The lead author of a Wine Australia-funded study that provides a guide to rootstock suitability for vineyards challenged by root zone salinity has been honoured with the ASVO award for Viticulture Paper of the year 2020.
CSIRO Honorary Post Retirement Fellow, Affiliate Professor University of Adelaide and Adjunct Professor Charles Sturt University Dr Rob Walker – whose research on salt tolerance and rootstock salt exclusion is widely recognised nationally and internationally – led a project involving scientists from the CSIRO and the Australian Wine Research Institute that together wrote the paper Effect of rootstock on yield, grape composition and wine sensory attributes of Shiraz grown in a moderately saline environment.
In food, it is well known that a small amount of salt enhances flavour but too much has the opposite effect. In wine it is similar, but what was unknown was the threshold for when the salty flavour becomes a negative influence on the final wine.
The study is important as it provides an indication of where the palate recognition threshold for a salty taste in red wine may lie in terms of chloride concentration.
The capacity of salt-challenged grapevines to limit the uptake and transport of chloride and sodium to leaves and berries (termed ‘salt exclusion’) is one measure of salt tolerance. Other measures are yield, balanced growth, good fruit and wine composition and wine sensory attributes.
The study investigated the role of rootstock vigour in the salt tolerance response; and the sensory attributes of wines made from the rootstocks, including salty taste.
It also examined a salt tolerance index that combined several parameters for assessment of salt tolerance.
Several measures were used to compare the performance of Shiraz on eight rootstocks in a moderately saline vineyard environment in the south east of South Australia. The grafted vines – which included three low to medium vigour rootstocks bred by CSIRO – were drip irrigated with moderately saline water over two seasons.
Using yield as a measure, the team found Ramsey was best, followed by 140 Ruggeri, then Merbein 5489 and 110 Richter.
Using salt ‘exclusion’ from wine as a measure, 140 Ruggeri and 110 Richter led to lowest chloride, though Merbein 5489, 101-14, Merbein 5512 and 1103 Paulsen were similarly low in 2013, and all rootstocks were good sodium ‘excluders’.
Using wine colour density and anthocyanin as a measure, the team found 101-14 was best.
As part of the research, a new ‘salt tolerance index’ was developed, based on yield, leaf area index, chloride and sodium concentrations in juice, and wine colour density. This index showed there was little difference between most rootstocks in 2012, while in 2013, Merbein 5489 was best, followed by 110 Richter.
Rob said chloride concentration increased by an average of 2.2-fold from grape juice to wine; and the study findings suggested that the recognition threshold for salty taste in red wine could lie between a chloride concentration of approximately 380 and 480 mg/L. However, he said this would need to be verified by further sensory testing.
‘Given forecasts of a warming and more variable future climate for Southern Australia – which includes periodic drought, the possibility of water deficits and related effects on root zone salinity – rootstock choice will be increasingly important’, said Rob.
He said rootstocks that had tolerance to salinity – and also featured other essential positive attributes – would play an important role in protecting against yield losses and in minimising the threat of salty taste in wine.
The study is entitled: Walker, R.R., Blackmore, D.H., Clingeleffer, P.R., Holt, H., Pearson, W. and Francis, I.L. (2019) Effect of rootstock on yield, grape composition and wine sensory attributes of Shiraz grown in a moderately saline environment. Australian Journal of Grape and Wine Research, 25: 414–429. https://onlinelibrary.wiley.com/doi/full/10.1111/ajgw.12409