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[SHADEGARDENS] Photostasis

Friends, 
This posting discusses some very recent research results that bear on our discussion of ‘shade tolerant’ versus ‘shade loving’.

Photostasis is a new finding that animals and plants work to keep the amount of light captured each day constant.  It was discovered in the eyes of animals, but seems to be easier to demonstrate in plants.

Each plant leaf has a size and # of chloroplasts determined by lighting conditions at the time the leaf grows.  Once a leaf grows, it is static--the # of chloroplasts can’t be changed.  Each plant type tries to achieve constancy in light capture per day with each leaf.  It can do this by making smaller leaves in brighter light and/or by inserting fewer chloroplasts in brighter light.  Photostasis maintains maximum efficiency for photosynthesis.

These ideas lead to some interesting explanations:

1) Hostas get bigger and darker leaves in the shade -- they need to be capturing more light for photostasis.

2) Gold hostas are golder in more sun -- they need fewer chloroplasts and therefore are less green.

3) Hostas grown in more shade and then transplanted into more sun don’t like it -- they get leaf scald.  This is caused by oxidative damage.  Photosynthesis produces oxygen.  When a shade-grown leaf with its higher density of chloroplasts is exposed to more sun, too much oxygen is produced for the leaf cells to survive.  It doesn’t hurt the plant, but it makes the leaves look awful.  The reverse is okay because the lowered light just makes photosynthesis less efficient for the rest of the season.  Next season the leaves will be bigger.

4) Vernal leaves, the second flush, are often smaller and lighter in color on hostas.  This is because there is more light in the summer than in the spring, even in dappled shade and the leaves are adjusted accordingly as they are grown.  Often, the later leaves in the spring flush are larger because the leaves on the overhead canopy have come out and the light intensity has gone down.

5) Transplanting hostas in the spring before the leaves emerge will yield happier-looking plants because the leaves will adjust to the lighting conditions as they develop.

6) Overwhelming the plant’s ability to protect itself from oxidative damage is not too detrimental to the plant provided the leaves have sufficient substance--but the foliage looks awful.  Underwhelming the plant’s ability to have efficient photosynthesis leads to a tendency not to thrive.

NOW, ‘shade tolerant’ versus ‘shade loving’.  I think it is going to turn out that the range of leaf size and range of chloroplast density available to different plants are very different.  A plant with very restricted adaptability might be limited to either full sun to get enough photosynthesis done to survive or full shade to avoid getting oxidative damage to its leaves.  The first kind could legitimately be called ‘sun loving’ because it needs high light intensities to photosynthesize efficiently.  The second kind bould legitimately be called ‘shade loving’ because it needs low light intensities to photosynthesize efficiently.  Those with a wide range of adaptability will clearly grow faster in more light, but with smaller leaves.  These might be better called ‘shade tolerant’ rather than ‘shade loving’.

For those of you who have gotten this far, there is a book coming out later this year that will discuss photostasis in plants.  I’ll be certain to tell everyone the title, authors and publisher when it becomes available.

Cheers,
Steve Chamberlain
Manlius, NY
Zone 5-

PS.  Failure of photostasis in plants leads to scorched leaves or failure to thrive.  In humans, it leads to seasonal affective disorder.
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