Individual fitness versus whole-crop photosynthesis: solar tracking tradeoffs in alfalfa

Authors: DENISON, R FORD - University Of Minnesota; Fedders, James; Harter, Barry

Submitted to: Evolutionary Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2010
Publication Date: 8/3/2010
Citation: Denison, R., Fedders, J.M., Harter, B.L. 2010. Individual fitness versus whole-crop photosynthesis: solar tracking tradeoffs in alfalfa. Evolutionary Applications. 1752-4571:466-472.

Interpretive Summary: Some plants have characteristics that increase growth and competitive ability of the individual yet reduce crop yield when grown in pure stands. Manipulation of such characteristics, through selection and breeding, offer opportunities for improved yields. For instance, modern dwarf wheat and rice are the results of breeding programs that increased grain yield while reducing plant stature and ability to compete with weeds. Solar tracking, or the movement of leaves to follow the sun, may be one such characteristic that benefits the growth and competitive ability of individual alfalfa plants but reduces the yield of pure stands. This may occur if the increased photosynthesis due to solar-tracking in upper leaves is offset by decreased photosynthesis of heavily-shaded lower leaves. This idea was tested by comparing photosynthesis of potted alfalfa when the leaves were tracking or not tracking the sun. Solar tracking increased photosynthesis slightly when the potted plants had few leaves but solar tracking tended to decrease photosynthesis when plants had many leaves. Computer modeling that simulated the growth of pure alfalfa stands suggested that solar tracking could reduce yields by up to 5% percent. Breeding and selection of alfalfa for reduced ability to track the sun could increase yields of this important forage crop.

Technical Abstract: Natural selection in crops and their wild ancestors, mostly prior to modern plant breeding, is unlikely to have missed simple genetic improvements that would consistently have enhanced individual fitness. There may therefore be few if any tradeoff-free opportunities for further improvement of crop traits like photosynthetic efficiency or drought tolerance. Fortunately, opportunities linked to tradeoffs, such as adaptation to past versus present atmospheric CO2, may be abundant. Tradeoffs between individual-plant competitiveness and the collective productivity of plant communities have been key to past increases in yield potential. Solar tracking by upper leaves may involve such tradeoffs, if photosynthetic benefits to tracking leaves are offset by increased shading of leaves lower in the canopy. This hypothesis was tested experimentally, using rotation in the horizontal plane to disrupt solar tracking in alfalfa. Solar tracking increased net photosynthesis of sparse canopies with little self-shading, but rarely by more than 3%. As leaf area increased, solar tracking tended to decrease net canopy photosynthesis, despite edge effects in our 1-m2 artificial communities that probably exaggerated net photosynthetic benefits of tracking. A computer model suggests that a less-competitive solar tracking pattern could increase yield, assuming good weed control.