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Crop-SetTM and ISR 2000TM are products intended to enhance growth, yield responses and disease resistance in a variety of crops, including citrus. Responses such as fruit size and number increases, and internal quality improvement have been obtained by applying the products to a number of cultivars, including Valencias, Navels, mandarins and grapefruit, at various sites in the world. However, from a commercial perspective, considerable confusion exists as to the most desirable timing of applications for intended plant responses. In addition, the appropriate usage of the products may not always be understood. These issues are vital for the successful usage of the products.

An understanding of the growth pattern (phenological cycle) has been useful in a number of tree crops, and most notably in avocado (Whiley et al., 1988) for making management decisions. If the growth pattern of the tree is taken into account, and linked with the known constituents or perceived activity of applied compounds, then the optimal timing for desired effects can be determined.

The objectives of this paper are to outline the potential for applying the products to attain particular desired effects through using a generic phenological cycle for citrus tree growth and fruit development during a cropping season, by understanding the physiological state of the trees and matching the product applications to this. The theoretical timing and application of the products will be confirmed with known effects of Crop-SetTM and ISR2000TM, as demonstrated by experimental evidence, where this is possible.


The phenological cycle

Little has been done to outline the phenological cycle of citrus growth within any major production area. However, sufficient generic information relating to the order of growth for various plant parts is available, and can thus be used. With the exception of the fruit maturity rate, which can vary considerably with cultivar and environmental conditions, the cycles of vegetative growth, root growth and flowering are remarkably similar in most subtropical or Mediterranean areas of the world. There is almost no growth cycling in true tropical areas (Davies and Albrigo, 1994), and thus these regions will not be considered. Therefore, a generic phenological model can be used as a basis for management, suitably adapted in terms of time scale, for any production area. Such a generic phenological cycle, based on northern hemisphere dates, is shown in Figure 1.

A shoot flush is usually initiated in spring, when the temperature rises above 12.5°C (Davies and Albrigo, 1994) or after alleviation of water stress (Mendel, 1969). This is followed by a summer flush approximately three months later. Little substantial vegetative growth occurs during the rest of the year, particularly in subtropical or Mediterranean areas. The spring flush (March – April in the northern hemisphere) is usually far more intense, affecting more growing points than the summer flush (January– February).

Root flushes normally occur during the periods between vegetative growth flushes. Bevington and Castle (1985) found the major root flushes to occur from May through June and August through September (the latter being the major flush) in Florida. Stimulation of root growth is both temperature and water dependent, with a minimum temperature of 7°C considered a threshold, although rootstock may also play a role. Perhaps of more importance, however, is the physiological effect of root activity.

Wilcox and Davies (1981) reported considerable increases in hydraulic conductivity as root temperature increased from 10 to 30°C. Nutrient uptake in particular is affected. Thus in late winter or early spring if root temperature is still low, yellowing (chlorosis) may occur, which will undoubtedly affect photosynthetic capacity of the tree and thus fruit set, growth and retention.

In addition, plant growth regulators are affected, with less movement of cytokinins from the roots to shoots and thus less vegetative bud break. The consequences will be further discussed in relation to flowering. Poor root activity and thus water uptake and movement, may also increase the levels of the plant growth regulator abscisic acid (ABA) with resultant stomatal closure (Zeevaart, 1999), and consequential decreased photosynthetic activity, and altered nitrogen metabolism (King and O’Donoghue, 1995), which may change flowering patterns (Lovatt et al., 1998) through elevated ammonia levels.

Late winter or early spring, just prior to bud break, therefore presents an opportunity to intervene and thus modify the roles of available endogenous tree nutrition factors and plant growth regulators such as cytokinins, thus antagonising the effects of ABA.





Citrus flowering has been extensively reviewed (Davenport, 1990; Krajewski and Rabe, 1995). Flower bud induction commences during the winter rest period due to cold or dry conditions. The degree of induction is proportional to intensity of the inductive conditions (Southwick and Davenport, 1986). During this period, reversal to a vegetative state is possible should gibberellic acid be applied (Davenport, 1990). However, once differentiation occurs, the buds will not revert (Lord and Eckard, 1987). The rate of bud development depends on climate (degree days) and inflorescence type (Lovatt et al., 1984), but it can be assumed that macroscopic differentiation occurs during December – January, and visible buds in February (northern hemisphere). Anthesis then occurs in April.

An important characteristic of flowering shoots in citrus is that the inflorescence can range from floral only, to predominantly leafy, with mainly new flush leaves and few flowers. The more leafy inflorescences result in greater fruit set or retention (Monselise, 1986). This could be due to the carbohydrate contribution of these leaves to early fruit growth, or better vascular development due to plant growth regulators produced by the leaves (Erner and Bravdo, 1983). This in turn could enhance the movement of carbohydrates, water and plant growth regulators, which are of particular importance (Erner et al., 2000) to the fruit sink strength and thus growth.

Stress periods, when gibberellic acid levels are low or when auxins decrease in the fruit as cell division slows (Cleland, 1999) would be of note. Under these conditions, ABA may predominate, resulting in fruit abscission. The degree of leafiness appears related to the stress (water or temperature) intensity of the induction phase (Lovatt et al., 1998). Manipulation of leafiness characteristics is clearly an opportunity for increasing fruit set.

Once anthesis has taken place, fruit growth occurs. Citrus fruit has a sigmoidal growth pattern. The first phase is one of cell division, which takes approximately six weeks, and is characterised by two drop periods, one from flowering for three to four weeks, followed by a second (June drop in northern hemisphere) as cell division begins decreasing. This is probably related to plant growth regulator changes. Gibberellins may play a vital role in fruit set and early growth (Cleland, 1999) and are particularly important in decreasing fruit drop (Talon et al., 2000) while cytokinins promote cell division and with auxins enhance vascular development. This would link with the effects of inflorescence leafiness.

After the cessation of cell division, fruit cell differentiation takes place, followed by a period of cell elongation (phase 2 growth). Application of auxins at the end of the cell division phase enhances pulp growth (Agusti, 2000) resulting in larger fruit. This is due to expansion of the juice vesicles (Agusti et al., 1995), which relies on maintenance of turgor, which in turn requires inflow of sugars to the fruit. The cell expansion phase is indeed the period when loading of sugars occurs, mainly from adjacent leaves (Koch and Avigne, 1984). Agusti (2000) states that application of auxins at the start of this phase enhances sucrose deposition in the fruit, which not only increases sugar content, but also ensures turgor and larger fruit size. The cell enlargement phase varies with cultivar and climate, but is typically three to four months (Davies and Albrigo, 1994). Thereafter fruit maturation occurs.

Fruit maturation is characterised by a decrease in juice acid, small increase in sugars and change in rind colour. The time taken for this process varies considerably with climate and cultivar, with temperature being the predominant factor. Plant growth regulators play a role. Application of gibberellins decreases the rate of chlorophyll loss (Agusti, 2000), while other unpublished results appear to indicate that auxin applications at physiological drop enhance colour development. Auxin applications near harvest can delay fruit senescence and prolong hanging of fruit, as is done in some countries. As rind maturation progresses, so susceptibility to postharvest pathogens increases (Angioni et al., 1998). Application of gibberellins or cytokinins (anti-senescence) may slow this process.


Opportunities for Crop-SetTM usage

Crop-SetTM is a natural product derived from an extract primarily of the yucca plant. As such, natural plant growth regulator activity such as expected from cytokinins, gibberellins and auxins are present. In addition, the product has been fortified with micronutrients.

Considering the growth pattern of citrus as outlined, the first window of opportunity would be after flower induction has already occurred, but before visible buds are present. This could be in approximately December – January in the northern hemisphere. The objective at this stage would be to increase the leafiness of the spring growth flush, and thereby improve the potential for fruit set (retention) and growth. The cytokinin-like activity in the product could enhance the potential for bud break, as well as cell division, while gibberellins would enhance leafiness and decrease flowering (Guardiola et al., 1982).

The nitrogen and micronutrient content of the product could also be beneficial for fruit set, if applied at this stage. Care needs to be taken that application is not too early, as bud reversal from flower to vegetative could occur (Davenport, 1990). The appropriateness of the application will also depend on season and cultivar. The product should be used where it is expected that heavy flowering will take place, so as to enhance the leafiness of the blossom (El-Otmani, 2000) and as a consequence improve fruit set or retention.

The second window of opportunity is logically at flowering. Work done by Medina (2003) on navel oranges in Brazil showed that an application of Crop- SetTM at the end of blossoming (approximately the end of cell division) resulted in larger fruit size without any decrease in fruit number. Navels are not sensitive to gibberellin applications at fruit set (Agusti et al., 1982) although many other cultivars are. Cytokinins applied to fruit shortly after set (Guardiola, 2000) enhanced fruit retention. Thus, an application of Crop-SetTM at 100% petal fall could be advantageous in retaining fruitlets.

For similar reasons, the third opportunity for Crop- SetTM application is at the end of cell division. At this stage, intense competition for available carbohydrates occurs, and application of compounds with cytokinin-like activity may aid carbohydrate flow to the fruits (Kreidemann, 1968). The effects of auxins are clear, as previously discussed, and the role of micronutrients such as zinc and boron should not be ignored. This is also a critical period in terms of adverse effects of environmental stress.

A product such as Crop-SetTM could mitigate the effects of excess ethylene production during stress periods, thus improving the chances of fruit set and growth. The issue of carbohydrate movement toward fruits is also important, considering that the next phase of fruit growth (cell enlargement) coincides with the start of sugar accumulation, which therefore relates to final fruit brix content. Work done by Bower (unpublished data) showed that applications of Crop-SetTM on Valencia oranges at six weeks post-petal fall (end of cell division) or at six weeks plus at 11 weeks postpetal fall, substantially enhanced final fruit number. Application also changed size distribution to a larger size grouping, as indicated by the number and percentage of total fruits in the most economical size range of 40 to 88 (15 kg carton). Results are shown in Table 1.

Similar research was conducted in South Africa on grapefruit, where fruit size is a considerable problem, enhanced by the presence of citrus tristeza. Results are shown in Table 2 for the Star Ruby cultivar, which is particularly susceptible to production of small fruit. Further, similar results from South Africa and Florida have been published by Marais and Frank (2003).

Opportunities for Crop-SetTM usage later in the fruit growth cycle are questionable. A third spray in the South African Valencia trial at 16 weeks post-petal fall had no positive effects, and in fact resulted in a poorer fruit count distribution than the control. The reasons are unclear, but in considering the phenological cycle (Figure 1), application would have coincided with the summer growth flush. The product could, as previously explained, have stimulated this flush (as it would have done to the leafy blossom). If this were correct, then carbohydrates would have been diverted to the leaf flush rather than the fruit, resulting in poorer size and internal quality. Thus, it is considered that a mid-summer application of Crop- SetTM should not be considered.


Opportunities for ISR 2000TM usage

ISR 2000TM is a product containing Crop-SetTM plus a yeast cell wall component containing a mannan oligosaccharide. The latter is believed to create an induced resistance response in plants. Bower (2003) showed that the product, when applied to navel orange trees three weeks prior to harvest, did indeed induce a response that inhibited the growth of the postharvest pathogen Penicillium digitatum. While the inhibition was not sufficient to be commercially acceptable on its own, efficacy was statistically indistinguishable from that of Imazalil fungicide treatment when combined with a postharvest yeast application (Table 3).

Considerable potential exists for the use of the product in eliminating use of traditional fungicides, to which consumers are becoming increasingly averse (Lopez-Garcia et al., 2000), and solves the problem of poor competitive inhibition of the pathogens by most of the biological products presently available for postharvest pathogen control.

In addition, the product may, by the nature of its constituents, have a positive effect on fruit integrity, by slowing the senescence process. Thus longer shelf life, in addition to enhanced resistance to postharvest decay, can be expected. For these purposes, application should be after colour break, and two to three weeks prior to harvest. The product will, however, require further testing on cultivars other than Navels. Depending upon the rate of fruit maturity, caution needs to be exercised should application time coincide with flower bud induction, as the Crop-SetTM component may decrease blossom. This may, on the other hand, be advantageous if stimulation of a leafy blossom is desired. This is indeed probably the case in Navels, due to a naturally high flower number where there may be benefits to decreasing flower number. Should this not be the case in other cultivars, an alternative product, without the component of Crop-SetTM could be used.


Table 1 - Click here to see the image


Table 2 - Click here to see the image





Conclusions

The phenological cycle for citrus can be constructed for any environment, and can therefore be a useful tool for management decisions, which may vary from place to place and indeed from season to season as well as with cultivar and rootstock combination. By following the phenological cycle, growers can target the use of Crop-SetTM and if desired, ISR 2000TM, in a manner which matches their needs to manipulate the tree and fruit growth to advantage. Because tree physiology is correctly matched with the constituents of the products, the desired responses, when required, are likely. Should growers follow this scheme, enhanced fruit size and quality can be achieved, without the potential for failure due to inappropriate product usage, as only the physiologically correct applications will be made.

A summary of potential application windows is shown in Table 4.


Table 4 - Click here to see the image



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