| Allelopathy plays an important role in disturbing the forest regeneration and decreasing the undercrown biodiversity, especially in the plantation ecosystems. These years, the ecological problems of Eucalyptus grandis plantations arouse much attention. Indeed, the leaf litter of E. grandis was reported to contain numerous secondary metabolites, and many of which were believed allelopathic to other species. So realizing and utilizing the allelopathic effects of E. grandis leaf litter correctly is of great benefit to the sustainable management of E. grandis plantations.Litterbags and the gas chromatography-mass spectrography (GC-MS) were used in present experiment to study the main allelochemicals of E. grandis leaf litter and their dynamic in its initial decomposition in the soil. Meanwhile, a pot experiment simulating the natural decomposition of E. grandis leaf litter by mixing it with soil was conducted for the bioassay of its allelopathy. Dactylis glomerata was selected the receptor for the observation of the responses of morphology traits, photosynthetic physiology and resistant physiology during the initial decomposition of E. grandis leaf litter. Additionally,40d after the germination of D. glomerata, urea was applied in each pot of different leaf litter treatments to know the effect of nitrogen on the expression of the allelopathy of E. grandis leaf litter. Main results showed as follows,(1)33small molecule organic components with the terpene in the majority were identified from the leaf litter of E. grandis. Of them, the eucalyptol and a-pinene were the two with the highest relative content, and most of the components that own higher relative content belonged to the eucalyptus oils. It was suggested that terpenes such as eudesmol (i.e.,1,8-eucalyptol), a-pinene, a-terpineol and2-carene were probable allelochemicals of E. grandis leaf litter, which had great potential to be allelopathic.(2)6,10, and15components, mainly terpenes, were lost in the leaf litter that decomposed for30,60and90d, respectively, compared with the original leaf litter sample, indicating that the potential allelochemicals were released gradually with the decomposition of the leaf litter and likely to affect the species nearby. The decay of leaf litter was indeed a very pathway for E. grandis to release its allelochemicals to the surroundings.(3) Application of E. grandis leaf litter exerted significant influence on the gas exchange parameters of leaves of D. glomerata. Concretely, the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and water use efficiency (WUE) all fell, while the intercellular CO2concentration (Ci) rose with the increase of the leaf litter. Apparently, the allelochemicals from the decomposition of the leaf litter affected the stomatal aperture to some extent, however, nonstomatal limitations (e.g., the decrease of the photosynthetic activity of the mesophyll cells) were believed responsible for the most of the decrease of the Pn in present paper. Additionally, the photosynthetic pigments reduced gradually when the leaf litter increased, which suggested that the biosynthesis of the photosynthetic pigments was inhibited, or their degradation was accelerated by application of the leaf litter. But both the ratio of the chlorophyll a to the chlorophyll b (Chla/Chlb) and the ratio of the chlorophyll to the carotenoids (Chl/Car) presented an uptrend. That meant the decomposition products had a greater impact on the Chlb than on the Chla, and a greater impact on the Car than on the Chl. The light adaptability of D. glomerata was also observed affected because that the apparent quantum yield (AQY), maximum net photosynthetic rate (Pmax), light compensation point (LCP), light saturation point (LSP), dark respiration rate (Rd) and the interval of available light intensity all descended with the increase of the leaf litter. It could be conjectured that the allelochemicals from the decomposition of the leaf litter damaged the photosynthetic reaction centre of D. glomerata, suppressed its photosynthetic production and became the main cause of its height increment and biomass accumulation being seriously inhibited.(4) The osmotic equilibrium and reactive oxygen species (ROS) metabolism were both influenced by the decomposition of the leaf litter, i.e., the releasing allelochemicals might be an obstruction for the root of D. glomerata to absorb water and made it suffer osmotic stress. In case of that, D. glomerata primarily accumulated the proline (Pro) and soluble sugar for the osmoregulation of its cells. On the other hand, the antioxidative system of D. glomerata was also impacted remarkably as follows. In the early stage of its growth, the activity of peroxidase (POD) and catalase (CAT) was inhibited, in addition that the superoxide dismutase (SOD) did not response positively in time, the malonaldehyde (MDA) content increased to some extent as a result, indicating the obvious lipid peroxidation of its cells.90d after its germination, the activity of POD and CAT was still suppressed, while the SOD was of help to resist ROS and alleviated the lipid peroxidation under the treatment LI (30g·pot-1) and L2(60g·pot-1). However, when the leaf litter increased in the soil (the treatment L3and L4, i.e.,90and120g·pot-1,respectively), these antioxidative enzymes were all suppressed. In a word, it seemed that an adverse situation formed for D. glomerata during the decomposition of the leaf litter, and the releasing allelochemicals were likely to bring the seedlings significant oxidation damage and osmotic stress, thus led to the retard of their growth. However, when the decomposition duration prolonged, or the seedlings grew older, the inhibition got weaker probably due to the adaptability of the seedlings got stonger.(5) In terms of the growth of D. glomerata, the decomposition of the leaf litter also showed apparent allelopathic effects through the inhibition of its height increment and biomass accumulation. As stated above, the turbulence of the physiology metabolism and the decrease of the photosynthetic production had to be responsible for that. Although the growth of the seedlings were suffering obvious inhibition from the treatment L3and L4during the90-day experiment, the inhibition of the treatment L1and L2got weaker after60d. Nitrogen application promoted the growth of D. glomerata, but failed to eliminate the allelopathic effect of the leaf litter. It was suggested that fertilization might not affect the expression of the allelopathy of the leaf litter, on the contrary, the nitrogen use ability of the seedlings was likely to be affected.In summary, the leaf litter of E. grandis contains a large amount of potential allelochemicals, which are released gradually with the decomposition of the leaf litter. D. glomerata, the receptor, the growth, resistant physiology, photosynthetic physiology of which were all apparently influenced by the decomposition of the leaf litter. So it is suggested in present paper that the leaf litter of E. grandis has great potential to be allelopathic, which should be utilized rationally in the practice. |
PDF Full Text Request