Introduction

Both overall concentrations and composition of plant secondary metabolites (PSMs) may vary strongly not only among different plant species or different plant individuals within a species, but also within a single individual plant over time. We distinguish two types of temporal changes in plants: ontogenetic and seasonal. Ontogeny refers to genetically programmed developmental changes that take place during a plant’s life; the terms ‘maturation’ and ‘phase change’ are also sometimes used to describe the same process (Kozlowski, 1971; Poethig, 1990). Commonly recognised ontogenetic stages in plants include seedlings, juvenile plants (saplings) and mature plants. Each stage is characterised by distinctive anatomical, morphological and biochemical features (e.g. dependence on seed reserves in seedlings, and ability to flower and set seeds in mature plants). Since plants are modular organisms composed of numerous semi-autonomous units (e.g. branches and leaves), in addition to ontogenetic changes at the whole-plant level, each plant module/organ also experiences ontogenetic changes as it develops and matures. For instance, leaves formed during a specific phase of shoot growth retain morphological and physiological features characteristic of that phase, resulting in variation in the character of structures along the axis of the shoot; this phenomenon is known as heteroblasty (Poethig, 1990; O’Reilly-Wapstra et al., Chapter 2). While these within-module ontogenetic changes are genetically programmed just as whole-plant ontogenetic changes are, they are not necessarily synchronised with the latter because new modules are produced continuously through the plant’s lifetime. As a result, long-lived plants like trees may represent a complex mosaic or a gradient of plant tissues at different ontogenetic phases of development (sometimes referred to as a ‘developmental stream’) (Kearsley & Whitham, 1998).

In addition to ontogenetic changes, in seasonal environments, plant modules experience a series of physiological changes over the season caused by changes in temperature and the availability of light (photoperiod), water and nutrients. We refer to these changes as seasonal changes. In annual and short-lived herbaceous plants, seasonal and ontogenetic changes are confounded, but in long-lived woody plants like trees they can be distinguished because both juvenile and mature ontogenetic stages last many seasons, and within-plant ontogenetic changes such as leaf expansion usually occur within a relatively short time interval during the season (e.g. a few weeks in spring in temperate ecosystems). Both ontogenetic and seasonal changes may affect PSM production in plants.