Old leaves need to die in time or they will bring a plant down

By using transgenic tobacco plants that do not shed their lower leaves, they were able to show that shaded old leaves become a burden to a plant because they no longer photosynthesize but still require energy to be maintained.

Moreover, the nutrients in these leaves can be more usefully employed by the plant when re-allocated to new leaves at the top of the canopy, where more light is available and higher photosynthetic rates can be attained.

Previously, theoretical modeling has been extensively used to investigate how plants should distribute their leaf area and nutrients to maximize their photosynthesis and fitness. However, a direct experimental test was lacking till now.

“Keeping up with the neighbors is important for plants in leaf canopies” Alex Boonman states, “because failure to project enough leaf area at the top of the canopy means that some other plant will do it, with shading and therefore diminished photosynthesis as the consequence.” The transgenics, which were originally developed by Susheng Gan and Richard Amasino at the University of Wisconsin, indeed produced less leaf area in the upper canopy layer than normal plants and performed less well in competition exeriments.