Probably the earliest measurable response of plants to boron deficiency is restriction in elongation of root tips of plants transferred to solutions lacking boron (1,5,7,13). Necrosis of the root tip is also soon evident, followed by the emergence of short, stubby laterals close to the tip (1,11,12). Why the growth of roots stops when boron is lacking is not known. Indeed, of the many physiological roles that have been suggested for boron (3,10), no one has yet been established by rigorous proof.Skok (10) 10-3 M Ca(NO3)2, 2.0 X 10-3 M K2S04, 5.0 X 10-4 M (NH4)2SO4, 9.2 x 10-6 M H3BO3, 1.8 X 10-5 M FeSO4, 1.6 x 10-7 M CuSO4, 1.0 X 10-7 M Na2MoO4, 1.5 x 10-6 M ZnSO4, 4.5 X 10-6 M MnCl2. Soft-glass containers were used for the preparation of solutions and for the culture jars. The nutrient solutions were routinely changed once a week prior to the start of the experiment.After 10 days, when the plants were about 9 cm tall, they were transferred to the test solutions. The root systems were gently washed with running distilled water before transfer to fresh nutrient solutions with or without added boron. During the 72-hour test period, the solutions were continuously aerated and were maintained at 27°, a temperature which preliminary experiments had shown to be optimum for growth of the roots.Measurements of Elongation of Root Tips. Elongation of the root tips was measured from India ink marks placed 5 mm from the tips as reference points. The India ink was applied with a Chinese wool brush, the most satisfactory of several devices tried. All root elongation measurements were estimated to the nearest 0.5 mm with a celluloid ruler with mm spacing. Data for elongation as presented are averages of at least 20 root tips from each of 4 plants in a given treatment.Procedure for Shading. When shaded, plants were grown under 2 layers of 15-mesh green "Saran" screening. Under this shade the light intensity was 33 % of that received by unshaded plants.Sampling. Two to 4 replicate samples were taken from each treatment for each type of analysis. For carbohydrate determinations, each sample included a hundred 10-mm root-tip sections; for protein-N, 50 similar root-tip sections.Carbohydrates. Samples were placed in 1 ml of nearly boiling 80 % (v/v) ethyl alcohol in a 5 ml calibrated test tube and briefly heated. Sugars soluble in the 80 % alcohol were then separated from the solid material by centrifuging at 1000 rpm for 10 minutes and washing the residue twice with 1 ml hot 80 % alcohol. The combined extract and washings were then evaporated to dryness under reduced pressure at 300 or less. This dried material, and the dried residue which had been extracted with 80 % alcohol, were separately heated at 1000 for 2 hours with 1 ml of 1.5 N H2SO4. After cooling, the acid hydrolysate was neutralized with solid Ba(OH)2, centrifuged, and the precipitate washed twice with metal-free water; the supernatant fraction was used for analysis by paper chromatographic separation of individual sugars. The sample was run 312
