Effect of Selected Nitrogen-bases and Boron on the Ribonucleic Acid Content, Elongation, and Visible Deficiency Symptoms of Tomato Root Tips.

In a previous paper Albert (2) reported the RNA content of root tips of intact tomato plants decreased and boron deficiency symptoms occurred concurrently wlhen boron (B) was withheld from nutrient solutions in which the plants were growing. It was also observed that elongation of root tips stopped 6 to 12 hours and the RNA content of tips decreased 24 to 48 hours after B was withlleld. The DNA content of the tips did not change significantly during the experimental period. Using tomato plants in a similar experimental system, Yih and Clark (3) reported that root elongation ceased after 24 hours but that protein and carbohydrate content of minus-B root tips (10 mm) did not decrease within 72 hours. A decreased RNA content, therefore, appears to be one of the earliest biochemical changes associated witlh the development of visible symptoms of B-deficiency after elongation in tomato root tips stops. Thus, if B is involved in some aspect of RNA metabolism, the feeding of appropriate metabolites or their analogues may reveal information about such a role. Compounds selected for investigation were adenine, cytosine, iguanine, thymine, uracil, orotic acid, barbituric acid, and 6-azauracil. Tomato plants (Lycopersicont escutlentumn Mill. var. Rutgers) were grown from seeds (W. Atlee Burpee Company) planted in Perlite and watered with a Hyponex nutrient solution (Hydroponic Chemical Company, Inc.). When the seedlings had 2 small leaves (the largest about 1 cm long), they were transplanted to an aerated, modified Shive's nutrient solution contained in light-tight soft-glass gallon jars, one plant per jar (1). Deionized-distilled water was used throughout. All plants received an adequate amount of B (0.1 mg/liter) throughout their growing period and nutrient solution's were changed every 7 to 10 days. The plants were approximately 15 cm in height (cotyledons to apex) with 6 to 8 leaves when they were used for experiments. Net root elongation was determined by placinlg India 'ink marks 10 mm from the tips and measuring ssubsequent elongation at the end of the experimental period (1). Prior to marking the tips, the roots and nutrient solution jar covers were thoroughly rinsed with deionized-distilled water. After marking the tips the plants were put into their respective treatment solutions which consisted of fresh nutrient solutions to which the appropriate compounds were added in the presence of B (0.1 mg/liter) or to solutions from which B was withheld. All plants were placed in the greenhouse for the experimental treatment period. The n-itrogen-bases, adenine, cytosine, guanine. thymine, uracil, orotic acid, baebituric acid, and 6-azauracil (Mann Research Laboratories, Inc.) were dissolved at a concentration of 0.1 mM, except where noted otherwise, in the nutrient solutions 'witlh the aid of heat (60408) and, wlhen necessary, by adjusting the solutions with concentrated H1Cl to a pH of 1 or 2. On cooling, the solutions were readjusted to a pH of 4.7 (±+ 0.1) with NaOH. The pH of regular nutrient solution was 4.7. At the end of the experimental period root elongation measurements were made and tips were harvested for nucleic acid analysis. The terminal 3 mm of the tips were excised (15 tips per,plant), blotted, quickly weighed, and stored in 2 ml of 80 % (v/v) ethanol at 20 until nucleic acid analyses were performed using a modified Schmidt-Thannhauser method (2). Typical results from ex'periments in which B and selected nitrogen-bases were added alone and in combination with each other, or withheld from nutrient solutions are given in table I. These data indicate that thymine, guanine and cytosine stimulated root elongation when B was withheld, while uracil, orotic acid, and adenine were less effective. Barbituric acid and 6-azauracil inhibited root elongation in both the presence and absence of B. Total RNA content of root tips decreased when B was withheld from nutrient solutions. However, when B was withheld and thymine, guanine, or cytosine were present the RNA content of the root tips was only slightly less than the plus-B treatment. When uracil, orotic acid, or adenine was present in minus-B nutrient solutions root tips had an equal or slightly greater amount of RNA than did minus-B tips. Barbituric acid and 6-azauracil treatments resulted in a decreased RNA content of root tips for both plus and minus-B treated