Quote: [EMAIL PROTECTED] said: >I believe that the size of the upper part of the orchid is limited by >the root size below. The primary limitation is water.
As usual, it all depends... But one really important point about roots is that they are the source of cytokines - a class of hormone - for the plant. Most plants exist naturally on the edge of a state of senescence, and this is prevented by the free flow of cytokines. Sick roots or starved roots do not produce as much of these necessary compounds as will a healthy one, and such a plant will either become senescent - yellow, dropping leaves and so forth - or become static and quasi-dormant. You can see the affects of this most strongly in annual grassy plants, such as wheat. This occurs because of a rather beautiful dance, which goes something as follows. The wheat leaves produce sugars by photosynthesis. This energy source is transported around the plant, to new leaves that are expanding, to the elongating stem and to the roots. Each part of the plant "sucks" as hard as it can on the phloem transport system, and the hydraulics of this work out such that the roots get enough sugar in order to grow and maintain a flow of cytokines. There is some dedication between particular leaves and parts of the root system, so that shaded leaves may become unable to feed "their" roots. When this happens, then these roots stop exporting cytokines and the leave sin question drop into senescence. A number of genes turn off, and others turn on as a result of the changing hormone balance, so creating enzymes which proceed to dismantle the leaf's cellular components, exporting them to other parts of the plant. After a few days, what is left is a dry cellulose husk, with everything that can be mobilized exported to sinks elsewhere. This is a precursor for what happens when the seeds set. Wheat seeds are a ferocious sink for sugar, and they take so much that the roots are largely starved. They stop making cytokines, and so the entire plant drops into senescence and begins to breaks down its tissues. It exports these released materials up to the seeds, which may end up holding over half of all the dry matter in the plant. The entire structure then dries up and the seeds scatter, or are harvested. This is much more dramatic than happens in orchids, although I suspect that the Autumn tuber formation in some terrestrials creates a similar collapse of the upper parts of the plant. Dendrobium growers will have seen the growth spurt that happens right at the end of the season: reserves are built up in old wood, and then the seemingly static plant hurls up a new pseudobulb in a fortnight. Whether this is due to hormones from the roots I cannot say, but I am quite sure that a hormone trigger is involved. It may well be that the "sullen" plant, the one that just sits there looking dull green and doing nothing, is in fact waiting for the right hormonal cue. _______________________________________________ the OrchidGuide Digest (OGD) [email protected] http://orchidguide.com/mailman/listinfo/orchids_orchidguide.com
