Insect herbivory in relation to dynamic changes in host plant quality*

Evidence that chemical changes in plants following insect feeding can lead to reduced grazing levels, enhanced insect movement and selective leaf avoidance is briefly reviewed. A simple model is constructed in which changes in damaged and/or adjacent leaves lead to effects on herbivore performance. The model reveals that as the density of herbivore larvae/plant increases from one to twenty-four, wound-induced changes in the leaves reduce larval survival by up to 40%, treble the number of movements of the larvae and increase their development time by c. 10%. The distribution of grazing between leaves changes in the direction of more leaves with lower grazing levels but overall grazing levels are not greatly affected by the above changes in larval performance. The model's output is discussed in relation to recent views concerning the relative roles of intra-specific competition and predation in regulating insect herbivore numbers.     

Leaf trichome responses to herbivory in willows: induction, relaxation and costs

Direct measurement of the carbon (C) 'cost' of mycorrhizas is problematic. Although estimates have been made for arbuscular and ectomycorrhizal symbioses, these are based on incomplete budgets or indirect measurements. Furthermore, the conventional model of unidirectional plant-to-fungus C flux is too simplistic. Net fungus-to-plant C transfer supports seedling establishment in c. 10% of plant species, including most orchids, and bidirectional C flows occur in ectomycorrhiza utilizing soil amino acids. Here, the C cost of mycorrhizas to the green orchid Goodyera repens was determined by measurement of simultaneous bidirectional fluxes of 14C labelled sources using a monoxenic system with the fungus Ceratobasidium cornigerum. Transfer of C from fungus to plant ('up-flow') occurs in the photosynthesizing orchid G. repens (max. 0.06 microg) whereas over five times more current assimilate (min. 0.355 microg) is simultaneously allocated in the reverse direction to the mycorrhizal fungus ('down-flow') after 8 d. Carbon is transferred rapidly, being detected in plant-fungal respiration within 31 h of labelling. This study provides the most complete C budget for an orchid-mycorrhizal symbiosis, and clearly shows net plant-to-fungus C flux. The rapidity of bidirectional C flux is indicative of dynamic transfer at an interfacial apoplast as opposed to reliance on digestion of fungal pelotons.     

Biotic and abiotic consequences of differences in leaf structure

Both within and between species, leaves of plants display wide ranges in structural features. These features include: gross investments of carbon and nitrogen substrates (e.g. leaf mass per unit area); stomatal density, distribution between adaxial and abaxial surfaces, and aperture; internal and external optical scattering structures; defensive structures, such as trichomes and spines; and defensive compounds, including UV screens, antifeedants, toxins, and silica abrasives. I offer a synthesis of selected publications, including some of my own. A unifying theme is the adaptive value of expressing certain structural features, posed as metabolic costs and benefits, for (1) competitive acquisition and use of abiotic resources (such as water, light and nitrogen) and (2) regulation of biotic interactions, particularly fungal attack and herbivory. Both acclimatory responses in one plant and adaptations over evolutionary time scales are covered where possible. The ubiquity of trade-offs in function is a recurrent theme; this helps to explain diversity in solutions to the same environmental challenges but poses problems for investigators to uncover numerous important trade-offs. I offer some suggestions for research, such as on the need for models that integrate biotic and abiotic effects (these must be highly focused), and some speculations, such as on the intensity of selection pressures for these structures.     

Defensive role of leaf trichomes in resistance to herbivorous insects in Datura stramonium

This study assessed the role of leaf trichome density as a component of resistance to herbivores, in six populations of Datura stramonium. Phenotypic selection on plant resistance was estimated for each population. A common garden experiment was carried out to determine if population differences in leaf trichome density are genetically based. Among population differences in leaf trichome density, relative resistance and fitness were found. Leaf trichome density was strongly positively correlated to resistance across populations. In 5 out of 6 populations, trichome density was related to resistance, and positive directional selection on resistance to herbivores was detected in three populations. Differences among populations in mean leaf trichome density in the common garden suggest genetic differentiation for this character in Datura stramonium. The results are considered in the light of the adaptive role of leaf trichomes as a component of defence to herbivores, and variable selection among populations.     

中国16个森林站点叶片植食格局及其调控因子

植食性昆虫对植物叶片的植食作用,在生物相互作用的研究中占有重要地位。然而,关于我国典型森林植食性昆虫对叶片的植食作用格局及其调控因子仍然不清。以海南、广东、江西等省区16个森林站点的98种乔木为研究对象,采用野外调查与室内统计分析相结合的实验方法,研究植食性昆虫对叶片的采食情况,量化了叶片植食率的区域格局及其环境决定因素。结果表明,36科98种植物的29834片叶片的平均植食率为3.82%。叶片植食作用随纬度的升高而下降,其中海南尖峰岭最高,黑龙江呼中最低,分别为7.77%和1.09%。年均温、年降水量、气温年较差、最冷月份最低温度、最暖月份最高温度、温度季节性变化、降水季节性变化、最冷季度降水量在很大程度上决定了植食性昆虫对叶片的植食作用(P<0.05),而叶片成分以及比叶重与植食率之间无显著联系(P>0.05)。研究表明,我国森林叶片植食率的纬向格局很大程度上是由气候因子决定的,这为揭示我国植食率格局及其驱动因子提供了定量依据。