Citation

Verbruggen H. (2001)
The ecology of phosphate uptake by macrophytes, with emphasis on Chara globularis (in Dutch)
Licentiate thesis. June 2001. Catholic University of Leuven.

Summary

Phosphorus is a major nutrient acquired primarily via high-affinity inorganic phosphate transporters. In this thesis, it is shown that phosphorus uptake activity in Chara globularis decreases at external phosphate concentrations exceeding 65 µM. It is suggested that, at these high concentrations, the high-affinity transport system is inhibited.

The first major aim of the present study was to link seasonal patterns in abiotic and biotic factors from a pool dominated by the submerged macrophyte C. globularis to the phosphate uptake kinetics in this species. The phosphate uptake could not be linked to the phosphorus concentrations of either tissue or water. It rather seemed dependent on the developmental stage of the plants, showing high uptake rates when spores were formed and young shoots were intensively growing. C. globularis seemed to have a large impact on the ecosystem. As the plants died off, the water became anoxic, and most probably anoxic phosphate release from the sediment occured.

It seems the C. globularis from the studied pool does not use the sediment as a source for phosphorus. To explain this, two hypotheses were formulated. Firstly, a lack of oxygen at the sediment surface could cause uptake rates to be low, and secondly, basal decay of the plants could inhibit the transport of phosphorus taken up from the sediment to the actively growing parts of the thallus.

A second aim was to investigate the influence of abiotic factors on the uptake rate. As Na+/Pi cotransport was recently shown in C. corallina, Na+ was given most attention. The influence of medium pH on uptake rate was also investigated.

Although it could not be proven directly, three observations provide evidence for the existence of Na+/Pi cotransport in C. globularis. Firstly, the uptake of Pi was stimulated by the presence of Na+ in the medium, with an optimal concentration in the range 1-2 mM. Secondly, Na+ starvation of the plants enhanced the phosphate uptake rates. Thirdly, addition of Pi to the uptake medium did not increase its pH, suggesting uptake does not occur through proton-phosphate cotransport.

In both C. globularis and Nitella flexilis, phosphate uptake was maximal in the central part of the studied pH range, at pH 6 to 8. Preference for weakly ionised Pi (H2PO4- is obviously lower than in certain other studied species. The patterns the uptake rate showed as a function of pH changed with increasing phosphate concentrations in the uptake medium. This phenomenon stays unclear. Nitella showed lower uptake rates than Chara. This was probably due to the large difference in tissue phosphorus content and not to species-specific differences.

The third aim of the study was to investigate the influence of the tissue nutrient status on phosphate uptake. For this purpose two macrophyte species (C. globularis and Elodea nutallii) were subjected to phosphorus starvation and enrichment.

The uptake rate was correlated with the tissue phosphorus content for both species. Elodea seems more susceptible to phosphorus stress than Chara. It increases its uptake rate as the internal phosphorus content decreases below 0.3% of dry weight. Chara only starts showing increased uptake rates until its phosphorus content decreases below 0.15% of dry weight. The N:P ratio of the tissue is rather constant in Elodea and considerably lower than in Chara. This seems to agree with the growth strategies of both species.

Phosphate uptake responds more quickly to changing conditions in Elodea. Probably this species can produce great amounts of phosphate transporters at a fast rate. Most probably the turnover rate of membrane proteins too, is higher in Elodea.

On supply of phosphorus to the plants, Elodea is better able to convert this into higher growth rates. This means the plants will be competitively advantaged on eutrophication of the water. Although Chara hardly succeeds to convert higher phosphorus supply rates into an increased growth rate, it is able to keep the phosphorus concentration of natural waters low. This is in great amount due to the accumulation of phosphorus in plant tissue. In the experiments no upper limit to the phosphorus content of Chara could be observed. The phosphorus concentration of the plant tissue increased to 0.78% of dry weight in one of the experiments. Chara can, due to its capacity to store phosphorus to large amounts, efficiently stabilize moderate eutrophication of natural waters.

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