Chlorophyll doesn’t simply wear out or fade. A healthy leaf keeps its green steady by making new pigment as fast as the old pigment breaks down.1PubMed Central. Metabolic Reprogramming in Chloroplasts under Heat Stress in Plants Leaves turn yellow when the plant deliberately switches on a breakdown program, and that switch, not damage, is the cause. Usually the plant reads a signal from its surroundings, such as the length of the day, and passes it along with chemical messengers inside the plant. The care is warranted: loose chlorophyll in sunlight is dangerous to the plant’s own cells.2Chemistry Letters. Risk Management by Organisms of the Phototoxicity of Chlorophylls
Shorter Days Start the Autumn Color Change
In trees like aspen, the main trigger for autumn leaf breakdown is days getting shorter.3PubMed Central. The control of autumn senescence in European aspen In one closely tracked aspen, breakdown began on a single September date set by day length alone, then ran steadily regardless of weather.4PubMed Central. A Cellular Timetable of Autumn Senescence A warm spell in early fall usually won’t stop yellowing once it has started.
A tree uses day length rather than temperature because the length of the day on a given date is the same every year, while temperature jumps around. A tree that relied on warmth or cold could be fooled into shedding during a cold snap in August. Day length lets it start reclaiming its leaves well before the first hard frost kills them. Temperature then acts as a fine-tune once the process is underway.
Trees still vary widely in when they start, even neighbors in the same stand.5PubMed Central. The control of autumn senescence in European aspen Trees whose ancestors came from farther north tend to start earlier, matching their native latitude.6PubMed Central. The control of autumn senescence in European aspen So two maples on the same street can change at different times: each carries its own inherited timing, and a nursery tree raised from northern stock may color earlier than a local one. Microsites matter too, since a tree near a warm wall or a streetlight may lag.
Once breakdown begins, cold temperatures and longer nights make it go faster.7PubMed Central. The control of autumn senescence in European aspen Trees that start late tend to change color faster, catching up.8PubMed Central. The control of autumn senescence in European aspen In larch seedlings kept at 8°C, needles lost their chlorophyll in about 22 days, versus about 54 days at 15°C.9Plant, Cell & Environment. Effects of air temperature, photoperiod and leaf age on foliar senescence of western larch (<i>Larix occidentalis</i> Nutt.) in environmentally controlled chambers
What you see as the green goes depends on what’s left behind. Yellow and orange pigments were in the leaf all summer, hidden under the green, and they show through as chlorophyll disappears. Reds are different, because many trees make new red pigments in autumn.
Leaf Aging Is a Planned Shutdown
Chlorophyll loss is the most visible step in the planned aging of a leaf. Plant hormones guide every stage of it, from the start through the final phase.10PubMed Central. Leaf senescence: progression, regulation, and application The pigment is held in protein complexes that contain a lot of the leaf’s nitrogen, which the plant wants back before the leaf drops.
The leaf cells stay alive and working during most of the color change. They are actively running the breakdown and shipping nutrients out, and the leaf itself dies at the end, after the useful material is gone.
The payoff is real. Nitrogen from the dismantled chlorophyll-protein complexes is carried out of the leaf through the plant’s plumbing and stored in stems, roots or buds. In trees, that nitrogen is reused the next spring to build new leaves, so the autumn shutdown is partly an investment in next year’s growth. A leaf dropped while still green hands all that nitrogen to the leaf litter instead, a real loss for the plant.
The other reason for the careful teardown is that chlorophyll freed from its normal place is extremely toxic to cells in light.11Chemistry Letters. Risk Management by Organisms of the Phototoxicity of Chlorophylls Chlorophyll is built to capture light energy. When it isn’t wired into its usual machinery, that energy has nowhere safe to go and can pass to oxygen, making a reactive form that damages cell parts. So plants convert the pigment step by step into colorless compounds that no longer react with light.12Chemistry Letters. Risk Management by Organisms of the Phototoxicity of Chlorophylls Being able to clear this pigment safely appears to have been a key step that let plants move onto land about 500 million years ago.13bioRxiv. Evolution of chlorophyll degradation is associated with plant transition to land
Ethylene and Other Hormones Carry the Signal
Some hormones speed leaf aging and others slow it. Ethylene, a gas plants make themselves, pushes it forward, along with jasmonic acid, abscisic acid and salicylic acid.14PubMed Central. Leaf senescence: progression, regulation, and application Another hormone group also promotes it.15PubMed Central. Leaf senescence: progression, regulation, and application Cytokinins, gibberellins and auxin delay it.16PubMed Central. Leaf Senescence: The Chloroplast Connection Comes of Age Cytokinins can even keep a cut-off leaf green by preventing chlorophyll breakdown.17PubMed Central. Leaf Senescence: The Chloroplast Connection Comes of Age
No leaf is under the control of one hormone. It responds to the mix, and the ratio between them is what matters. Cytokinins arriving from the roots hold the brakes on, while rising ethylene and abscisic acid push the accelerator. As a leaf ages, the braking signals fade and the pushing signals build, so the shift to yellowing is usually gradual rather than a single flip.
Ethylene works by activating switch proteins that turn on chlorophyll-destroying genes directly.18PubMed Central. New Advances in the Regulation of Leaf Senescence by Classical and Peptide Hormones The same ethylene-driven switch has been found in crops such as maize.19PubMed Central. New Advances in the Regulation of Leaf Senescence by Classical and Peptide Hormones
This is also how outside conditions get translated into action. The plant senses short days or drought and responds by shifting its hormone balance. Abscisic acid, for instance, rises in drought, a sign of stress. Those hormones then turn on the breakdown genes.
The same logic explains why a cut flower or harvested lettuce yellows fast. Cutting removes the roots, a main source of the cytokinins that keep leaves green, so without that supply the aging signals win. Damaged produce also makes extra ethylene, and storing ethylene-releasing fruit near greens can speed yellowing.
Heat, Drought, Darkness and Hunger Start It Early
Heat stress commonly shows up as leaf yellowing, because chlorophyll content drops.20PubMed Central. Metabolic Reprogramming in Chloroplasts under Heat Stress in Plants In geraniums, heat caused new leaves to yellow and the plants to grow less.21HortScience. Heat Stress Affects Plant Morphology, Leaf Chlorophyll Content, and Photosynthetic Characteristics in Pelargonium Heat on top of an existing drought hurts the leaf’s light-capturing machinery more than either does alone.22PubMed Central. Combining Heat Stress with Pre-Existing Drought Exacerbated the Effects on Chlorophyll Fluorescence Rise Kinetics in Four Contrasting Plant Species So in a summer heat wave, a plant that’s also short on water is the one most likely to yellow.
Darkness works through a different route. Keeping leaves in the dark turns on several chlorophyll-breakdown genes, including the main one that starts the process.23PubMed. PHYTOCHROME-INTERACTING FACTOR 5 (PIF5) positively regulates dark-induced senescence and chlorophyll degradation in Arabidopsis A light-sensing protein in the plant helps switch those genes on when light is missing.24PubMed. PHYTOCHROME-INTERACTING FACTOR 5 (PIF5) positively regulates dark-induced senescence and chlorophyll degradation in Arabidopsis A houseplant in a dim corner, or lower leaves shaded by upper growth, can yellow for this reason.
Hunger does it too. When algae run short of nitrogen, they lose much of their chlorophyll and other light-capturing pigments.25PubMed. Low light intensity and nitrogen starvation modulate the chlorophyll content of Scenedesmus dimorphus The cells take apart nitrogen-rich pigment proteins to reuse the nitrogen.26Aquatic Botany. Effects of nitrogen starvation on the photosynthetic physiology of a tropical marine microalga Rhodomonas sp. (Cryptophyceae) Land plants short of nitrogen likewise yellow, usually in older leaves first.
Giving up chlorophyll is a sensible response. When heat or drought has damaged the light-capturing machinery, keeping all the chlorophyll can do more harm than good, since it keeps absorbing light the damaged machinery can’t use safely. Taking it apart lowers that risk and lets the plant recover some nitrogen.
The pattern of yellowing offers clues. Older lower leaves yellowing first can point toward nitrogen shortage or shading. Yellowing on new leaves after a heat wave can point to heat stress. Wilting plus yellowing can suggest water problems, though these patterns are clues rather than proof. A few of the oldest leaves yellowing slowly is often just normal aging.
Fixes follow from the cause. Water deeply before a forecast heat wave rather than after. Move a houseplant closer to a window if its lower leaves are yellowing in dim light. If pale older leaves suggest hunger, feed with a nitrogen-containing fertilizer. Already-yellow leaves rarely regreen, so judge success by new growth.
Why Bananas and Citrus Lose Their Green
In citrus peel, ethylene increased a chlorophyll-removing enzyme about 12-fold.27PubMed. Ethylene induces de novo synthesis of chlorophyllase, a chlorophyll degrading enzyme, in Citrus fruit peel The peel builds new copies of that enzyme in response, and the enzyme hardly exists without ethylene.28PubMed. Ethylene induces de novo synthesis of chlorophyllase, a chlorophyll degrading enzyme, in Citrus fruit peel In calamondin, the effect took about 6 to 12 hours of ethylene exposure to appear.29HortScience. Effect of Ethylene on Chlorophyllase Activity and Chlorophyll Content in Calamondin Rind Tissue Gibberellins and cytokinins block the enzyme from being made.30PubMed. Ethylene induces de novo synthesis of chlorophyllase, a chlorophyll degrading enzyme, in Citrus fruit peel
Bananas follow the same rule. An ethylene-controlled switch turns on the gene that starts chlorophyll removal.31PubMed Central. Ethylene Response Factor MaERF012 Modulates Fruit Ripening by Regulating Chlorophyll Degradation and Softening in Banana An anti-ethylene storage treatment at high doses can block that switch, leaving bananas that ripen without properly yellowing.32PubMed Central. Ethylene Response Factor MaERF012 Modulates Fruit Ripening by Regulating Chlorophyll Degradation and Softening in Banana In many fruits the peel is a working leaf-like tissue early on, so it uses the same breakdown machinery as aging leaves.
That is why a commercial orange can be green-skinned but ripe. In warm climates citrus can be ripe inside while the peel stays green, because cool nights are what normally trigger the color change. Packers may expose fruit to ethylene to degreen the peel. This changes the color, not the sweetness.
At home, you can use the same gas. A paper bag traps the ethylene fruit gives off, and adding a ripe banana or apple adds more. A warm room helps, while fridge cold slows ripening. Expect results over a day or more, not hours.
The First Step Inside the Leaf
Chlorophyll sits in protein complexes inside the chloroplast, and these must be loosened first.33PubMed Central. The senescence-induced staygreen protein regulates chlorophyll degradation A protein called stay-green binds those complexes and pulls them apart.34PubMed Central. The senescence-induced staygreen protein regulates chlorophyll degradation It then removes the magnesium atom at chlorophyll’s center, the first committed step of breakdown.35PubMed Central. Crystal structure and reaction mechanism of a bacterial Mg‐dechelatase homolog from the Chloroflexi Anaerolineae Once the magnesium is out, the pigment is set on a one-way path.
Chlorophyll is a ring with a magnesium atom in the middle and a long tail. The magnesium helps give it its bright green hue, so without it the pigment becomes a duller olive-brown compound. That’s the first visible sign in a leaf just beginning to fade, and it’s also why cooked green vegetables turn drab olive. The ring is still intact at that stage, so some color remains.
The green disappears entirely when another enzyme breaks open chlorophyll’s ring-shaped core.36PubMed Central. Chlorophyll breakdown: pheophorbide a oxygenase is a Rieske-type iron-sulfur protein, encoded by the accelerated cell death 1 gene This step’s enzyme becomes more active as leaves age.37PubMed Central. Chlorophyll Breakdown in Senescent Arabidopsis Leaves. Characterization of Chlorophyll Catabolites and of Chlorophyll Catabolic Enzymes Involved in the Degreening Reaction The end products are colorless compounds found in aging leaves of many plants.38PubMed. A novel blue fluorescent chlorophyll catabolite accumulates in senescent leaves of the peace lily and indicates a split path of chlorophyll breakdown
The order matters. Each piece must be freed before the next enzyme can reach it, and leaving half-broken pieces around would be dangerous in light. The cell moves the pieces out of the chloroplast to store them.
Plants That Stay Green
Plants that fail to break down chlorophyll are called stay-green. Gregor Mendel’s green-seed peas carried exactly this kind of breakdown defect.39PubMed. Stay-green protein, defective in Mendel’s green cotyledon mutant, acts independent and upstream of pheophorbide a oxygenase in the chlorophyll catabolic pathway The defective stay-green protein can’t loosen chlorophyll from its holders.40PubMed Central. The senescence-induced staygreen protein regulates chlorophyll degradation
Green and yellow dried peas differ by this one trait. Yellow peas broke down their chlorophyll as the seeds matured, and green peas couldn’t.
Green isn’t always healthy, though. In some stay-green plants, the leaf stays green but has already stopped working.41PubMed. Stay-green regulates chlorophyll and chlorophyll-binding protein degradation during senescence
Breeders value the stay-green plants that keep photosynthesizing longer, since these can help grain fill in drought. Only the kinds that stay working are useful.