Investigating peduncle colour evolution and chemistry during ripening in Vitis vinifera L. cv Shiraz

Abstract

This thesis describes an investigation of how the various sections of the grape stem (peduncle and rachis) in Vitis vinifera L. cv Shiraz evolve in terms of their colour and chemical attributes during the ripening period and is compared to the prototypical berry ripeness parameters (⁰Brix, pH, TA, etc) over three seasons. The grape peduncles turned from green at veraison, to predominantly brown at harvest, whilst the rachises remained green during the maturation period. Certain peduncle and rachis chemical changes were also uncovered during these studies and shed light on the relationships between peduncle colour and chemical composition. Statistical modelling was conducted to examine the relationship between the peduncle colour and chemical composition, and a clear co-development was observed between peduncle evolution (colour and moisture) and the berry ripening continuum. Such observations are yet to be reported and provide for the opportunity to develop a new simple platform to assist in predicting grape berry ripeness and therefore aid in harvest decisions. Chapter 1 of this thesis comprises a detailed introduction of our current understanding on how both grape berries and their stems evolve over the ripening period and sets the scene by pinpointing key aspects for this exciting research journey. Chapter 2 details a large study on how the change in morphology (size and colour) of both the grape peduncles and rachises evolve in eight distinct patches of Shiraz from the same vineyard over two seasons. A semi-automatic method was developed to quantify the peduncle morphological details exploiting digital image analysis. The berries associated with the peduncles from the same bunches were also analysed for their prototypical berry ripeness parameters, namely the sugar content (⁰Brix), acidity (pH and TA), total anthocyanins and total phenolics. The image analysis showed that the overall peduncle colour evolution from green to brown was well reflected in the change of a* and b* values in the CIEL*a*b* colour system coupled with their polar parameters hue and chroma. Importantly, the change in the colour coordinates of the peduncles was found to be in parallel with the change in berry ripeness parameters thus providing a new means to assist in harvest decisions. This work was submitted to the Australian Journal of Grape and Wine Research in 2014. Chapter 3 evaluates a range of key chemical changes in Shiraz peduncles and rachises from veraison to harvest over two seasons. During ripening, peduncles experienced significant decrease in moisture and pigment (chlorophylls and carotenoids) levels while the total phenolics and antioxidant capacity (DPPH) levels did not change significantly. The levels of these chemical traits were always higher in the rachises than the peduncles; an observation yet to be reported. Moreover, it was found that the peduncle moisture content was correlated with the peduncle colour hue value in a strong linear fashion and was negatively associated with the pigments ratio (total chlorophylls / total carotenoids). Finally, the results showed that peduncle moisture content also co-developed with the prototypical berry ripeness parameters during ripening, an observation which excitingly provides for a new hitherto unknown approach to predicting berry ripeness and harvest date via peduncle moisture detection. This work was submitted to the Journal of Agricultural and Food Chemistry in 2014. Chapter 4 evaluates the spatial and temporal variation of peduncle colour in five Shiraz patches from the same vineyard during the 2013/14 season. Peduncles were collected from the same positions based on a regular grid sampling design for each patch of Shiraz; an enormous but fulfilling task! The within-patch variation among individual peduncle colour was found to be mostly driven by the berry maturation stages. It was found that at veraison, the peduncle colour was more homogeneous and displayed a green state, then the peduncle colour became more variable during ripening whilst less heterogeneous at harvest with the majority of peduncles in brownish colour. We also observed that actual fruit harvesting within these vineyard patches began when over 40% of the sampled peduncles appeared visually predominantly brown, providing the sample size was no less than 100 from that particular patch. Thus a simple visual assessment of the proportion of brown peduncles could be used as an easy and quick way to help predict time for harvest. Moreover, the linear relationship between peduncle colour hue value and moisture content was verified and extended to the individual bunch level during the 2014 vintage. Again these observations strengthen the potential use of monitoring peduncle colour evolution to predict harvest date, and pave the way for the development of non-destructive methods of measuring peduncle colour in the field. This work has been prepared in publication style for the Australian Journal of Grape and Wine Research in 2014. Chapter 5 completes these exhilarating research works by summarising the key new findings and their potential benefits to the wine industry and provides suggestions for future research directions.