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

Abstract

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.

Dynamics of carbon allocation in a deep-water population of the deciduous kelp Pleurophycus gardneri (Laminariales)

“…Although this study revealed no evidence of major seasonal photoacclimation in Pleurophycus gardneri, differences in the magnitude of physiological parameters (α and chl) between the deep population studied here and the intertidal plants measured by Germann (1989) were evident. These deep-water plants exhibited higher light utilization efficiencies than intertidal plants on a biomass basis, but lower efficiencies on a chlorophyll-specific basis.…”

“…This pattern represents a classic photoacclimation response common to shadeadapted algae. The P/R ratio was slightly higher in this deep-water population than in the intertidal one studied by Germann (1989); however, it was not higher than in other kelps such as Macrocystis pyrifera (Zimmerman & Kremer 1986, Table 4). …”

“…Pleurophycus gardneri (Laminariales) is a deciduous perennial (Germann 1986) or pseudoperennial (Sears & Wilce 1975) kelp, and the only one known to undergo programmed defoliation characterized by the synchronous abscission of the photosynthetic blade each winter. Germann (1989) suggested that abscission of the blade may be more advantageous than its retention, because the high photosynthetic efficiency and low respiration of the stipe and the holdfast may compensate for loss of the greater photosynthetic surface area of the blade, which has large respiratory requirements.…”

“…Pleurophycus gardneri (Laminariales) is a deciduous perennial (Germann 1986) or pseudoperennial (Sears & Wilce 1975) kelp, and the only one known to undergo programmed defoliation characterized by the synchronous abscission of the photosynthetic blade each winter. Germann (1989) suggested that abscission of the blade may be more advantageous than its retention, because the high photosynthetic efficiency and low respiration of the stipe and the holdfast may compensate for loss of the greater photosynthetic surface area of the blade, which has large respiratory requirements.The distribution of Pleurophycus gardneri is commonly reported to be from Montague Island, Alaska, to Fort Bragg, California, USA, in the low intertidal to upper subtidal (Abbott & Hollenberg 1976). Individuals are common in the northern part of the range, but are thought to be comparatively rare in the south.…”

“…Carbon content of the stipe decreased during spring, perhaps due to remobilization and utilization of carbohydrates for growth of the blade. Germann's (1989) suggestion, that translocation from the blade to the stipe was not possible due to the lack of blade growth in complete darkness, assumed that blade growth only required the presence of carbohydrate reserves. It did not consider the presence of a growth-initiating trigger, which may be controlled by photoperiod (Lüning & Kadel 1993).…”