South Texas Fall Color (Not Really)

Helietta parvifolia , also known as Beretta, is a hyper-drought tolerant member of the Citrus Family (Rutaceae) that grows in rocky areas of South Texas and Northern Mexico. The leaves turn yellow when drought-stressed, but don't fall off.

When the leaves turn yellow it's because as the plant is entering a state of water stress it js breaking down the chlorophyll molecules and reabsorbing the nutrients they contain, like nitrogen. This "unmasks" the yellow pigments, which are always there but unable to be seen because they are overshadowed by the greener chlorophyll. This is the same thing that plants in colder climates do, except unlike plants in cold climates Helietta doesn't drop its leaves. Abcising a set of leaves and then having to grow them again is calorically very expensive. Much easier to just turn off photosynthesis temporarily but keep the machinery.

When the chlorophyll is reabsorbed, this effectively shuts down photosynthesis, And the plant basically enters a state similar to torpor in animals - a means of conserving resources until the rains come again.

I have a Helietta in my yard and it stays a glistening green all year, as my yard is a much milder and more music environment than these rocky hills and lomas of the South Texas borderla

nds.

The thing is : it's not just Helietta that does this. Plenty of plants from tropical areas with extended dry seasons do this as well. Well there are many plants from the same climate that are drought deciduous, there are also evergreen plants that never drop their leaves but shut down photosynthesis by reabsorbing the chlorophyll and turning yellow.