Leaves change color in the fall when they start to break down chlorophyll, which gives them their green color. Chlorophyll is used for photosynthesis, a process by which plants synthesize glucose using the energy of sunlight; photosynthesis also removes CO2 from the atmosphere and generates oxygen. Chlorophyll breakdown recycles precious building blocks that trees use as they shut down their metabolism in preparation for winter. Once chlorophyll levels drop, we can start to see the yellow and orange pigments from the carotenoids. Chlorophyll and carotenoids are always present in leaves but trees also synthesize a new red pigment called anthocyanin in the fall. Anthocyanin is thought to protect fall leaves from high light intensities, acting as a sort of sunscreen. Neither carotenoids nor anthocyanins contain a lot of the building blocks trees scavenge from chlorophyll. The resulting varying levels of chlorophyll, carotenoids and anthocyanin result in a color palette that ranges from pale green to yellow, orange, red and dark maroon, beautiful to behold in the fall.
This color display of broad-leaf trees is not equally stunning each year, nor does it happen at exactly the same time. Plants sense what season it is by measuring both the length of the day and the ambient temperature. Foliage changes are mainly triggered by the shortening of day length in the fall. Importantly, anthocyanin biosynthesis requires sunlight and sugars, so the display of red is most stunning in sunny falls. Cool nights (low 40s) also contribute to increased anthocyanin production by trapping sugars in the leaves. The optimal combination of short, sunny, days and cool night means that the majority of the trees in the Northeast produce anthocyanin, while very few do so in the Southern or Western parts of the country. Leaf color displays may be less stunning if the climate changes, for example by causing warmer fall nights in the Northeast and Midatlanic regions. Another factor that can impinge on the fall foliar color display is premature leaf drop triggered by summer droughts, storms in early fall with high winds and rain, or early frost.
Why do some trees -such as broad-leaf trees- drop their leaves in the fall to begin with? Clearly many trees do not drop their leaves including conifers (pine trees or firs) or trees with tough small leaves (like California live oaks). Leaves lose a lot of water to the atmosphere during photosynthesis, with broad, fleshy, leaves incurring much larger water losses than pine needles or small scaly leaves. The lost water is replenished from the soil via water uptake in the roots and water transport through the tree trunk and branches to the leaves. If photosynthesis were triggered in broad-leaf trees on a sunny winter day, the water lost not could not be replenished if the ground and the water in the tree is frozen. This would cause tree embolism, presence of air bubbles in the water transport system that block water transport even after a thaw. Therefore, broad leaf trees play it safe and drop their leaves in the fall to resume their cycle of growth come spring. Prolonged droughts, as have been experienced in California in recent years, also causes tree embolisms, again water is lost from leaves that cannot be replenished from soil water, leading to loss of foliage and even tree death.
For more coverage on this topic see 11/24/2021 PennToday Article:
Today we gleefully celebrated Dr. Sarah Matar’s (possibly the best baker in Wagner Lab history) birthday with two delicious chocolate cakes. Getting innovative with cake traditions she blew out her candles using an ensemble method of allowing the wind to blow them out to waving her hand back and forth. A very impressive birthday scientist indeed!
For this years spooky season we celebrated by carving pumpkins, drinking pumpkin soup, and eating pumpkin pie. For many of us it was our first time carving pumpkins, and we had so much fun despite the wind.
Bieluszewski, T., Xiao, J., Yang, Y., & Wagner, D. (2021). PRC2 activity, recruitment, and silencing: a comparative perspective. Trends in Plant Science.
Polycomb repressive complex (PRC)-mediated gene silencing is vital for cell identity and development in both the plant and the animal kingdoms. It also modulates responses to stress. Two major protein complexes, PRC1 and PRC2, execute conserved nuclear functions in metazoans and plants through covalent modification of histones and by compacting chromatin. While a general requirement for Polycomb complexes in mitotically heritable gene repression in the context of chromatin is well established, recent studies have brought new insights into the regulation of Polycomb complex activity and recruitment. Here, we discuss these recent advances with emphasis on PRC2.
Related External Link
After a busy year of sheltering in place and self-quarantines, Dr. Doris Wagner and Dr. John Wagner treated the lab and loved ones to a joyful dinner in her backyard. This past year+ was a bit quirky, but we made the best of what we could. Who knows what the future may bring, but for now, we are so thankful to be able to feast and socialize together again.
With so many new faces joining the Wagner lab over the last year, about half the lab has only known our lab meetings as virtual. However, now that all the members of the Wagner lab have been fully vaccinated, we can finally return to lab meetings in-person. While virtual meetings had their pros (its easier to see the minute details of a slide on your own computer) and cons (having to constantly monitor your mute button), having our first in-person lab meeting felt like an accomplishment that warranted a group selfie.
Senior at University of Pennsylvania, Adam Konkol, an undergraduate who worked in the Wagner lab, was awarded a Churchill Scholarship along with December graduate Abigail Timmel. Featured in PennToday, the Wagner Lab is excited to have been a part of Konkol’s journey and look forward to what the future has in store. For more information, see the PennToday article at: https://penntoday.upenn.edu/news/two-churchill-scholars-penn
The Wagner Lab is delighted to receive yet another fantastic addition to our team. Dr. Sarah Matar joins us after defending her PhD thesis on the vernalization-driven transition to flowering in winter rapeseed in the lab of Dr. Christian Jung of the Plant breeding institute in Kiel University in Germany. In the Wagner Lab, we look forward to her work on understanding the antagonistic roles of FT and TFL1.
This Valentines Day weekend the Wagner Lab welcomed the newest member of our lab, Dr. Sandhan Prakash, to the City of Brotherly Love! Dr. Prakash flew all the way from the Indian Institute of Science (IISc) in Bangalore, India where he received his Ph.D. In the Wagner Lab, he plans to study the function of epigenomic regulators in cell fate reprogramming and plant survival mechanism during abiotic stress response.