Linking stress with macroscopic and microscopic leaf response in trees: New diagnostic perspectives

Abstract

Visible symptoms in tree foliage can be used for stress diagnosis once validated with microscopical analyses. This paper reviews and illustrates macroscopical and microscopical markers of stress with a biotic (bacteria, fungi, insects) or abiotic (frost, drought, mineral deficiency, heavy metal pollution in the soil, acidic deposition and ozone) origin helpful for the validation of symptoms in broadleaved and conifer trees. Differentiation of changes in the leaf or needle physiology, through ageing, senescence, accelerated cell senescence, programmed cell death and oxidative stress, provides additional clues raising diagnosis efficiency, especially in combination with information about the target of the stress agent at the tree, leaf/needle, tissue, cell and ultrastructural level. Given the increasing stress in a changing environment, this review discusses how integrated diagnostic approaches lead to better causal analysis to be applied for specific monitoring of stress factors affecting forest ecosystems.

Introduction

Observing visible symptoms in the foliage of plants belongs to basic human experience about the natural environment and this still plays an important role in the diagnosis of plant disease. Aside from local previous reports, e.g. on silver smelters in ‘naturalis historia’ by Plinius secundus (23–79 after Christ), the toxic effects of anthropogeneous air pollution on trees started to attract attention in the United States of America with the observation of mottling on needles of declining Pinus ponderosa in the San Bernardino Mountains, which were attributed to air pollution and/or drought and experimentally tested with elevated ozone concentrations (0.5 ppm, Miller et al., 1963). Since the 1970s in the USA macroscopic leaf and needle injury symptoms have been systematically monitored, with a variety of different survey approaches, and compared to measurements of ozone levels, as in the case of Pinus ponderosa and P. jeffreyi in California (Arbaugh et al., 1998). In Massachusetts, the occurrence of macroscopic symptoms observed in leaves of Prunus serotina increased in stands with higher site moisture and correlated with growth decline over a period of 31 years (Vollenweider et al., 2003b). Forest monitoring programs started all over Europe in the 1980s, using equal but not specific parameters, i.e. discoloration, foliage loss and tree growth. Field surveys of forest condition based on a specific ozone signature rather than on the stress-unspecific crown defoliation were implemented only at the beginning of this century by the Task Force of ICP Forests aiming to relate macroscopic ozone injury in the leaves and needles of natural forest vegetation in varying climates to different ambient ozone doses. Other stress symptoms such as drought have not been monitored on a large area scale in natural vegetation until now.

Unspecific symptoms, such as discoloration or increased crown transparency, reflect the general health status of trees resulting from the cumulative effects of stress events over the long term and, consecutively, do not indicate the nature of the particular stress agents involved (Godbold et al., 1993, Bussotti and Ferretti, 1998). As a consequence, the effects of anthropogenic influences, e.g. air pollutants, cannot be differentiated from those resulting from other environmental constrains (Badea et al., 2004). Therefore, the diagnosis of stress effects often remains basic and limited to paradigms like “All environmental stress leads to chlorophyll degradation” (Munné-Bosch and Alegre, 2004), or “If no pathogen can be found, an abiotic cause for the disease must be considered. However, abiotic stress factors interfere with normal physiological processes, causing unspecific symptoms and making it difficult to make a diagnosis, unless the history of the environmental conditions is known” (Agrios, 2005). Compilations of symptoms in textbooks (Skelly et al., 1990, Hartmann et al., 1995, Nienhaus et al., 1996, Sinclair and Lyon, 2005) allow practitioners, nevertheless, to identify some among the most common and easily identified diseases or deficiencies. The diagnostic efficiency is, however, reduced by limited understanding regarding plant responses underlying the symptoms. The development of diagnostic tools to discriminate between different stress factors is thus still required.

All different stresses eventually manifest themselves in macroscopic leaf symptoms. We have recently reviewed the diagnostic criteria, including the symptom expression at leaf, branch and tree level, for visible symptoms with an abiotic or biotic cause (Vollenweider and Günthardt-Goerg, 2006). The integrative evaluation of microscopic injury symptoms (causing macroscopic symptoms) for their potential to diagnose the causing agent still awaits exploration, because the macroscopic and microscopic levels have most often been studied independently from each other. This paper therefore intends to exploit the potential of connecting knowledge of macro to that of microscopic leaf, tissue and cell injury for a better understanding of the cause/effect relationship of symptoms in trees from the temperate (central European) zone. Indeed these long-living organisms present specificities important for stress diagnosis (as compared to herbaceous plants): (1) a vegetative development independent of the flowering activity letting symptoms better reflect the cumulative effects of an increasing stress dosage (Novak et al., 2003, Bassin et al., 2004), (2) sizable development of support tissues conveying important detoxification functions to cell walls for, e.g. oxidative stress or storage of heavy metal contaminants (Polle, 1997, Vollenweider et al., 2006) and (3) defense relying on a partly different foliage chemistry, noteworthy, with a more frequent involvement of condensed tannins (Waterman and Mole, 1994, p. 43). In the following sections the reader shall be introduced to background physiological changes occurring in foliage and how the leaves and needles express them structurally. Referring to these basic considerations, new perspectives using consistent structural markers of several natural and anthropogeneous stress factors will show the potential of microscopic symptoms for a differential diagnosis of the causing agents in trees. To facilitate and initiate new insights, results from relevant recent references (since 1990) are condensed in Table 1, Table 2. They have been selected for their illustrations, subject relevance and connection with the visible symptoms; they are completed with figures derived from original microscopic work by the authors. This paper aims to promote new integrated approaches for better diagnosis of abiotic and biotic stress factors causing structural changes in leaves and needles by improving the causal analysis of stress in trees.