An Explanation Of Epinasty

An Explanation of Epinasty by Paul D Pruitt MA - Biology University of Pennsylvania - 1986 Epinasty - "A downward bending of leaves or other plant parts, resulting from excessive growth of the upper side." - http://www.answers.com/epinasty&r=67

Epinasty is a strange phenomena in plants that occurs, among other instances, when its roots are flooded. The plant leaves will grow actively from being perpendicular to the sun's rays, to being more parallel. This odd behavior has perplexed scientists. What is plant trying to accomplish moving it's leaves to this new position? One could say that passive forces are enough to explain why the leaves are moved to this new position. That is, perhaps the leaf being more parallel to the stem gets more water vapor evaporated at the leaves, because the leaf is blown up and down by the wind and this causes more evaporation and greater transpiration and thus an emptying of water from around the roots. More interestingly though, what if the up and down action of the leaf is actually acting like a water pump? That is the angled leaf is the handle, the wind is the pumper and the backflow valve is the stomates or some newly grown valve induce by flooding or some other epinastic situation. The extra evaporation by being in the winds way could be part of the story but maybe not the full one. So if we believe the normal transpiration rate sometimes needs to be supplemented, which situations would these be? Maybe the simple answer is when there is too little water or too much. If this last statement is true then: 1. ABA should lead to epinasty. Scientists know when a plant is in need of water it makes ABA and ABA shuts down the guard cells under the leaf where water normally evaporates. So maybe interestingly, wilting is not just due to lack of water pressure in the leaves due to dehydration, but also is due to some epinasty that plant engages in to get more water into the leaves from the root. This may especially be true when the guard cells are shut down and transpiration can't function normally. 2. Ethylene should lead to epinasty. Scientists already know this, but maybe not why it occurs. My explanation is that Ethylene is an indicator of anoxia or lack of Oxygen that occurs in the roots during flooding. The epinasty that occurs is an active attempt to pump extra water up through the plant and thus away from the flooded roots letting air back into the spaces around the drowning roots. Roots apparently need to breathe. Perhaps the action of epinasty as indicated is partly as a water pump or maybe solely as an increaser of vapor diffusion in the wind. 3. Salicylic Acid should lead to epinasty. I believe Salicylic Acid (SA) is an indicator of water abundance (a healthy/growth producing amount of water actually). Thus if a plant is interested in its stem and leaves growing it may want to supplement

the normal process of transpiration with the extra diffusion produced by being in the way of the wind and possibly additionally in actually pumping the water skyward with a water pumping system.