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Girdling--ringing the genetic changes
Growers of several types of fruit tree, including peach and various citrus species, traditionally girdle the trees prior to flowering. This means removing a ring of bark from the main trunk so that phloem transport to the lower trunk and to the roots is prevented, increasing the relative sink strength of the fruit (which is already very high) and, as mentioned by Li et al. (Hebrew University of Jerusalem, pp. 137-143) often causing severe starvation of the roots. Nevertheless, the desired effect is seen in more successful fruit set and greater fruit size, presumably because of the diversion of nutrients to the upper parts of the tree, including the reproductive organs. However, as we have noted previously in these pages, some plants respond to plenty in unexpected ways. What then is the situation with girdled citrus trees? The authors used a variety that fruits every other year and were thus able to study the accumulation of soluble carbohydrate and starch in the leaves, bark and roots of girdled trees in both ‘on’ and ‘off’ years. Here we focus on leaves and bark. Girdling certainly leads to an abundant carbohydrate supply above the ring but in ‘on’ years the fruit act as such a strong sink that there is no accumulation in stems or leaves. In ‘off’ years when the soluble carbohydrate is not mopped up by the fruit, it is converted to starch in both leaves and bark. Further, this starch accumulation is associated with increases in the levels of mRNAs that encode several enzymes involved in starch biosynthesis. These increases in gene transcription do not occur in ‘on’ years when little or no soluble carbohydrate is available for starch biosynthesis. Thus gene expression is sensitive to carbohydrate status, adding further weight to the view that sugars may act as signalling molecules.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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