On 1/3/2010 2:10 PM, Frank wrote:
On Dec 29 2009, 11:56 am, "David E. Ross"
wrote:
On 12/22/2009 11:03 AM, Frank wrote:

Sunleaves are the large leaves at the base of and associated
with secondary branches.

I assume their role is to feed the secondaries with products
of photosynthesis and release of stored mobile nutrients such
as N, P, K.

That being understood, what is the effect of pruning out
one or the other, as when making a plant into a bonsai?
If we're trying to arrest stem elongation by pinching back,
does it leave the plant stronger to remove just the tip in
question without removing the sunleaves?

I don't think "sunleaves" are any different from other leaves. They are
larger primarily because they are on branches that are sufficiently
mature to sprout side branches. When the side branches mature, they too
will have larger leaves. All leaves participate in photosynthesis once
they have opened up from their buds.

Leaves -- even the larger leaves on mature stems -- do not usually store
or release N, P, and K. Leaves use those nutrients to produce sugars.
Only fleshy leaves (e.g., on succulents) might store any nutrients other
than sugar.

What accounts for the various spots and blanching on older
leaves specific to a given deficiency? I would have thought
nutrient translocation to be the cause.

Several things account for spots and other markings on older leaves.

First of all, older leaves have had more time to suffer damage from
sunburn, fungus, insects, and other environmental sources.

Many nutrient deficiencies can affect the growth and development of
leaves, including their color and markings. Once the damage is done,
the availability of nutrients that had previously been lacking cannot
undo the damage to a mature leaf.

Leaves have finite lifespans. For deciduous plants, lifespans usually
end with dormancy. For evergreens (conifer and broad-leaf), lifespans
might be long or short, depending upon the species. As a leaf
approaches the end of its lifespan, it shows changes caused by
senescence. This can happen one leaf at a time (especially for
evergreens) or for many leaves all at once, again depending upon the
species.

Leaves often lose their chlorophyll at the end of their lifespans,
causing hidden colors to show; this is the source of fall colors on
deciduous trees. The colors were there all along, but they were masked
by the green of chlorophyll. The chlorophyll does not translocate to
other leaves. Instead, the chloroplast cells die; and the chlorophyll
they contained decomposes. For some plants, the loss of chlorophyll
might be slow, giving the effect you describe.

--
David E. Ross
Climate: California Mediterranean
Sunset Zone: 21 -- interior Santa Monica Mountains with some ocean
influence (USDA 10a, very close to Sunset Zone 19)
Gardening diary at http://www.rossde.com/garden/diary