Occurrence of sugar alcohols determines boron toxicity symptoms of ornamental species

The phloem mobility of boron (B) in plants varies dramatically among specie s. Variations in phloem B mobility occur as a consequence of the presence o f sugar alcohols (polyols) in some species but not in others, and these dif ferences in phloem B mobility profoundly affect the expression of B toxicit y symptoms. Twenty-four species including common ornamental species varying in sugar alcohol content, were selected to test their response to B toxici ty. Species that do not produce sugar alcohols exhibited previously describ ed B toxicity symptoms that include accumulation of high concentrations of B in, and burning of, the tip and margin of old leaves. In the sugar-alcoho l-producing species these symptoms were absent, and B toxicity was expresse d as meristematic dieback and an accumulation of B in apical tissues. These symptoms have not previously been associated with B toxicity in these spec ies and hence may have been frequently misdiagnosed.