Green plants contain chlorophyll, a green pigment that absorbs light for photosynthesis. There are several reasons why green plants can turn brown:
Loss of chlorophyll
The most common reason is that the plant loses chlorophyll as it ages or becomes stressed. Chlorophyll breaks down over time, especially when the plant is exposed to too much sun, not enough water, disease, or cold temperatures. As the chlorophyll fades away, other pigments that were masked by the green color become visible. These include carotenoids, which are yellow, orange, and brown.
As leaves age, chlorophyll production slows down. In autumn, deciduous trees stop making chlorophyll in preparation for winter dormancy. The green fades away, revealing the yellows and oranges of carotenoids. Evergreen trees keep their leaves for multiple years, so old leaves turn yellow or brown while new growth remains green.
Stressors like drought, flooding, pests, and disease disrupt the plant’s metabolism, preventing it from producing enough chlorophyll. Physical damage to leaves can also destroy chlorophyll. As the chlorophyll diminishes, the underlying pigments show through.
Production of other pigments
While loss of chlorophyll is the most common cause of greens turning brown, some plants actively produce additional pigments that change their color.
Some plant leaves turn red or purple due to production of anthocyanins. These red/blue pigments act as antioxidants that protect leaves from sun damage. Certain environmental stresses like drought, cold, or nutrient deficiency stimulates anthocyanin synthesis.
Unlike anthocyanins, tannins and polyphenols are brown plant pigments. They are often produced as a defense response to insect attack. The unpalatable tannins deter herbivores from consuming the leaves. Polyphenols also act as antioxidants.
Oxidation reactions
Enzymatic or non-enzymatic oxidation reactions can also turn plant leaves brown:
– Polyphenol oxidase is an enzyme that oxidizes polyphenols into brown melanin pigments. This protective mechanism is triggered by leaf injury.
– Iron oxidation occurs when iron-containing proteins are exposed to oxygen. This happens during senescence, disease, or physical damage. The iron turns brown upon oxidizing.
– Non-enzymatic oxidation of polyunsaturated fatty acids and pigments by reactive oxygen species leads to polymerization and browning. This type of oxidation is exacerbated by stresses like drought and air pollution.
Factors that accelerate browning
Certain environmental conditions and stresses speed up the browning process in plants:
– Age: Older leaves have lower chlorophyll content and are more prone to oxidative browning.
– Light: Excess light causes photooxidation of chlorophyll and stimulates production of anthocyanins and other pigments.
– Temperature: Extreme heat denatures leaf proteins and pigments. Cold temperatures break down chlorophyll.
– Water: Drought stress accelerates chlorophyll loss and antioxidant production. Overwatering promotes fungal or bacterial diseases.
– Air pollution: Oxidant pollutants attack leaf pigments and metabolites.
– Nutrient deficiency: Lack of nitrogen, iron, magnesium, or other essential elements impairs chlorophyll synthesis.
– Physical damage: Bruising, herbivory, or mechanical wounding destroys chlorophyll and exposes polyphenols to oxidation.
– Pathogens: Bacterial, fungal, or viral diseases disrupt leaf metabolism, cell integrity, and pigment production.
| Factor | Mechanism of Browning |
|---|---|
| Age | Loss of chlorophyll over time |
| Light | Photooxidation of pigments |
| Temperature | Heat denaturation, cold breakdown of chlorophyll |
| Water | Drought stress, disease from overwatering |
| Air pollution | Oxidative damage to leaf metabolites |
| Nutrient deficiency | Impaired chlorophyll synthesis |
| Physical damage | Destruction of chlorophyll, polyphenol oxidation |
| Pathogens | Disruption of leaf metabolism and pigments |
Strategies to prevent browning
Browning of green leaves and needles is a natural process, but gardeners can slow it down with proper care:
– Water plants regularly during drought to reduce moisture stress. Improve drainage in poorly-draining soils.
– Avoid overfertilization, especially with excess nitrogen which causes rapid leaf growth and premature aging.
– Prune branches to increase air circulation and light penetration, preventing fungal diseases.
– Protect plants from harsh sun using shade screens or positioning. Gradual light acclimation minimizes photooxidation.
– Monitor for pests and diseases. Remove infected material and use recommended fungicides/insecticides if necessary.
– Apply masking agents like anti-transpirants and reflective clay particles to protect against photooxidation and heat stress.
– Remove accumulated debris like dead leaves or needles to limit disease inoculum. However, avoid excess pruning which stresses plants.
– Grow resistant cultivars that can better tolerate environmental and physiological stresses leading to browning.
Conclusion
Browning of green leaves is an unavoidable process in plants caused by natural aging, environmental stresses, or disease. The underlying mechanisms include loss of chlorophyll, production of other pigments, and various types of oxidative browning reactions. While browning cannot be prevented completely, good horticultural practices like proper watering, fertilization, and pest management can slow the process and prolong the lifespan of green leaves. Targeting the specific stress factor, whether it be drought, nutrient deficiency, or pathogens, is key to reducing browning. With appropriate care, the magnificent greens of the plant world can better persist before giving way to autumnal colors.
