Effects Of Leaf Litter Leaching Solution And Plant Polyphenols On Mangrove Germination And Seedling Growth Of Mangrove Trees

Kandelia obovata(Ko), Aegiceras corniculatum(Ac) and Avicennia marina(Am) are common and dominant plant species, and form a mixed community in mangrove swamps in South China. However, how these species interact and how one species affects the germination and growth of the other have seldom been reported. Mangrove plants are known to have high concentrations of tannins but their effects on species interactions are also unclear. The present study aimed to evaluate the effects of leaf litter leachates(LLLs) from Ko(LLLs-Ko) and Am(LLLs-Am), and the purified condensed tannins(PCTs) from Ko on the germination of propagule and growth of Ac based on field investigation, pot and bench experiments. Seedlings of Ac and leaf litter of mangrove in a mixed community containing three species(Ko+Ac+Am) were investigated by a sample method with three random quadrats of 5 m ×5 m and then each quadrats set up three sampling plots of 1 m×1 m. Replicate pots containing five different levels of LLLs(0, 2, 10, 20 and 50 g L-1) of Ko and Am in artificial seawater at a salinity of 10 parts per thousands(ppt) and PCTs(0, 10, 100, 200 and 600 mg L-1) of purified tannins in 10 ppt artificial seawater were separately prepared. Healthy propagules of Ac were placed in each treatment for three months. Leaching experiment was also conducted by shaking the leaf litter of Ko, Ac, Am and Sonneratia apetala(Sa) in artificial seawater at a salinity of 10 ppt on a shaker for nine days. The key findings from this study were summarized as follows:1. Large amounts of litter were found on mangrove floor under mature trees, especially Ko. In the three random sampling plots, the number of seedlings of Ac were 8, 3 and 1 in total, respectively. The size of the seedlings in three quadrats was comparable, with an average height of 0.18 m and each seedling had 2 to 6 leaves.2. The germination and growth of Ac seedlings decreased gradually with increasing concentrations of PCTs, especially when the levels were ? 200 mg L-1. Roots were injured by higher levels of PCTs as shown by darker, smaller, shorter and twisted roots than those in the control or low levels of PCTs. Shoot was generally more sensitive to PCTs than root. On the other hand, low concentrations of PCTs had stimulatory effects. In 10 mg L-1 PCT, the length and biomass of root, as well as the number and biomass of leaf, increased. The length and biomass of stem and the number of nutritive root were increased in100 mg L-1 PCT. It is clear that condensed tannins or plant polyphenols had stimulation on the germination and growth of Ac at low concentrations but became inhibitory at high concentrations.3. The effects of LLL treatments were more complex than that of PCTs. The germination of Ac propagule decreased gradually with increasing concentrations of LLLs-Ko and LLLs-Am, and both leachates had same trends of effects on root germination. For shoot germination, the inhibition started from 10 g L-1 in LLLs-Am but from 20g L-1 in LLLs-Ko. The growth indicators, including the biomass of root, stem and leaf, the length of root and stem, and the number of root and leaf, also changed with levels of leachates and the effective levels differed between two plant species. It is clear that LLLs, similar to PCTs, were stimulatory at low levels but were inhibitory at high levels. Between Ko and Ac, the stimulatory effect of LLLs-Ko on growth of Ac seedlings was higher than that of LLLs-Am, but LLLs-Ko also had higher inhibition when the concentration was high.4. Leaf litter leachate not only contained condensed tannins(ECT), other chemicals such as total phenols(TP), total nitrogen(TN), ammonia nitrogen(NH3-N), nitrate and nitrite nitrogen(NHx-N), total carbon(TC) and total organic carbon(TOC) were also found, but they did not show any significant inhibition on the germination and growth of Ac. The pH of leachate also did not pose any negative effect. However, the effects of LLLs were not exactly the same as that of PCTs, suggesting that some other unknown chemicals in leachate might affect the germination and growth of Ac, and needed further research.5. The changes of the concentrations of TP and ECT in the leaf litter leachate were similar among Ko, Ac, Am and Sa, all increased at the beginning then decreased with leaching time. At the end of 9-day leaching experiment, there were significant losses in the mass of litter and the amounts of TP and ECT in litter. The temporal trends of bound condensed tannins(BCT) differed among species. In litter of Ko and Ac, BCT increased with time. It is possible that ECT in Ko and Ac may be combined with the protein or cellulose in litter and converted to BCT. However, BCT continuously decreased in Sa, the reason was not sure and deserved more research work.