FLOWER INITIATION, FRUIT DEVELOPMENT AND
potential crop. However, more branching can occur around the time of budbreak after the dormant season, or whenever environmental conditions allow (May, 2000).
Factors affecting branching
Because buds in the varying node positions go through the process of flower cluster initiation at different times, changing environmental factors that affect this process will also affect the potential crop for each bud in the following season.
Fig. 3.5. A bifuricated shoot (centre), where a new shoot tip arises from a node position opposite a leaf.
Temperature is one such factor, as it affects all biological processes. Cool and excessively hot temperatures discourage the initiation process, though absolute values for minimum temperatures necessary for any initiation to occur vary from cultivar to cultivar (Buttrose, 1969). The optimum for maximizing potential crop is thought to be in the range of 30⫺35°C, but this also appears to vary with cultivar (Buttrose, 1969). In many cool climates, achieving this range of temperature during flowering, when flower cluster initiation is going on, is relatively rare, hence the chance for high crop loads is already limited by this factor.
Light is also important, as it affects the rate at which photosynthesis can occur and hence photoassimilate supply. The carbohydrate supply near bloom is an important factor affecting the number and potential size of the flower clusters being initiated (May, 1965; Sommer et al., 2000), but Bennett et al.
(2005) have also shown that vine carbohydrate status following fruit set will also affect flower cluster number and size in the following season. Hence, overly vigorous shoots are associated with fewer flower clusters because vigorous vines generally have shadier canopies, and also the growing points of a vigorous shoot are much better at drawing carbohydrates away from the developing flower clusters.
Plant growth regulators
As with many aspects of plant development, plant growth regulators play an important role in flower cluster initiation. The two most involved are gibberellic acid (GA) and cytokinins (CKs).
GA enhances the development of an uncommitted primordium that forms to the side of the shoot apex, encouraging the development of tendrils (Srinivasan and Mullins, 1980) but inhibiting the formation of inflorescences (Srinivasan and Mullins, 1980; Palma and Jackson, 1989). CKs will encourage the formation of flower clusters from the tendrils (Srinivasan and Mullins, 1980), so the net effect of applying GA to a grapevine shoot during the flower cluster initiation period will be to promote the production of tendrils rather than flower clusters.
The net effect of applying CKs to the shoot would be more flower clusters.
The role that each of these plant growth regulators plays has been elucidated in part through the use of chlormequat, which has been used as a herbicide. This chemical blocks the synthesis of GA (primarily, though it may have some effect on CK production as well) (Skene, 1968; Lang, 1970).
Cytokines are also important later on in flower development, as their presence promotes the formation of flower primordia on the inflorescence near budbreak in the following spring, and can improve fruit set during flowering as well (for a review, see Srinivasan and Mullins, 1981). Analysis of the bleeding sap from cuts on vines before budbreak show that CKs are being transported from the roots to the upper parts of the vine at this time of year (Skene and
Kerridge, 1967), which demonstrates the potential importance of soil tempera- ture early in the season, as warmer soils will have more root growth and potentially more CK production. Work by Woodham and Alexander (1966) showed that, for each of two 10°C increments (from approximately 10°C), if the root temperature was higher than that of the shoot, there was an approximate increase of 18% in flower number per inflorescence.
Flower development and anthesis
Differentiation of the flowers on the inflorescences starts near the time of budburst (Srinivasan and Mullins, 1981) as soil and air temperatures begin to rise, which also means that the full cropping potential of the vine has been realized.
The pollen sacs in the anthers mature shortly before capfall (Staudt, 1999), as does the receptivity of the style itself. Thus it is thought that the mechanical disruption caused by the movement of the cap off the ovary and style, with probable transfer of pollen from the anthers to the style (a process called pollination) as well as other factors, means that grapes are mostly self- pollinated (Lavee and Nir, 1986).
Capfall has been associated with a certain number of nodes on the flower cluster’s shoot, though the number seems to vary between cultivars. However, it does not seem to vary much within a cultivar or between seasons (Pratt and Coombe, 1978), which means that counting the number of nodes on shoots can be a way of predicting flowering date, as long as the number of days it takes to form a node is known. Some research suggests this is on the order of 3 days per node (Lovisolo and Schubert, 2000), though this rate depends on environmental conditions.
The duration of flowering is also highly dependent on the environment at the time. Cool, overcast weather, associated with rainfall, lengthens the flowering period, whereas warm and sunny conditions hasten it. Thus, flowering can occur over a period of a few days to longer than a month. This is thought to contribute to variation found in the fruit through the rest of the season, although there is also considerable variation found within the flower cluster itself (May, 1986; Friend et al., 2003).
Capfall occurs mainly in the morning hours (Staudt, 1999), with the highest rate occurring between 7.00 and 9.00, and the final ones falling by 12.00. This is thought to be a result of changes in turgor pressure within the cells in the calyptra’s abscission zone (Swanepoel and Archer, 1988).
Fruit set
Once viable pollen lands on a receptive stigmatic surface (see Plate 13), a germ tube emerges from the pollen grain within a short period of time, as little as
30 min (Staudt, 1982). It then begins to grow down through the style to the micropyle, following a ‘J’-shaped path, and on to the nucellus (see Fig. 3.3).
There is some thought that the size of the flower, and thus the length the pollen tube must grow, is a factor in the success of fruit set. This is because the speed at which the pollen tube grows is very much dependent on the temperature, with colder temperatures resulting in drastically lower rates of elongation (Staudt, 1982). If the growth is slowed to the point where the ovules degenerate before the pollen tube reaches them, fertilization cannot take place.
Plant growth regulators are also involved in this phase of reproductive growth, as auxins are released from the pollen tube (Taylor and Hepler, 1997) as it grows, which stimulates growth of the ovarian tissues.
If the pollen tube does reach past the micropyle, the generative nucleus in the pollen unites with the synergids in the ovarian embryo sac, causing a rapid increase in metabolic activity (Kassemeyer and Staudt, 1981) that results in further development of the now fertilized embryo. The ovule becomes the seed, and the ovule wall enlarges to become the berry pericarp, or flesh.
Commonly, one or two seeds are found per berry in V. viniferafruit, usually more than two on average for V. labrusca(Ebadiet al., 1995), though there is considerable variation between cultivars (Forsline et al., 1983). Berry size seems to be driven initially by the presence or absence of seeds, and then by the mass of seed within the berry rather than just the number of seed (Scienza et al., 1978; Ebadi et al., 1996; Trought and Tannock, 1996; Roby and Matthews, 2004; Friend, 2005). In terms of vine yield, it is most important to have at least one viable seed.
Seedlessness
Evolutionarily speaking, the seeds within the fruit are the most important part of the vine: the vine produces seed to propagate and spread the offspring. So how have seedless berries come about? After all, if no seeds are produced, then how can the vine propagate itself ? As with the appearance of clones, random mutations can cause a vine to arise that has seedless berries. In addition, some crosses of plants can result in sterile offspring, so a seeded cultivar pollinating another seeded cultivar may result in a seedless offspring.
In most cases seedless cultivars are not, in fact, really seedless. Many of the popular seedless table grapes are stenospermocarpic. In this case, the flowers are pollinated and the embryos fertilized, but soon after the embryo aborts;
however, in the time that the fertilized embryo is developing, it produces enough plant growth regulators to encourage growth of a large berry. If a seedless grape is cut apart, often a small remnant is visible inside, which is the aborted seed (see Fig. 3.6). In some cases there may be something that looks like a viable seed, but is in fact the lignified shell of a seed with no embryo within. ‘Einset Seedless’ (developed at the NYSAES, Geneva) is one cultivar that is prone to having these crunchy seed remnants.
Parthenocarpy, a process that requires pollination but not fertilization to set fruit, does occur with cultivars like ‘Black Corinth’ (syn. ‘Zante Current’,
Plate 14). These vines have very small berries, from a lack of cell division, in comparison with seeded ones (Coombe, 1973), as without the seed present, division and growth of the cells of the ovarian wall are not encouraged (Olmo, 1946).
Factors influencing fruit set
There are many factors that will affect the percentage fruit set in grapevine.
Among the most important are the availability of light, moderate temperatures and dry weather. The effects of temperature on pollen tube growth and root- produced plant growth regulators have already been mentioned. However, light is at the core of many vine functions as it is the source of carbohydrate energy for the plant. If conditions are overcast, the vine will not be able to produce enough photosynthates to feed the growth of shoots and all other sinks, and so flower clusters, being a weak sink, will not attract a great deal of them. This correlates with how the grapevine grows in the wild, as the vine is vegetative until it has grown up the tree trunks and emerged into the higher light levels in the canopy above.
As the vine is a perennial plant, the amount of carbohydrate stored is also a factor ⫺ if the vine has sufficient reserves in its root and other permanent wood, then it is better able to supply energy to the growing parts of when Fig. 3.6. Seed remnants within a seedless table grape (‘Perlette’).
photosynthesis falters. This is used as evidence to support the notion that older vines (having more permanent wood than younger vines) perform better (Howell, 2001). In addition, since the early-season shoot growth is highly dependent on stored reserves, if reserves are not sufficient then shoot growth will suffer and so, too, will the leaf area available to manufacture photoassimilates.
Recent research has demonstrated a positive link between vine carbohydrate levels pre-budburst and percentage fruit set in a cool-climate growing region (Bennettet al., 2005). This has led to the suspicion that stored carbohydrates are more important than previously thought when it comes to determining how many fruit will set at bloom.
Water must be available to the vine near flowering to allow photosynthesis to proceed at the optimal rate, as water stress has been found to be highly detrimental to fruit set (Hardie and Considine, 1976; Jackson, 1991). However, as flowering occurs early in the season, water is usually available in the soil profile, and stress must be actively avoided in only the driest of regions, or in those areas with soils of very low water-holding capacity.
One physiological disorder that is probably connected to a lack of available carbohydrate is early bunchstem necrosis (syn. inflorescence necrosis). This fruit set problem results in the loss of individual flowers, branches of flower clusters or even entire flower clusters in the weeks leading up to and including the fruit set period (Jackson and Coombe, 1988; Ibacache, 1990; Plate 15). It is thought that this problem is a result of a build-up of toxic ammonium in the flower cluster tissues because of an inability of those tissues to convert ammonium to the amino acid glutamate ⫺ a process that requires carbo- hydrates (Jordan et al., 1993). Its appearance is sporadic and difficult to predict, but can have a significant effect on yields.
Deficiencies in certain nutrients can also reduce fruit set, chief among them being zinc and boron. Zinc is necessary for activation of many enzymes, as well as for the production of the plant growth regulator indole acetic acid (Marschner, 1986), and boron is necessary for the formation of cell walls (Thellieret al., 1979), which is involved in all aspects of plant growth.
In fact, any significant stress the vine is under, be it abiotic (e.g. nutrient or water) or biotic (disease or insect pest damage), can reduce the potential of the vine to set fruit (Creasy, 1991).
Berry development and maturation
Once the embryo is fertilized in grapes, the vine is committed to maturing the fruit, unlike some other fruit crops where there is fruit abscission and thus a chance to reduce what might otherwise be too high a crop. Fertilization results in immediate and rapid cell division (from 1 to 2.5 per cell on average) to move from approximately 200,000 cells at anthesis to a maximum of 600,000 by véraison (Harris et al., 1968). However, the highest activity in terms of cell
division occurs before flowering, where about 17 cell divisions are responsible for attaining that figure of 200,000 in the first place.
In Phase 1 of growth there is no significant accumulation of sugars in the berry, as much of the photoassimilate is used for cell division and expansion.
However, there is an accumulation of organic acids, primarily malic and tartaric (Hrazdina et al., 1984), the latter of which is relatively unusual in fruit crops (McGovern et al., 2004). The duration of Phase I seems to be similar for most grape cultivars (Nakagawa and Nanjo, 1965; Coombe, 1976), while that for Stage II can vary considerably depending on cultivar, management and environment (Winkler and Williams, 1935; Coombe, 1960). Many characteristics in the grape change in the period leading up to maturity.
Grape composition
The primary component of mature grapes is water, making up about 75⫺85%
of their weight (see Table 3.1). Approximately 15⫺25% is in the form of sugar, a higher percentage than in many other fresh fruits. The organic acids tartaric, malic and citric make up 0.5⫺1.0% of the fruit, pectin about 0.25% and there is a long list of other nutritional components. If all of these are totalled they amount to over 99% of the weight of grapes but, if they were the only contributors, we would not be able tell grapes apart from many other fruits.
Fleshy fruits like grapes are made up of living cells and therefore have all the ‘primary metabolites’ of all plants. Primary metabolites are the proteins, amino acids, nucleic acids, fatty acids, cellulose and others found in all plant cells that facilitate respiration, photosynthesis and, to all intents and purposes, life. These primary metabolites make up a higher percentage of the total mass in leaves, buds, flowers and other non-reproductive parts compared with fruit.
Most of the attributes that make grapes desirable to us (and to certain pest species!) accumulate or develop in Phase III of berry development. One of the most important of these is sugar, in the forms of glucose and fructose.
The currency of the phloem, which is the network of conduits that moves photoassimilates from one place to another in the vine, is sucrose, which is not the same as that for photosynthesis or for ripening grapes. Photosynthesis creates glucose, which is freely inter-converted with fructose. One each of these molecules is combined into sucrose prior to their journey through the phloem and then on to the sink (growing point, fruit, etc.), where it’s taken out of the phloem and converted back into glucose and fructose. Therefore, very little sucrose (which is the form of sugar we’re most familiar with, as it is what makes up the granulated sugar we put in much of our food) is found in ripe grapes (0.15% sucrose compared with 7 and 8% by weight glucose and fructose, respectively (see Table 3.1).
Grapes are high in carbohydrates and not a particularly good source of dietary fibre. However, they are a useful source of many minerals and vitamins B6, C, E and K. They are also a source of antioxidant compounds through the phenolics in their skins and possibly seeds (Yilmaz and Toledo, 2004).
Table 3.1. Raw grape composition, comparing V. labruscawithV. vinifera(from US Department of Agriculture, Agricultural Research Service, 2005; USDA Nutrient Database for Standard Reference, Release 18, http://www.ars.usda.gov/ba/bhnrc/ndl (used with permission)).
Composition/100 g
Nutrient Units V. labrusca V. vinifera
Water g 81.3 80.5
Energy kcal 67.0 69.0
Protein g 0.63 0.72
Total lipid (fat) g 0.35 0.16
Ash g 0.57 0.48
Fibre (total dietary) g 0.9 0.9
Carbohydrate (by difference) g 17.1 18.1
Sugars (total) g 16.2 15.5
Minerals
Calcium (Ca) mg 14.0 10.0
Iron (Fe) mg 0.29 0.36
Magnesium (Mg) mg 5.0 7.0
Phosphorus (P) mg 10.0 20.0
Potassium (K) mg 191 191
Sodium (Na) mg 2.0 2.0
Zinc (Zn) mg 0.04 0.07
Copper (Cu) mg 0.04 0.13
Manganese (Mn) mg 0.72 0.71
Vitamins
Vitamin C (total ascorbic acid) mg 4.0 10.8
Thiamin mg 0.09 0.07
Riboflavin mg 0.06 0.07
Niacin mg 0.30 0.19
Pantothenic acid mg 0.02 0.05
Vitamin B6 mg 0.11 0.09
Folate (total) mcg 4.0 2.0
Vitamin A IU 100.0 66.0
Vitamin E (alpha-tocopherol) mg 0.19 0.19
Vitamin K (phylloquinone) mcg 14.6 14.6
Others
Amino acids g 0.50 0.57
Carotenoid pigments mcg 131 111
What is fruit maturity?
Fruit maturity and ripeness are often-used terms in the grape industry, but it is difficult making the linkage from these to the quality of the grapes. This is because quality is, by its very nature, a qualitative term, not a quantitative one
⫺there are very few absolute measures of quality, as one person’s quality may differ from another’s. Today, industries talk about suitability for end use to try to get around this elusive concept of quality.
There are several measurable parameters in grapes that relate in some way to quality factors. One of these is some measure of sugar concentration, which usually is accomplished by estimating the amount of dissolved compounds in the juice. As the vast majority of dissolved compounds are sugars (glucose, fructose and sucrose), this ends up being a pretty accurate representation.
There are several different and commonly used measures. Baumé is a con- venient term for winemakers as the number corresponds to the approximate alcohol percentage once the grapes have been fermented to dryness. The °Brix value corresponds to the approximate percentage sugar in the solution and is an adaptation of the percentage soluble solids, which is used in many industries to measure sugar in liquids. Specific gravity is an actual measure of the density of a solution, commonly determined through the use of a hydrometer. Its value is expressed with respect to the density of water, as a solution with dissolved sugar will be heavier than water. Oechsle is a derivative of specific gravity where the value of one is subtracted from the value for specific gravity and the result multiplied by 1000. Table 3.2 shows the relationship between these different measures. Note that the specific gravity of a liquid changes with its temperature, so this is a factor that must be held constant or corrected for when comparing units.
Because many parameters associated with quality are linked with changes to sugar in the berry, °Brix (or other measures related to sugar concentration) is a commonly used measure linked to meeting minimum requirements for harvest. For example, ‘Concord’ grapes harvested for juice production may have to reach a minimum of 15 °Brix to be accepted by the processing plant.
Bonuses can be associated with exceeding this target, or there may be deductions from a base pay rate if the crop comes in below the target.
As the degree of sweetness is affected by the amount of acid mixed in with the sugar, and vice versa (McBride and Johnson, 1987), the amount of acid in grapes is also often measured. This is most often performed by titrating a sample
Table 3.2. Equivalent measures of sugar concentration for four scales used in the grape industry. Note that the relationships between the measurements are altered with changes in temperature of the liquid.
Baumé °Brix Specific gravity Oechsle
12.5 22.5 1.094 94