Relationship between gibberellins and carbohydrates in vegetable products

Gibberellin is a vegetable hormone widely used in agriculture and horticulture. The objective of this review was to show the relationship between gibberellins and carbohydrates in vegetable products. Thus, the study concludes that the involvement of substances may be related with the terminal stages of development of various plants. However, the interactions of carbohydrate content and gibberellin are still discussed.


INTRODUCTION
Commercially use, the term regulator that is nothing more than substances that reduce the effects of hormones produced by plants (Yamada, 1992).The benefits from the knowledge of the activity of hormones on the various processes of plant development are undeniable.Its discovery has brought great advances in physiology, leading to understanding and controlling cell differentiation, culminating in the emergence of tissue culture isolates in vitro, which together with molecular biology, was one of the most important tools for the development agriculture (Taiz and Zeiger, 2004).
In this respect, the objective of this review is to show the relationship of the vegetal hormone, gibberellins, with one of the substances that may be related to senescence in plants, such as carbohydrates.

HORMONES
Hormones regulate source/drain ratios, probably in combined action; where a hormone acts as an event promoter and another as inhibitor of such event.Thus, the plant can control the whole process of absorption, transport and utilization of carbohydrates step by step, making a fine adjustment according to the instant environmental variations, such as contents of CO 2 or H 2 O, to maximize the plant production process.Probably none of these processes is controlled by a single hormone but rather by their balance; as example the control of stomatal opening (Mansfield and Mcainsh, 1995).The floral stimulus, for example, is believed to be produced simultaneously on different leaves, through phytochromes, sent and amplified in cascade reactions in two levels, with cooperation between intracellular microfunctions and intercellular macrofunctions and this is in connection with the production and distribution of energy and genetic activation, associated with changes in the balance of phytohormones.Such a response is also dependent on the changes that occurred in the existing *Corresponding author.E-mail: m.r.s.v@hotmail.com.
structures and enzymatic activity that remained in the vegetative growth (Jaegher and Boyer, 1990).

Gibberellins
In the 1950's, gibberellin was characterized as phytohormones.Phytohormones are organic compounds, nonnutrients, produced by plants and in low concentrations (10 -4 M) they promote, inhibit or modify physiological and morphological processes of vegetables (Davies, 2004).The most important gibberellin is GA 1 and most of the others are precursors of GA 1 , except GA 3 , GA 5 and GA 6 .It has a complex structure, being chemically isoprenoid.It is synthesized by a branch in the route of terpenoids, which are synthesized by the route of mevalonic acid and by the route of metileritritolfosfato (MEP).
The differences between many gibberellin are the number and the localization of the double connections and the hydroxyls group (Taiz and Zeiger, 2004).The gibberellin group covers a large number of compounds, where 1/3 are gibberellin with 20 carbons and the others are gibberellin with 19 carbons, being more active with the GA 1 , GA 3 , GA 4 , GA 7 , GA 9 and GA 20 .

CARBOHYDRATES
The knowledge of phenology and how the reserve carbohydrates content varies during the development of plants have been the basis to developing phenological models of production.These models have been used in the assessment of management techniques in numerous plant species that have been successfully grown around the world (Corsato et al., 2004).
Carbohydrates are a group of compounds including simple sugars and more complex molecules consisting of subunits of simpler sugars, whose main function is to supply energy to the chemical processes of the cell life cycle (Brum et al., 1994;Moalen-Beno et al., 1997).Carbohydrates also present various functions, such as osmotic regulator (Bieleski, 1993) and as a metabolic precursor of other molecules (Sood et al., 2006).
Starch and soluble sugars (sucrose and derivatives) are the main reserve carbohydrates derived from glucose formed during photosynthesis.The first is still in the cell, being synthesized in the chloroplasts of photosynthetic tissues, while the second has high mobility in cells and tissues being discharged into the phloem and cytosol of the cells.Once translocated to non-photosynthetic drains, sucrose is rapidly converted into fructose and glucose (Dennis and Blakeley, 2000).Once in these organs, these sugars can be used in three ways: a) in the breath for the synthesis of energy and other compounds; b) be stored in the vacuole for subsequent use in breathing or c) be converted inside the reserve organs for use in situation of low photosynthetic activity in shoots (Taiz and Zeiger, 2002).By the time that precedes, the start of photoassimilates export by the photosynthetic tissues newly formed in the shoot for every new cycle of the plant is observed; all developing organs are totally dependent on the carbohydrate reserves present in the storage organs.This dependence decreases with the increased amount of assimilates exported by the expanding canopy (Darnell and Birkhold, 1996).
The carbon metabolism in the source and drain, as well as transportation between these tissues is regulated by a "feedback" control by the levels of intermediate and final metabolites.The "feedback" control is performed directly by the metabolite at transcription level of specific enzymes of their metabolism, or mediated by hormones, which control these enzymes.When there is no use of these metabolites their formation decreases (Foyer and Galtier, 1996).
The movement of photoassimilates from the synthesis site, in the source, to the place of use or accumulation in the drain can be regulated at several points.The concentration gradient between the source and drain is generally accepted as the main determinant of the rates of transport and partitioning among organs.Such processes are also controlled in part by hormones that act as messengers among cells, tissues and organs, promoting the activation of specific genes or metabolic processes (Foyer and Galtier, 1996).However, to evaluate the control of source/drain ratios, it is important to first understand where these hormones act and if possible, their place of origin, which are generally in the very drains (HO et al., 1989).According to Kock et al. (1996), the carbohydrate metabolism of plants has a complex signaling mechanism involving sugars and hormones capable of controlling the enzymes involved in the synthesis/degradation of starch and sucrose accumulation.
In the literature were found studies showing the relationship between carbohydrates and gibberellins in vegetable products.In potato, the gibberellins are involved in dormancy and initiation of tuber development, associated with levels of inhibitors of carbohydrate available and environmental factors such as light, temperature and photoperiod (Tamas, 1995).In vines, gibberellins increase the activity of sucrose-phosphate synthase, stimulating the export of sucrose from the leaf, being considered to induce the reproductive development (Brenner and Cheikh, 1995).Spraying with gibberellic acid (GA 3 ) decreased the diameter of the pseudobulbs of D. nobile, probably because this is the main storage organ for water, carbohydrates and mineral nutrients of epiphytic orchids (Zimmermann, 1990).This author also reports that gibberellins act on the metabolism of carbohydrates mainly stored in the pseudobulb, causing decreased pseudobulb diameter due to the hydrolysis of its reserve substances.Furthermore, the accumulation of carbohydrates in the tissue increases the osmotic pressure, making the stored water flow, especially in the pseudobulb; occur more rapidly in the interior of the cell, promoting its expansion (Cordeiro, 1979;Daykin et al., 1997;Pires, 1998).Martinez-Cortina and Sanz (1991), studying the carbohydrate content in Citrus, found increased levels of carbohydrates in buds and leaves on oranges pears treated with GA 3 , and was related to a possible increase in the photosynthetic capacity of leaves or a modification of the model of assimilates distribution in the plant.In the view of these authors, the GA 3 seems to have especially important role as a regulator of hydrolytic enzymes.In an experiment with postharvest of papaya fruits stored under refrigeration, Coneglian (1994) noted that it was more effective when the plant growth regulators auxin, gibberellin and cytokinin were used in combination, since in this way higher levels of total soluble carbohydrates were obtained.
There are few studies that report the changes in carbohydrate contents with gibberellins use in flowers, especially cut flowers.Coorts (1973) reports that glucose is the main constituent of the tissues of rose petals and the accumulation of glucose is due to the fact that, when removed from the plant, the flower acts as a drain where translocation from leaves to the tissues of the petals occur.This transport occurs in the form of sucrose (sugar transport) which form glucose when they reach the flowers (Taiz and Zeiger, 1998).Adachi et al. (2000) noted that high temperatures decreased the concentrations of glucose and fructose in chrysanthemum 'Seiun', but did not contribute to the premature aging of the species.This is due to high temperatures that influenced the complete development of ligules.Vieira et al. (2010) studying the cut chrysanthemum cultivar Faroe' observed decreased total carbohydrate content in the leaves and flowers during the lifetime, especially during senescence.Larcher (2000) mentioned that the distribution of sugars in the plant has been reported in some cultivated species; the leaves closer to the soil supply the root system and those closer to the apex supply the meristems of the shoot and especially the flowers.Vieira et al. (2010) also reported that gibberellin was not effective in altering the carbohydrate content in leaves and chrysanthemum flowers at the concentrations and method of application used.Several studies showed the relationship between these two substances, but also there is need for better discussion.