Calcium is considered a plant macronutrient. In their book “Plant Physiology 4th edition,” Frank Salisbury and Cleon Ross define a macronutrient as any element with a concentration above 1,000 mg/kg or 0.1 percent in the plant tissue. Secondly, calcium is not mobile; it cannot be moved through the plant’s vascular tissues because it is locked away in structural compounds of the plant.
In the book “The Use of Nutrients in Crop Plants,” the authors reviewed a variety of studies that suggest plants exhibit a linear increase in total calcium concentration in their tissues as they grow, which suggests that calcium is a physiological requirement throughout the entirety of a plant’s life cycle.
With these assertions in mind, what is the function of calcium in plants and how much is required for a plant to be healthy and productive? Let’s take a dive into the inner workings of plant physiology.
Physiological Roles of Calcium
What follows is by no means an exhaustive review because a large body of research is currently being undertaken to better understand the complex and numerous chemical and electrical relationships among calcium and other cellular components.
Calcium and the Cell Wall
Calcium is an important constituent of pectin—the outer-most layer of the cell wall. Also called the middle lamella, this layer of complex polysaccharides (carbohydrate molecules) cements adjacent cells together and is responsible for the rigidity of plant tissues. The biochemistry involved in the synthesis of pectin is rather complex, but it occurs in the late phases of mitosis (cell division) when the cell plate is being formed between the two daughter cells.
Recall that calcium is considered an immobile nutrient. The reason for that is due to the element being “locked-up” in molecules that are structural in nature. This means those molecules cannot be degraded to release calcium for other purposes during a plant’s growth cycle. For this reason, most deficiency symptoms occur in the newly formed tissues of the plant.
The bottom line: Calcium is a constituent of the cell wall (i.e., pectin layer), which is constructed during cell division and is responsible for plant rigidity. Because calcium is part of the cell wall, it is not possible for it to be remobilized for use in other parts of the plant.
Calcium and Cell Division
Numerous authors cite calcium’s importance in the migration of chromosomes during the anaphase portion of mitosis. Anaphase is the period of cell division where the chromosomes are aligned with the cell’s central axis. This phenomenon occurs with the assistance of the mitotic spindle, which is a series of proteins that segregate the chromosomes for allocation to daughter cells. The formation of the mitotic spindle is thought to be controlled by the relative concentration of calcium in the cytoplasm.
In his review paper entitled “Calcium: A Central Regulator of Plant Growth and Development,” Peter K. Hepler cites other authors’ studies that suggest calcium concentration in the cytosol (internal cell fluids) is tightly controlled; the plant cell will actively pump calcium from the cytosol to storage areas (e.g., endoplasmic reticulum or network of membranous tubules in the cell’s cytoplasm), which leads to the formation of the mitotic spindle.
With that in mind, it might be counterintuitive to think of low calcium as a detriment to cell division; however, one must consider that cell processes are tightly controlled. Considering the ideas of homeostasis and calcium deficiency, the resulting logical outcome is that a cell that is unable to modulate (balance) cytosolic calcium concentrations would also be incapable of controlling the development of the mitotic spindle. This would result in uncontrolled cell division in concert with poor cell wall development (i.e., lack of calcium to build pectin), leading to weakly formed and (potentially) genetically compromised young tissues.
The bottom line: Calcium plays a significant role in the organization of genetic material during the process of cell division. An appropriate concentration of calcium in the cytosol triggers the chromosomes to be allocated between the two new cells that form during the cell-division process.
Calcium and Cell Membrane Permeability
The membrane of plant cells, which is comprised of phospholipids, plays an important role in cell homeostasis. It is believed that calcium plays an important role in maintenance of cell membrane structure and function. In his review paper, Hepler also cites that calcium binds to the phospholipid bilayer, which improves the cell’s ionic balance and structural integrity.
J. B. Hanson in his chapter, The Functions of Calcium in Plant Nutrition, of the book “Advances in Plant Nutrition” cites studies that suggest calcium, by binding with the phospholipid bilayer, controls the cell membrane’s permeability. To characterize the relationship between bilayer integrity and calcium, Hanson describes studies where plant cells are cultured in low-calcium solutions. In those studies, plant cells were not able to absorb and retain nutrients, which indicates a key role for calcium in cell membrane structural integrity and cellular homeostasis.
Furthermore, an ultrastructural study conducted by N.G. Marinos in 1962 (“Studies on Submicroscopic Aspects of Mineral Deficiencies”: 1. Calcium Deficiency in Shoot Apex of Barley) found that low calcium concentrations resulted in structural discontinuities in the nuclear, plasma and mitochondria membranes, which the author suggests would lead to compromised cell-regulation processes or immediate cell death. Interactions among calcium and the phospholipid bilayer are extremely important in the life of a cell because the structural integrity and homeostatic processes of the cells are controlled by those critical relationships. Finally, if calcium availability is limited, cellular membranes become leaky, and normal physiological processes are compromised. This also could lead to cell malfunction or cell death.
The bottom line: Calcium interacts with the membrane structures of the cell, which stabilizes cell structure and affords the living cell the ability to control its ionic concentrations. Calcium allows the cell to maintain homeostasis.
Calcium and Cell Signaling
The calcium-signaling pathways in plant cells has been described as an ever-evolving story with new and changing processes. The complexity of these systems makes it impossible to provide an exhaustive review, but below are four important points to keep in mind:
- Calcium cell-signaling pathways involve a unique set of proteins called calcium-binding proteins.
- Calcium is stored in organelles (e.g., endoplasmic reticulum, mitochondria and vacuole) where it is actively pumped in or out to modulate the cytosolic calcium concentration.
- Cytosolic calcium concentrations are tightly controlled because elevated levels can cause a precipitation of calcium phosphate inside of the cell, which causes disruptions to cellular processes or cell death.
- Calcium binds with proteins and triggers cascades of other intercellular reactions that control hormone activity, energy generation, gene expression and enzyme activities. Despite the numerous roles that have been detected for calcium in the cell, it remains unclear how the cell tailors the intercellular responses to subtle changes in the cytosolic calcium concentration.
The bottom line: Calcium participates in a wide range of intercellular processes through a diverse set of specialized proteins. Calcium modulates hormone and enzyme activities, energy generation and gene expression.
What It All Means
Calcium is an extremely important plant nutrient due to its many functions, which includes membrane structural integrity, maintenance of homeostasis, segregation of genetic material during cell division, gene expression, energetics and enzyme activities. The full picture of calcium-mediated physiological processes has not been fully described here nor clarified in academic research; however, researchers do know that calcium is immobile in plants and that it is a constant requirement throughout all growth phases.
The upper tolerance of calcium for plants is not known, and most fertigation programs call for a concentration between 150 and 300 ppm extractable from the growth media or in solution. Be mindful: Calcium will interact with phosphate in solution and, therefore, should not be applied in feeds containing phosphorus.