不同生境条件下核桃凋落叶次生代谢物质的差异及其分解初期对小白菜光合生理特性的影响

采用气质联用技术(GC-MS)分析了秦巴山区大巴山南坡和川西高山峡谷区两个不同生境条件下生长的核桃(Juglans regia)凋落叶中次生代谢物质的差异,并采用盆栽试验,研究了这两个地区核桃凋落叶分解对小白菜(Brassica chinensis)生长和光合生理特性的影响。试验设置F0(0 g/pot)、F1(30 g/pot)、F2(60 g/pot)、F3(120 g/pot)和F4(180 g/pot)5个凋落叶施用水平,分别将各处理的凋落叶与8 kg土壤混合后装盆,播种小白菜。分别于播种后20、45和75 d测定小白菜的生长指标,待凋落叶最高添加量处理组F4植株的第3片真叶完全展开后,测定小白菜的光合生理指标。结果表明:(1)采用GC-MS共检测出8类21种相对含量1%的次生代谢物质,其中,川西高山峡谷区核桃7类15种,相对含量总计56.03%,秦巴山区大巴山南坡核桃5类12种,相对含量总计51.46%。川西高山峡谷区核桃白菖油萜、茅苍术醇、β-桉叶醇、6,10,14-三甲基-2-十五烷酮、亚麻油酸、硬脂酸、十八醇、γ-谷甾醇、豆甾-4-烯-3-酮等为报道较多的潜在化感物质,其相对含量高于秦巴山区大巴山南坡核桃,而角鲨烯、δ-生育酚、黑麦草内酯等潜在的化感物质相对含量则低于秦巴山区大巴山南坡核桃;(2)播种后20 d和45 d,两个地区各添加量核桃凋落叶分解对小白菜生物量积累均产生显著的抑制作用,但75 d后效应减弱;(3)在凋落叶分解初期,小白菜叶片光合色素合成受到明显抑制,且随着凋落叶添加量增加抑制作用增强,与此同时,净光合速率(Pn)、气孔导度(Gs)和蒸腾速率(Tr)也明显受到核桃凋落叶分解的抑制;(4)总体来看,川西高山峡谷区核桃凋落叶化感作用大于秦巴山区大巴山南坡核桃凋落叶,这可能是由于前者经受较多的环境胁迫,体内积累了更多的酮类、烯烃、醇类和甾族化合物等化感物质造成的。

Effect of secondary metabolites in Juglans regia leaf litter from different habitats on photosynthetic physiological characteristics of Brassica chinensis during early decomposing stage of leaf litter

Walnut (Juglans regia) has been regarded as an allelopathic tree species for a long time. In the management of agroforestry systems which are able to take good advantage of land resources, the allelopathic effect of walnut on interplanting crops elicits increased concerns. However, the environmental factors may alter markedly the allelopathic effect of a plant. Gas chromatography-mass spectrometry (GC-MS) was used in the present study to identify and compare chemicals in walnut leaf litter collected from two different habitats, i.e., Qinba mountainous area (QMA) and high-mountain gorge area of West Sichuan (HGWS). A container experiment was performed to investigate the effects of the walnut leaf litter from these two regions on growth and photosynthetic characteristics of pakchoi (Brassica chinensis) during the early decomposing stage of the litter. Walnut leaf litter was applied at the rate of 0, 30, 60, 120, and 180 g of leaf litter per pot, resulting in five treatments: F0 (no leaf litter), F1, F2, F3, and F4. Each container was filled with 8 kg of soil mixed with corresponding amount of walnut leaf litter, and the pakchoi seeds were sowed. The growth indices of pakchoi were determined 20, 45, and 75 days after sowing, and the photosynthetic parameters were determined when the 3^rd euphylla had emerged in almost all pakchoi seedlings exposed to treatment F4. The results were as follows: (1) Twenty-one compounds with a relative content no less than 1% were identified by GC-MS, which were classified into eight categories. Of them, 12 compounds with a relative content no less than 1% from five categories were identified from the QMA walnut leaf litter, representing 51.46% of the total chemicals, and 15 compounds with a relative content no less than 1% from seven categories were identified from the HGWS walnut leaf litter, representing 56.03% of the total chemicals. Some chemicals which were frequently reported to be allelochemicals, such as calarene, hinesol, beta-eudesmol, hexahydrofarnesylacetone, linolic acid, stearic acid, 1-octadecanol, gamma-sitosterol, and beta-sitostenone were detected in higher quantities in the HGWS walnut leaf litter compared to the relative content; while some other chemicals, such as squalene, delta-tocopherol, and loliolide were higher in the QMA walnut leaf litter. (2) The biomass of pakchoi significantly decreased in the treatments with walnut leaf litter regardless of the amount at 20 and 45 days after sowing; however, at 75 days after sowing, the inhibition effect of the litter decreased. (3) At the early decomposing stage, the contents of all photosynthetic pigments decreased progressively as the amount of leaf litter increased; simultaneously, the net photosynthetic rate (P_n), the stomatal conductance (G_s), and the transpiration rate (T_r) were significantly inhibited by the decomposing walnut leaf litter. (4) On the whole, the allelopathic effect of the HGWS walnut leaf litter was stronger than that of the QMA walnut leaf litter. It is hypothesized that walnut trees accumulate more allelochemicals (ketones, alkenes, alcohols, steroids, etc.) in the high-mountain gorge area of West Sichuan because of the more stressful environmental conditions, thereby exerting stronger allelopathic effect via decomposition of its leaf litter.