Originally Posted by Nicolas Naranja

I just read over this:

The laminae of banana leaves fold in response to environmental stimuli.

This movement does not reflect wilting of the leaf because if the leaf is excised and inverted, the laminae do not flop apart.

These leaf movements are due to differential turgor of cells within the pulvinar bands caused by water movement accompanying ion movements (Satter, 1979; W. Robertson, unpublished data).

The folding of banana leaves typically follows a diurnal rhythm with the leaves more horizontal during the night, and early morning, becoming more vertical during periods of bright sunlight, and returning to more horizontal positions in late afternoon.

These changes in turgor within the cells of the pulvinar bands are similar to changes in stomatal aperture that is controlled by the turgor of guard cells.

In addition, the laminae of banana leaves fold in response to soil drought (Milburn et al., 1990; Thomas and Turner, 1998; Turner and Thomas, 1998) suggesting leaf folding may reflect leaf water status, albeit somewhat unreliably.

However, Lu et al. (2002) did not find any link between leaf folding and soil water deficit.

Lu et al. (2002) measured water use of banana plants using a sap flow system calibrated against gravimetrically determined water loss.

This system measured water use by insertion of heat sensing probes into the central cylinder of the banana corm that conducts water from the root system to the shoot.

It is a widely used technique to measure water use of trees, and has the advantage over other systems that measure whole plant water use in that the water sensing unit (typically heat based) is localized thus causing less damage to the plant being measured and hence greater reliability of data.

Alternate systems can overheat and greatly damage or kill banana plants thus rendering measurements useless (D.S. Thomas, unpublished data).

Originally Posted by Darkman

Having read all that I stand by my premise that the external stimuli of bright sunlight, which could cause sunscald, results in the plant, thru yet undetermined methods, moving ions that have been proven to effect the turgor of certain cells which allows the leaf to fold. This is supported by the fact that leaves will fold if the plant suffers a severe water loss from soil dehydration but not reliably with less than severe water loss and additionally the leaves fold when there is no dyhydration and soil conditions are hydrated fully. This has been documented thru the use of sophisticated and reliable instrumentation that are not known to effect the test results thru variances in their own operating parameters.

I think the banana is smart enough to know when it needs to protect irself from extreme sunlight.

That's my story and I'm sticking to it!

Originally Posted by Nicolas Naranja

The problem with your hypothesis is that the first poster's plant was folding its leaves in indirect sunlight. The stomatal guard cells respond to atmospheric moisture by closing if it is too dry. My guess is that the leaf folds up to conserve water. The air on the inside of the folded leaf should be more moist.