Why Tissue Culture Plants Turn Brown, Glassy or Stop Growing
Not every tissue culture problem is contamination. Plants can remain perfectly sterile while still growing poorly. Browning, glassy tissue, yellow leaves and stalled growth are all common symptoms that can provide clues about what is happening inside the culture vessel.
Browning
Freshly cut plant tissue can release phenolic compounds. When these compounds oxidize, the explant and surrounding medium may turn brown. Some species produce much more phenolic material than others. Severe oxidation can damage tissue and eventually kill the explant.
Browning may become worse after:
- aggressive cutting
- strong sterilization
- tissue injury
- prolonged stress
Reducing unnecessary tissue damage and transferring affected explants to fresh medium can sometimes help.
Hyperhydricity
Hyperhydricity is often described as glassy, watery or translucent growth. Affected shoots may look swollen and unusually soft. Leaves can become thick, brittle or malformed.
Hyperhydricity can be associated with:
- excessive cytokinin
- poor gas exchange
- very high humidity
- overly wet medium
- low agar concentration
- certain nutrient conditions
Reducing cytokinin exposure or improving the physical conditions of the culture can sometimes improve later growth.
Yellow leaves
Yellowing may indicate several different problems. Possible causes include:
- nutrient imbalance
- declining medium quality
- excessive salt concentration
- unsuitable pH
- aging leaves
- light stress
- poor root function
The pattern of yellowing is often more informative than the symptom alone.
Shoots that remain very small
High multiplication rates are not always desirable. Strong cytokinin exposure may produce many tiny shoots that fail to elongate. If shoots remain extremely compact, reducing cytokinin concentration during the next subculture may help them develop more normally.
Cultures that stop growing
Growth can slow when the medium becomes depleted or the plant has remained in the same vessel for too long. Other possibilities include:
- inappropriate hormone balance
- unsuitable basal salt concentration
- insufficient light
- low temperature
- physiological stress
- latent contamination
A fresh subculture can often help determine whether the problem lies with the old medium.
One symptom can have several causes
Tissue culture troubleshooting is rarely as simple as:
“Brown = one specific problem.”
Instead, look at several variables together.
Ask:
- When did the symptom begin?
- Did it happen immediately after inoculation?
- Is every culture affected?
- Was anything changed in the last batch?
- Is the medium the same?
- Is the temperature stable?
- Are other cultures growing normally?
Good troubleshooting is essentially controlled experimentation.
Make one change, observe the response, and keep records.