The dynamics of δ13C and δ18O in musts during berries development

Aim: Many processes or reactions that occur in plants involved isotopic discrimination. Water availability, for example, affects the isotopic ratio of carbon (δ¹³C) and oxygen (δ¹⁸O). In viticulture, δ¹³C is used in experiments related to water relations and irrigation in vineyards. δ¹⁸O is used much less but it could be a good complement to δ¹³C. The aim of this study was to generate knowledge on how these isotopic ratios, measured in musts, could help to better understand the water behavior of grape varieties. 

Methods and Results: The present work was carried out in 2019 with a set of seven varieties selected to monitor the aforementioned ratios in musts obtained during berries development: three of them red (Bobal, Tinto Velasco and Syrah), managed with irrigation, and the other three white (Airén, Malvar and Albillo Real) kept without irrigation; the seventh, Tempranillo, was managed with or without irrigation. 

Monitoring the dynamics of isotope ratios was undertaken through sampling of grapes carried out periodically, from shortly before closing cluster to maturity. In obtained musts, δ¹³C and δ¹⁸O were determined by mass spectrometry of isotope ratios.

The small changes observed between samples in δ¹³C in a specific variety did not seem to follow any pattern. In most cases, the comparisons of means performed showed no significant differences between samples. However, differences were observed between the two management systems: irrigated and rainfed; in dry conditions, with the stomatal closure, ¹³C isotopic discrimination declined during photosynthesis, and the ratio then increased.

This was not so with δ¹⁸O, where the comparisons of means always showed significant differences between samples. Dynamics of δ¹⁸O seemed to adapt, in this case, to a double curve pattern (cubic polynomial): the intense increase in the ratio of the first stages of fruit development was followed by a phase of slight decline, which lasted up to 15 or 20 days before harvest, at which point the ratio increased again. There were both inter-varietal and between management system differences: musts in early harvest varieties showed higher δ¹⁸O values than late varieties, while the isotopic enrichment was lower for this isotopic ratio in irrigated vines.

Conclusions:

Differences in the narrow margin in which δ¹³C values of the grapes are maintained throughout their development seemed to respond more to the crop management practice than to the variety. However, the notable changes in δ¹⁸O values seem to be due to a complex mechanism that involves the discharge of water in the grapes from the phloem at beginning of ripening and the loss of water due to transpiration through the skin.

Significance and Impact of the Study: In the search for the genotypes with the highest water efficiency that effectively respond to the proliferation and dilation of drought periods that are expected in many regions, it is urgent to explore the existing genetic variability. In this sense, δ¹³C and δ¹⁸O could be useful tools to take into account in any research related to water use by cultivars at physiological or agronomic levels.