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Growth phenology of the monotypic northeast Pacific kelp Pleurophycus gardneri was assessed between 1982 and 1984. The study gives evidence that Pleurophycus is a deciduous perennial. Pleurophycus shows complete and synchronous defoliation during a short period of time in late fall and commences new blade growth after about 2 weeks of quiescence. Pleurophycus sheds the lamina along an abscission zone at the transition between stipe and blade. The abscission zone is macroscopically evident through a change in color and a slight indentation in senescent plants. After separation, outer cortical cells constitute a protective layer which increases in thickness as new blade outgrowth progresses. Temperature of 14 to 15 °C is assumed to be a triggering factor for the induction of blade senescence. Warm water intrusion, associated with E1 Ninõ, induced premature blade senescence and blade abscission in 1983. Growth of the new blade is completely inhibited in darkness. The induction of new outgrowth is not mediated through a photoperiodic effect and showed no day-length saturation.

Pre-abscission changes in the deciduous kelp Pleurophycus gardneri involve a gradual decrease in dry weight, mannitol and laminaran content. These carbon compounds decreased from about 18% (dry weight) to 4 % prior to blade abscission, while the perennial structures, stipe and holdfast, showed a concomitant increase in mannitol and dry weight towards autumn. In November, at the time of new blade outgrowth, mannitol content peaked in stipe and holdfast with 12 and 1 8 % , respectively. Laminaran accounted for less than 2 % in all thallus parts and showed no measurable seasonal variation. Pigment levels of the blade increased with progressing senescence and were at maximum just prior to blade abscission. The photosynthetic performance of the blade revealed saturating irradiances of about 50pE m-2 S-' and a compensation irradiance of 2.5 to 3~t E m-2 S-' measured in April. h g h t saturation of the stipe was at 38 pE m-2 S-' and of the haptera at 23 pE m-2 S-' while compensation irradiances were at roughly 8 FE m-2 S-' for both perennial parts. It is suggested that new blade outgrowth requires 11ght-carbon fixation. No acropetal translocation of the abundant carbon store of the perennial structures was observed The deciduous habit of P. gardneri reflects a shade-adapted growth strategy. The perennial parts are involved In the overall physiology of the plant and have similar seasonal fluctuations in their constituents to those of the blade. The described growth strategy is novel among perennial kelp species.

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