Phloem anatomy traits predict maximum sugar accumulation rates

Abstract

Context and purpose of the study

Heat and water stress can accelerate berry sugar accumulation and lead to excessive sugar-to-acid ratios at harvest, producing bland, overly-alcoholic wines. Selecting grapevines for slower sugar accumulation could help maintain wine quality under future, hotter conditions, but these efforts have been stymied by our limited understanding of the traits determining sugar accumulation rates. Here, we measured traits characterizing the structure and anatomy of the sugar transport system – the phloem – in 16 winegrape cultivars and tested for relationships with sugar accumulation rates and cultivar climate classifications. We expected hot-climate cultivars to delay sugar accumulation through traits that increase resistance to phloem transport.

Material and methods

We measured mature vines of 8 hot-climate (red: Syrah, Montepulciano, Mourvèdre, Tempranillo, Zinfandel, and Anglianico; white: Fiano and Verdelho, 7 warm-climate (red: Barbera, Cabernet Sauvignon, Merlot, Carignane, and Nebbiolo; white: Chardonnay and Sauvignon Blanc), and 1 temperate-climate (white: Riesling) cultivars growing in an experimental vineyard block on the UC Davis campus (N = 3 – 4 vines/cultivar). We measured berry total soluble solids (TSS) every 2 – 3 weeks from Jun – Sep 2020 and calculated the maximum sugar accumulation rate for each cultivar as the maximum slope of the relationship between TSS and growing degree days (GDD). We sampled leaves and berries in Sep 2020 and used light microscopy to measure total and mean phloem area area and scanning electron microscopy to measure sieve plate porosity and sieve element area in the leaf midvein, petiole, and berry pedicel.    

Results

The maximum sugar accumulation rate was significantly correlated with the total phloem sieve element area in the pedicel (r² = 0.25, p = 0.046, N = 16) and petiole (r² = 0.48, p = 0.004, N = 15). Maximum rates of sugar accumulation were faster in the cultivars with more phloem area. The total phloem area in the pedicel and the petiole was significantly smaller, and sugar accumulation was slower, in the hot-climate than the warm-climate red cultivars (ANOVA, p < 0.05). Mean sieve element area and sieve plate porosity were not significantly different between the climate groups or correlated with sugar accumulation rate (p > 0.05). These findings show that heat-adapted cultivars may avoid excessive sugar accumulation through phloem traits that reduce the capacity for sugar transport. Future work should test whether reduced phloem area also contributes to a water-saving strategy, by impeding sugar export from the leaves and activating sugar-induced signalling for stomatal closure. These findings also suggest a potential application for petiole phloem area in screening for rates of sugar accumulation, since petioles could be sampled years before vines are mature enough to produce fruit.