The sugarless grape trait characterized by single berry phenotyping

Abstract

For grape production, an important driver for the selection of varieties better adapted to climate fluctuations, especially warming, is the balance between fruit sugars and acidity. Since the past decades, temperature during ripening has constantly raised causing excessive sugars concentrations and insufficient acidity of the wine grapes in warmest regions. There is thus an increasing interest in breeding new cultivars, able to ripen at lower sugar concentration while preserving fruit acidity. However, the phenotyping of berry composition challenges both methodological and conceptual issues. Indeed, most authors predetermine either average harvest date, ripening duration, thermal time or even hexoses concentration threshold itself, to compare accessions at an hopefully similar ripe stage. Here, we have phenotyped the fruit development and composition of 6 genotypes, including 3 new disease-tolerant varieties known to produce wines with low alcoholic contents. The study was performed at single berry level from the end of green growth stage to the arrest of phloem unloading, when water and solute contents reach a maximum per berry. The results confirmed that sugarless genotypes achieve fruit ripening with 20-30% less hexoses than classical varieties, Grenache N and Merlot N, without impacting berry growth, total acidity or cations accumulation. Sugarless genotypes displayed a higher malic acid/tartaric acid balance than other genotypes with similar sucrose/H+ exchanges at the onset of ripening. Data suggest that sugarless phenotype results from a specific plasticity in the relationship between growth and the turgor imposed by organic acid accumulation and sugar loading. This opens interesting perspectives to understand the mechanism of grapevine berry growth and to breed varieties better coping with climate warming.

Ripening dynamics revisited: an automated method to track the development of asynchronous berries on time-lapse images

“…The automated tracking of the asynchronous ripening of individual berries within one grapevine bunch allowed us to revisit the basic growth rates of ripening berries, and to propose, for the first time, an analysis of their growth and colour kinetics for a statistically significant number of observations, with an unprecedented time resolution (less than one day). In line with preliminary reports on other cultivars [ 5 , 42 ], we found that the ripening duration of individual berry does not exceed three weeks, which differs from the 32 to 56 days duration of sugar loading reported in a panel of 36 international cultivars, following random sampling and averaging 50 berries over time [ 45 ]. This confirms that by neglecting berry asynchrony as a confounding variable, the ripening duration of daily averaged ‘mean berry’ could be up to two fold overestimated compared to individual fruit kinetics.…”

“…8 C; grey dotted line), and underwent a more or less intense shrivelling period once phloem unloading in berries definitively stopped [ 41 ]. Such a relative expansion rate is in line with the approximate doubling of berry volume during the ripening of most V. vinifera cultivars [ 4 , 5 , 22 ], which took three weeks to complete on individual fruits of Meunier, Syrah, Zinfandel or ML1 [ 42 ], Cabernet Sauvignon and Pinot [ 15 ]. Further studies are needed to establish if the slightly shorter growth duration and expansion of Alexandroouli’s berry is truely of genetic origin, or is the result from tests on fairly young own rooted potted plants in greenhouse conditions.…”

“…bananas, apples or mangoes) which accumulate sufficient starch reserve to achieve post-harvest ripening, the grape berry necessarily ripens on the vine, at the rate of translocation of water and sucrose through the phloem. Ripening involves the sudden activation of the apoplastic pathway of phloem unloading [ 52 ], which leads to the second growth phase during which each berry accumulates about 1 M hexoses and becomes coloured [ 5 , 22 , 36 , 42 ]. Then, the definitive stop of phloem unloading triggers a more or less pronounced shrivelling period known as overripening [ 30 , 41 ].…”

“…Moreover, scarce studies on single berries recently revealed how chimerical these population-relevant samples are, on both the phenology and metabolic point of view. For example, the fact that the developmental lag between two berries can be almost as long as their growth duration leads to a two fold overestimation of the duration of the second growth period, when considering the average evolution among several berries [ 5 , 42 ]. Furthermore, mixing growing and shrivelling berries leads to the averaging bias that a constant volume is maintained during late ripening, and that excess water from the phloem mass flow must be released into the xylem backflow [ 25 ].…”

The LowSugarBerry trait phenotyped at population and single fruit levels

“…Firmness monitoring -Firmness was appreciated by hand as explained in [2] (Tables 1, 2 and 3). In Table 5, rmness was monitored with a digital penetrometer [5].…”