Module 1: Berry Development

Following anthesis (flowering), berry fresh weight \(W(t)\) rises not monotonically but along a double-sigmoid with two distinct growth episodes separated by a short quiescent plateau. Coombe (1976, 1992) fit the trajectory with two additive logistic terms:

Stage I: Cell Division and Expansion

From fruit-set (~3–5 days after anthesis, DAA) to ~45 DAA the pericarp undergoes a rapid expansion driven by mitotic proliferation of the mesocarp for the first ~3 weeks, then by post-mitotic cell enlargement. The berry is firm, intensely green, photosynthetically active (contains chloroplasts in mesocarp and exocarp), and accumulates organic acids (malate, tartrate) by both in-berry metabolism and phloem import. Chlorophyll density in the skin is high; sugar content is low (1–3 °Brix). The seed is growing rapidly and entering the maturation phase.

Lag Phase

A ~10-day plateau follows Stage I at ~45–60 DAA, during which fresh-weight gain essentially stalls. Biochemically the seed endosperm desiccates, seed-coat pigmentation darkens, and the berry completes the shift from vegetative-sink physiology to fruit-sink physiology. The lag phase is the cue commonly used by viticulturists to schedule green cluster-thinning (adjust crop load to the expected leaf-area-fruit-mass ratio).

Stage II: Veraison

Veraison (from the French véraison, “turning of the color”) marks the ignition of Stage II. Within 3–5 days of onset: (i) anthocyanin accumulation begins in red varieties (skin color break from green to pink to deep red or blue-black), (ii) chlorophyll is degraded, (iii) phloem unloading switches from symplastic to apoplastic (Zhang 2006), and (iv) hexose (glucose+fructose) begins to accumulate rapidly in the mesocarp vacuole. Brix rises from ~5 to ~22–26 in the ensuing 6–8 weeks.

Stage III: Ripening

Ripening encompasses sugar loading to commercial target, malic-acid respiration and decarboxylation, cell-wall softening via pectin methyl-esterase (PME) and polygalacturonase (PG), and biosynthesis of varietal volatile aromas (monoterpenes in Muscat, thiol precursors in Sauvignon Blanc, methoxypyrazine degradation in Cabernet Sauvignon, rotundone in Syrah). Color intensity deepens as anthocyanins are co-pigmented with flavonols and tannins.

Stage II expansion is almost entirely driven by water influx into the vacuole, which occupies >95% of the mesocarp cell volume at harvest. Water crosses the plasma membrane and the tonoplast via PIP and TIP aquaporin channels; the dominant vacuolar aquaporin in ripening grape is VvPIP2;1(Chervin 2008), whose transcript rises sharply from veraison onward.

The Lockhart Equation

Cell expansion against a turgor-yielding wall follows the Lockhart (1965) extension, which couples wall mechanics to water uptake:

Simulation 2 integrates this coupled ODE system for a single mesocarp cell, forcing sugar loading post-veraison. The model reproduces the empirical observation that cell volume grows ~30× from fruit-set to harvest, with most of that expansion concentrated in Stages II–III.

Hydrostatic and Osmotic Pressure Budget

At harvest, mesocarp vacuolar osmotic pressure reaches ~2–3 MPa (dominated by glucose and fructose near 1 M combined), with cellular turgor falling paradoxically to ~0.1–0.3 MPa as walls soften. This turgor collapse underlies the perceived “sagging” of ripe berries at harvest and is an active biophysical transition, not mere desiccation.

Grape berries are sink organs: they cannot fix significant carbon themselves (their chlorophyll is largely degraded by veraison) and instead import sucrose from source leaves via the phloem. The peduncle phloem of the rachis terminates in the receptacle, from which sieve elements penetrate through the berry pedicel and ramify into the dorsal and ventral bundles of the pericarp.

Pre-Veraison: Symplastic Unloading

Before veraison, sucrose moves cell-to-cell through plasmodesmata in a symplastic pathway: sieve-element / companion cell → phloem parenchyma → mesocarp vacuole. Solute gradient is maintained largely by dilution into a rapidly growing sink tissue. Concentrations are modest (1–3 °Brix) because the solute is largely diluted by continuous water influx.

Zhang 2006: Apoplastic Switch

Zhang et al. (2006) showed that at veraison the plasmodesmata of the phloem-mesocarp interface close and sucrose unloading switches to an apoplastic route. Sucrose exported from sieve elements is cleaved in the apoplast by cell-wall (vacuolar) invertase (VvGIN/CWINV) to glucose + fructose, which are then co-transported into mesocarp vacuoles via proton-coupled hexose transporters (VvHT family). The apoplastic step thermodynamically uncouples phloem delivery from mesocarp storage, allowing the mesocarp vacuole to reach ~1 M hexose without creating a reverse gradient that would stall phloem unloading.

Hexose Storage in the Vacuole

Glucose and fructose are stored in equimolar ratio (~1.0 : 1.0 at harvest for most cultivars; Chardonnay slightly glucose-rich, some Muscats fructose-rich) in the vacuolar sap, typically reaching concentrations of 1 M or higher—equivalent to ~2 MPa of osmotic pressure and roughly 200 g/L total sugar. The mass accumulation of sugar between veraison and harvest is on the order of 1 g per berry, or roughly 300–400 mg of hexose per mesocarp cell.

The transition from the hard, green Stage I berry to a yielding, fleshy Stage III berry depends on extensive remodelling of the primary cell wall and the middle lamella. Grape wall polysaccharides are dominated by pectins (homogalacturonan, rhamnogalacturonan I and II) and hemicellulose (xyloglucan), with cellulose microfibrils contributing tensile strength.

Pectin Methyl-Esterification Reversal

Newly synthesised homogalacturonan is highly methyl-esterified; carboxyl groups on GalA residues are capped with methyl esters that prevent Ca^2+ cross-linking. At veraison, VvPME expression rises sharply, stripping methyl groups:

Polygalacturonase Depolymerisation

The now-accessible polygalacturonate is attacked by endo- and exo-polygalacturonases (VvPG1, VvPG2) that cleave (1→4)-alpha-D-galacturonan bonds, fragmenting the pectin network. Simultaneously, pectate-lyase activity and expansion of the xyloglucan endotransglucosylase/hydrolase (XTH) family loosen the cellulose-hemicellulose network, and apoplastic wall-loosening proteins such as expansins facilitate wall creep. The combined result is a reduction of wall modulus and a tissue that yields readily to light pressure—the basis for machine-harvest compatibility.

Berry Softening and Firmness Measurement

Fruit firmness can be measured by compression with a Durofel penetrometer or by the force-deformation response of a single berry. Coombe (1992) showed that firmness declines from ~1.0 N (pre-veraison) to ~0.1 N at commercial maturity, with the inflection tightly tied to veraison onset.

From the plant’s perspective the fleshy berry is a reward to the dispersing vertebrate; the biologically essential organ is the seed. A normal V. viniferaovule is tetracarpellate with up to four ovules; in practice most harvested berries carry 1–3 mature seeds, with tetraseeded berries occurring in Cabernetand tri-seeded berries in Merlot. Parthenocarpic (seedless) berries such as Thompson Seedless develop from ovule abortion after fertilisation; seedless commercial table grapes are produced either from genetic stenospermocarpy or by exogenous gibberellin sprays that induce ovule abortion.

Seed-Driven Berry Size

Berry final fresh weight correlates strongly with seed number: typical slope is ~0.3 g of additional pericarp mass per mature seed. Seeds are the source of auxin (IAA) and gibberellins that signal pericarp expansion during Stage I; seedless berries therefore tend to be smaller unless gibberellin-treated. Seed-coat phenolic content (condensed tannins, catechins) is cultivar-specific and contributes significantly to the bitter-astringent backbone of red wines.

Seed-Berry Ratio and Wine Style

Seed-to-pulp weight ratio is roughly 3–7% fresh weight, and seed phenolic extraction into must during fermentation scales with maceration intensity (punch-down frequency, pump-over, extended maceration). Viticulture selects for moderate seed contribution to the final wine via pre-fermentation cold soak and adjustment of cap management protocols.

Varietal aroma compounds accumulate late in Stage III and continue into the first days of over-ripeness. Major classes:

• Monoterpenes (linalool, geraniol, nerol, alpha-terpineol) peak at ~24–26 °Brix in Muscat and Gewürztraminer.
• C13-norisoprenoids (beta-damascenone, beta-ionone, TDN) are cleaved from oxidised carotenoids; they rise throughout Stage III and dominate the aroma of aged Riesling.
• Methoxypyrazines (IBMP, sec-IBMP) decline photochemically during sun exposure; retained under dense canopies.
• Volatile thiol precursors (glutathionyl-3MH, cysteinyl-3MH) accumulate but are not themselves volatile until cleaved by yeast lyases during fermentation.
• Rotundone (sesquiterpene, black-pepper note) accumulates late in Syrah in cool mesoclimates.

Many of the most distinctive “varietal” characters are cryptic: they exist in the grape as non-volatile glycosylated precursors and are released to free volatile form either by acid hydrolysis during bottle aging or by enzymatic hydrolysis during fermentation. The post-harvest trajectory of volatile aroma can therefore be decoupled in time and space from the grape-berry trajectory.

The double-sigmoid growth curve is not unique to grape. Apple (Malus domestica), peach (Prunus persica), and other Prunus stone fruits exhibit similar Stage I–III patterns, but tomato (Solanum lycopersicum) shows a single-sigmoid curve without a pronounced lag. Veraison is specific to non-climacteric fruits: grape (non-climacteric) ripens without the ethylene burst typical of climacteric fruits such as tomato, banana, and apple.

Climacteric vs. Non-Climacteric

Climacteric fruits (apple, banana, tomato, avocado) exhibit a sharp peak in respiration rate and ethylene production that initiates ripening in an autocatalytic fashion. Ethylene can be applied exogenously to ripen the fruit after harvest. Grape is non-climacteric: respiration declines monotonically from fruit-set through harvest, ethylene is not essential for ripening, and ripening cannot be induced post-harvest by ethylene application. The non-climacteric signature strongly affects storage, logistics, and wine-style management, because sugar cannot be further accumulated after the berries are severed from the vine.

Abscisic Acid as the Grape Ripening Hormone

In place of ethylene, grape berry ripening is coordinated by abscisic acid (ABA), whose tissue concentration rises sharply at veraison and triggers anthocyanin biosynthesis (via VvMYBA1 and VvMYBPA1) and sugar accumulation. Exogenous ABA application before veraison advances color development in red varieties and is sometimes used in problem climates to synchronise ripening.

Sucrose is transported from source leaves to the berry along the phloem sieve tubes by the Munch pressure-flow mechanism: active sucrose loading in leaves raises phloem osmotic potential, water flows into the sieve tubes by osmosis, and the resulting hydrostatic gradient drives bulk flow of phloem sap toward sink tissues (the berry). Phloem pressure in actively loading leaves reaches 1–2 MPa; delivery rates in ripening grape clusters are on the order of 0.03–0.10 g sucrose per cluster per hour.

Xylem-Phloem Competition

Stage I water and solute delivery is biased toward the xylem pathway, because xylem flow is driven by leaf transpiration and the berry is still functionally vegetative. After veraison, xylem becomes largely non-functional in the pedicel (Greenspan 1994, Choat 2009), probably due to embolism and wall occlusion, and phloem dominates water delivery. This xylem-phloem switch has practical implications for vineyard management: deficit irrigation post-veraison has a much smaller effect on berry water status than pre-veraison deficit, because phloem (not xylem) now dominates water import.

Grape berry development is coordinated by a sequential cascade of endogenous phytohormones whose relative concentrations swing dramatically through the four Coombe stages. The prevailing model (Bottcher 2011, Kuhn 2014) places auxin (indole-3-acetic acid, IAA) as the dominant hormone of Stage I, giving way to abscisic acid (ABA) as the master signal of Stage II (veraison) and Stage III (ripening). Ethylene, the dominant hormone of climacteric fruit such as tomato and apple, plays only a transient role in grape.

Auxin in Stage I

Auxin levels are high throughout Stage I, peaking around 20 DAA when cell division is most active. IAA is synthesised mainly in the developing seed integument and diffuses basipetally into the pericarp to drive cell proliferation and expansion. Synthetic auxin sprays (NAA, 4-CPA) have been used historically to increase berry size in table-grape cultivars and to delay ripening for climate adaptation (Bottcher 2011). Conjugation of free IAA to IAA-aspartate by GH3 enzymes at veraison terminates the auxin signal.

Gibberellins and Seedlessness

Gibberellins (GA1, GA3, GA4) are the hormone class most exploited commercially in viticulture. Pre-bloom GA3 sprays elongate the rachis and loosen clusters (reducing Botrytis pressure), while post-set GA3 sprays enlarge individual berries. The large-berry phenotype of Thompson Seedless for raisin production is almost entirely GA-induced: untreated Thompson Seedless berries are ~1 g; GA-treated are 5–7 g. GA also triggers stenospermocarpic seedlessness in many table-grape breeding lines.

ABA as the Veraison Signal

Abscisic acid concentration in grape berry mesocarp rises ~10-fold at veraison and remains elevated through Stage III. ABA is synthesised from cleavage of neoxanthin via 9-cis-epoxycarotenoid dioxygenase (VvNCED1, VvNCED2), whose transcripts are the earliest detectable molecular signal of imminent veraison. Exogenous ABA applied pre-veraison advances color development by 5–10 d and is permitted in some jurisdictions for vintage synchronisation in problem climates (Koyama 2010).

The Ethylene Puzzle

Grape ethylene production is very low compared to climacteric fruits (<1 nL g−1 h−1 vs. 10–100 nL g−1 h−1 in apple). A transient ethylene peak has been detected at veraison (Chervin 2004), and the ethylene perception inhibitor 1-MCP can delay color development, suggesting ethylene acts as an early ripening trigger. However, subsequent sugar accumulation is independent of ethylene; this distinguishes grape from climacteric fruit where the autocatalytic ethylene burst drives the full ripening cascade.

The double-sigmoid berry growth curve is a physical consequence of two distinct developmental programs: an early cell-proliferation program driven by auxin and gibberellin from the developing seed, and a late ripening program driven by abscisic acid and the associated coordinated expression of aquaporin, invertase, hexose-transporter, pectinase, and anthocyanin-biosynthesis genes. Between them lies a short lag phase that is the viticulturist’s window for thinning, canopy management, and irrigation adjustment. The biophysics of Stage II is a tightly coupled system of water flux across aquaporin-rich tonoplasts, sucrose unloading in apoplastic mode, and osmotic drive by vacuolar hexose accumulation. Stage III adds cell-wall softening and the biosynthesis of volatile aroma that will define the eventual wine style.

The timing of bud-break, flowering, veraison, and maturity is well-predicted by growing-degree-day (GDD) accumulation above a base temperature (commonly 10 °C for V. vinifera). Parker et al. (2011, 2013) parameterised a cultivar-specific phenology model with thermal requirements calibrated across European and New World vineyards:

• Bud-break to flowering: ~250–350 GDD (base 10 °C).
• Flowering to veraison: ~600–800 GDD.
• Veraison to harvest: ~350–500 GDD (target 22–24 °Brix).

Climate warming has advanced European bud-break and flowering dates by 10–20 days since 1990 (Jones 2005, van Leeuwen 2019), with direct consequences for must chemistry: veraison and harvest now frequently occur in the hottest weeks of summer, accelerating malic loss and increasing alcohol.