Botanist's Lens: Life Sans Senescence

Meenal Harankhedkar, Director of Horticulture
Nov 6, 2020
2 min read

Maidenhair tree (Ginkgo biloba)

Senescence is the last stage of plant development. Leaf buds grow into a leaf, mature and during senescence, the last stage occurs where it withers, falls and leaves the plant. Senescence and abscission (natural cutting off of plant part from the plant) together help in falling of leaves, flowers and fruits.

Without abscission how would the apple fall down from the tree? Then how would, Newton still discover gravity? Senescence and abscission are timed sequences that, even though they lead to death of a tissue, they are a key process for the plant to live on. Without fruits falling off, how would the seeds wither away? Without leaves withering in a timely manner, the plant would loose crucial building blocks to live on. Life sans senescence and abscission wouldn't be good for plants.

Maple 'Sango Kaku' (Coral Bark Maple)

Autumn senescence in plants is an important process in deciduous eastern forests. It is initiated by change in photoperiod first than change in temperature. Lower temperatures accelerate the green pigment chlorophyll degradation, but the onset is certainly due to change of day light hours. Senescence is a programmed process in which the plant almost reclaims its building blocks from the leaves, before they wither and fall away.

"Leaf senescence is induced as part of plant development but can also be prematurely induced as a result of environmental changes or harvesting. Premature senescence leads to reduced yield and quality of crops and this is likely to be of increasing concern in times of climate change and parallel population growth." - Source

Fan shaped Ginkgo tree leaves green pigments slowly diminish and unmask the bright yellow carotenoids.

In autumn when the leaves start changing colors - from green to warm colors of matted blotches of orange and yellow, red and yellow or just to pure gold yellow (such as with the Ginkgo), then it is a sign that the plant is salvaging all the possible building blocks before the leaf prematurely dies of a cold snap. Programmed autumn senescence is quite a complex process, in which the green pigments chloroplast reduce in number unmasking the hidden yellow and orange cytokinin and anthocyanin pigments giving the leaves beautiful warm colors. In other words, the leaves don't suddenly turn yellow/orange, they have those pigments in them from the time they are formed, but the green pigment conceals them completely.

Black gum tree (Nyssa) oranges in the backdrop are precious along with the feathery golden Amsonia. But the bright plum red Oak leaf Hydrangea, aptly takes center stage!

Autumn colors are plenty and warm. London Town gardens are currently displaying the stunning golden yellows of the Ginkgo tree, red-orange hues of Witch Hazels, Oak leaf hydrangeas and many more. Come and enjoy them before the gusty winds blow them away.