Effect of 2,4-D on Apparent Photosynthesis and Developmental Morphology of Velvet Bean

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Abstract
Studies were conducted to determine the effect of 2,4-D on the photo-synthetic activity and developmental morphology of velvet bean plants. There was a sharp reduction or cessation of photosynthesis in velvet bean leaves within 5 hrs. after treatment with 0.1% or 0.05% 2,4-D applied as a spray or dip. This was associated with extensive damage and destruction of the meso-phyll leaf tissue. Plants treated with this concn. died before any pronounced modifications occurred in the tissues of the stem. The rate of photosynthesis declined within 5 hrs. after treatment with 0.01% 2,4-D. The rate was further reduced within the next few days, but several days elapsed before CO2 assimilation ceased and the plants died. The treatment caused very little damage to the leaf, but extensive cell proliferation occurred in the tissues of the stem, with eventual differentiation of root primordia. Sieve tubeB and companion cells were crushed by the proliferation of new cells, and xylem formation was inhibited and presumably also water conduction. The rate of photosynthesis of plants sprayed with 0.001% 2,4-D was not appreciably affected until a week or more after treatment. Although growth ceased, the plants were not killed, and photosynthesis continued at a reduced rate for several weeks after treatment. Associated with this reduction was a sequence of morphological developments and cell changes in the stem, similar to, but much less extensive than that which occurred with the 0.01% treatment.