Calcium (Ca) is an essential nutrient for plants. After being absorbed by the roots, it is transported via the xylem following the transpiration flow and is basically distributed by the same route to all plant organs. This nutrient is involved in numerous functions within the plant, acts as a messenger and provides structural stability to the cell wall and membrane.
In terms of its requirement by higher plants, Ca is classified as a secondary nutrient. However, it is involved in numerous biochemical and morphological processes.
Calcium is implicated in more than thirty economically important physiological disorders of crops basically due to maldistribution of the element in organ tissues (Shear, 1975). In tomato, for example, localized calcium deficiency causes apical rot and favors fruit cracking, while excess induces the formation of golden blotch. Calcium plays an important role in plant resistance to disease based on the protection of the cell wall from disintegrating enzymes secreted by pathogens.
In cultivated plants, calcium deficiency symptoms are rarely observed. However, every year there are important losses due to physiological disorders as a result of inadequate concentration of this nutrient in leaves, fruits, roots or tubers.
Calcium has received considerable attention in recent years not only because of its relationship with physiological disorders, but also because of its beneficial effects, particularly in fruits, in which it can reduce respiration, delay ripening, increase post-harvest life, as well as improve firmness and vitamin C content. Several experiments indicate that the increase of calcium in the cell walls of plant tissues reduces the presence or severity of diseases (Villegas et al., 2007b).
Boron (B) is a micronutrient required for the nutrition of all plants. The main functions of boron are related to cell wall development and resistance, cell division, fruit and seed development, sugar transport and hormone development. Although the essentiality of B as a micronutrient for all vascular plants in obtaining high and good quality yields in agricultural practices is well established, knowledge about its metabolic functions in vegetables still remains incomplete. Some research has helped to greatly improve the understanding of some processes in plants regarding its uptake and transport (Takano et al., 2006), cell wall formation (O’Neally et al., 2004), cell membrane functions (Goldbach et al., 2001) and antioxidative defense (Cakmak and Römheld, 1997).
CALCIUM UPTAKE AND DISTRIBUTION IN THE PLANT
Ca2+ is transported apoplastically into the xylem. Ca transport in the xylem occurs by mass flow of free Ca2+, some organically complexed Ca, and by chromatographic movement along Ca exchange sites in the xylem walls. Competition between demand sites is intensified when the Ca2+ concentration in the xylem is low and transpiration is high (Clarkson, 1984), which can lead to nutritional deficiency.
The Ca concentration in plants fluctuates between 0.2 and 3.0 % of leaf tissue dry weight, with sufficiency values of 0.3% to 1.0% in the leaves of most crops. The highest concentration is found in older leaves. It has been suggested that total Ca concentration is not a reliable value in relation to sufficiency, because it accumulates in some plants as calcium oxalate crystals. Therefore, extractable Ca may be a better indicator for sufficiency (Jones et al., 1991).
ROLE OF CALCIUM IN CELL STRUCTURE
Ca acts as a second messenger in the regulation of a wide variety of physiological and anatomical processes. Regulation of cellular Ca is an essential function, performed by a set of complex processes collectively termed Ca homeostasis.
Ca functions predominantly as a structural component in cell walls and in maintaining plasma membrane integrity. Deficiency of this nutrient increases membrane permeability, middle lamella dissolution and associated cell wall changes. Most of the Ca that enters the plant is accumulated in cell walls and membranes. In the cell wall, accumulation is facilitated by binding with pectin polymers, particularly of the middle lamella, to form a cell wall network that increases mechanical strength (Gerasopoulos and Chebli, 1999).
Ca constitutes a structural element in membrane architecture: electrostatic bridges formed by Ca and components of the membrane lipid layer allow anchoring and stationarity of integral proteins. Membrane stability, microviscosity and lipid phase state can be controlled by Ca (Leshem et al., 1992). Ca preserves membrane integrity in two ways: one, by delaying changes in membrane lipids due to senescence; and the other, by increasing restructuring processes (Gerasopoulos and Chebli, 1999).
Although other cations can replace Ca in the binding sites, they are not able to replace its function in membrane stabilization. The absence of Ca in the membrane causes it to become porous and solutes are lost from the cytoplasm. With Ca deficiency there is a disintegration of the membrane structure. Disorders occur primarily in meristematic tissues such as the root apex, growing points of the upper parts of plants and storage organs (Kirkby and Pilbeam, 1984).
Ca deficiency modifies the selective action of cations, induces ultrastructural changes and alters the process related to plasma membrane fusion, and also causes the separation of the phospholipid phase in artificial membranes (Marmé, 1983).
EFFECT OF HIGH CALCIUM CONCENTRATION IN PLANTS
Ca is a relatively non-toxic cation in large quantities and plants can adapt to a wide range of supply. Toxicity appears slowly and can usually be attributed to an indirect effect involving other ions (Nonami et al., 1995). Among the effects that can occur in plants when subjected to high calcium concentrations are: growth arrest based on preventing cell wall extension, rigidity of the cell membrane and increased insoluble deposits in walls and vacuoles (Hanson, 1984). High Ca2+ concentration beyond physiological tolerance decreases photosynthesis and disruption of K+ fluxes (Marschner, 2002).
In contrast to the other macronutrients, a high proportion of total calcium in plant tissues is often localized in the cell walls due to the large number of binding sites found in the cell walls, as well as the reduced transport of calcium to the cytoplasm. In the lamina media it is bound to the carboxyl groups of polygalacturonic acids (pectins) in a relatively interchangeable form. In dicotyledonous plants, which have a high cation exchange capacity (CEC), more than 50% of the total calcium is bound to pectates (Marschner, 2002).
CALCIUM DEFICIENCY DISEASES
Problems associated with Ca deficiency in plants are characterized in two areas:
- those related to the inability to absorb Ca from solution due to low absolute concentration or by low Ca/other cation ratios; 2. those related to inadequate Ca distribution to active growth tissues after uptake.
- those related to inadequate distribution of Ca to actively growing tissues after uptake. The upward movement of Ca in the xylem and the final distribution are considerably dependent on the mass flow associated with transpiration. Ca transport through the xylem is controlled by the density of negative charges in the vessels, the concentration of other cations in this tissue, and the ability of adjacent cells to remove Ca from exchange sites (Marschner, 2002).
Disorders associated with Ca deficiency include diverse symptomatology. Apical rots begin at the pole opposite the peduncle, with the formation of small and numerous necroses that end up forming an almost circular, depressed spot with well marked edges, which can cover half of the fruit. Apical rot, which correlates with the collapse of the middle lamellae of the pulp cells, is related to low Ca concentration in the distal tissue and rapid fruit growth (Ho et al., 1999). The symptoms of fruit cracking consist of cracks emerging from the insertion with the calyx, appearing almost always in ripe fruit and sometimes during ripening. It is usually triggered by a water deficiency with high ambient temperature followed by a rapid change in the humidity supplied to the plants. However, this phenomenon can be increased due to the reduction of cell wall rigidity by low Ca concentration (Resh, 2001).
THE ROLE OF CALCIUM IN PLANT RESISTANCE TO BIOTIC DISEASES
The infective capacity of microorganisms that develop in plant cells is determined by the ability to hydrolyze cell walls. Without this condition, the penetration of an infective agent into a set of cells affected by a wound would not be accompanied by a progression of the infection towards neighboring cells; for this reason, a very important aspect in the pathogenesis of plant diseases is the chemical mechanism of penetration of the pathogen and the colonization of its tissues (Cornide et al., 1994).
Pathogens secrete enzymes that cause softening of cell walls. The role of these enzymes in pathological processes is of indisputable importance, especially in young plants, since the primary wall of plants is rich in pectins, while the secondary wall, characteristic of mature tissues, is more abundant in cellulose and hemicellulose (Cornide et al., 1994).
Several experiments indicate that by increasing Ca in the cell walls of plant tissues it is possible to reduce the presence or severity of diseases.
In a study in which the role of Ca in the protection of pumpkin fruit tissues against Botrytis cinerea infection was evaluated, it was determined that Ca applied to the fruit increased the concentration of this element in the cell walls and thus decreased the digestion of pectins by the pectinolytic enzymes of the fungus (Chardonnet and Doneche, 1995).
Increases in Ca concentration in potato significantly improved quality and storage time by decreasing the damage caused by Erwinia carotovora (Conway and Gross, 1987). In fruit tissues with high Ca concentration, changes in cell wall composition were generally less due to infection compared to those observed in fruit with low Ca content. The results of this investigation indicated that the effect of Ca in reducing decay is associated with the stability of cell wall structure (Tobias et al., 1993).
BORON IN THE CELL WALL AND CELL MEMBRANE
The cell wall is fundamental in determining plant cell growth and development, which involves a dynamic and continuous modification during cell differentiation (Perez Almeida and Carpita, 2006), where boron is important in forming borate bridges for the formation of the B-RGII dimer, a fundamental component of cell wall architecture (Goldbach and Wimmer, 2007). The role of B is also correlated with the development and lignification of cell walls (Matsunaga et al., 2004).
Extensive studies have demonstrated the importance of B for the complete functionality of the different processes at the cellular level in plants, where a diversity of enzymes and other plasma proteins participate, in addition to the transport processes through the membrane and its integrity (Bronw et al., 2002). B deficiency alters membrane potential (Blaser-Grill et al., 1989), reduces ATPase activity in proton pumping, and consequently the proton gradient across the plasma membrane (Obermeyer et al., 1996), and reduces Fe-reductase activity (Ferrol and Donaire, 1992).
OTHER PROCESSES INVOLVED IN THE BORON-PLANT RELATIONSHIP AT THE CELLULAR LEVEL
Some studies have shown that B deficiency affects the process of photosynthesis in plants. The primary mechanisms of B performance in photosynthesis are not known, but it could affect functions at the chloroplastic membrane level by disruption of electron transport and energy gradient across the membrane, resulting in photoinhibition (Goldbach and Wimmer, 2007).
Other studies indicate the existence of a close relationship between B and Ca where both co-act at the cell membrane level by interactions still unknown (Bolaños et al., 2004). In this aspect, the evidence obtained from different investigations indicates that this relationship is a determining factor in gene expression (Redondo Nieto, 2002), in addition to the fact that the participation of Ca is important in the stabilization of B complexes (Wimmer and Goldbach, 1999). Additionally, Ca reduces the effects of B deficiency on nodule development (Redondo Nieto et al., 2002) even under salt stress (El-Hamdaoui et al., 2003).
BORON MANAGEMENT IN AGRICULTURAL SYSTEMS
Imbalances caused by B deficiency and toxicity are problems that exist in many agricultural regions of the world and need to be identified and corrected not only through a good knowledge of the processes involved in its uptake, mobilization and distribution in the plant (Brown and Hu, 1998).
In general, sampling techniques to diagnose B status in plants are based on the premise that B is immobile, not mobile in the phloem, as is the case in most species. However, it is now known that B is mobile in those species that use polyols (simple sugars) as a primary photosynthetic metabolite with high affinity to bind B for subsequent transport in the phloem to areas of active accumulation, such as vegetative or reproductive meristems (Brown et al., 2002). In species that do not produce significant amounts of polyols, B once translocated with the flow of transpiration to the leaves, remains immobile without being able to reenter the phloem, accumulating in the terminal parts of the leaf veins (Brown and Hu, 1998).
The difference in mobility of B influences the diagnosis of its status to correct its deficiency and toxicity in plants, taking into account its mobility in the phloem for the choice of the tissue to be sampled. This is due to the fact that B does not accumulate in the oldest leaves, but in the youngest leaves of species where it is mobile; while on the contrary, in species where it is immobile, its accumulation is greater in the oldest leaves, with respect to the youngest, due to greater transpiration (Brown and Hu, 1998).
B fertilization should be managed very carefully so as not to create contamination problems in the crop, taking into account the mobility patterns in the plants. According to experimental evidence, foliar-applied B is retranslocated to growing organs in species where it is mobile (Christensen et al., 2006). However, in species where B is immobile, its foliar application does not translocate it from the applied site, and its requirements cannot be supplied in the still unformed tissues. In this sense, the correction of the deficiency is achieved by direct application in the sites of interest. Thus, in fruit trees where B is immobile, but essential for the flowering process, applications are effective directly to the flower buds or flowers (Brown and Hu, 1998).
The study of calcium from the point of view of plant physiology is fundamental to understand more precisely the role it plays in the development and production of crops, which allows the design of methodologies in which the knowledge generated can be applied to improve plant response and have an impact on increasing yields and crop quality. The absolute concentration of calcium and its relationship with other ions in the nutrient solution is fundamental to reduce physiological disorders caused by the poor distribution of this element in plant organs (Villegas et al., 2007a).
Calcium increases plant resistance to diseases based on the protection of the cell wall against the action of disintegrating enzymes secreted by pathogens (Villegas et al., 2007b).
Boron stands out as a dynamic element that affects an exceptionally large number of biological functions involved in a broad spectrum of plant processes (Malavé and Carrero, 2007).
It is essential to know the relative mobility of boron in the species for tissue sampling whose analysis will indicate the status of boron in the plant and the consequent strategy of applying or not fertilization, taking into account the narrow margin between deficiency and toxicity (Malavé and Carrero, 2007).
In Cultifort we have a wide range of zero residue nutritional formulations, manufactured with high quality raw materials that guarantee the maximum stability of the product, its highest possible assimilation and the improvement of the physiological processes of the plants.
Within our range of organic amendments and deficiency correctors, you can find CULTIFORT CALCIUM and CULTIBORO PLUS.
CULTIFORT CALCIO is a liquid fertilizer with high calcium content complexed with organic acids and formulated with organic matter and carbohydrates. The organic acids facilitate the assimilation of calcium by the plant (via foliar and root) and the organic matter and carbohydrates guarantee its efficient translocation inside the plant, avoiding nutritional imbalances caused by high transpiration rates (high temperatures) that divert the sap flow towards the leaves causing nutritional deficiencies in the fruits.
CULTIBORO PLUS is a liquid formulation of boron complexed with ethanolamine and reducing sugars, which guarantee its rapid assimilation, both by foliar and root application. CULTIBORO PLUS helps regulate plant hormone levels, improving root growth and cell division in stems and leaves. It also ensures the transport of sugars in the plant and facilitates respiration.
The relationship between CULTIBORO PLUS and CULTIFORT CALCIUM (B/Ca) plays a very important role at the structural level and in signal transduction. Both formulations reciprocally improve their translocation inside the plant, multiplying the efficiency of the applications.
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