Effect of Chemical Control of Stomata on Transpiration of Intact Plants.

In the preceding communication' it was shown that phenylmercuric acetate sprayed upon detached leaves brings about closure of the stomata and thereby diminishes the diffusion of water from the leaves and of CO2 into the leaves. In 9 out of 11 experiments, induced closure of stomata reduced transpiration relatively more than CO2 assimilation as predicted from diffusion theory. This work led to experiments in transpiration control in whole plants grown in containers in the greenhouse. Success in the greenhouse required, of course, that the plants be sufficiently turgid, well-watered, and illuminated to permit the opening of the stomata in the untreated plants. It also required that the compound reduce transpiration without destroying the leaf. The experiments reported employed the effective 3.3 and 10 X 10-5 Al phenylmercuric acetate solutions sprayed in the afternoon upon a species whose stomata close uniformly. Methods.-Each lot consisted of nine tobacco plants (so-called shade-type) 25 to 35 cm tall, growing in about 600 gm of fine sandy loam in waxed paper cups and arranged in a Latin square. The soil surface was covered by paraffin film. The initial soil moisture was the quantity retained after drainage; in some cups the water diminished to the wilting percentage before restoration to the initial state. The transpiration for one day was determined before spraying, and the later transpiration was expressed as a fraction of this reference amount. The containers were set on shields that rested on a bed of wet sand. Experiments had shown that the stomata of control plants were not fully open in the dry air of the greenhouse, and, as expected under these circumstances, transpiration was scarcely reduced by treatment. In the experiments to be presented, the plants were enclosed in a transparent plastic tent to ensure high humidity and open stomata in the unsprayed plants. Results.-The plants of the experiment of Table 1 were sprayed on February 27. The next day was cloudy, transpiration was slight, and no treatment effects were seen. During the next four days the sun shone, transpiration was high, and the 45 per cent reduction between plants sprayed with 0 and 10-4 M inhibition was significant. The next three days were cloudy; nevertheless, transpiration was reduced. The second week of observation had five clear days, and transpiration was reduced by 40 per cent.