Photosynthetic responses to biotic stress

Introduction to Plant Biotic Stress: Agricultural and native plants are subject to a myriad of biotic stresses inflicted by other living organisms, from viruses to mammals, and many of these damaging agents affect photosynthesis, either by altering its underlying metabolism (primary photochemistry, electron transport, Calvin cycle) or gas diffusion, or by reducing photosynthetic leaf area. Pathogens (fungal, bacteria or viral agents) and animal pests causes on average, a 15% and 18% reduction in crop yield, respectively (Oerke and Dehne, 2004). Biotic stresses on plants have had enormous repercussions for humanity. For example, the potato blight (Phytophthora infestans) caused widespread famine in England, Ireland and Belgium, or the introduction of grape phylloxera (Daktulosphaira vitifoliae or Phylloxera vastatrix) from America in the mid 19th century nearly put an end to the French wine industry. In addition, it is predicted that plant-pathogen interactions will favour pathogens under the high CO₂ conditions expected for the next decades (Lake and Wade, 2009). This chapter reviews the effects of parasitic plants, pathogens (virus, bacteria and fungi) and arthropods on photosynthesis. While competition among plants clearly is a ‘biotic interaction’, we have not included it in this discussion as competition often manifests itself through reduced availability of growth-limiting resources and does not affect photosynthesis per se. The underlying mechanisms by which biotic agents affect photosynthesis vary widely and it may be useful to classify these agents into different ‘damage guilds’ (Table 22.1), although it has been claimed that biotic stresses generally downregulate photosynthesis genes (Bilgin et al., 2010). Chewing insects, for example, reduce carbon gain primarily by reducing leaf area, where virus infections rarely reduce leaf area but instead depress the rate of photosynthesis per unit leaf area. Among fungi, leaf-disease fungi (rust, mildew, etc.) behave like herbivores in their ability to reduce photosynthetic leaf area, where vascular-wilt fungi compromise plant water transport and reduce photosynthesis by inducing stomatal closure. In addition to exploring how different biotic agents affect photosynthesis, this chapter concludes with a brief discussion of different methods for detecting the effects of biotic stress on photosynthesis, sometimes in advance of the development of visual symptoms.