丛枝菌根真菌对羊草耐牧性影响的初步研究
摘要
植物的耐牧性是植物与草食动物长期相互作用过程中形成的适应性特征。耐牧植物可以通过生物量或者繁殖器官的补偿性生长来弥补草食动物采食作用造成的损失。环境资源状况、植物竞争、动物采食和菌根真菌均能对植物的耐牧能力产生深刻的影响。
植物菌根(Mycorrhiza)是在陆地生态系统中广泛存在的一类互惠共生体。大约80%的陆生植物能够被菌根真菌定植形成菌根共生体。菌根真菌通过其菌丝吸收土壤中的水分和矿质营养促进寄主植物的生长,同时依赖宿主植物供给必要的碳水化合物以保证其生命活动,完成其生活史。羊草作为我国东北松嫩草地的主要优势种,是一种多年生根茎型禾本科牧草,其根系内丛枝菌根真菌(Arbuscular mycorrhizal fungi, AMF)侵染率非常高。虽然大量研究表明,羊草在被刈割或放牧后具有很强的补偿性生长能力(主要体现在耐牧性方面),但是从菌根共生体的角度出发,探讨其对植物补偿性生长的作用的研究还很少。因此,本论文以羊草为研究对象,采用盆栽控制实验,研究了不同刈割强度下, AMF侵染对羊草耐牧性的贡献及其时间动态。实验得出如下主要结果和结论:
(1)刈割处理对AMF总的侵染率(P < 0.001)以及丛枝(P = 0.007)、泡囊(P = 0.011)的数量均有显著影响。轻度刈割虽然一定程度上降低了AMF的侵染率,但总体侵染率随时间仍呈上升的趋势;重度刈割则明显抑制了菌根真菌在植物根系内的定殖。轻度刈割对菌根的定植和发展影响不大;而重度刈割作用则可能导致植物对菌根真菌碳水化合物的供应不足,限制菌根生长。
(2)羊草耐牧策略随刈割强度变化具有很强的可塑性。无刈割时,无菌根和有菌根的羊草的根冠比分别为1.78和2.68,表明AMF能够明显提高了羊草根系碳水化合物的分配比例(P = 0.004),进而提高羊草耐牧性。轻度刈割时,羊草耐牧性主要表现为地上组织的补偿性生长(根冠比分别为1.78和1.65),而随着刈割强度的加重,羊草则可能选择优先扩大其繁殖潜力的方式适应环境变化的需要(根冠比分别为2.08和1.77)。
(3)无刈割(P = 0.02)和轻度刈割(P < 0.000)处理时,AMF均显著提高羊草生物量的积累;而重度刈割时,菌根处理与对照组间羊草生物量无明显差异(P = 0.80)。此外,轻度刈割条件下,AMF处理组羊草的相对生长速率最高,菌根响应指数也最大,说明轻度刈割促进了AMF对羊草生物量补偿性积累的贡献。重度刈割时,菌根依赖性指数为-0.05,对生物量积累贡献不显著,但是仍有提高繁殖潜力的趋势(P = 0.054)。
(4)氮、磷元素的分析结果显示:轻度刈割(P = 0.02)和重度刈割处理时(P = 0.03),接种菌根羊草的含氮量均显著高于无菌根羊草;磷元素的分析结果也呈现相同的趋势(P < 0.001, P=0.001),尤其是在25%刈割水平下,有菌根植物的磷浓度几乎是无菌根植物的磷浓度2倍。说明在各个刈割水平上,菌根真菌均表现出对植物氮、磷吸收的促进作用,重度刈割没有限制菌根对植物矿质营养吸收的贡献。
综上所述,本论文的实验结果表明,AMF与植物之间的互惠关系与植物自身的耐牧策略具有一定的相关性。不同的模拟动物采食强度下,菌根对植物的贡献并未改变,只是随着羊草自身耐牧策略的变化而表现不同。轻度刈割下,羊草的耐牧性以补偿性生长为主,菌根的作用功能是促进植物生物量积累;重度刈割时,羊草耐牧性主要表现为提高繁殖潜力或者其它未知途径,此时菌根具有提高植物繁殖潜力的趋势。
Herbivory tolerance is an important plant defensive trait in long term interaction of plant-herbivore. Plant can reduce the herbivory damage through the compensatory vegetative or reproductive regrowth. Many studies showed that plant herbivory tolerance was influenced by factors such as hebivory types, grazing period, grazing intensity, available nutrient level in the environment and plant competition intensity, etc. Moreover, abiotic factors in the habitat such as mycorrhiza also had an important impact on plant herbivory tolerance.
Mycorrhiza is a nearly ubiquitous mutual symbiosis in terrestrial ecosystems, established in the plant-soil–interface zone. In excess of 80% of all plants form symbiotic associations with mycorrhizal fungi in which the plant partner supplied carbon fixed through photosynthesis to the fungus in return for mineral nutrients supplied by the fungus to the host plant. Leymus chinensis (Trin.) Tzvel. is a perennial rhizomatous grass which is widely distributed in the eastern region of the Eurasian steppe zone. It dominates the meadow and dry steppe from arid to semi-arid areas in northern China, and eastern Mongolia and Siberia with good tolerance to grazing, drought, cold and alkali. Like many plants, L. chinensis is profusely associated with arbuscular mycorrhizal (AM) fungi in the natural grassland ecosystems. Over the past decades, many studies highlighted defense strategies of L. chinensis in response to herbivory or clip-simulated herbivory, and tolerance was deemed as the most import pathway since this species has very good compensation or over-compensation regrowth after herbivory. However, to date, little attempt has been taken to investigate the role of AM fungi in mediation of plant-animal interaction, especially in this region. Therefore, we conducted a greenhouse experiment with L. chinensis. Our main aims were to investigate (1) the dynamics of the colonization rate of AM fungi when host plant were subjected to intensity of defoliation influence, and (2) does the contribution of AM fungi to L. chinensis changed consistent with their colonization level along the defoliation intensity gradient. Finally, we got key results and conclusions as follows:
(1) Defoliation presented significant influences on total colonization level of AM fungi, frequency of arbuscular and vesicles (P < 0.000, P = 0.007, P = 0.011). Light defoliation somewhat decreased the AM fungi colonization level, but it still climbed up as time went by. Heavy defoliation suppressed the development of AM fungi in plant roots. Therefore, mycorrhizal colonization level was not consistent with varied defoliation intensity.
(2) Plant defensive strategies are most likely to change along defoliation gradient. Firstly, we noticed that AM fungi significant increased host plants’R/S ratio (1.78 for non-mycorrhizal plant and 2.68 for mycorrhizal plant respectively; P = 0.004). Plants with or without mycorrhiza which were subjected to heavy defoliation held higher R/S ratio (2.08 and 1.77 for plants with and without mycorrhiza respectively), and significantly re-grew slower but reproduced more tillers and held a higher RP value than those with light defoliation in non-mycorrhizal treatments (R/S ratio were 1.78 and 1.65 for plants with and without mycorrhiza respectively). It suggested that plant defense strategies might shift from tolerance to avoidance along the defoliation gradient.
(3) In conditions of non-defoliation or light defoliation, plants with mycorrhiza always significantly produced more biomass than those without mycorrhiza (P = 0.02, P < 0.000 respectively). When plants were subjected to heavy defoliation, however, there were no significant differences in biomass between plants with and without mycorrhiza (P = 0.80); the mycorrhizal responsiveness index was even below zero (-0.05). Then, in terms of biomass accumulation, heavy defoliation indeed suppressed the beneficial effect of AM fungi, even turned it to be negative. Alternatively, AM fungi tended to benefit plants in contents of fitness when host plants were subjected to heavy defoliation (P = 0.054).
(4) We also measure plant nitrogen and phosphorus concentration in L. chinensis, and found that AM fungi both increased plant nitrogen and phosphorus concentration when plants were subjected to light (P = 0.02, P < 0.001 respectively) or heavy defoliation (P = 0.03, P=0.001 respectively). Particularly, plant P concentration was almost doubled in mycorrhizal plants which were subjected to light defoliation. It suggested that AM fungi are always a beneficial partner to host plant and did not change its role with varied intensity of defoliation.
In conclusion, we propose that the mutualistic relationship between AM fungi and plant is coherence with host plant’s tolerance strategy. The benefit of AM fungi to their host did not changed a lot along the defoliation gradient, but just was represented in different ways. Under condition of light defoliation, AM fungi mainly make contribution to their host plant by means of biomass accumulation when host plants take compensatory growth as their prior choice after grazing. Under conditions of heavy defoliation, however, plant tolerance may shift from vegetative growth to reproductive growth or other underlying pathways, and then AM fungi potentially benefit their host plant in terms of reproduction or other unknown pathways.
植物菌根(Mycorrhiza)是在陆地生态系统中广泛存在的一类互惠共生体。大约80%的陆生植物能够被菌根真菌定植形成菌根共生体。菌根真菌通过其菌丝吸收土壤中的水分和矿质营养促进寄主植物的生长,同时依赖宿主植物供给必要的碳水化合物以保证其生命活动,完成其生活史。羊草作为我国东北松嫩草地的主要优势种,是一种多年生根茎型禾本科牧草,其根系内丛枝菌根真菌(Arbuscular mycorrhizal fungi, AMF)侵染率非常高。虽然大量研究表明,羊草在被刈割或放牧后具有很强的补偿性生长能力(主要体现在耐牧性方面),但是从菌根共生体的角度出发,探讨其对植物补偿性生长的作用的研究还很少。因此,本论文以羊草为研究对象,采用盆栽控制实验,研究了不同刈割强度下, AMF侵染对羊草耐牧性的贡献及其时间动态。实验得出如下主要结果和结论:
(1)刈割处理对AMF总的侵染率(P < 0.001)以及丛枝(P = 0.007)、泡囊(P = 0.011)的数量均有显著影响。轻度刈割虽然一定程度上降低了AMF的侵染率,但总体侵染率随时间仍呈上升的趋势;重度刈割则明显抑制了菌根真菌在植物根系内的定殖。轻度刈割对菌根的定植和发展影响不大;而重度刈割作用则可能导致植物对菌根真菌碳水化合物的供应不足,限制菌根生长。
(2)羊草耐牧策略随刈割强度变化具有很强的可塑性。无刈割时,无菌根和有菌根的羊草的根冠比分别为1.78和2.68,表明AMF能够明显提高了羊草根系碳水化合物的分配比例(P = 0.004),进而提高羊草耐牧性。轻度刈割时,羊草耐牧性主要表现为地上组织的补偿性生长(根冠比分别为1.78和1.65),而随着刈割强度的加重,羊草则可能选择优先扩大其繁殖潜力的方式适应环境变化的需要(根冠比分别为2.08和1.77)。
(3)无刈割(P = 0.02)和轻度刈割(P < 0.000)处理时,AMF均显著提高羊草生物量的积累;而重度刈割时,菌根处理与对照组间羊草生物量无明显差异(P = 0.80)。此外,轻度刈割条件下,AMF处理组羊草的相对生长速率最高,菌根响应指数也最大,说明轻度刈割促进了AMF对羊草生物量补偿性积累的贡献。重度刈割时,菌根依赖性指数为-0.05,对生物量积累贡献不显著,但是仍有提高繁殖潜力的趋势(P = 0.054)。
(4)氮、磷元素的分析结果显示:轻度刈割(P = 0.02)和重度刈割处理时(P = 0.03),接种菌根羊草的含氮量均显著高于无菌根羊草;磷元素的分析结果也呈现相同的趋势(P < 0.001, P=0.001),尤其是在25%刈割水平下,有菌根植物的磷浓度几乎是无菌根植物的磷浓度2倍。说明在各个刈割水平上,菌根真菌均表现出对植物氮、磷吸收的促进作用,重度刈割没有限制菌根对植物矿质营养吸收的贡献。
综上所述,本论文的实验结果表明,AMF与植物之间的互惠关系与植物自身的耐牧策略具有一定的相关性。不同的模拟动物采食强度下,菌根对植物的贡献并未改变,只是随着羊草自身耐牧策略的变化而表现不同。轻度刈割下,羊草的耐牧性以补偿性生长为主,菌根的作用功能是促进植物生物量积累;重度刈割时,羊草耐牧性主要表现为提高繁殖潜力或者其它未知途径,此时菌根具有提高植物繁殖潜力的趋势。
Herbivory tolerance is an important plant defensive trait in long term interaction of plant-herbivore. Plant can reduce the herbivory damage through the compensatory vegetative or reproductive regrowth. Many studies showed that plant herbivory tolerance was influenced by factors such as hebivory types, grazing period, grazing intensity, available nutrient level in the environment and plant competition intensity, etc. Moreover, abiotic factors in the habitat such as mycorrhiza also had an important impact on plant herbivory tolerance.
Mycorrhiza is a nearly ubiquitous mutual symbiosis in terrestrial ecosystems, established in the plant-soil–interface zone. In excess of 80% of all plants form symbiotic associations with mycorrhizal fungi in which the plant partner supplied carbon fixed through photosynthesis to the fungus in return for mineral nutrients supplied by the fungus to the host plant. Leymus chinensis (Trin.) Tzvel. is a perennial rhizomatous grass which is widely distributed in the eastern region of the Eurasian steppe zone. It dominates the meadow and dry steppe from arid to semi-arid areas in northern China, and eastern Mongolia and Siberia with good tolerance to grazing, drought, cold and alkali. Like many plants, L. chinensis is profusely associated with arbuscular mycorrhizal (AM) fungi in the natural grassland ecosystems. Over the past decades, many studies highlighted defense strategies of L. chinensis in response to herbivory or clip-simulated herbivory, and tolerance was deemed as the most import pathway since this species has very good compensation or over-compensation regrowth after herbivory. However, to date, little attempt has been taken to investigate the role of AM fungi in mediation of plant-animal interaction, especially in this region. Therefore, we conducted a greenhouse experiment with L. chinensis. Our main aims were to investigate (1) the dynamics of the colonization rate of AM fungi when host plant were subjected to intensity of defoliation influence, and (2) does the contribution of AM fungi to L. chinensis changed consistent with their colonization level along the defoliation intensity gradient. Finally, we got key results and conclusions as follows:
(1) Defoliation presented significant influences on total colonization level of AM fungi, frequency of arbuscular and vesicles (P < 0.000, P = 0.007, P = 0.011). Light defoliation somewhat decreased the AM fungi colonization level, but it still climbed up as time went by. Heavy defoliation suppressed the development of AM fungi in plant roots. Therefore, mycorrhizal colonization level was not consistent with varied defoliation intensity.
(2) Plant defensive strategies are most likely to change along defoliation gradient. Firstly, we noticed that AM fungi significant increased host plants’R/S ratio (1.78 for non-mycorrhizal plant and 2.68 for mycorrhizal plant respectively; P = 0.004). Plants with or without mycorrhiza which were subjected to heavy defoliation held higher R/S ratio (2.08 and 1.77 for plants with and without mycorrhiza respectively), and significantly re-grew slower but reproduced more tillers and held a higher RP value than those with light defoliation in non-mycorrhizal treatments (R/S ratio were 1.78 and 1.65 for plants with and without mycorrhiza respectively). It suggested that plant defense strategies might shift from tolerance to avoidance along the defoliation gradient.
(3) In conditions of non-defoliation or light defoliation, plants with mycorrhiza always significantly produced more biomass than those without mycorrhiza (P = 0.02, P < 0.000 respectively). When plants were subjected to heavy defoliation, however, there were no significant differences in biomass between plants with and without mycorrhiza (P = 0.80); the mycorrhizal responsiveness index was even below zero (-0.05). Then, in terms of biomass accumulation, heavy defoliation indeed suppressed the beneficial effect of AM fungi, even turned it to be negative. Alternatively, AM fungi tended to benefit plants in contents of fitness when host plants were subjected to heavy defoliation (P = 0.054).
(4) We also measure plant nitrogen and phosphorus concentration in L. chinensis, and found that AM fungi both increased plant nitrogen and phosphorus concentration when plants were subjected to light (P = 0.02, P < 0.001 respectively) or heavy defoliation (P = 0.03, P=0.001 respectively). Particularly, plant P concentration was almost doubled in mycorrhizal plants which were subjected to light defoliation. It suggested that AM fungi are always a beneficial partner to host plant and did not change its role with varied intensity of defoliation.
In conclusion, we propose that the mutualistic relationship between AM fungi and plant is coherence with host plant’s tolerance strategy. The benefit of AM fungi to their host did not changed a lot along the defoliation gradient, but just was represented in different ways. Under condition of light defoliation, AM fungi mainly make contribution to their host plant by means of biomass accumulation when host plants take compensatory growth as their prior choice after grazing. Under conditions of heavy defoliation, however, plant tolerance may shift from vegetative growth to reproductive growth or other underlying pathways, and then AM fungi potentially benefit their host plant in terms of reproduction or other unknown pathways.
引文
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