WINETECH Technical Yearbook 2019

Chone et al ., 2006). Traditionally, vineyard nutrition was carried out by adding fertilisers to the soil to be absorbed by the roots of the plant. Due to climate change and more frequent summer droughts, soil fertilisation is no longer the best solution to increase the nitrogen levels of the must and vines (Fischer, 2007; Laget et al ., 2008; Keller, 2010). Foliar spray fertilisation is a widely-used technique on various crops and can lead to a quick nutrient uptake through the leaves (Christensen & Smart, 2005; Jreij et al ., 2009; Lasa et al ., 2012). Nitrogen foliar fertilisation is only effective in plants approaching nitrogen deficiency and deficiencies can be overcome temporarily (Delas, 2000). Aroma compounds in wine contribute directly to the quality of wine (Marais, 1994). Therefore, an ongoing goal of winemakers is to improve aromatic expression and complexity of wines (Loubser, 2008). Some aroma compounds in wines are influenced by the amino acid composition of the initial grape juice. Grape juice with low YAN (amino acids and ammonia) can lead to low yeast populations, poor fermentation vigour and increased risk of sluggish or stuck alcoholic fermentations (Monteiro & Bisson, 1991). Various viticultural factors, such as canopy management and nutrition, can influence the juice YAN and therefore the quality and aromatic expression of wines (Chone et al ., 2006; Lacroux et al ., 2008).

Nutrition levels can be adjusted or supplied to the vines through soil fertilisation or by applying foliar fertilisation sprays, meanwhile DAP or complex nutrients can be added in the cellar to grape musts (Lorenzini & Vuichard, 2012). Previous research proved that nitrogen foliar fertilisation can result in increased levels of amino acids in the must (Fisher, 2007; Lacroux et al ., 2008). Therefore, nitrogen with or without sulphur foliar nutrition can enhance the aromatic expression in wines (Chone et al ., 2006; Lacroux et al ., 2008). It has been shown that foliar fertilisation with nitrogen (N) and nitrogen with sulphur (N+S) at véraison positively impacts the berry chemical composition, yeast growth and metabolism, and produces more aromatic wines. Most foliar research studies have mentioned and have proven positive effects when fertilisation applications were performed prior to and at véraison, due to the vine’s nutrient uptake patterns and requirements (Lacroux et al ., 2008; Dufourcq et al ., 2009). By doing nitrogen and sulphur foliar applications at véraison, the concentrations of N- and S-containing compounds can be increased. The studies show that N and N+S applications can positively affect various volatile and non- volatile compounds in grapes, musts and resulting wines. Increased levels of N and S compounds, such as YAN, FAN, amino acids and volatile thiols, were measured following the treatments (Jreij et al ., 2009; Lasa et al .,

2012; Lacroux et al ., 2008; Dufourcq et al ., 2009, Kelly et al ., 2017; Mundy et al ., 2009; Hannam et al ., 2014; Geffroy et al ., 2016; Koestel et al ., 2017; Gutiérrez-Gamboa et al ., 2017; Helwi et al ., 2014). CHENIN BLANC AND SAUVIGNON BLANC In South Africa, Chenin blanc and Sauvignon blanc are two of the most planted white wine cultivars. By means of renewed interest by researchers and the industry, Chenin blanc wines have increased in quality and aroma styles in the past few years. Research by Lawrence (2012) on Chenin blanc investigated analytical methods and aroma compounds, such as esters, monoterpenes, higher alcohols and fatty acids. Only recently volatile thiols levels in Chenin blanc have been reported by Wilson (2017). Many studies have been performed on Sauvignon blanc with the key focus on aroma compounds that influence aroma expression. A combination of odour-active aroma compounds gives the specific aromatic character of Sauvignon blanc. They are the methoxypyrazines that are found in grapes, and the thiols and major volatiles that are formed from precursors during alcoholic fermentation (Fisher, 2007). Most foliar nutrition research has focused widely on thiols (Lacroux et al ., 2008; Geffroy et al ., 2016; Dufourcq et al ., 2007). Recent studies with N and S foliar fertilisation applications resulted in Sauvignon blanc juice

and wine having higher volatile thiol levels and improved aromatic potential (Lacroux et al ., 2008; Jreij et al ., 2009; Dufourcq et al ., 2007). Increased glutathione (GSH) levels were obtained where soil nitrogen, as well as foliar nitrogen and sulphur foliar applications, were done (Lacroux et al ., 2008). Due to its antioxidant properties, GSH plays an important role in Sauvignon blanc wines by protecting the aroma compounds, such as volatile thiols (Dubourdieu & Lavigne, 2004). The various foliar fertilisation studies have been performed on Sauvignon blanc in different locations in the world (usually cool climate areas) and varied in application products, rates and times. Chenin blanc is one of South Africa’s most important and planted white cultivars, and to date no foliar fertilisation trials have included this cultivar. Only two soil fertilisation studies have been done on Chenin blanc (Conradie, 1981; Conradie & Saaiman, 1989) both of them in South Africa. In general since few research studies regarding foliar fertilisation have been published and most trials have been conducted under European conditions on various white and red wine cultivars (Jreij et al ., 2009; Lasa et al ., 2012; Lacroux et al ., 2008; Dufourcq et al ., Verdenal et al ., 2016; Garde-Cerdan et al ., 2017; Geffroy et al ., 2016). It was of interest to us and the industry to perform N and N+S foliar