Better Red than Dead!!!

David, one of our newer readers, asked why his red-stemmed roses seem to be more cold hardy than the green-stemmed cultivars.  So today’s blog will be dedicated to a brief discussion of why it’s better to be red than dead.

The brilliant red, blue, and purple colors seen in flowers and fruits are due to anthocyanins (and the closely related betacyanins).  These water-soluble, non-photosynthetic pigments are also commonly found in stems, leaves and other vegetative tissues.  In 1999 I wrote a review article exploring the reasons that leaves and stems might turn red.  A few years later I wrote another review, more specifically looking at how anthocyanins might influence plant water relations.  (This last phrase is plant physiology-geek jargon, and I have to admit that the class I took on this topic during my PhD work was the hardest, and probably most hated, of all the classes I took.  And now it’s turned out to be one of the most valuable.  Go  figure.)

While you hard-core types can read the review articles that I’ve hot-linked above, what I’ll try to do is summarize my hypothesis for why leaves (and stems) turn red.  Some leaves are red when young, then turn green when older.  Green, deciduous leaves turn red before they fall off in the autumn.  And some plants are genetically programmed to have red leaves all their lives.

The environment can also influence leaf reddening.  Drought, nutrient deficiency or toxicity, salts, heavy metals in soils, cold temperatures, low soil oxygen, whew!  All of these environmental factors have been attributed to temporary reddening.  What do these factors have in common?

It turns out that all of these environmental stresses directly or indirectly affect the ability of plants to take up and/or retain water. Because anthocyanins are water-soluble, they effectively dilute the concentration of water in the plant.  Look at it this way: any limited area will only hold so many water molecules.  A test tube of pure water has the maximum number of water molecules possible.  A test tube of water plus sugar (or salt, or anthocyanins for that matter) will have fewer water molecules, because the other substances take up space, too.  So effectively, anthocyanins reduced the apparent concentration of water in plant tissues.

Why is this important?  Well, anthocyanins in leaves helps reduce water loss, because the concentration of water in the leaves is reduced and evaporation slows down.  They also could serve as antifreeze compounds, allowing red leaves (and stems, David!) to be more cold hardy.  And if anthocyanins aren’t amazing enough already, they also (1) bind and transport sugars during fall leaf color change, (2) protect tissues against high levels of solar radiation, and (3) are natural antioxidants.  (That’s why you’re supposed to eat red fruits!)

I could go on and on, but I hope this might help explain why David’s red stemmed roses might be more cold hardy than the green variety. (And my thanks to my daughter Charlotte for allowing me to use her photos here.)

Foliage fun flaunted!

Not much activity on the Friday quiz!  It was a tricky one.  Take a look at our photos in total:

As you can see, these aren’t plant “problems” in the strictest sense.  (The “landscape” in question is a retail nursery.)  They are cultivated anomalies – little mutations that have been discovered and propagated.  There are several points to this exercise:

1)  Be sure you know your plant material!  Many peope mistakenly assume that plants such as these are diseased, pest-ridden, or lacking some nutrient and need to be “fixed.”  Personally, I don’t care for yellow cultivars; like Lisa B and Deb, I think they look chlorotic.  Without identifying tags, though , it would be hard to know these are not deficient in nitrogen or some other macronutrient.  I guess I would wait until leaves emerge in the spring:  if they were yellow then and stayed yellow, I would presume the plant was a yellow cultivar.

2)  Many of these cultivars are not particularly vigorous.  A plant that’s missing much of its foliar chlorophyll does not photosynthesize efficiently and would probably not survive in nature.  In our managed landscapes, however, we can nurture these oddities so they aren’t out-competed by other plants.

3)  Cultivars such as these often revert to the wild form (remember Bert’s quiz last week?).  The natural form (green vs. yellow leaves, or normal vs. dwarf stature, for example) is nearly always more vigrous than the mutation, and given the opportunity plants will outgrow these limitations.  Thus, many cultivars require careful maintenance to remove “sports” before they overtake the plant.

Autumn color puzzler

Here’s a photo I took in Buffalo about 20 years ago.  Buffalo, like many places in the northeast part of the country, has fantastic color changes in the fall.  This maple seems to have changed its mind part of the way through the process:

What do you think caused part of this tree to retain its green leaves?  Answer and another photo on Monday!