develop a non-destructive method for assessing bud fruitfulness, X-ray micro-computed tomography (micro-CT) was used to visualize buds and assess morphological traits of IP. Results showed that the interior microstructure of the bud samples were captured in detail by the scanning and three dimensional morphometric parameters of IP were comparable with conventional methods. Unpredictable seasonal variations of grapevine yield continue to cause major economic problems in the wine industry. This study improved our understanding of the early development of yield components at a molecular level and the influence of environmental conditions on these components. Bud fruitfulness was manipulated in field by shoot thinning and the effects were reflected in yield components and bunch architecture in the subsequent season. These insights may be used by practitioners to make more informed vineyard management decisions when manipulating fruitfulness and resultant yield.