| Abstract
| - It is well established that defoliation induces changes in numerous metabolic processes in forage species, including alterations in enzyme activities and in the size of corresponding substrate pools, and mobilization of C or N compounds, these being under the control of modified source/sink relationships. Such integrated physiological responses suggest that regulation occurs at the molecular level. This hypothesis was tested with a genetically uniform line of Lolium temulentum L, an annual species, after confirming with continuous 15N labelling that the same basic processes (N uptake, N mobilization) occurred during regrowth after defoliation as previously reported in L. perenne L. Results showed that the initial availability of N reserves fully accounted for the difference in regrowth yield. In vitro translation products of RNA extracted from different tissues of L. temulentum and L. perenne after defoliation were compared with those from intact plants, and clearly showed that mRNA profiles were finely modulated, and dependent upon both the tissue (meristem, sheath or laminae) and the time elapsed after defoliation. Changes in the pattern of translation products were detected using 2-dimensional electro-phoresis as soon as 1 h after defoliation, but the major up-or down-regulations of transcript abundance occurred between 12 h and 72 h after shoot removal. The results suggested that the early appearance of new messages (< 1 h) may be involved in the regulation of medium-term (>12 h) physiological adjustments to defoliation. These medium-term modifications may be functionally related to post-defoliation changes in enzyme activities and kinetics reported in the literature.
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