Some recent research that’s been carried out for commercial growers may be interesting for you. Basically, plants like raspberries and peppers are grown close together and the lower leaves are quite shaded. It’s been found that under these circumstances the lower leaves are not contributing very much in the way of photosynthetic production. Removing the lower leaves has the effect of reducing the water burden on the plant (since it cuts the amount that the plant can transpire through the missing leaves) while not affecting the plant’s ability to be productive very much.
An article in December 2009’s “Commercial Greenhouse Grower” reports that in peppers the top 40cm of the plant do most of the work while the second 40cm makes a reasonable contribution with the bottom 60cm being pretty useless. Removing the leaves from the bottom 80 cm of a 2.4m plant made no apparent difference to either plant growth or yield.
The effect on transpiration was to reduce humidity in the glasshouse by between 6-9%. As managing humidity (due to danger of fungal infection) is important in a commercial glasshouse environment there are obvious savings to be made in terms of water costs and the energy costs of the fans which reduce humidity especially during September.
An interesting practical piece of research – how far these observations could be used to improve cultivation of crops like runner beans or glasshouse tomatoes and cucumbers is of interest to the practical gardener.