The Impact of Phloem Feeding Insects on Leaf Ecophysiology Varies With Leaf Age.

Sylvain Pincebourde,Jérôme Ngao

doi:10.3389/fpls.2021.625689

Abstract

Herbivore insects have strong impacts on leaf gas exchange when feeding on the plant. Leaf age also drives leaf gas exchanges but the interaction of leaf age and phloem herbivory has been largely underexplored. We investigated the amplitude and direction of herbivore impact on leaf gas exchange across a wide range of leaf age in the apple tree–apple green aphid (Aphis pomi) system. We measured the gas exchange (assimilation and transpiration rates, stomatal conductance and internal CO2 concentration) of leaves infested versus non-infested by the aphid across leaf age. For very young leaves up to 15 days-old, the gas exchange rates of infested leaves were similar to those of non-infested leaves. After few days, photosynthesis, stomatal conductance and transpiration rate increased in infested leaves up to about the age of 30 days, and gradually decreased after that age. By contrast, gas exchanges in non-infested leaves gradually decreased across leaf age such that they were always lower than in infested leaves. Aphids were observed on relatively young leaves up to 25 days and despite the positive effect on leaf photosynthesis and leaf performance, their presence negatively affected the growth rate of apple seedlings. Indeed, aphids decreased leaf dry mass, leaf surface, and leaf carbon content except in old leaves. By contrast, aphids induced an increase in leaf nitrogen content and the deviation relative to non-infested leaves increased with leaf age. Overall, the impacts of aphids at multiple levels of plant performance depend on leaf age. While aphids cause an increase in some leaf traits (gas exchanges and nitrogen content), they also depress others (plant growth rate and carbon content). The balance between those effects, as modulated by leaf age, may be the key for herbivory mitigation in plants.

Highlights

Plant gas exchanges are at the forefront of ecosystem functioning, as they are measurements of heat and mass exchange between the plant and the atmosphere
The leaf assimilation rate (Amax), transpiration rate (Tr), stomatal conductance, and internal CO2 concentration (Ci) differed between infested and non-infested plants when controlling for leaf age variability (Table 1; analysis of covariance (ANCOVA): P < 0.015 for all)
The growth rate of apple seedling impacted the measure of Amax and gsmax (Table 1; ANCOVA: P < 0.014 for both), but not that of Tr and Ci (Table 1; ANCOVA: P > 0.05 for both)

Summary

Introduction

Plant gas exchanges are at the forefront of ecosystem functioning, as they are measurements of heat and mass exchange between the plant and the atmosphere. The impacts on plant gas exchange of injuries caused by herbivore insects can be roughly of similar amplitude than the influence of climatic variables like temperature, irradiance, and humidity (Jarvis, 1976; Welter, 1989; Peterson, 2000). This comparison denotes the potential importance of herbivore injuries on the functioning of vegetation-atmosphere. In phloem feeders like aphids, photosynthesis of the host plant can be dramatically lowered (Macedo et al, 2003), while sap feeders such as scale insects can induce an increase in leaf assimilation rate (Retuerto et al, 2004). The cascading consequences of this suppression for photosynthesis remain unclear

Objectives

Methods

Results

Conclusion