Ribonucleic Acid Content, Boron Deficiency Symptoms, and Elongation of Tomato Root Tips.

Boron is recognized as an essential micronutrient for higher plants, but the precise role of this element in plant growth and development is not clear. In Gaiigh and Dugger's (3) comprehensive review they presented and discussed a wide spectrum of proposed roles and effects of boron on higher plants. From their own work and their interpretative review of the literature they concluded that "boron may be essential for sugar transport in higher green plants," and "boron deficiency symptoms are an expression of sugar deficiency in the cambia, stem tips, root tips, and flowers and fruits." From reports that have appeared since their review was published in late 1954, it appears that the role of boron in sugar translocation per se must be indirect (5, 8, 9, 13, 17). that sucrose does not readily alleviate the symptoms of boron deficiency (1, 6, 7, 15), and that as much or more carbohydrate was found in boron-deficient root tips as in boron sufficient tips (5, 16). Although the exact function of boron in plant metabolism is unknown, a common feature on which many investigators agree is that boron is essential for the normal growth and functioning of apical meristems (1, 7). Whittington (14) reported that a deficiency of boron caused a cessation of cell division and later suggested (15) that in the absence of boron, cell division ceases because abnormalities in the formation of the cell wall prevent the cell from becoming organized for mitosis. However, Skok (13) concluded from radiosensitivity studies with sunflower plants that boron is required "for some process or processes concerned with cellular maturation or differentiation rather than with cell division." Albert and Wilson (1) found that external symptoms of boron deficiency of roots of intact tomato plants were detected within 24 hours after boron was withheld from the nutrient solution. These symptoms consisted of the cessation of root elongation as early as 6 hours after boron was withheld and the subsequent development of a brown color and loss of fluorescence in the terminal portion (2mm) of the root tips. Brown root tips did not resume elongation when again supplied with an adequate amount of boron. However, roots that ceased elongation in miinus boron solutions will resume elongation if boron is supplied before the tips turn brown. As the brown color develops, the intensity of the characteristic white fluorescence from this same area of the root tip decreases and finally disappears. Internal symptoms were observed to occur in the post-meristematic region of the tip and were apparent as a darker staining of the cytoplasm followed by its disintegration, resulting in empty, apparently dead cells. Cell walls of some, but not all, of the dead cells were thicker than cell walls of cells retaining their cytoplasm. These observations emphasize that boron is essential for cellular activity and that the absence of boron from the nutrient medium results in early symptoms of boron deficiency in root meristems. They also indicate that boron is necessary for cellular processes which are essential to the generation or differentiation of new cells. While this investigation was in progress several reports appeared relating to boron and RNA in plants. Matter and Turian (4) observed that high concentrations of boric acid had antimitotic activity at the apex of the radicle of Lactuca sativa. At high and moderate concentrations of boric acid the nucleolus was enlarged, and boric acid was detected in the nucleolus by cytocheniical means. Sherstnev and Kurilenok (12) found that adenine-8-C14 supplied in the nutrient solution to sunflower plants was incorporated into RNA of the upper-most leaves and roots of plants supplied with boron to a greater extent than in boron-deficient plants. These results supported previous reports from their laboratory that a lowering of nucleic acid content occurs when plants are grown in nutrient media deficient in boron. Also, Shkolnik and Kositsyn (11) followed the incorporation of p32 into nucleic acids in the presence and absence of boron and concluded that boron deficiency strongly inhibits the synthesis of nucleic acid and supports their hypothesis that boron plays an important role in the synthesis of nucleic acids. The present investigation developed from an attempt to isolate and identify the fluorescent material in healthv tomato root tips which disappeared and was replaced by a brown color as boron deficiency symptoms progressed. It was found that the RNA content of the root tips of tomato plants decreases and boron deficiency symptoms of roots (loss of fluorescence and browning) occur concurrently when boron is withheld from the nutrient solution. However, root elongation ceases before the decrease in RNA content occurs. 1 Received December 15, 1964. 2 Supported by National Science Foundation Grant G-21452.