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Hormonal control of grape berry growth and ripening
Regulation of seed and pericarp growth responses are believed to be hormonal (Coombe 1972). Hormones are chemical regulators that act as messengers: their function is to communicate between plant parts and to integrate the responses of one part of the plant with another. The precise mode of action of hormones in fruit is unknown, but they function by stimulating metabolism, cell division, cell enlargement and cell maturation (Davies and Böttcher 2009). Five major groups of phytohormones have been found in tissues of Vitis species: auxins, gibberellins, cytokinins, abscisins and ethylene (Coombe and Hale 1973, Coombe 1976). These endogenous hormones are involved in all aspects of reproductive and vegetative growth of the grape vine. They reach a maximum concentration just before veraison and then decrease sharply along ripening (Coombe 1992).
The growth hormones auxin, cytokinin, and gibberellin produced by the embryos and released into the pericarp reach high concentrations early on and then decrease during the first growth period (Scienza et al. 1978). In addition, import of the germination-inhibiting hormone ABA prevents seed abortion and promotes normal embryo development (Nambara and Marion-Poll 2005). Consequently, the ABA concentration in the berry is high during early development but declines as the berry expands (Davies and Böttcher 2009).
During the lag phase, although the pericarp grows only insignificantly, the concentration of the growth hormone auxin peaks briefly and then declines sharply (Nitsch et al. 1960). The influx of ABA and its production in the berry itself increase toward the end of this phase, suppressing further embryo growth by blocking gibberellin production in the seeds (Pérez et al. 2000). ABA also seems to induce changes in the expression of many genes that ultimately bring about fruit ripening (Gambetta et al. 2010).
Unlike in climacteric fruits, the hormone ethylene does not play a prominent role in grape ripening (Coombe and Hale 1973). ABA acts in a positive feedback loop with sugars, whereby sugars stimulate ABA production and ABA promotes sugar accumulation (Castellarin et al 2007a, 2007b). The concentration of ABA increases rapidly after veraison, peaks during seed maturation, then decreases and is relatively low in mature seeds and berries (Scienza et al. 1978).
Chemical composition of the ripe grape berry
The grape berries physical and chemical composition at harvest is responsible for the fruit quality characteristics and, consequently, the quality attributes of the wine or grape juice produced from the fruit (Keller 2015). As berries ripen, they undergo a multitude of physical and chemical changes, which, in turn, are coordinated by the interplay and cooperation of several genes (Keller 2015 and the references therein). The beginning of grape ripening is recognized by the change of skin color and the sudden softening of the berry. The change in color occurs due to the degradation of the green chlorophylls (Hardie et al. 1996) and due to the simultaneous accumulation of anthocyanin pigments in dark-skinned cultivars (Keller 2015). Softening, which coincides with the beginning of sugar accumulation (Coombe 1992), occurs due to the gradual disassembly of the mesocarp cell walls and the decline in mesocarp cell turgor during the pre-veraison lag phase of berry growth (Thomas et al. 2008). Cell wall loosening in the pulp seems to be responsible for berry softening, and soon afterward cell wall loosening in both the pulp and the skin enables berry expansion (Keller 2015 from Huang and Huang 2001, Huang et al. 2005a). This occurs despite the decline in mesocarp turgor pressure just before veraison (Thomas et al. 2006) due to the accumulation of sugars and other solutes (Keller et al. 2006). Although the berry volume increases during ripening, it cannot do so indefinitely because the skin sets a limit to mesocarp expansion (Matthews et al. 1987). Following a temporary increase at veraison, the extensibility of the skin later decreases. The phenolics may be deposited in the skin cell where they are bounded to cell wall polysaccharides and proteins. This fact stiffens the cell walls and, consequently, limits cell expansion (Keller 2015). As the grape berry is a fleshy fruit, most of the chemicals in grapes that contribute to wine are in water solution and water is the berry’s principle compounds (Ollat et al. 2002, Coombe and Iland 2004). Of the non-water components the largest proportion is made up of the solutes glucose and fructose. The important chemical components of the grape berry are stated by Hulme (1970, 1971) to be sugars, acids, phenolics and flavor compounds. A schematic overview of changes in relative concentration of principal berry metabolites from flowering to senescence is displayed in figure 3.
Origin of variability of grape berry mass and relationship with grape composition
Final berry mass is determined by the number of cell divisions that occur before and after bloom, by the cell expansion degree after bloom and by the possible variation of berry mass by dehydration or hydration just before the harvest (Keller 2015 and the references therein). These phenomena are affected by biotic and abiotic factors (Fernandez et al. 2006, Houel et al. 2011).
Variety is certainly one of the major factors determining the difference in size of the berry as a result of specific genetic characteristics associated with growth and the relative proportion of the components (flesh, seeds and skin) and their relationships (Matthews and Nuzzo 2007, Attia et al. 2010, Barbagallo et al. 2011, Dai et al. 2011). Genetic determines composition and has an influence on the ability of the variety to accumulate compounds, on the way in which photosynthetic products are distributed within the plant and on their influence on secondary metabolism (Dai et al. 2011, Ferrer et al. 2014). Many other factors impact on berry mass. Some of them are intrinsic to the berry itself. This is the case of seed number per berry (Scienza et al. 1978) and seed mass per berry (Roby and Matthews 2004). Carbon balance of the vine can also impact berry mass (Coombe 1962). Finally, environmental conditions are directly involved in determining the size and the composition of the berries. Among this external factors, vine water status (Matthews and Anderson 1988, van Leeuwen and Seguin 1994, Ojeda et al. 2001, Roby et al. 2004, Chaves et al. 2007, Ferrer et al. 2008, Girona et al. 2009) and vine nitrogen status (Choné et al. 2001a, Trégoat et al. 2002, van Leeuwen et al. 2007) play certainly a key role, affecting berry growth and development and, directly or indirectly, the grape composition (Roby et al. 2004, Roby and Matthews 2004, Walker et al 2005). The impact of factors depends on their intensity and/or the development period at which they act (Ojeda et al. 1999). Hence, because berry mass is the result of the combined effect of all these impacting factors, the final berry mass is a parameter highly variable at all scales (Gray 2002, Dai et al. 2011). This variation is greatest early in the berry developmental cycle and declines as berries resynchronize their growth during the second period of growth (Gray 2002, Pagay and Cheng 2010). This means that the major source of variation are early events (Coombe 1976).
Even when all vineyard management practices are uniform and properly executed, it still extremely difficult to obtain uniform berry diameter and composition under field conditions (Pisciotta et al. 2013). This variability, in fact, may result from parcel heterogeneity, such as soil characteristics, graft combination, plant material quality, node number per shoot, shoot number per cane, bunch number per plant, bunch position, etc. (Di Lorenzo et al. 2007, Hunter et al. 2010, Pisciotta et al. 2013). As a result, variability of berry mass and composition can be observed at different scales: (i) between vines within the vineyards, (ii) between bunches within the vine and (iii) between berries within the bunch (Coombe and Iland 2004, Pagay and Cheng 2010, Dai et al. 2011, Pisciotta et al. 2013). Depending on the considered scale, the hierarchy of these factors may vary. At the parcel scale, grapevine cultivar is likely to be the dominant factor. At intra-parcel scale, the variability may be related to variations in soil characteristics. Finally, at bunch scale, the differences between berries could be related to internal factors (Scienza et al. 1978, Carwthon and Morris 1982, Walker et al. 2005, Roby and Matthews 2004).
Berry seed content
The maximal seed number per berry is four, although the average seed number is less than two (Ollat et al. 2002). Fruit and seed growth and development are two highly dependent phenomena (Coombe and McCarthy 2000). This relationship has been the object of several earlier studies. After bloom, cell division rate and cell expansion degree depend on the number of fertile seeds per berry. Their effect on berry development is primarily related to the growing substances that they issue during the first phase of berry growth (Coombe 1972). These substances are hormones, such as auxins, gibberellins, cytokinins and abscisic acid, which stimulate the division and the expansion of cells (Scienza et al. 1978, Lavee and Nir 1986, Coombe 1992). According to Ojeda et al. (1999) it is more likely that seeds stimulate the cell division, rather than cell expansion. Hence, final berry mass and volume are proportional not only to seed number but also to their fertility (Scienza et al. 1978, Roby and Matthews 2004, Walker et al. 2005, Friend et al 2009).
May (2000) showed that the stimulant effect of an individual seed on pericarp development decreases as the number of seed per berry increases. As a result, the relationship between berry volume and berry seed number is not linear but quadratic.
Conversely, the influence of berry seed content on processes linked to the berry maturation, such as accumulation of primary and secondary metabolites, is not so clear. In grape berry, seeds complete their development and maturity at the veraison (Ristic and Iland 2005). At this stage, it is possible to observe a change in hormone levels, especially of auxins and abscisic acid (Davies and Böttcher 2009). At veraison, a phenological variability between berries belonging to the same bunch can be observed. This means that each berry is independent to another and not all berries enter into to the second phase of growth simultaneously. This result could be related to the fact that flowering and/or fertilization of berry are not absolutely synchronized (Friend et al. 2009). Gouthu and Deluc (2015) showed, in fact, that a higher seed number delay veraison and, consequently, maturity. As a result also the final concentration of sugar in berry is reduced. This could be related to the higher concentration of auxins observed in berries containing a higher number of seed, which limit the sugar accumulation (Sundberg et al. 2009). Conversely, higher level of ABA, stimulating the sugar transport into the berry (Castellarin et al. 2007a), were found into the berries containing a lower seed number. However, different observations were made by Carwthon and Morris (1982), who found a lack relationship between berry seed number and auxin and abscisic acid concentration.
Berry size and grape and wine composition
Berry size is a major quality factor in wine production. One of the most widely accepted ideas in winemaking is that large diameter fruit would have a greater solvent to solute ratio as a result of the lower surface to volume ratio compared to smaller fruit (Singleton 1972, Matthews and Anderson 1988, Kennedy 2002). The underlying idea is that the higher concentration of important compounds localized in the skin is favorable for the quality of red wines (Matthews and Anderson 1988). This principle is based primarily on the geometric assumption that grape berry is a sphere.
Hence, according to surface-volume relationship of a sphere, as the berry radius increases, the skin to flesh ratio decreases. (Roby and Matthews 2004, Roby et al. 2004, Walker et al. 2005, Matthews and Nuzzo 2007). Consequently, the concentration of skin solutes would be increasingly diluted with increasing berry size (Matthews and Nuzzo 2007).
In fact, implicit in this believe is the assumption that the proportion of berry tissues remain constant while berry size changes. However, several studies have found that the proportion of skin and flesh did not vary according to the relationship between the surface and volume of a sphere. (Roby and Matthews 2004, Walker et al. 2005, Barbagallo et al. 2011). In other words, the skin does not stretch around a larger flesh but grows with it. Hence, although the relative amount of berry tissues can vary depending on variety and environment (Keller 2015), among berries growing under similar environmental conditions, berry skin and seed tissue development are coordinated with flesh growth (Barbagallo et al. 2011) and, as a result, there may be little variation in the skin to flesh ratio (Matthews 2015). However, some environmental conditions, such as water deficit, alter that general relationship. Roby and Matthews 2004 reported that water deficits inhibit more flesh growth than skin growth, increasing, as a result, the skin-to flesh ratio. This effect can result in higher concentration of skin solutes.
Most grape growers agree also that there is a fixed amount of primary and secondary metabolites in each berry and that the variable in berry size is the water. Consequently, this fixed amount of skin solutes is increasingly diluted by the flesh of ever bigger berries. Effectively, if these assumptions are true, smaller berries will have higher concentrations of solutes. However, Coombe (1987) and Roby and Matthews (2004) showed that the amount of solutes per berry increases linearly with berry size.
The relationship between berry size and grape composition is complex and still far from being fully understood. Actually, there are contrasting conclusions among researchers regarding differences in berry composition when comparing berries strictly on size.
Some authors did not find a significant difference between the composition of small and large berries (Barbagallo et al. 2011). Other studies reported that the sugar concentration is higher in smaller berries (Scienza et al. 1978, Carwthon and Morris 1982). In contrast, Glynn (2003), measuring the sugar content of Cabernet-Sauvignon and Chardonnay berry by berry, did not find a relationship between °Brix and berry size. Similar results were obtained by Walker et al. (2005) on Shiraz berries. Roby et al. 2004 reported that berry sugar content (g/berry) depended on berry mass, while berry sugar concentration (g/L) did not change with berry mass. Similar relationships were observed by Roby and Matthews (2004), when studying the effect of berry mass on anthocyanins. Anthocyanins per berry were proportional to the size of the berry, while the concentration of anthocyanins decreased with increasing berry size, indicating the possibility of producing different wine styles from berries with different sizes. These results were not confirmed by Ferrer et al. (2014) who reported that total anthocyanin content or concentration was independent of berry size.
Similarly to grape berry composition, there is no consensus among researchers on whether smaller berries make superior wines. Gil et al. (2015) demonstrated that smaller grapes produced wines of deeper colour and that size is inversely correlated with the concentration of phenolics, such as anthocyanins and stilbenes. In contrast, comparing wines from « small » and « large » berries, Walker et al. (2005) came to the conclusion that smaller berries do not produced superior wines. Other researchers found that there is no simple linear relationship between grape composition and wine quality (Johnstone et al. 1995).
The absence of a consensus among researcher could be due to the fact that the final berry composition (physical and chemical) is a result of interactions among factors impacting its growth and development.
Berry composition is dependent on physiological processes other than growth. The way in which berry mass is reduced seems to be more important than the berry mass itself: hence, wine improvement is not due just to berry size, but to changes in vine metabolism provoked primarily by factors like cultural practices or annual weather conditions, which may also impact berry mass (Matthews and Anderson 1988, Roby et al. 2004, Walker et al. 2005, Holt et al. 2008). However, the interaction among factors, under field condition, make difficult the study of berry size impact on grape composition.
Table of contents :
Chapter I: General introduction
Berry morphology and anatomy
From flower to berry
Grape berry growth
Hormonal control of grape berry growth and ripening
Chemical composition of the ripe grape berry
Origin of variability of grape berry mass and relationship with grape composition
Berry size and grape and wine composition
Objectives
Chapter II: Hierarchy of factors impacting grape berry mass. Separation of direct and indirect effects on major berry metabolites
Chapter III: Hierarchy of factors impacting grape berry mass and its consequences on grape composition at intra-bunch and intra-plant scale
Chapter IV: The impact of berry mass on wine quality
Chapter V: Effect of water deficit on berry mass and skin to flesh ratio
Chapter VI: General discussion and conclusion
Literature cited .
