Swaying gracefully in the gentle breeze, a tattered leaf took the spotlight. Though old and torn, the sunlit leaf, adorned by vivid colours from nature’s palette, stood out bright and resplendent like a beautiful piece of art. The colours, patterns, and intricate details had the power to spark one’s imagination.
As I stood in admiration I began to wonder: What was my favourite artist, Nature, telling me? What did the shades of yellow, orange, and brown amidst the receding green mean? Was colour being lost or gained? Why aren’t leaves simply dropped when their time is up? Why is there a showy display while going from leaf to litter?
Behind the flamboyance of dying leaves lies a complex yet beautiful story. A story unique to plants. In the animal world, senescence or the process of deterioration with age ultimately leads to death of the whole being. But in the plant kingdom, leaf senescence, is an extraordinary evolutionary feat that ensures death is not in vain. Here, senescence contributes to the plant’s upkeep.
Leaf senescence is the last stage of leaf development. It is not a chaotic, unintended, or unfortunate end to a life well lived. But it is the unfolding of a highly regulated, genetically programmed death in response to internal cues such as age information and/or unfavourable environmental conditions (like drought, extreme temperatures, nutrient limitation, pathogens, shading by other plants, among others).
Vivid Display
When occurring naturally, leaf death begins at the tip or margins, and progressively wipes out green as it marches in a coordinated manner towards the leaf base. However, when triggered prematurely by environmental factors that unevenly target leaves, affected leaf regions die earlier than other parts. Despite the manner in which leaves senesce, this stage in the life cycle of a plant is often illustrated by a splendid display of colour changes in leaves from greens to yellows, oranges, and/or reds to browns.
Leaves get their vivid colours from organic compounds called pigments housed within chloroplasts: tiny cellular structures inside plant cells where photosynthesis occurs. Chlorophyll is one such pigment that puts time spent sunbathing to good use. It soaks up sunlight in the red-blue wavelengths, while reflecting blue-green to green wavelengths. Being an integral component of the photosynthetic machinery, the light energy it absorbs triggers a series of crucial chemical reactions that also utilise water and carbon-dioxide to churn out glucose — a simple sugar that plants snack on while expelling life-sustaining oxygen.
Yellow-Orange Sunlight Sponges
In addition to chlorophyll, all leaves contain pigments called carotenoids. They are responsible for yellow and orange hues. For e.g. a carotenoid known as beta-carotene is responsible for colouring carrots orange. In leaves, carotenoids are not mute spectators to chlorophyll’s doings, but are in fact partners in crime. They too help absorb sunlight, but in the blue to green wavelengths that chlorophyll does poorly in. They pass on the light energy they absorb to neighbouring chlorophyll molecules thereby increasing light harvesting efficiency. Yet, in healthy leaves, chlorophyll is clearly the star of the show. Being the stronger pigment, it masks others. But, carotenoids break down more slowly when compared to chlorophyll. Thus, in senescing leaves as chlorophyll breaks down and greens fade, the yellow and orange pigments finally take centre stage.
The Red-Magenta Sunblock
Reds and magentas result from pigments called anthocyanins. Unlike the green, yellow and orange pigments, not all plants produce anthocyanins. Colouration by anthocyanins is transiently seen in certain species. It is expressed in young, growing leaves or old, ageing leaves. At times, it is seen in mature leaves in response to environmental stress. Anthocyanins are known to protect leaves from a variety of threats: leaf chomping herbivores, excess light, UV radiation, and low temperatures to name a few. As far as senescing leaves are concerned, the prevailing theory is that red pigmentation acts as a protective sunblock while photosynthetic machinery continues to break down in dying leaves.
Zero Waste
Although these colourful displays during senescence are aesthetically pleasing one might wonder, why all the fuss? Why spend time and energy carefully breaking down and (at times) protecting a dying leaf? Why not just sever the connection? Why not just drop leaves?
Leaves are the major photosynthetic organs of plants. They are treasure troves of nutrients in the form of cellular material that has accumulated over the growth period. For example, chloroplasts contain a substantial amount of leaf protein. Chlorophyll itself is a large pigment that is energetically expensive to produce. Simply dropping leaves would be wasteful. And nature wastes nothing.
Slow Death
During senescence, anabolic cellular processes (processes that build molecules) such as photosynthesis cease. Instead, a slow coordinated self-destructing process or genetically programmed cell death is activated. Leaf cells before being killed off are carefully and purposefully dismantled.
The earliest and most significant of changes occur in the chloroplast. An increase in catabolic activity (processes that break down complex molecules into smaller molecules) helps convert cellular materials such as chlorophyll, protein, and other molecules into exportable nutrients. These are shipped off to supply developing seeds and other growing organs, or stored by plants to be used later.
In contrast, parts of the cell such as the nucleus and mitochondria that play an important role in the mobilisation of nutrients remain intact until the very last stage of senescence. Leaf senescence is thus an active recycling of nutrients and energy that was initially invested in growing and maintaining leaves and their photosynthetic machinery.
Slow degeneration and production of protective sunscreens are strategies that enable plants to recover any last remaining nutrients before bidding farewell to leaves. Although the process is deleterious to the leaf organ as it is killed off slowly cell by cell, it has evolved to serve a larger purpose — contributing critically to the continued success of the whole plant. Hence, leaf senescence is often described as an “altruistic death”.
What a beautiful story Nature tells us. Of dying leaves embracing their end as if it were a new beginning.
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