Calcium Dosing Unlocks Healthier In Vitro Almond Rootstock and Stronger Roots

One of the most stubborn problems in commercial plant tissue culture is a strange disorder that makes shoots look water-soaked, glassy and brittle — a condition known as hyperhydricity. For nursery operations that rely on cloning elite woody plants in vitro, hyperhydric shoots translate directly into lost plants, wasted labor and unreliable rooting. A new study published in BMC Plant Biology suggests that a surprisingly simple lever — the amount of calcium chloride added to the culture medium — can make the difference between a tray of malformed, glassy shoots and a batch of robust plantlets ready to take root.

The research, conducted by Heydem Ekinci of the Department of Horticulture, Faculty of Agriculture, Harran University in Şanlıurfa, Türkiye, focused on Garnem, a commercially important almond × peach hybrid rootstock (Prunus dulcis × Prunus persica) widely used in stone fruit orchards. The central question was whether supplementing the standard Murashige and Skoog (MS) medium with additional CaCl₂·2H₂O could suppress hyperhydricity and, crucially, whether the optimal dose would shift depending on the developmental stage of the culture — from shoot initiation through proliferation to rooting.

The logic behind the experiment rests on calcium’s fundamental role in plant architecture. Calcium ions cross-link pectin molecules in the cell wall, stabilizing its structure; they also preserve membrane integrity and act as a signaling intermediate in morphogenesis. Tissue-culture conditions — high humidity, elevated cytokinin levels and sealed vessels — are notorious for disturbing these processes, which is one reason hyperhydricity plagues woody plant micropropagation. Because calcium availability influences cell-wall rigidity and water relations, the researcher hypothesized that boosting calcium could restore structural normality to fragile shoots.

Single-node explants were grown on hormone-free MS medium supplemented with 2 mg L⁻¹ of the cytokinin 6-benzylaminopurine (BAP), the standard cocktail for inducing shoot formation in Garnem. On top of the calcium already present in the MS formulation, treatments received either 100, 200 or 300 mg L⁻¹ of supplemental CaCl₂·2H₂O. A control treatment received BAP without extra calcium. The effects were assessed statistically by analysis of variance followed by Tukey’s multiple comparison test, with significance thresholds from p < 0.05 down to p < 0.0001.

The results during the initial culture stage were striking. Modest supplementation at 100 mg L⁻¹ delivered the best shoot performance: the highest shooting rate, at 95.83 percent, and the largest number of leaves per plantlet, at 8.13. This shows that a light calcium boost supports organogenesis — the initiation of new shoots and leaves — without pushing the medium into a regime that slows bud break. For nurseries establishing new lines of rootstock from mother plants, that early phase is where every percentage point of shooting success compounds into economic advantage.

Hyperhydricity told a different, equally important story. In the BAP control, 62.50 percent of plantlets showed the characteristic glassy symptoms. As calcium climbed, the disorder receded sharply: hyperhydricity fell to 20.83 percent at 200 mg L⁻¹ and dropped further to 10.42 percent at 300 mg L⁻¹ supplemental CaCl₂·2H₂O. In other words, the higher doses cut the incidence of the disorder by roughly two-thirds to more than four-fifths relative to the control. The interpretation fits calcium’s biochemistry — reinforced cell walls and more stable membranes resist the waterlogging and structural collapse that define hyperhydric tissue.

But calcium’s benefits were not monotonic, and this is where the study’s stage-dependence finding becomes valuable. When cultures entered the first subculture, the sweet spot shifted upward: 200 mg L⁻¹ supplemental calcium produced the highest number of proliferated shoots, at 2.23 per plantlet, and the highest number of nodes, at 1.60 per plantlet. Too little calcium limited the structural and signaling support needed for rapid multiplication; the data suggest an intermediate dose best balances proliferation demand with tissue quality.

The true test of any micropropagation protocol, however, is what happens at rooting — the stage at which many tissue-cultured plantlets fail even after looking healthy in the multiplication vessel. In this experiment, all treatment groups were transferred to a common rooting medium containing 1.5 mg L⁻¹ indole-3-butyric acid (IBA), with no mixing of calcium treatments at that stage, so that any rooting differences would reflect the plants’ calcium history during the earlier stages. The verdict was clear: plantlets that had received 300 mg L⁻¹ supplemental CaCl₂·2H₂O during both the initial stage and the first subculture achieved the highest rooting rate, at 84.13 percent, along with the longest roots, at 3.49 cm, and the greatest fresh root weight, at 0.95 g. Root number was also significantly increased by the 200 and 300 mg L⁻¹ treatments.

The pattern that emerges — low calcium for shoot initiation, intermediate for proliferation, high for quality and rooting — is the study’s most consequential insight. It challenges the one-size-fits-all approach to medium formulation that dominates much of commercial micropropagation. Instead of fixing a single calcium level for the entire propagation cycle, growers could stage calcium supplementation to match each developmental objective, first coaxing buds to break, then multiplying shoots, then hardening tissue so that it roots aggressively once exposed to auxin. Such stage-specific optimization could raise the efficiency and quality of Garnem micropropagation without new hormones, new genotypes or expensive equipment — a rare win in an industry where improvements often come at high cost.

For the broader field of woody plant biotechnology, the findings reinforce a growing appreciation of calcium as a morphogenic regulator rather than merely a bulk nutrient. The demonstration that a single, inexpensive salt can simultaneously suppress a quality-eroding physiological disorder and enhance downstream rooting capacity offers a template that could be tested in other Prunus species and hyperhydricity-prone crops. As clonal rootstock demand grows alongside global stone fruit orcharding, protocols built on stage-specific calcium tuning may well become standard practice, turning one of tissue culture’s oldest headaches into a manageable — even exploitable — variable.

Subject of Research: Optimizing calcium supplementation to reduce hyperhydricity and improve micropropagation and rooting of the Garnem almond × peach hybrid rootstock.

Article Title: Effects of different calcium concentrations on hyperhydricity, morphophysiological characteristics and rooting responses of Garnem rootstock cultured in vitro

Article References: Effects of different calcium concentrations on hyperhydricity, morphophysiological characteristics and rooting responses of Garnem rootstock cultured in vitro. (n.d.). https://doi.org/10.1186/s12870-026-09990-w

Image Credits: AI Generated

DOI: 10.1186/s12870-026-09990-w

Keywords: micropropagation, calcium, hyperhydricity, Garnem rootstock, Prunus, plant tissue culture, rooting, shoot proliferation, 6-benzylaminopurine, plant physiology, BMC Plant Biology, clonal propagation