Effects Of Decomposing Walnut Leaf Litter On The Growth Of Lettuce

In order to investigate the allelopathic effect of Juglans regia on crops, a pot experiment was conducted to study the impacts of naturally decomposing leaf litter of walnut (Juglans regia) in soil on the resistance physiology, photosynthetic and growth of lettuce (Lactuca sativa). Experimental design included control (CK, without applying leaf litter) and three treatments (with 30,60,90 g-pot’1 leaf litter fragments, respectively, corresponding denoted as T1, T2, T3), The leaf litter of each treatment mixed with soil (8 kg-pot-1) respectively, then lettuce seeds were sowed. Physiology and growth indexes were measured at 80,100,120,140 d after sowing, meanwhile, the chemical composition in walnut leaf litter were analyzed by GC-MS. The results showed that:(1) 16 kinds of organic molecules extracted from hexane in walnut leaf litter were identified, respectively, including alkenes, phenols, alcohols, alkanes, terpenes and ketones. The organic substances in a relatively high content from the extraction were:tocopherol (30.82%), squalene (26.74%) and sitosterol (16.17%), in addition, substances possessed of allelopathic potential like eucalyptol (5.47%), Zhu Luan sesquiterpenes (2.52%) and so on, which indicated that walnut leaf litter has allelopathic potential.(2) The activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) of lettuce treated with leaf litter were promoted at 80 d, restrained at 100 d, while began to recover at 120 d and has returned to normal levels basically at 140 d, with the most sensitive response to the allelopathy in SOD, malondialdehyde (MDA) content of each treatment was significantly higher than that of control at 100 d. So, the intensity of allelopathic effects caused by walnut leaf litter decomposition in the soil on antioxidant enzyme systems of lettuce leaf gradually increased (80-120 d) then weakened (120-140 d) as decomposition time prolonged.(3) Soluble protein (SP) content was promoted at 80 d, inhibited at 100,120 d, while the soluble sugars (SS) and proline (Pro) content are promoted at 100,120 d. That has indicated the leaf litter decomposition on osmotic adjustment of lettuce leaves played an obvious role on SP rather than SS and Pro before 80 d, however, it just performed conversely in the period of 100-120 d.(4) The chlorophyll content of all treatments were less affected at 80 d, but apparently lower than that of the CK until 100 d and later, besides, it showed a gradual downward trend with the increasing of the amount of the leaf litter at 120 d, but this difference disappeared at 140 d. Carotenoid (Car) synthesis seemed not to suffer inhibition of leaf litter decomposition, even with a content which was significantly higher than that of CK at 120 d (P<0.05). The net photosynthetic rate (Pn) received a significantly (P<0.05) inhibition, stomatal conductance (Gs) showed a downward trend, intercellular CO2 concentration (Ci) and stomatal limitation value (Ls) affected relatively weak at 100 and 120 d. Indicating that the declined photosynthetic rate may be related to the decreased chlorophyll content, and little to do with stomatal factors.(5) Using three models to fit Pn-light response process of lettuce leaf and the merits of fitting results were as follows:rectangular hyperbolic correction model> non-rectangular hyperbolic model> rectangular hyperbolic model. By fitting the value of the best model showed that with the increase in the amount of leaf litter been applied, dark respiration rate (Rd) showed a downward trend, and each treatment were significantly lower than CK, which indicated that lettuce was able to lower consumption of photosynthetic products by slowing breath to maintain growth as a physiological adaptation characteristics under allelopathic stress. Light compensation point (LCP) showed a rising trend, and reached a significant level, indicating that allelopathic effect reduced the ability of lettuce to utilize low light. The maximum net photosynthetic (Pn max) and light saturation point (LSP) decreased, suggesting that allelopathy caused lettuce more prone to light saturation. Apparent quantum yield parameters of lettuce leaves showed a decreasing trend, and each treatment was significantly lower than the CK, which may reduced the ability to convert light into energy, greatly inhibited the use of low-light ability when walnut leaf litter releasing allelochemicals in decomposition.(6) The height, aboveground biomass and the proportion in total biomass of lettuce were significantly suppressed after 120 d of sowing, while the suppression began to alleviate and returned to normal at 140 d (with no significant difference to CK (P> 0.05)). Analysis with all indicators integrated showed that before 120 d, The decomposing walnut leaf litter in the soil released eudesmol, sesquiterpene and other allelochemicals, which might produce a strong allelopathic impact on growth and resistant physiology of lettuce, with an obvious inhabitation on morphology indexes.120 d later, allelopathic intensity gradually weakened, also resistant and photosynthetic physiology returned to control level, while morphological indexes recovered at 140 d. On the whole, allelopathy intensity of walnut leaf litter in the soil gradually increased then weakened with the extension of decomposition time, overall, no significant difference among treatments.