Calcium is an essential plant element. With a hydrated ionic radius of 0.412 nm it’s a relatively large divalent cation. Calcium is firmly bound and incorporated in plant structures like cell walls, but some Ca2+ is exchangeable at the cell walls (pectins) and at the exterior surface of the plasma membrane. The vacuoles contain high calcium concentrations, whereas cytosol calcium concentration is low. Moreover, Ca mobility in the symplasm and in the phloem is low.
Calcium is required for stability of the cell wall and membranes, it is a counter-cation for inorganic and organic anions in the vacuole, and the cytosolic Ca2+ concentration is an important intracellular messenger coordinating response to numerous developmental and environmental cues. Transport from and export to the vacuole can facilitate plant osmoregulation. Low Ca2+ concentrations in the cytosol are essential for (i) prevention of phosphate precipitation, (ii) competition with Mg2+ for binding sites, and (iii) as a prerequisite for the function of Ca2+ as a second messenger in the cell. Chloroplasts can also contain large amounts of Ca (6.5–15 mM total Ca, mostly bound to thylakoid membranes). Without calcium supply, root extension ceases within a few hours. Calcium can counter balance the harmful effects of high concentrations of other cations. Not only the roots but also root hairs and pollen tubes rely on the availability of apoplasmic Ca. Calcium plays an important role in protein stimulation. For example, high (millimolar) Ca concentrations can stimulate of α-amylase activity in germinating cereal seeds.
Growing conditions, species and the measured organ result in different Ca concentration (1 and 50 g kg-1 ). Monocotyledons need lower amounts of calcium than in dicotyledons. That is calcium concentrations for optimal growth are 2.5 μM for the monocot ryegrass and 100 μM for the dicot tomato. This 40 fold difference is caused by a different Ca demand at the tissue level, i.e. ryegrass needs 0.7 mg g kg-1 and tomato 12.9 g kg-1. In general differences in Ca requirements between species are explained by Ca2+-binding sites in the cell walls, i.e. the cation-exchange capacity CEC (White and Broadley 2003)
An important factor for Ca requirement is the concentration of the other cations in the soil or substrate nutrient solution. Calcium requirement increases with increasing external concentrations of other cations like: Mg, K, Al, Na, heavy metals and protons (acidity). These cations can replace calcium at its plasma membrane binding sites. Acidified soil require a higher Ca concentration to counteract the adverse effect of high H+ concentrations on root elongation.
Higher Ca concentrations in the rhizosphere often increase the Ca concentration in the leaves. However, this is not necessarily the case for low-transpiring organs such as fleshy fruits or tubers, which are supplied predominantly via the phloem. On the one hand the low Ca mobility in the phloem can protect these organs against excessive Ca accumulation. On the other hand, high growth rates of low-transpiring organs increase the risk of local Ca deficiency. As calcium levels fall below critical level required for cell wall stabilization and membrane integrity common physiological disorders appear such as: blossom end rot in tomato, tipburn in lettuce, black heart in celery, or watermelon, and bitter pit in apple. Plant organs also tend to senesce faster. Low Ca concentrations in fruits and tubers do not only result in physiological disorders, it makes plants also more susceptible to fungal infections.
Calcium fertilisation can be supplied at high concentrations. Disorders are often a secondary effect of cation imbalances and soil pH increase due to usage of calcium carbonates. If these adverse secondary effects are managed properly calcium concentration can exceed 10% of the dry weight, without a display of toxicity symptoms or serious inhibition of plant growth. That is because plants can generally manage high Ca concentrations by accumulating excess Ca in the vacuole. There are big differences between species and absorbance of calcium as is showed in Table 5.2.
Table 5.2: Calcium content in selected vegetables and its absorbability in premenopausal women (Copied from: Dayod et al. 2010)
Dayod M, Tyerman SD, Leigh RA, Gilliham M. 2010. Calcium storage in plants and the implications for calcium biofortification. Protoplasma 247: 215–231. DOI: 10.1007/s00709-010-0182-0.
White PJ, Broadley MR. 2003. Calcium in Plants. Annals of Botany 92: 487–511. DOI: 10.1093/aob/mcg164.