Phenology is the study of events or growth stages of a plant or animal that recur seasonally and their relations with various climatic factors, including temperature, solar radiation and day length (Mullins et al., 1992). With respect to grapevines, it can be used as a predictive tool, allowing the manager to plan vineyard operations in advance. In combination with other plants’ phenological data, it can help to sequence events even though the weather patterns in the current year may be substantially different to those of previous years.
It is useful to track the growth stages of grapevines so that development can be monitored with respect to other seasons (or some other benchmark), and so that pesticide sprays can be applied in a timely fashion. In discussing grapevine phenology, it is useful to use a starting point prior to budbreak.
Before any visible signs of growth, the vine begins to come out of dormancy at about the same time as the soil temperatures starts to rise (due to increased solar radiation and air temperatures). The roots reactivate, and the phloem and xylem tissues start to function again. Visibly, this leads to sap exuding from fresh cuts on the vine, as root pressure builds to fill the empty xylem vessels (see Fig.
3.1; Sperry et al., 1987). The volume of liquid that can exude from the cut end of a cane can be high ⫺Bennett (2002) collected 174 ml in a 24 h period.
Actual breaking (or appearance of green tissue through the bud scales) of the dormant buds has usually been said to occur when the air temperature reaches approximately 10°C (Winkler et al., 1974; Williams et al., 1985), though Moncur et al. (1989) report no physiological basis for this threshold.
Indeed, their experiments on potted cuttings revealed that the base temperature for bud development in grapevines was as low as 0.4°C (depending on cultivar), which more closely matches values for other woody perennials (Anstey, 1966;
Richardsonet al., 1975).
The progression of the budbreak process is illustrated in Plate 7. Buds are at their most hardy prior to budbreak with respect to both physical and cold temperature damage. The first shoots start to grow powered by energy derived from stored carbohydrate, as there is no photosynthesis yet occurring (Winkler et al., 1974; May, 1986). The pre-formed leaves expand, as do the internodes.
Any flower clusters present also develop, rapidly forming individual florets (see Fig. 3.2). Shoots can grow as fast as 2⫺5cm/day (4 cm/day, Creasy, 1996;
1.8 cm/day, Keller et al., 2005; 5 cm/day, Wolf and Warren, 1995), forming a new internode every 2⫺3 days (Lovisolo and Schubert, 2000). Photosynthesis occurs as soon as there is green tissue on the shoots; however, due to the high Fig. 3.1. Part of a vine trunk cut with a saw late in the dormancy period. Xylem sap, flowing from the roots and the cut vessels, oozes out; it contains low concentrations of sugars (Bennett, 2002) ⫺in the range of 5⫺6mg/ml, which supports the proliferation of fungi and bacteria, making the sap here appear white.
metabolic activity and use of stored carbohydrates, there is no net production of photoassimilates until several leaves have fully expanded (Hale and Weaver, 1962; Winkler et al., 1974).
Unlike many other perennial woody crops, grapes flower long after budbreak. As it takes some time for the shoot to develop leaves that are capable of supplying the carbohydrate needs of the rest of the vine, it is critical that enough stored carbohydrate is available to support the development of shoots, roots and flower clusters. If there is not, then it is the flower clusters that suffer the most, as they can drop off the vine due to a lack of available carbohydrate (Ollat, 1992).
When the shoots have approximately 15⫺17 nodes formed on them (Pratt and Coombe, 1978), the flowers begin to open and the calyptra fall from the rest of the flower (see Plate 8). The flowering and fruit set process in grapes is very weather dependent. Pollination (transfer of the pollen from the anther to the stigma) is mostly by wind, though insects may also contribute. Self-pollination, Fig. 3.2. Young shoot showing the first leaves and two flower clusters. Individual florets have already differentiated by this stage.
occurring as the cap comes off the ovary, is the norm (Winkler et al., 1974;
Kimura et al., 1998), though some studies have suggested that cross-pollination results in bigger fruit and higher seed counts (Sampson, et al. 2001; Milne et al., 2003). Airborne pollen counts have been used as a predictor for fruit set in some grape-growing areas (Cristofolini and Gottardini, 2000; Cunha et al., 2003) as there is a good correlation between pollen in the air and fruit set, but this may be due to more pollen being released when the weather is warmer and dry, which is generally conducive to fruit set.
Fertilization occurs 2⫺3 days after pollination, as the pollen tube must grow (a highly temperature-dependent process) down through the style and up into the micropyle in a ‘J’ shape (see Fig. 3.3) in order to reach the embryo.
Even relatively brief spells of cool temperatures cause degeneration of embryos and decrease the chance of fruit set (Ebadi et al., 1996), which goes some way to explaining why this is a problem in cool climate grape-growing areas.
Typical success rates for fruit set may be in the 30⫺45% range. Flower clusters may contain from just a few to more than 1000 flowers (Woodham and Alexander, 1966), depending on conditions at flower cluster initiation and cultivar.
Fig. 3.3. Growth of pollen tubes (lighter paths through the style) from stigmatic surface to the micropyle, shown using a fluorescent dye (image courtesy of M. Longbottom).
Flower cluster initiation occurs about 18 months before the fruit from those flowers is harvested. At about the time of flowering, the flower cluster primordia are being initiated in the compound buds located in the axils of the basal leaf petioles (Winkler et al., 1974). Both temperature and light are important determinants of flower cluster size and number (Buttrose, 1969). As the season progresses, the initiation of flower cluster primordia works its way up the shoot, meaning that along different parts of the shoot (next season’s cane) there can be significantly different levels of fruitfulness (this has implications for methods of pruning, see Chapter 6). Individual florets do not form until after budbreak the following season, but the the amount of branching, and thus cluster size and total floret number, is largely determined in the current season.
Lateral shoots arising from axillary buds may also have fruit on them, which is termed second set. Generally, these are quite far behind in terms of development compared with the primary crop and, as well as being smaller in size than the primary crop, are rarely used in production. However, in tropical climates, there would be the opportunity for this crop to be harvested. In most cases this fruit is a nuisance, as hand-pickers can mistake it for ripe fruit, and machine-harvesters harvest all grapes in the canopy, regardless of maturity.
Once fruit is set on the vine, it is unlikely that the vine will lose it. Apple and other tree fruit crops have one or more times of the year when the crop abscises naturally (Westwood, 1993; Bangerth, 2000; Wertheim, 2000); however, with grape the fruit cannot be dropped, and the vine is pretty much committed to bringing it to maturity. As such, after fruit set it becomes the most important destination for the vine’s carbohydrate supply, as carbon radio-tracer studies have shown (Hale and Weaver, 1962; Quinlan and Weaver, 1970).
Berry expansion is rapid following fertilization, due to both cell division and cell expansion. The growth in terms of weight, diameter, volume or other measurements can be described as a double sigmoid curve (see Fig. 3.4), particularly for seeded berries ⫺for seedless ones the curve is less pronounced (Coombe, 1960; Winkler et al., 1974; Friend, 2005). The first period of rapid growth is often called Phase I, which is followed by Phase II, a time of relatively little growth as the seed matures and begins to lignify (develop a hard outer coating). At the end of this period the berry undergoes an amazing trans- formation, softening, becoming translucent, beginning to colour (if a red cultivar) and increasing in size rapidly (Phase III). On the inside, the berry beings to metabolize malic acid, accumulate sugar and develop characteristic flavour and aroma compounds. The French use the term véraisonto describe the shift in colour, which signals that all these other changes are taking place, too. It is not known exactly what triggers or even regulates this massive and rapid change in berry physiology, but the term engustation (Coombe and McCarthy, 1997) has been proposed to describe the great upheaval of events.