Descent of Frost: Leaves turn yellow and fall

April 29th, 2012
by Jo Law

This pentad began on a windy Anzac day public holiday. The maximum wind gust recorded at Bellambi (AWS) was 80km/h at 08.28am. This 5-day period ended with a wet and grey wintery day. Relative humdity on average has fallen slightly but there is still plenty of moisture in the air and we remain ever vigilent against the prescence of ticks in the backyard. During this period, we were finally able to make a push in the garden (thanks to our friend, Michelle, who babysat Hollis for the morning) and started cleaning up areas favoured by the dreaded arachnids.I picked the last cucmbers off the vines to make a last batch of relish. The tomatoes bushes have crept out of their bed and colonised the surrounding areas. Their leaves are still very green, some are bearing small immature fruit with little yellow flowers further up the stems. The ripening of any fruit at this time would have to compete against the lessening daylight. Our proximity to the escarpment means that we are loosing daylight earlier. We decided to cut our loses and harvested the unripe fruit to make green tomatoes relish.

As mentioned elsewhere, the chemical, ethylene, play a key role in both the dropping the ripend fruit and falling of leaves. Known as abscission, this mechanism in temperate dedicious plants is preceded by the reduction in (and later complete cessation of) chlorophylls production, triggered by the lengthening of darkness and corresponding degration of photosynthesis. The green colour of the foliage fades as chorophylls are withdrawn, unmasking the yellows, oranges, and browns produced by carotenoids in the leaves. The pigment, anthocyanin, produced earlier in some plants protecting the leaves from the sun’s harmful ultraviolet radiation, create the brilliant reds and purples. Abscission in decidious plant species is a  conservation mechanism that defends the organisms from water loss in dry winters. Leaf fall also occurs in tropical and sub-tropical species but without the ‘autumn colours’.

The autumnal foliage displays of decidious trees are enjoyed and celebrated by different cultures that inhabit temperate climatic zones. In Japan, the autumnal display of the bright red momiji (maple) and luminous yellow of the ichou (ginko) are met a quiet awe when compared to the feverish hanami (cherry blossoms viewings) of springtimes. Nowhere is the appreciation for empheral beauty, transciency and impermenance so venerated.

In our garden, the almond tree has already lost most of its leaves while the maple is yet to change colour. Surprisingly, the mulberry still has most of its green foliage. We worked in the yard to pile up sticks and branches (from prunings) to dry for firewood under the white cedar which is freely shedding its browning leaves. Our autumnal celebration is simply be marked by the collection of leaves and sticks.

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