Phototropism

From: ted.held at us.henkel.com (ted.held at us.henkel.com) on 2008.03.14 at 18:39:21(17179) Dear List, As many of you know, from time to time I come across a botanical thought that perplexes me. Today's thought concerns phototropism, that stalwart of Botany 101 students worldwide. The aroids I grow (Cryptocoryne) often have very elongated, rather weak petioles. When growing in crowds (their typical habit) the crisscrossing petioles and leaves make for a veritable swarm of vegetation floating on water surfaces. Among this leafy mess is considerable movement on the time order of days. A time-lapse movie of a patch of Crypt leaves would be a seething turmoil. It is only because of our own time sense that plants seem to be passive, quiescent beings. I assume that much or most of this movement pertains to the struggle for light. A new leaf emerges and starts to shade out some existing ones, which then bend and turn to avoid the dreaded shade. I figure this is a manifestation of positive phototropism, the leaves seeking to maximize their uptake of light energy. Fine. Every once in a while I have to manage the plants and I can never seem to get all the leaves back in a situation where the upper surfaces are happily pointed to the sky. Some leaves always get turned over because the petioles are soft and can twist, exposing the underside of the leaf to the sky and the incoming light. While you can see that the plants attempt, in their way, to get the leaf turned back the right way, often this is not possible. Even though the leaf is bathed in light, this light is shining onto what is anatomically the underside and the plants do not like it this way. If the leaf does not get turned back upright, it dies. Why is that? My first theory is that it has something to do with stomata, those little openings with which plants maintain their air/moisture balance. Everybody from Botany 101 knows that the bottom side of the leaf is where the lion's share of these gizmos lie. If the underside is exposed to the sun and weather the leaf gets off-kilter and the plant shuts it down. That's a pretty good theory even if we are dealing here with aquatic plants where the relative humidity (at least in my setup) is 100%. My second theory is that the photosynthetic apparatus in plants is directional, meaning that reception of photons for photosynthesis is a one-way mechanism. By this theory photons coming in through the backside, as it were, are not efficiently captured, basically rendering it the same as if that leaf were in the dark. A leaf in the dark is a wasted leaf, so the plant gives orders to withdraw the sap-soluble goodies and abandon that leaf in favor of producing a new one that can orient itself properly. Any ideas from the botanists? Ted. +More -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.gizmoworks.com/mailman/private/aroid-l/attachments/20080314/fd990e4f/attachment.htm

From: abri1973 at wp.pl (Marek Argent) on 2008.03.14 at 19:44:47(17181) Hello,

From: ted.held at us.henkel.com (ted.held at us.henkel.com) on 2008.03.18 at 16:03:33(17187) Dear Marek, Your comment:

From: abri1973 at wp.pl (Marek Argent) on 2008.03.19 at 22:12:45(17193) Hello, I'm not so into Cryptocoryne, but these bought in a shop really look like like growing in a shallow water where the leaves flow free. They couldn't be grown emerse, then they would be more stiff Thanks for so long info Marek +More This is a different sort of phenomenon than what I am speaking about. Some, perhaps most species in this genus do become very different in leaf morphology when grown under different lighting and planting conditions. The again, sometimes just the opposite happens; the leaves and petioles elongate from what you buy at the store or at an auction. In general, however, Crypt leaves tend to look more like your "before" pictures. My bet is that those had been growing emerse (with leaves into atmosphere), while yours look submerged. That alone is enough to account for how different they look now. If you look at the right-hand picture of the newly-purchased plants on your site (the ones in the mesh baskets) http://www.wschowa.com/abrimaal/araceum/cryptocoryne/wendjahn.htm you will see the sort of situation that can lead to a "flip-over". If that plant were planted in a shallow-immersed situation, where the leaves tend to splay across the water surface, that middle leaf could well end up with its underside exposed to the sky. We are not speaking here of an event where the petiole has folded, kinked, or twisted. Those conditions would obviously result in a physical problem for the conduction of sap containing soluble foodstuffs that could well lead to the plant sacrificing that leaf. But because the petioles tend to be rather flexible in this genus, the chances of ending up with an inverted leaf are good. Judging from the lack of responses I am thinking that the list does not know the answer. If I think of all the leaves I have ever seen, unless a plant has suffered damage, all the leaves want to be more or less horizontal. There is an upper leaf surface and a lower leaf surface. The upper leaf surface may have a thicker skin, perhaps with a waxy coating, or a darker color to deal with the stress of being in direct light and more exposed to the elements. The underside is specialized by having gas-transport structures, perhaps hairy features, and who knows what else. The leaf may be exactly horizontal or may prefer to orient itself at some angle (up or down from horizontal). Or it may even change as the leaf matures. But this presentation to the world is a characteristic of the plant. And it is always directional in higher plants. Algae and such may be different. At the same time leaves are typically translucent. This means that if you place a leaf between you and a light source you can detect the light coming right through the tissue. To be sure, some wavelengths have been filtered out, which is why the leaf looks green (or reddish, or brownish, etc.). But the same sort of filter is in place if you look through the other leaf surface. It still looks green, meaning that light has been gathered and filtered. Is light coming up through the leaf underside useful for photosynthesis? If not, why not? Don't think I'm pressuring anyone for answers. These are just the questions I throw out because they bug me. There are many unexplained phenomena in the world. Sometimes I think there are more mysteries than there are answers. I am curious. Ted. ------------------------------------------------------------------------------ _______________________________________________ Aroid-L mailing list Aroid-L at www.gizmoworks.com http://www.gizmoworks.com/mailman/listinfo/aroid-l __________ NOD32 Informacje 2954 (20080318) __________ Wiadomosc zostala sprawdzona przez System Antywirusowy NOD32 http://www.nod32.com lub http://www.nod32.pl -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.gizmoworks.com/mailman/private/aroid-l/attachments/20080319/8d7b0339/attachment.htm

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