Summer squash plants {Cucurbita pepo L., cv. ‘Early Prolific Straightneck’) hydroponically cultured under optimal boron nutrition (+ B)3 until age 5 d were transferred to hydroponic solutions supplemented with excess boron (+ + B)3. As boron accumulated in the leaves with time, leaf conductance to water vapour, 14CO2 fixation, and chlorophyll content of the oldest leaf became significantly less than in the + B-control plants of the same age; shoot growth essentially ceased. Boron also accumulated in the roots; concomitant inhibition of root elongation and lateral root development resulted. These metabolic changes all occurred prior to the accumulation of sufficient boron to cause the appearance of any symptoms characteristic of boron toxicity.
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Lovatt et al. (1984) studied this question.