A comparative study of the seedling-juvenile and flowering stages of the life cycle in an annual and a perennial species of Senna (Leguminosae; Caesalpinioideae)

Various growth parameters and the requirements for flowering of Senna (=Cassia) marilandica, an herbaceous polycarpic perennial native to the eastern United States, and Senna (=Cassia) oblusifolia, a pantropical-warm temperate annual weed, were compared in greenhouse and growth chamber studies. Relative growth rate (RGR) of seedlings-juveniles of S. marilandica and S. obtusifolia did not differ. However, after growth (from seeds) in the greenhouse from 28 May 1991 to 18 September 1991: (1) dry mass of S. obtusifolia plants was nearly twice that of S. marilandica plants; (2) root/shoot ratio of S. marilandica plants was more than four times that of S. obtusifolia plants; and (3) reproductive effort (dry mass of fruits/dry mass of whole plant) of S. obtusifolia plants was 27%, whereas plants of S. marilandica did not flower. Compared to the control, shoots of cold-treated S. marilandica plants emerged earlier, and a higher percentage of them flowered. Senna marilandica was confirmed to be an obligate long-day plant, and S. obtusifolia was shown to be a facultative short-day plant. Results of this study and of others clearly show that S. obtusifolia is a more strictly r (sensu MacArthur and Wilson) or R (sensu Grime) species than is S. marilandica.