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?
That's a very interesting question. Since nobody has answered as yet, why don't you try some of the Bonsai groups and post back with the answer. TIA
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.
Plants will, of necessity, use what energy and nutrients they have for their meristems (growing tips). As the plants get larger, and they have more meristems, more resources are diverted from the earlier leaves, to their detriment, to the benefit of the new growth.
Quite correct. Nutrients are diverted to the meristems, but they are not depleted from the older leaves. It's just that the older leaves fail to receive as much nutrients as they received while still growing.
In the meantime, the older leaves still receive enough nutrients to engage in photosynthesis, producing sugars that are transported by sap to the meristems and elsewhere within the plant. Other plant tissues convert the sugars into starches, cellulose, and other carbohydrates. And still other plant tissues combine the sugars with other nutrients to produce proteins and oils. It does not take large basal leaves to produce sugars; any leaf containing chlorophyll can do this. Even stems containing chlorophyll can do this.
Still, new growth up above, means shadows below, at least for plants that I have grown.
Which is why I prune my roses and peach tree to an "open vase" pattern, with the new growth directed outward to allow the sunlight to penetrate to the center.
Hmmm, an "open vase" pattern would seem to be an esthetically radical step for a bonsai. (See Common Styles:
There is a trade-off in opening up plants to sunlight. It increases the surface area exposed to sunlight, which would increase photosynthesis and decreases the chance of mold and mildew. Conversely, it increases the chance of sunburn on fruit and its' attendant loss of production. Pays yo' money, and takes yo' chances.
Most of my experience is with annuals (tomatoes, peppers, cucumbers, squash) where an arbor (cage) or trellis seem to offer the advantages of more sunlight, without the pruning.
On the matter of nutrients, plants, especially leafy plants such as lettuce and spinach, will store nitrogen in their leaves when abundantly supplied with nitrogen, as with rapidly absorbed chemical fertilizers, which will make their leaves targets for insects.
The leaves do produce and other compounds besides glucose, such as anthocyanins, which are translocated, and responsible for the colors of the flowers and the fruit.
In grapevines, when the adhesion forms between the petiole and the branch, these anthocyanins are trapped in the leaf, causing it to change color, and gives the wine country its' Fall colors.
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