Fruit and shoot growth in avocado

BACKGROUND

The avocado exhibits rhythmic growth, with two or more sproutings per year, alternating with short rest periods. Growth flows can be vegetative or reproductive. Vegetative sprouting allows the tree to increase or renew its growth. photosynthetic capacity and also generate new buds which subsequently give rise to reproductive flows. Reproductive flows are those shoots or growths that give rise to flowering; they can be indeterminate or determinate (Alcaraz et al., 2013). Indeterminate shoots consist of the inflorescence and its corresponding leaves, while determinate shoots lack leaves.

The avocado grows mainly in three climatic zones: cold, semi-arid climates, with rains predominantly in winter (California, Chile, Israel); humid subtropical climates with predominant rainfall in summer (eastern Australia, Mexico and South Africa), and tropical and semitropical climates with rains predominantly in summer (Brazil, Florida, Indonesia). It is also divided into three ecological races: Mexican, Guatemalan and AntilleanCultivars within each race generally exhibit similar responses to climatic and soil conditions. However, there are differences between races and cultivars in their adaptability to environmental conditions (Whiley and Shaffer, 1994), as is the case with cv. 'Hass,' a hybrid between the Mexican and Guatemalan races, which exhibits intermediate characteristics.

Broadly speaking, the ideal temperature conditions for this species they are around 25-30ºC for daytime temperatures, and between 15 and 20ºC for nighttime temperaturesTemperatures above 36ºC cause serious damage, particularly to fertilization and fruit set, and it is important that a cold period (around 10ºC) occurs in winter to stimulate floral induction (Galán, 1990).

Photosynthetic activity is an indicator of crop growth and productivity. Growth and production depend heavily on carbohydrate partitioning. Increasing production in subtropical species with terminal polyaxial fruiting, such as avocado, poses a challenge for agronomic management, since The tree has a natural tendency towards vegetative growth, which results in a greater allocation of dry matter to it, to the detriment of the development of reproductive organs. (Wolstenholme, 1990). Environmental factors such as light, temperature and CO concentration₂, affect photosynthesis, respiration and carbohydrate distribution.

The partitioning of photoassimilates is regulated by source-sink interactions. Sources are net exporters and sinks are net importers of photoassimilates (Ho, 1988). The priority order of demand depends on the growth rate (sink activity) and the size of the sink (number of fruits). The order is generally: seed > fruit pulp = shoot and leaf tips > cambium > roots > storage tissues (Wolstenholme, 1990). Young leaves, while they are expanding, are strong sinks that compete with other demanding organs of the plant until they reach their final size, at which point they become net exporters. (Ho, 1988).

The availability of incident light is probably the factor that exerts the greatest influence on photosynthesis. In an orchard. In avocado, towards the end of spring shoot growth, light transmission to the fruiting zone is reduced to 40% of that under full illumination, and at distances of 0,5 and 1,0 m within the canopy from the fruiting zone, it is reduced to 14% and 10%, respectively. Towards the end of summer shoot growth, light transmission to the fruiting zone under full illumination has decreased to 13%, and at internal points (0,5 and 1,0 m) to 9,7% and 6,3%, respectively (Whiley et al., 1992).

The intensity and duration of lighting are determining factors of flowering (Coutanceau, 1964), and it is widely known that Flowering and fruiting are less abundant in the shade than in full light. (Meyer, 1960). When the lighting is low, regarding their requirements, the vegetative growth is reduced, both in number and length of shoots, as well as in the size of the leaves, resulting in a less tree development and lower photosynthetic activity. Thus, numerous branches develop inside the tree, and the density of external twigs reduces illumination, and therefore, flowering within the tree; only the outer part of the canopy, with adequate illumination, produces satisfactory flowering (Coutanceau, 1964).

PHENOLOGY AND DEVELOPMENT

The avocado is characterized by rhythmic monopodial growth, that is, with growth from a terminal vegetative bud from the central axis of each shoot that remains and continues its development year after year, and is an example of the Rauh architectural model, one of the most common in temperate and tropical zones.

The trunk forms branches that are morphogenetically identical to the trunk and the flowers originate laterally without having an effect on the growth of the shoots, although in some shoots there are flowers in a terminal position, with subsequent growth being sympodial (Thorp and Sedgley, 1992).

The buds can be axillary or apical. The tree grows mainly from the apical buds, because the axillary buds either become detached or remain dormant. (Calabrese, 1992). The vigor of full tree growth and fruit production depend on the timing and extent of phenological events, which are controlled by carbon and energy availability and distribution (Wolstenholme and Whiley, 1989) in response to environmental conditions (Scholefield et al., 1985). The leaves require about 40 days from bud break to transition from sink to source. (Whiley, 1990). During this period they can compete for photoassimilates with the developing fruits. (Cutting and Bower, 1990).

Throughout the year, the avocado can have one or more vegetative cycles followed by a period of root growth.. Roots begin to grow when the first vegetative growth begins to decline. A second period of vegetative growth then begins, thus reestablishing the balance between a root growth phase and a vegetative phase (Calabrese, 1992).

The avocado is a fruit tree with a marked crop alternation effect, where optimal climatic conditions for setting will produce a year of high harvest, which will be followed by a year of low production and, on the contrary, a year of climatic conditions that produce a high abscission of flowers and/or fruits, will give rise to a year of low production and, subsequently, the following year, increase flowering and therefore the harvest (Lovatt, 2004).

Flowering and fruit growth use large amounts of carbohydrates, so a deficiency would limit these processes and result in a year of poor production. In other words, A high fruit load would be the main cause of the reduction in carbohydrate accumulation during autumn and winter, resulting in low flowering and a large vegetative development the following year. On the contrary, during a year of low production, trees would accumulate higher levels of carbohydrates during the fall and winter, resulting in intense flowering (Paz Vega, 1997).

Scholefield et al. (1985) state that In autumn, the concentration of carbohydrates is minimal, at the same time as a decrease in vegetative activity takes place., which translates into a less competition between vegetative and reproductive developmentThese authors point out that, accordingly, floral initiation in avocado occurs when carbohydrate levels are minimal, thus establishing a close relationship between carbohydrate levels and floral initiation. Low carbohydrate levels could be the cause of the cessation of vegetative growth and this factor is the determining factor, to a greater degree, of floral initiation.

In avocado, Vegetative growth occurs just when reserve levels in the branches are very lowThe highest levels of starch reserves in wood are found in winter, coinciding with the cessation of vegetative growth. These reserves decline rapidly during flowering, reaching their lowest levels during summer, when fruit abscission occurs (Whiley and Wolstenholme, 1990). This accumulation of starch in winter determines the maintenance of fruit on the tree.

Scholefield et al. (1985) state that In autumn, the concentration of carbohydrates is minimal, at the same time as a decrease in vegetative activity takes place., which translates into a less competition between vegetative and reproductive developmentThese authors point out that, accordingly, floral initiation in avocado occurs when carbohydrate levels are minimal, thus establishing a close relationship between carbohydrate levels and floral initiation. Low carbohydrate levels could be the cause of the cessation of vegetative growth and this factor is the determining factor, to a greater degree, of floral initiation.

In avocado, Vegetative growth occurs just when reserve levels in the branches are very lowThe highest levels of starch reserves in wood are found in winter, coinciding with the cessation of vegetative growth. These reserves decline rapidly during flowering, reaching their lowest levels during summer, when fruit abscission occurs (Whiley and Wolstenholme, 1990). This accumulation of starch in winter determines the maintenance of fruit on the tree.

In indeterminate inflorescences, the leaves compete for photoassimilates with the flowers and the developing fruit until they reach 2/3 of their total expansion. (Whiley, 1990). Once the fruits begin to develop, the direction of photoassimilate transport changes in favor of fruit growth (Ho, 1992).

An additional fact used to support the hypothesis that carbohydrate availability could be the regulator of the alternating-bearing cycle is the avocado's high oil content, the storage of which is energetically very costly. Compared to other fruits, such as apples, oranges, and bananas, the amount of sugars stored is much lower, but their high oil content could explain the drastic reduction in carbohydrate reserves in a year of high production.

It is also accepted that Alternating bearing is related to the role of growth regulators (PGRs) produced by the seed in inhibiting flowering the following year. (Paz Vega, 1997). In avocado, gibberellic acid produced by the fruit endocarp has been linked to decreased flowering and fruit production (Martens et al., 1994). The role of carbohydrate reserves is, from this perspective, questioned (Liu et al., 1999).

Scholefield et al. (1985) mention that The cessation of vegetative growth is related to a decrease in the production of gibberellins, allowing the initiation of flower buds, and, on the other hand, the fruits are characterized by their intense metabolic demand.

In general, we can conclude that many factors make The study of crop alternation and its relationship with vegetative growth is difficultEg nutritional, hormonal and climatic factors and other factors derived from endogenous reactions involved in the competition between growing organs. What we can do is prevent its effect from becoming more pronounced by acting on factors that we can control, such as nutritional factors.

Finally, it should be noted that starch and soluble sugars are the predominant reserve carbohydrates for providing energy for growth (Dey and Dixon, 1985).

RECOMMENDATION

For all the above reasons, from the Cultifort technical department, It is recommended to work nutritionally during the stages of highest demand for avocado, so that vegetative growth and flowering and fruit setting, they look so alone affected by those factors beyond human control.

For more information on Avocado cultivation: expert advice another article that may interest you Preventive treatments against fruit drop

From Cultifort we offer a Wide range of biostimulants, deficiency correctors, organic amendments and other specialty products, to satisfy the demand for avocado in each of its phenological states.

In the current phenological phase, growth of shoots and fruits, we must take into account what was stated above, that There is high competition for photoassimilates or carbohydrates between growing fruit and young leaves and shoots. This competition can also lead to increased physiological fruit drop if not properly controlled. It is therefore highly recommended correct any type of nutritional deficiency and work at the level of biostimulation at this critical stage.

As a basis for this period, it is recommended improve soil conditions, since we have already seen that for the vegetative growth is efficient, this has to be linked to a good root development. MICROVITAL – L, liquid formulation rich in organic acids, flavonoid molecules, magnesium and micronutrients, it's a  biological soil activator of plant origin that in addition to improve the physical-chemical parameters, acts as a prebiotic product, stimulating the growth and development of soil microfaunaThese effects are achieved thanks to:

• The organic acids, in forms directly assimilated by the plant (unlike other organic acids)
• And flavonoid molecules, which as polyphenols that they are, possess antioxidant, helping the plant to remain photosynthetically more active, delaying leaf senescence and exercising a protective action against different types of abiotic stress, such as ultraviolet radiation. They also possess chelation properties of iron and other metals, so the use of MICROVITAL – L contributes to the nutrient mobilization present in the soil.

What's more, MICROVITAL – L, Corrects and prevents deficiencies of magnesium, boron, iron, manganese and zinc. Making the plant stay greener, more metabolically active, with better development… in general, healthier and stronger.

Within the range of deficiency correctorsCultifort offers all types of nutritional solutions: magnesium, calcium, iron, boron, zinc, manganese, molybdenum, etc., formulating all of them with the more efficient technology for its correct and maximum assimilation, using different chelates and complexing agents, along with carbohydrates, organic polyacids, polycarboxylic acids, polysaccharides and reducing sugars.

At the level of biostimulation, FOLITON is a Biostimulant with a high content of proteinogenic L-amino acids, both free and combined in the form of peptides and polypeptides, which allows the avocado to save energy in protein formation, especially when there are high nutritional requirements. It is a liquid formulation of rapid assimilation and translocation in the plant. Its application represents a strong stimulation of plant metabolism at times when increased vegetative activity is required. The joint action of the components in its formulation They promote the synthesis of proteins and carbohydrates, promote the start of the plant's physiological activity and stimulate the formation of leaves and flowers, reducing the competitive relationship between them.

SPIRALIS Long Life It is a formulation of special organic acids together with a complex of selected peptides, related to red microalgae (Gellidium) and green (spirulina). Its functionality is based on induce and enhance the local and systemic endogenous increase of molecules with high defensive capacity (alkaloids, thionines, phytoalexins, PR proteins, etc.). On the other hand, induces structural changes in the cell walls of plants at the level of their lignification, thus constituting a physical barrier against abiotic stress. It especially stimulates the response levels of Acquired Systemic Resistance (ASR) and Induced Systemic Resistance (ISR) of the plant against fungi and bacteria, among others. Its use is recommended prior to risk situations and/or while favorable conditions for disease development are maintained. Its broad-spectrum antifungal and antibacterial effect, at the prevention level, and its effect on improving the post-harvest life of the fruits, make SPIRALIS Long Life in one of the most complete and technical products in the Cultifort catalogue.

But it's also important to protect the avocado from various types of stress that may occur during this phase, whether at the root or leaf level.

OXIFORT It is a product that, when applied together with irrigation water, releases oxygen slowly, improving aeration in the root zone, stopping root asphyxia processes and the development of anaerobic microorganisms. Also improves soil structure, giving it fluffiness, making air and water circulate better through the porous space. Increases fertilization performance, especially nitrogenous and promotes the development of beneficial microorganisms that participate in the transformation and mobilization of soil nutrients. OXIFORT It is very important because it prevents stress caused by root asphyxiation. Remember that avocado crops are extremely sensitive to excess soil moisture. Root asphyxiation occurs either due to poor irrigation management or excessive rainfall during a time of lower water demand. In any case, there is a period of approximately 90–100 days after flowering and fruit set in which this crop is especially sensitive to excess moisture, and can suffer root asphyxiation stress, causing massive fall of fruits directlyTo avoid such situations as much as possible, it is recommended to plant avocados in loose soils with a macropore volume greater than 17% (24% is ideal), of good drainage, with a minimum depth, planning well the water outlets of the plot avoiding accumulations in the lowest parts and carrying out a good irrigation management, optimizing water contributions and soil oxygenation.

CultisilK It is a potassium silicate-based formulation that also incorporates free amino acids. On the one hand, silicon will create a protective layer on the plant to cushion the thermal regime against excessive temperatures, also protecting against sunburn; on the other hand, potassium will act as regulator of good stomatal behavior, helping to maintain cellular turgor and thus avoiding excessive water loss through transpiration; and finally, free amino acids are focused on reinforcing the synthesis of chlorophyll activate photosynthesis, avoiding interruptions in the synthesis of assimilates and ensuring that the competitive relationships between the fruits and the growing shoots are not accentuated. As a source of potassium and silicon, Improves plant growth and strengthens its resistance to environmental factors, enhancing self-defenseSilicon is a structural element that reinforces the cell wall, strengthening the physical support of the plant and protecting it from attack by external agents. It also has synergies with calcium, magnesium and potassium, improving its absorption and transport in the plant.

In short, in Cultivator we offer a wide range of nutritional solutions with specific functions designed to improve crop yields in an efficient and environmentally friendly manner. Furthermore, our catalog only includes products “zero waste”.

REFERENCES

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Ho, LC 1992. Fruit growth and sink strength. In: Fruit set production: Aspects of development, environmental physiology and ecology. Marshall, C., and Grace J. (Eds.). Cambridge Univ. Press. Great Britain. pp: 101-124.

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Lovatt, C. 2004. Eliminating alternate bearing of “Hass” Avocado. Proceeding of the California Avocado Research Symposium, October 30, 2004. University of California, Riverside. California Avocado Commission. Pages 89-95.

Martens, D.A., S. Luck, and W.T. Frankenberger, Jr. 1994. Role of plant growth regulators in vegetative spring flush, flowering and fruit drop in avocado (Persea Americana Mill.). Spec. Rpt. Qual. Avocado Dev.Org., Calif. Avocado Soc., Saticoy.

Meyer, B. 1960. Introduction to Plant Physiology. Buenos Aires, Eudeba. 57 Op.

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Thorp; T, and Sedgley, M. 1992. Shoot growth and tree architecture in range of avocado cultivars. In: Proceedings of Second World Avocado Congress. California 1992. pp. 237-240.

Whiley, AW, 1990. CO2 assimilation of developing fruit shoots of cv Hass avocado (Persea Americana Mili.) – A preliminary report. SA Avocado Growers' Assn. Yrbk. 13,28:30-XNUMX.

Whiley, AW and Schaffer, B. 1994. Avocado. In: Schaffer, B., Anderson, P. (eds.). Handbook of Environmental Physiology of Fruit Crops, Vol 2. Subtropical and Tropical Crops. CRC Press, Boca Raton, Florida. Pp. 3-35.

Whiley AW, Schaffer, B. and Lara, SP 1992. Carbon dioxide exchange of developing avocado (Persea Americana Mill.) fruit. Tree Physiology 11, 85-94.

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Wolstenholme, B.N. and Whiley, A.W. 1989. Carbohydrate and phenological cycles as management tools for avocado orchards. South African Avocado Growers' Association. Yrbk. 12: 33–37.