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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 18%, 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.

Aspects of carbon metabolism in relation to autumnal blade abscission in the kelp Pleurophycus gardneri (Phaeophyceae, Laminariales