Defoliation-Induced Stress in Nodules of White Clover: I. CHANGES IN PHYSIOLOGICAL PARAMETERS AND PROTEIN SYNTHESIS

Nodule function and protein synthesis were studied in defoliationstressed white clover plants. Uncut control plants (C) werecompared with plants from two defoliation treatments: (1) continuousdefoliation (CD) where all leaves and petioles were removedeach day; and (2) defoliated/recovered (DR) where, after removalof all leaves and petioles, new leaves were then allowed toregrow. After a single defoliation N₂ fixation (acetylene reductionactivity) and nitrogenase-linked respiration declined by morethan 80% within 3 h and by nearly 100% by 24 h. DR plants beganto fix nitrogen again at a very low level 3 d later and thereafterrose to control levels by 15 d. Continuously defoliated plantsnever recovered N₂ fixation capacity. Nodule protein complementwas assessed by polyacrylamide gel electrophoresis. Major changesoccurred in buffer soluble protein band patterns by 6 d in CDplants, but few changes were evident in SDS soluble proteins.By 9 and 14 d significant disruption of all proteins was evident.The prominent host plant protein, leghaemoglobin (Lb) had disappearedby 14 d. In DR plants the intensity of staining was reducedbut no major changes in band patterns were evident and by 21d nodules were rejuvenated. [³⁵S]-labelled methionine was incorporated into nodule proteinsfrom all treatments throughout the experiment. However, continuousdefoliation caused increasing variability between replicatesin the labelled band patterns. By 21 d CD, much of the labelledprotein was present as amorphous low M_r material which suggestseither disruption of the protein synthesizing machinery or rapidhydrolysis by proteolytic enzymes. Surprisingly [³⁵S]-methionine was never found in Lb from nodulesof any treatment. It is possible that white clover Lb does notcontain any methionine residues or that no synthesis of Lb occurred. Key words: Trifolium repens, white clover, defoliation, protein synthesis, nodules