Nutritional Relationships of Boron and Indoleacetic Acid in Head Lettuce.

Lettuce, which is being grown in ever increasing quantities, suffers from a number of nutritional disorders which have been the subject of considerable speculation and study. Much of the difficulty with lettuce is evidently the result of the type of forcinig culture which is used to obtain the most firm and succulent heads. Among the micro-nutrient elements, boron has been given considerable attention in recent years since it is now known to be more widely deficient in the soils of the country than was believed earlier. For lettuce it has been shown that boron deficient plants show necrotic tissue near the leaf margins and over the growing point. Such plants have been photographed and described by McHARGUE and CALFEE (2) and others. Some have maintained that the symptoms are the same as those of the socalled tip-burn which has been the most serious disease of head lettuce in many localities. Perhaps that which the grower includes under the designation of tip-burn is in reality more than one physiological disorder. McHARGUE and CALFEE found that 0.5-1.0 p.p.m. of boron in the culture medium was optimum for the production of lettuce leaf tissue dry matter, although they noted that quantities of boron which were toxic for young plants, might give favorable results with older plants. The function of boron in the plant is unknown but several hypotheses as to its possible activities have been proposed. MCMURTREY (3), in his review of nutritional deficiency symptoms, points out the importance of boron for proper functioning of the growing points. SCHROPP and ARENZ (5) found that a deficiency of boron was associated with greater uptake of nitrogen. Severe deficiency symptoms resemble those of ammonia poisoning. These authors also suggested an indirect function of boron, that of preventing swelling of the plasma colloids which might interfere with sap rise in the stem. MINARIK and SHIVE (4) found that, lacking an optimum boron supply, there was a subnormal quantity of calcium in the tissues of soybeans. SHIvE (6) has also reported a direct relationship between the soluble boron in plants and quantity of soluble calcium in both monocotyledons and dicotyledons. MINARIK and SHIVE found that within the range of boron concentrations used, the percentage of water in the tissues of soybean leaves decreased with increasing concentrations of boron in the nutrient solution. A possible function of boron was suggested by EATON (1) as being a necessary element for the formation of an auxin, such as indoleacetic acid. He was of the opinion that those plants in his tests which were grown with a limited supply of boron showed an increased growth rate when indoleacetic acid was spraved on the leaves. Similarly SKOOG (7) found an inter-rela517