carrots grown in nutrient solution without the addition of boron were unable to complete their normal development and exhibited both external and internal deficiency symptoms:
I pnrt boric acid in 2 million was sutticient to maintain healthy growth when the culture solutions were renewed regularly.
The effect of a lack of boron at late stages of growth was determined by transferring plants to a boron-free solution after they had received an initial supply.
The chief deficiency symptom were:
- (a) E x t d. The leaves curled back, pointed downwards and often turned red or yellow. The laminae of the youngest leaves were much reduced and the growing apex of the shoot died. The flowering stage was not reached. A small tap root was formed but the laterals were abnormal m appearance, being short and thickened in places. When the boron de- ficiency only arose in the later stages of development, certain additional symptoms appeared, viz. the flowering heads bent over and died, those of the side shoots succumbing first. In some cases small wart-like excrescences arose on the outside of the tap root.
- (b) Intmrol. Although no marked degeneration within the tap root was evident to thenaked eye, microscopic investigation showed that most of the tissues of the shoot and root were af ected by a lack of boron. Breakdown began in the region of the growing point of the shoot, patches of irregular and blackened tissue appearing in the young leaves and in the vascular tissue close behind the tip, though the meristematic apex itself was not necessarily h t involved. Later, similar patches were found in the cortex and occasionally also in the pith. Hypertrophy usually preceded or accompanied the formation of these lesions. All cmponents of the vascular bundles were affected, blocking occurring in the xylem and phloem, and hypertrophy and breakdown in the thin-walled tissues. The formation of the wart-like growth on the tap roots and the swellings on the laterals was explained by the fact that in the absence of boron, repeated branching and radial extension of lateral roots contin, when elongation and emergence were prevented through death of the apices.
Recovery took place in both shoot and root when boron was supplied, even after prolonged growth in its absence. Traces of the earlier deficiency period could be detected in the anatomical structure of the older parts of the plants although the external appearance was quite normal.