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I live in Pittsburgh PA, the eclipse on Monday August 21 was only about 80% coverage here. It was notably darker, but less dark then an overcast day. The night of August 21, had a low temp of about 70F. On my daily walk the following day (Aug 22) there was notable amount of fallen leaves from the trees along my daily walk (mostly maple), by this morning (Wednesday Aug 23) the leaf fall has increased such that the grounds crews are out cleaning the leaves.

Did the eclipse make the leaves fall?

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  • $\begingroup$ I think the photo period has to change for longer than a few hours to cause leaves to drop. I would imagine cold could kill tender growth much faster. $\endgroup$ – ebrohman Aug 23 '17 at 18:46
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What said by ebrohman is correct. In fact there are two factor that led to leaves falling when fall approaching: photoperiod and the circadian rhythm (24 hours) of the plant.
In the circadian rhythm is possible observe oscillation in proteins expression. Some of the proteins that regulate leaves abscission (aka falling) have an oscillations in their expression and present a pic of the expression in the late afternoon. Moreover to be active this protein need to be on "a dark" condition because the light will lead to proteins degradation.

For exemple

Imagine that in summer there is a long day condition with sunlight present from 7am until 20am (photoperiod) and that the proteins that lead to leaves abscission is expressed by the plant around 6pm (circadian rhythm). Due the presence of the sunlight the proteins will be degrade. When the hour of sunlight decrease (with fall approaching) the proteins is day by day less degraded and this could led to the plant to prepare itself for the winter, also with the leaves abscission.

If you are interested to know more about the argument I can suggest you an interesting review write by Pin and Nilsson (2012)

Cold Is possible that a decrease in the temperature lead to a leaves falling. However I think that temperature has to decrease significantly over more day to allow any changes in plants structure.

I hope that my answer could help, if you do not understand something let me know. I will try to explain it in a different way.

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