Juvenile Reddening: Red Tips & Purple Leaves on New Growth Help Plants Manage Stress

Southwest Yard & Garden by Dr. Marisa Thompson

   

Reddish-purple coloration on young leaves, aka juvenile reddening, is caused by pigments that protect tender tissue from cold, heat, drought, and UV stress. From top left: rose bush, pomegranate (redness only in tiny new leaves), tree of heaven (redness only in newest leaflets). From bottom left: redbud tree (in my front yard), lamb’s quarters, chinquapin oak. Photos taken in Albuquerque and Los Lunas on May 25, 2020, by Marisa Thompson.

Question: Why is new growth at the tips of some plants distinctively redder than older leaves?

-          Yours Truly, Los Lunas, NM

Answer: I’ve been noticing young reddish-purplish leaves on otherwise green plants, and it makes me wonder what benefits these colors might have for the plants themselves. Past columns on other pigment-related questions have been the most liked, clicked, and shared compared to any other topic. In the most popular weekly column I’ve published so far, “What Does Red or Green Really Mean? - Phytochemical Coloration in Chiles,” assistant professor of horticulture at NMSU Dr. Ivette Guzman helped me understand why pigments are so cool in chile peppers: “The diverse colors on peppers are indicators that they are rich in phytochemicals, whether they be sweet, hot, or sweet and hot. Three main classes of phytochemicals are responsible for these pretty pigments. Chlorophyll is commonly associated with greenness. Carotenoids are the phytochemicals causing the colors to get warmer as they turn from yellow to orange to red. And anthocyanins create those dynamic purple and blackish colors. The relative concentrations of these phytochemicals as they accumulate in the fruit tissue dictate the color. These pigment chemicals also affect the nutrient content and use as natural dyes.” (Color is not, however, linked to pepper spiciness.)

The same pigments (chlorophyll, carotenoids, and anthocyanins) are also found in leaves of all kinds of plants. Red tip photinia is the first that comes to mind. But chlorophyll is the pigment associated with the photosynthetic process, and we know that process is crucial for plants to be able to turn sunlight, carbon dioxide, and water into sugars for food that are needed for plant growth. So why would new young leaves be red? Wouldn’t it make more sense if they were brighter green?

Well, on many plants the new growth is a nice bright green, but in plants with colored tips, the key is that just because we see red or purple leaves doesn’t mean there isn’t also chlorophyll hiding in there somewhere. In the column “Deciding Factors for Deciduous Color,” I explained that in the fall chlorophyll that breaks down in the leaves doesn’t get replenished. “As the green color fades, other pigments that were there all along, like yellow and sometimes orange, can be seen.”Answer: I’ve been noticing young reddish-purplish leaves on otherwise green plants and it makes me wonder what benefits these colors might have for the plants themselves. Past columns on other pigment-related questions have been the most liked, clicked, and shared compared to any other topic. In the number one most popular weekly column I’ve published so far, “What Does Red or Green Really Mean? - Phytochemical Coloration in Chiles,” assistant professor of horticulture at NMSU Dr. Ivette Guzman helped me understand why pigments are so cool in chile peppers: “The diverse colors on peppers are indicators that they are rich in phytochemicals, whether they be sweet, hot, or sweet and hot. Three main classes of phytochemicals are responsible for these pretty pigments. Chlorophyll is commonly associated with greenness. Carotenoids are the phytochemicals causing the colors to get warmer as they turn from yellow to orange to red. And anthocyanins create those dynamic purple and blackish colors. The relative concentrations of these phytochemicals as they accumulate in the fruit tissue dictate the color. These pigment chemicals also affect the nutrient content and use as natural dyes.” (Color is not, however, linked to pepper spiciness.)

The same pigments (chlorophyll, carotenoids, and anthocyanins) are also found in leaves of all kinds of plants. Red tip photinia is the first that comes to mind. But chlorophyll is the pigment associated with the photosynthetic process, and we know that process is crucial for plants to be able to turn sunlight, carbon dioxide, and water into sugars for food that are needed for plant growth. So why would new young leaves be red? Wouldn’t it make more sense if they were brighter green?

Well, on many plants the new growth is a nice bright green, but in plants with colored tips, the key is that just because we see red or purple leaves doesn’t mean there isn’t also chlorophyll hiding in there somewhere. In the column “Deciding Factors for Deciduous Color,” I explained, in the fall chlorophyll that breaks down in the leaves doesn’t get replenished. “As the green color fades, other pigments that were there all along, like yellow and sometimes orange, can be seen.”

Redbud leaves collected from the same tree get 

progressively greener and older from 

top to bottom. Photo credit M. Thompson.

In the 1999 review article “Environmental Significance of Anthocyanins in Plant Stress Responses,” Dr. Linda Chalker-Scott, the Extension Urban Horticulturist for Washington State University, reported that anthocyanin pigments responsible for juvenile leaf reddening have many protective roles, such as “preventing damage caused, directly or indirectly, by cold temperatures, drought, and UV radiation.” In some ways, these purple-red compounds near the leaf surface act as a sunscreen of sorts by physically shielding chlorophyll deeper in the leaf tissue from too much sun. 

As described in my November 2018 column on fall foliage, here’s a fun color activity you can do at home: Collect different-colored leaves and stick them in the freezer before they dry out too much. Cut a paper towel or coffee filter into strips. Keeping the different-colored leaves separate, rip them up and then mash them with a little rubbing alcohol using a mortar and pestle, the blunt end of a wooden spoon, or a blender to rupture the leaf cells and let the chlorophyll (and other pigments!) run out, turning the alcohol bright colors. (Freezing the leaves before trying to smash the colors out helps rupture the cells and break down the tissues.) Pour the pigmented alcohols into different cups and dip one edge of the paper towel or coffee filter in the liquid. If your paper strip is long enough—or the cup is short enough—you can drape the paper over the edge and let it sit. The pigments are made up of different molecules that run through the paper at different rates, so on green leaves, you’ll likely see a yellow or orange color (carotenoids) separate from the green (chlorophyll). And in the purplish-red leaves, you’ll see the longer blur of anthocyanins lower on the filter strip, with yellow on top and bright chlorophyll in between.

Setting up the experiment with rubbing alcohol in jars, leaves of different colors, a wooden spoon for mashing, and filter paper to watch the pigments separate as they soak up. Photo credit M. Thompson.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the NM Desert Blooms Facebook page (@NMDesertBlooms)

Please copy your County Extension Agent (http://aces.nmsu.edu/county/) and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms (http://desertblooms.nmsu.edu/) and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505- 865-7340, ext. 113.