Effect of the formation of a sugar-borate complex on the growth inhibition of pollen tubes ofCamellia sinensisand cultured cells ofNicotiana tabacumby toxic levels of borate

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Abstract
The relationship between exogenously supplied sucrose, glucose, and fructose and growth inhibition of tea pollen tubes cultured in a liquid medium and of cultured tobacco cells by toxic levels of borate was investigated. In the medium containing 0.2 M sugar at pH 5.8, tea pollen tube growth was strongly inhibited by 20 mM borate with sucrose, and by 10 and 20 mM borate with glucose, but not inhibited even by 20 mM borate with fructose. The growth inhibition of the pollen tubes by 20 mM borate was obviously alleviated when the concentration of fructose exceeded 0.2 M and the pH of the medium exceeded 5.4. Twenty millimolar borate inhibited the growth of tobacco cells in the media with sucrose and glucose, but not with fructose at pH 5.8. The ability of sugars to form a borate complex was in the decreasing order of fructose, glucose, and sucrose based on the decrease of the pH of the mixtures of sugars and borate. It is, therefore, suggested that the toxicity of boron was reduced by the formation of a sugar-borate complex, especially a fructose-borate complex, when the concentration of fructose was much higher than that of borate.