Effect of severity of defoliation on the viability of reproductive and vegetative axillary buds of Trifolium repens L

This glasshouse experiment was performed to assess the effects of a range of constant defoliation regimes applied to cuttings of a single large-leaved genotype of Trifolium repens L. on the viability of its axillary buds. Plants were established to comprise a single main stolon (axillary branches were removed) and defoliation treatments were applied by removing the older (basal) leaves until leaf complements of 1.0, 1.5, 2.0, 2.5, 3.0 or all leaves (control) remained. Basal leaves were subsequently removed as necessary to maintain the target leaf complements. Only severe defoliation (leaf complements of 1.0 and 1.5) induced a loss of viability in axillary buds. Loss of viability was greatest in reproductive buds present within the apical bud when the treatments were first imposed. Although the most severe treatment (leaf complement 1.0) resulted in death of half the plants, in plants surviving that treatment, death of vegetative axillary buds was restricted to 21% of the vegetative buds at the three youngest node positions within the apical bud at the time of treatment application. No other treatment induced any loss of viability of vegetative buds. There was no loss of viability of axillary buds at nodes formed after the treatments were imposed. The frequency of initiation of inflorescences at nodes formed after treatments were imposed decreased as defoliation severity increased. Severe defoliation resulted in marked changes in plant morphology indicative of a sharp decrease in availability of intraplant resources. It was concluded that under severe defoliation: (1) the potential for vegetative growth (as represented by viable vegetative axillary buds) was maintained at the expense of reproductive growth; and (2) that the loss of viability of axillary buds was associated with the sudden changes in physiological processes induced by defoliation as there was no loss of viability in buds formed after plants had adjusted their phenotype to one of smaller size.