Primary and secondary host plants differ in leaf‐level photosynthetic response to herbivory: evidence from Alnus and Betula grazed by the alder beetle, Agelastica alni

Abstract

Field-grown trees of Alnus incana (L.) Moench, Alnus glutinosa (L.) Geartner and Betula pendula Roth displayed pronounced differences in responses of light-saturated net photosynthesis (A sat ) to herbivory by the alder beetle (Agelastica alni L., Galerucinae), a specialized insect which primarily defoliates alders. We found that photosynthetic rates of grazed leaves increased following herbivory in Alnus but not in Betula. Area-and massbased A sat of grazed leaves declined linearly with increasing amount of leaf perforation in B. pendula, by as much as 57 %. By contrast Alnus glutinosa and Alnus incana increased area-based rates of A sat by 10-50 % at all levels of leaf grazing. Given increased A sat in the remaining portion of grazed leaves, a net reduction in photosynthesis per leaf occurred only when the proportion of leaf area grazed exceeded 40 % for Alnus incana and 23 % for Alnus glutinosa. Since vein perforation by Agelastica alni was observed much more frequently in leaves of Betula than in Alnus, we hypothesized that declining A sat in herbivorized Betula was related to this disruption of water transport. A field experiment with artificial leaf perforation demonstrated a greater decline in A sat in veinperforated Betula leaves than perforated leaves with midrib veins intact. However, regardless of leaf perforation regime, birch never showed post-perforation increases in A sat . In all species, rates of transpiration of grazed leaves linearly increased and water-use efficiency decreased with increased amount of leaf perforation. In grazed Alnus incana leaves, increasing leaf area consumption by Agelastica alni resulted in an increase of total phenols, a reduction in starch content and no changes in nitrogen concentration in the remaining portion. The increase in photosynthesis in Alnus incana might be related to declining leaf starch concentration or increasing stomatal conductance, but was unrelated to leaf nitrogen concentration. These gas exchange and leaf chemistry measurements suggest that in contrast to B. pendula, Alnus incana and Alnus glutinosa, which are the major host species for Agelastica alni, possess leaf-level physiological adaptations and defence mechanisms which can attenuate negative effects of herbivory by the alder leaf-beetle.

Leaf palmate venation and vascular redundancy confer tolerance of hydraulic disruption

“…The decline in function caused by severing the midrib for pinnately veined species was consistent with previous studies of at least 13 species (22,(28)(29)(30)(31)(32)(33)(34). We found that damage tolerance was greater for leaves with greater 1°vein redundancy.…”

“…Nevertheless, major vein density might have hydraulic consequences in damaged leaves. Although leaves can often survive midrib vein damage while remaining green and turgid (27), they may suffer reduced K leaf , stomatal conductance, and photosynthetic rate, immediately, and weeks later, after the damage has healed (22,(28)(29)(30)(31)(32)(33)(34). The reticulation of venation has been hypothesized to reduce the impact of disruption by providing transport around damaged veins (35,36).…”

Cascading effects of induced terrestrial plant defences on aquatic and terrestrial ecosystem function

“…The physiological responses resulting in the increased C : N among the herbivory treated trees are quite possibly different among the fertilized versus unfertilized trees (plants growing on N-poor soils often use C-based defences and vice versa) [43]. A study simulating herbivory only using mechanical damage in Alnus incana and A. glutinosa found no change in % N content [44]. This may indicate this response is unique to A. rubra or that by using methyl jasmonate, our study invoked a more complex defence response.…”

Impact of folivory on photosynthesis is greater than the sum of its holes

“…2), specific location (e.g., proximity to a vein; ref. 3), induced production of allelochemicals that can be costly to produce (4), and the potential for compensatory increases of photosynthetic rates in intact leaf tissue (5).…”