p-香豆酸对西洋参的化感作用及生理机制

Allelopathic effect of p-coumaric acid on American ginseng and its physiological mechanism

西洋参(Panax quinquefolium L.)栽培中存在严重的连作障碍现象,前期发现p-香豆酸在以滤纸片为基质的条件下,能够显著抑制西洋参胚根的生长。为了明确p-香豆酸在土壤基质中对种胚的化感活性以及对成株西洋参生长的作用及生理机制,以自然土壤为基质,观察p-香豆酸作用后种胚的生长情况;采用室内水培试验,观察p-香豆酸作用下2年生西洋参种根从出苗至结果期的生长及部分生理指标的变化。种胚生长实验在土壤中分别添加0.0024、0.012、0.06、0.3、1.5、7.5 mg/g的p-香豆酸,处理7 d后测定西洋参种胚的胚根长和胚芽长。水培试验中全营养液中分别添加0.012 mg/m L、0.06 mg/m L、0.3 mg/m L p-香豆酸,处理后每隔5 d测定植株叶片展开情况、株高、冠幅等生长指标;于展叶期(10 d)、现蕾期(20 d)、结果期(30 d)测定地上部分及新生须根的生物量,同时测定新生须根苯丙氨酸解氨酶(PAL)活力;叶片完全展开后测定植株净光合速率(Pn)、表观电子传递速率(ETR)和最大光化学效率(Fv/Fm)等光合特性参数。结果表明,土壤中添加0.0024—7.5 mg/g p-香豆酸西洋参胚根长度降低28.52%—100%,胚芽长度降低1.09%—100%,并呈现一定的剂量抑制效应。实验浓度内的p-香豆酸可显著抑制西洋参植株地上部分生长,推迟展叶期;结果期地上部生物量比对照降低17.17%—54.55%(P<0.05,Dunnett-t test);同时,叶片的Pn、ETR受到抑制(P<0.05),但Fv/Fm不变;对须根的影响主要表现为0.06 mg/m L p-香豆酸处理组在展叶期PAL酶活力提高69.05%,之后PAL活力和生物量均比对照下降,浓度增加至0.3 mg/m L时整个培养期内PAL酶活力和生物量均低于对照。由此推论,根系环境中的p-香豆酸在自然土壤中对西洋参种胚具有显著抑制其生长的化感作用;对成株西洋参的作用主要为抑制地上部分生长,并通过降低成株西洋参叶片光合能力,从而表现出明显的化感作用,0.06 mg/m L p-香豆酸诱导须根PAL酶活力先升高再降低并最终降低生物量的结果也表明p-香豆酸是西洋参根系生长的胁迫因素。结果证实p-香豆酸对西洋参种胚和成株的生长均具有自毒作用,其抑制生长的生理机制在于抑制叶片的光合作用。

The cultivation of American ginseng（ Panax quinquefolium L.; Araliaceae） is frequently hindered by problems during replanting. Our previous studies showed that p-coumaric acid significantly inhibited the radicle growth of American ginseng embryos grown on filter paper. The objectives of the study were to determine the allelopathic effects of p-coumaric acid on embryos of American ginseng grown in soil,to evaluate the effects of p-coumaric acid on adult plant growth,and to identify the physiological mechanism involved. The role p-coumaric acid played in the growth of American ginseng embryos was investigated using natural soil to mimic real conditions in the field. We also studied how p-coumaric acid affected the growth of 2-year-old American ginseng roots grown in nutrient solution. During the seedling emergence and fruit set stages,we monitored changes in plant growth and physiological index. In the experiment with embryos,American ginseng seeds were planted in soil to which either 0. 0024,0. 012,0. 06,0. 3,1. 5,or 7. 5 mg p-coumaric acid per gram of soil had been added; after 7 days of treatment,the radicle and shoot lengths of the embryo were measured. In the hydroponic experiment,the nutrient solution was amended with 0. 012,0. 06,or 0. 3 p-coumaric acid per m L of solution. Plant growth data,including expansion of leaves,plant height,and canopy growth,were collected at 5-day intervals starting from the day p-coumaric acid was added to the solution. The aboveground biomass,the new grown fibrous root biomass and the activity of phenylalanine ammonia lyase（ PAL） in the fibrous roots were measured at the leaf expansion（ 10 d）,blooming（ 20 d）,and fruit set（ 30 d） stages. Moreover,photosynthetic parameters,including net photosynthetic rate（ Pn）,apparent photosynthetic electron transport rate（ ETR）,and maximum photochemical efficiency of photosystem II（ Fv / Fm）,were determined after leaf expansion. The results showed that in soil containing 0. 0024—7. 5 mg / g p-coumaric acid,the radicle and shoot lengths of American ginseng embryos decreased by 28. 52% —100% and 1. 09% —100%,respectively,and this inhibitory effect was dose dependent. p-Coumaric acid significantly inhibited the above-ground growth of American ginseng plant at all tested concentrations（ P0. 05,Dunnett t-test） and also delayed leaf expansion. Furthermore,the aboveground biomass was significantly reduced by 17. 17% —54. 55% at the fruiting stage（ P0. 05）. Moreover,Pn and ETR of leaves were also significantly reduced（ P 〈0. 05） by p-coumaric acid; however,the values of Fv / Fm were not affected. For fibrous roots,their PAL activity in the 0. 06 mg / m L p-coumaric acid treatment was 69. 05% higher than in the control at the fruiting stage,and then both their biomass and PAL activity were lower than in the control after this growth stage. When p-coumaric acid was at concentration of 0. 3 mg / m L,the PAL activity and biomass of American ginseng fibrous roots were lower than in the control in all the three growth stage. These observations indicated that p-coumaric acid in the natural-soil rhizosphere of American ginseng embryos exerted profound allelopathic inhibitory effects,and it also inhibited the growth of adult American ginseng plants above ground while decreased the photosynthetic capacity of leaves. The fact that PAL activity in fibrous roots was higher than the control at seedling emergence stage and lower than the control at blooming stage in 0. 06 mg / m L p-coumaric acid treatment,associated with fibrous root biomass was decreased at blooming stage,also demonstrated that p-coumaric acid may be a potential stress factor during the growth of American ginseng root systems. In conclusion,our data showed that p-coumaric acid exerts autotoxic effects on both embryos and seedlings of American ginseng and that these allelopathic effects might be partially achieved via the inhibition of photosynthesis in leaves of adult plants.