It is well known that chlorophylls, associated with the green colour of leaves, are key components of photosynthesis and therefore essential for plant physiology. However, they are not the only key pigments for plant survival. Less popular than chlorophylls, but still essential, carotenoids are a family of pigments that exhibit a range of colours and functions and can be found in different organs including roots (e.g., carrots), fruits (e.g., tomatoes) and leaves.
In plants, carotenoids are involved in several fundamental processes. In photosynthesis, carotenoids can absorb light wavelengths that chlorophylls are not able to absorb (around 500 nm). Thus, carotenoids increase the spectrum absorption during photosynthesis working as accessory pigments. Moreover, due to their physicochemical properties, carotenoids can disperse the excess of energy produced during photosynthesis, exhibiting a photoprotective role during this essential process.
The biosynthetic pathways of chlorophylls and carotenoids share some molecules. This means that their production is interconnected, and their abundance can influence each other under certain circumstances.
Moreover, carotenoids are precursors of plant hormones including Abscisic Acid (ABA) and strigolactones. ABA is a well-established key player in abiotic stress resistance, seed dormancy and organ size. On the other hand, strigolactones have been less studied, but they have been described as involved in the control of plant development (e.g branching) and facilitating the growth of arbuscular mycorrhizal fungi in the soil. Carotenoids are also relevant in some flowers, playing a role in attracting seed-dispersing organisms.
Overall, carotenoids have been associated with a wide range of biological functions, crucial for plant physiology and normal plant development.
Since chlorophylls and carotenoids participate in essential physiological functions, these pigments can be used as indicators for plant health. Reduced levels of chlorophylls and/or carotenoids are normally a consequence of a nutritional deficiency and/or stress exposure. Deficiencies of macronutrients (e.g. nitrogen), micronutrients (e.g. Fe, Mg or Zn), waterlogged roots or pathogen infection, are examples of causes for a reduction in chlorophyll level. This results in leaves exhibiting a yellowish colour. Moreover, plants with reduced levels of carotenoids will be sensitive to abiotic stimuli such as drought and light exposure. Severe reductions in these pigments could be ultimately lethal for the plant.
A reduction in both chlorophylls and/or carotenoids will have a negative impact on the plant development, and a concomitant reduction in yield. Therefore, maintaining these levels within the normal range has to be a priority in order to ensure and maximize production. Monitoring levels of these pigments represent a powerful agricultural tool to identify problems in plants and amend any deficiencies and/or stress in time to reduce the negative effects.