If there’s a question that comes up consistently in TC lab troubleshooting, it’s this: should I be using MS or WPM? A protocol isn’t working as expected — growth is slow, browning is occurring, multiplication rates are lower than published data suggests — and the medium formulation is the first variable to examine.
The honest answer is that MS works well for the majority of species that TC labs work with. But for a specific group of plants — woody species, highland species, acid-adapted carnivorous plants, and certain orchids — WPM consistently outperforms MS, and understanding why helps you make the right choice before losing months of trial-and-error.
What MS Medium Is and Why It Works So Well
The Murashige & Skoog formulation was published in 1962 and remains the most widely used tissue culture medium in the world. Its design was deliberately “universal” — high salt concentrations, high ammonium nitrate, complete vitamins, optimised for rapid cell division across a wide range of species.
The strength of MS is also the source of its limitations. The high ammonium content — 20.6 mM NH₄⁺ — that drives rapid cell division in herbaceous species is toxic at those levels to many woody and acid-adapted plants. The high total ionic concentration — around 60 mM — is too high for plants that evolved in nutrient-poor soils. The pH of 5.7–5.8, while appropriate for most species, is above the optimum for highland carnivores and certain ericaceous species that perform best below pH 5.5.
What WPM Is and When It Was Developed
Woody Plant Medium was developed by Lloyd and McCown in 1980 specifically for the micropropagation of woody species where MS was producing toxicity symptoms, poor multiplication, or complete culture failure.
The key differences from MS:
- Much lower ammonium: WPM contains only 5 mM NH₄⁺ versus MS’s 20.6 mM — a fourfold reduction critical for woody species with limited capacity to assimilate ammonium at the rates MS provides.
- Lower total salt concentration: roughly 25–30% of MS’s ionic strength, reducing osmotic stress on sensitive root systems.
- Higher calcium relative to potassium: the Ca²⁺/K⁺ ratio in WPM is more favourable for woody species, supporting cell wall integrity and reducing tip dieback.
Species That Consistently Prefer WPM
Nepenthes and Other Highland Carnivorous Plants
This is perhaps the clearest case for WPM in the Indian TC context. Nepenthes rajah, N. villosa, N. lowii, and other highland species on MS full or half strength consistently show browning at leaf margins within 2–3 transfers, declining multiplication rates over successive subcultures, and callus formation at the explant base instead of clean shoot proliferation.
On WPM at pH 5.5 with activated charcoal at 0.5 g/L, the same species typically show clean shoot multiplication at 4–5× per transfer, with no browning and consistent performance across 10+ generations of subculture.
Nepenthes khasiana (India’s Endemic Species)
India’s own pitcher plant shows particularly dramatic medium sensitivity. On MS full strength, N. khasiana typically shows leaf margin browning by transfer 3 and culture collapse by transfer 5–6. On WPM at pH 5.5, the same genotype maintains vigorous culture indefinitely. For anyone working with this conservation-significant species, WPM is the only appropriate medium.
Woody Trees (Teak, Sandalwood, Walnut)
Commercial TC of teak and sandalwood in India has largely settled on WPM as the base medium, often with modifications. These species on MS show chlorosis, poor root induction, and brittle shoot growth that doesn’t survive transfer to ex-vitro conditions well. WPM-grown shoots root more reliably and establish better post-hardening.
Species That Are Better on MS
- Banana (Musa): MS full strength is the standard, with BAP at 5 mg/L for Stage II. WPM offers no consistent advantage.
- Anthurium: MS half strength performs well for both Stage I and II. Browning in some cultivars is managed by diluting to half, not by switching medium.
- Orchids (Phalaenopsis, Dendrobium): Knudson C, Vacin & Went, and MS half strength are used depending on the stage. WPM is not the natural choice for most orchids.
- Vanilla planifolia: MS full with PVP and activated charcoal is the standard for Stage I. The browning issue in Vanilla is managed with anti-oxidants, not medium switching.
The Decision Framework
When deciding between MS and WPM for a species you’re working with for the first time:
- Is it a woody species or does it have significant secondary growth? Start with WPM.
- Does it naturally grow in nutrient-poor, acidic soil? WPM at pH 5.5 is the starting point.
- Is it highland-adapted, from elevations above 1000m, and/or from heath or bog environments? WPM, with pH adjustment to 5.0–5.5.
- Is it a herbaceous crop, ornamental, or commercially established TC species? Start with MS — published protocols usually specify MS for a reason.
- Seeing progressive browning or declining performance across multiple transfers on MS? Trial WPM before adjusting PGR concentrations.
Modifications That Make Both Media Work Better
- Activated charcoal (0.5 g/L): Adsorbs phenolic compounds released by wounded explants. Essential for Nepenthes, Vanilla, and most woody species.
- PVP-40 (250–500 mg/L): Prevents oxidative browning in high-phenolic species at Stage I.
- PPM (0.5–1.0 ml/L after autoclaving): Reduces contamination risk in high-risk protocols. Add after cooling — autoclaving destroys PPM.
- AgNO₃ (1–2 mg/L): Inhibits ethylene action — useful for reducing vitrification in banana Stage II and improving rooting in recalcitrant woody species.
The Practical Starting Point
If you’re setting up a new protocol and are uncertain: run a small parallel trial. Prepare 10 bottles on MS half strength and 10 on WPM with identical PGR concentrations and the same explant source. Assess at 28 days for contamination rate, shoot production per explant, and quality of growth. The result will be more informative than any literature recommendation — because it will be specific to your species, your facility, and your team.
Medium choice is the foundation that everything else is built on. Get it right at the beginning, and the subsequent work of optimising PGR concentrations, pH, and additives is refinement rather than rescue.