Stem Orientation in Two Populations of Acacia farnesiana from Mexico Under Controlled Environmental Conditions

The orientation of stem growth in upright plants from an inland population at Saltillo, Coahuila, and in prostrate plants from a beach population at Mazatlan, Sinaloa, was examined under several conditions of photoperiod, temperature, and light intensity. Prostrateness is enhanced by lower temperatures, shorter photoperiods, and higher light intensities in both populations but is induced to a greater degree and over a wider range of conditions in the naturally prostrate form. Under longer day lengths, 16 hr, and higher temperatures, 32-24 C, the prostrate form develops almost upright and is not significantly more prostrate than the naturally upright form. Regenerated stems produced under high light intensities were significantly more prostrate in the beach population than were stems of the inland population, but under low light intensities both populations produced upright stems. The results suggest that niche plasticity of the prostrate population offers selective advantage in a subtropical beach ecosystem.