Effects Of Atmospheric CO₂ Enrichment On Aquatic Macrophytes Of Different Life Forms

Comparative studies were conducted to get some insights into the effects of atmospheric CO2 enrichment (1000umol mol-1) on the aquatic macrophytes of different life forms. The main results are as following:1. Semi-closed CO, inflating method was applied in these studies. The results showed that CO2 concentration in control was 350umol mol-1 and simulated CO2-enriched environment 1000umol mol-1 and could be sustained constantly. Elevated CO, had little effect on the water temperature and dissolved oxygen but pH decreased. CO, and HCO3- concentration increased significantly. So the inflating method was reasonable and workable for studies on the effects of elevated CO, on aquatic macrophytes.2. Effects of elevated CO2 on the growth and leaf characteristics of free-floating macrophytes were investigated by exposing Spirodela polyrhiza and Lemna minor to 1000umol mol-1 CO2 for 20 days. Both S. polyrhiza and L. minor formed more fronds in CO2-enriched aquarium as the results of dramatically enhanced relative growth rates. Dry weight (DW), fresh weight (FW) and DW/FW ratio (DFR) of certain number of fronds presented difference responses to elevated CO2. In L. minor, since DW increased significantly while FW unchanged, FWR increased significantly. In S. polyrhiza, DW unchanged while FW increased so the DFR decreased significantly. Elevated CO, had positive effects on both frond sizes and the chlorophyll (a+b) content but negative on Chla/Chlb ratio, which might indicate increased photosynthetic capacity.3. The growth and physio-chemical responses of Vallisneria spiralis were studied comparatively in ambient and elevated CO, concentration. Young plants in elevated CO2 grew faster than their counterparts in ambient CO, at early stage, but this superiority did not persist throughout the experiment, and finally plants in CO2-enriched environment accumulated just only 11.6% more total weight than those in control. Total weight advantage was mainly due to leaf weight increment superiority. More turions formed in CO, enrichment. Root: Leaf ratio reduced in both elevated and ambient CO2, and no significant difference was detected. Total leaf area was not affected by elevated CO2 and the specific leaf area was persistently 7-30% lower. Elevated CO2 had positive effect on leaf width, but not on leaf number. Under elevated CO, chlorophyll and soluble protein content decreased, whereas total soluble sugar content increased significantly.3. Seedlings of Trapa pseudoincisan, a typical floating-leaved macrophyte accumulated more biomass after exposing 30 days in high CO,, and the biomass advantage was due to dry weight increase in root, stem, leaf and petiole. Elevated CO2 had little effect on biomass allocation with root weight ratio and leaf weight ratio tended to increase but stem weight ratio and petiole weight ratio decrease. In root, more assimilating root formed at the cost of decreased absorbing root at elevated CO2. Seedlings of T. pseudoincisan showed morphological variability in response to high CO,. Elevated CO, had positive effect on leaf area and leaf number but negative on specific leaf area. Stem length, swollen petiole length and diameter increased whereas petiole length shortened in elevated CO2 compared with their counterparts in ambient CO2.