Short-term physiological responses of the aquatic liverwort Jungermannia exsertifolia subsp. cordifolia to KH2PO4 and anoxia

Resumen

The effects of four increasing levels of KH2PO4 on the physiology of the aquatic liverwort Jungermannia exsertifolia subsp. cordifolia were analyzed in the laboratory in the short term (15 d). The accumulation of P and K in the liverwort tissues was influenced (ANOVA) by the level of KH2PO4-enrichment and was significantly higher in the more enriched culture solutions. However, only the concentration of P was influenced by the effect of time (ANOVA), and the gradual P accumulation throughout the culture period contrasted with the fluctuations observed in K accumulation; these were presumably due to the higher liability of K to be leaked from the cells. Our results suggest that the analysis of P in transplants of J. cordifolia may be a useful bioindicator of short term water eutrophication both in the spatial and the temporal scales, although the highest PO4 3- concentration used in this study (20 mg liter-1) may induce a P saturation in J. cordifolia tissue (0.53% DM). The rates of net photosynthesis showed a significant quadratic regression with the tissue P concentration, which might resemble the action curve of mineral nutrients. Using this regression as an indicator, there was no clear deficiency zone, probably due to the relatively high tissue P concentration initially found in the liverwort. The lack of stimulation of net photosynthesis with increasing tissue P could be due either to a deficiency in other mineral elements such as N, or to an intrinsic inability to use the excess of nutrients. The decline in net photosynthesis when the tissue P concentration exceeded 0.45% DM could be interpreted as a toxicity process. Chlorophyll concentration was not affected by P enrichment, but the decline in the chlorophyll a/b ratio and in the proportions of chlorophylls to phaeopigments, together with the increase in the proportion of carotenoids to chlorophylls, suggested also P toxicity. This phenomenon needs further investigation to be confirmed in other species and conditions, but it may help to explain the disappearance of certain aquatic bryophytes in eutrophicated water courses. The physiological effects of increasing tissue K concentration (decreases in the rate of dark respiration and in the chlorophyll a/b ratio) were slighter than those of P, probably because the accumulation of K was lower than that of P (1.44 and 1.96 times the initial value, respectively). In a different experiment in which J. cordifolia was cultured in P-enriched aerated and non-aerated solutions, anoxia caused a strongly diminished P accumulation in the first three days, probably because the mitochondrial respiration was blocked. Then, a clear net loss of P from the liverwort tissues was observed, maybe caused by membrane damage.