Gravitropism in higher plant shoots. VI. Changing sensitivity to auxin in gravistimulated soybean hypocotyls.

Although the Cholodny-Went model of auxin redistribution has been used to explain the transduction phase of gravitropism for over 60 years, problems are apparent, especially with dicot stems. An alternative to an auxin gradient is a physiological gradient in which lower tissues of a horizontal stem become more sensitive than upper tissues to auxin already present. Changes in tissue sensitivity to auxin were tested by immersing marked Glycine max Merrill (soybean) hypocotyl sections in buffered auxin solutions (0, 10(-8) to 10(-2) molar indoleacetic acid) and observing bending and growth of upper and lower surfaces.

Gravitropism in higher plant shoots. V. Changing sensitivity to auxin.

An alternative to the Cholodny-Went, auxin-transport hypothesis of gravitropic stem bending was proposed as early as 1958, suggesting that gravistimulation induces changes in sensitivity to auxin, accounting for differential growth and bending. To test the sensitivity hypothesis, we immersed marked, decapitated sunflower (Helianthus annuus L.) hypocotyl sections in buffered auxin solutions over a wide concentration range (0, 10(-8) to 10(-2) molar IAA), photographed them at half-hour intervals, analyzed the negatives with a digitizer/computer, and evaluated surface-length changes in terms of Michaelis-Menten enzyme kinetics.

Seasonal Shifts of Photosynthesis in Portulacaria afra (L.) Jacq.

Portulacaria afra (L.) Jacq., a perennial facultative Crassulacean acid metabolism (CAM) species, was studied under natural photoperiods and temperatures in San Diego, California.

Influence of Photoperiod and Leaf Age on Crassulacean Acid Metabolism in Portulacaria afra (L.) Jacq.

The possibility that Crassulacean acid metabolism (CAM) is subject to long day photoperiodic control in Portulacaria afra (L.) Jacq., a facultative CAM plant, was studied.