Effect of Boron Deficiency on Nicotine Formation in Tobacco.

ternal cooling of the leaf. If such an effect existed it should have been greatly reduced following illumination with 2,600 fc of blue light, for the blue filter (10 mm of 20 % aqueous CUSO4) reduced the external heating effect of the light markedly and should have reduced the internal heating even more. Average external leaf temperatures were as follows: white light-30.10, blue light-27.5°, dark-26.5° C. The initial and subsequent rates of respiration (based on the slopes of segments a and b as shown in figure 1) of a leaf of hybrid tobacco were measured for 20 consecutive cycles of light and dark during which white light at 2,800 fc and blue light at 2,600 fc were alternated. The mean a rates (in mg C02/min) were as follows: white-0.1437 ± 0.026 (standard error), blue-0.1348 ± 0.026. The slight difference between mean rates could have occurred by chance alone oftener than once in twenty trials and is considered fortuitous. The mean b rates were: 0.0345 and 0.0339, respectively. The ratios of mean a to mean b were: 4.7 and 3.98, respectively. These results show that the respiratory deceleration pattern could not have been the result of internal cooling of the leaf. The experiment was repeated through 20 cycles with each of two other plants from the same seed lot and planting with almost identical results. Subsequent experiments have shown that the CO2 evolution decelerated for approximately 6 to 9 minutes following darkening and that the final stable dark rate was about 75 % of that shown for segment b. Thus the mean initial rate was approximately 5.4 times greater than the basal dark rate. Obviously more work is needed before an explanation will be found which will demonstrate the compatibility of these findings and those of Brown (1).