1 BACKGROUND
The colour of fruit is generally associated with reproduction, specifically a means to promote seed dispersal by making the fruit visually attractive to herbivores/omnivores. Colour, therefore, is a key component of fruit development and intrinsically related to the ripening process. Fruit colours are derived from ‘secondary (or specialised) metabolite’ pathways. In evolutionary terms, these pathways in flowering plants and the breadth of their products have grown in number and complexity, in response to environmental and pathogenic stimuli and to adaptive changes for pollinator attraction. These colour pigments are largely derived from either the carotenoid and/or flavonoid pathways.
Colour helps to define the attractiveness and quality of fruits and strongly influences consumer demand. Hence, for many years, improvements or changes to fruit colour have been a major target of breeding programmes across a range of crops (Figure 1). Additionally, growers routinely implement a number of on-orchard management techniques to enhance fruit colour development (Wang et al., 2020). In this review, we will consider the impact of environmental stresses on fruit colour. While the term ‘stress’ usually conjures up negative connotations, moderate stress can enhance the concentration of numerous phytochemicals, including those that colour our fruits. The timing, duration and severity of any stress will determine the plant’s response and consequent impact on colour.