Boron deficiency and excess in cruciferous vegetables

Abstract

To ascertain the effects of boron on growth, boron and sugar accumulations, several cruciferous vegetables were grown in sand cultures with 0, 0.005, 0.01, 0.05, 0.3-0.5 and 3-5ppm of boron (B).
1. The symptoms of boron deficiency in the forms of curling, rolling and deforming were frequently found in the upper leaves of plants supplied with 0-0.005ppm B in most of the vegetables tested. In the middle leaves of Chinese cabbage and cauliflower, the inner surface of midribs were cracked crosswise, and turned brown.
Boron deficiency also inhibited growth of roots, especially development of lateral roots. In root crops such as turnip, radish, and Japanese radish, the surface of fleshy roots split and turned rusty brown because of being covered with cork. In addition, the shape of fleshy root of turnip and radish was transformed into a slender type as compared with the normal, round type. Only in the turnip roots, brown core was observed.
In boron deficient cauliflower and broccoli, water-soaked portions appeared inside of heads, and at the advanced stage in cauliflower, internal split or hollow stem was observed, and the surface of heads turned brown. In broccoli, healing with callus formation was slower on the cut end of stems after harvest of heads.
In seed production of Chinese cabbage, boron deficient plants supplied with 0.01ppm B were bushy, having flower buds with protruding stigmas. Pod set was reduced, and seeds were small. Consequently, seed yield was also remarkably lowered.
2. Injury due to excessive boron was shown by marginal yellowing and burning of the lower leaves in plants supplied with 3-5ppm B in all vegetables examined. Symptoms of boron excess in cabbage were slighter than in other vegetables.
3. Growth and yield of plants supplied with 0-0.005ppm B were considerably inferior to those of the control plants supplied with 0.3-0.5ppm B. The degree of growth reduction was more remarkable in fleshy roots or heads than in leaves.
Because of slighter deficiency symptoms and less growth reduction of plants supplied with 0-0.005ppm B, cabbage and broccoli seemed to be more tolerant of boron deficiency than other vegetables.
Significant reduction of growth and yield was not found in plants supplied with 3-5ppm B.
4. Boron concentrations of leaves, fleshy roots and heads which showed deficiency symptoms were less than 10ppm (dry weight basis).
Plants containing more than 20ppm in their leaves were normal and healthy. Boron concentrations of leaves manifesting injury due to boron excess were roughly higher than 100ppm.
There were small differences of boron concentrations among organs or leaf positions in boron deficient plants, but when sufficient boron was supplied, it was accumlated in leaves, especially in the lower ones.
5. There were no distinct difference in sugar concentration in leaves due to boron level in the solution. But sugar concentrations of both fleshy roots of root crops and heads of cauliflower were remarkably lower at 0-0.005ppm B applications as compared with the control. As for broccoli, there was no difference of sugar concentration in heads among the boron treatments.
Except in cauliflower heads, the sugars consisted mostly of reducing sugars.