TEACHING THE BIOLOGY OF FOOD CROPS
Despite the importance of plants for our everyday life, sometimes it’s difficult to grab the attention of biology students – often overwhelmed by the complexities of plant systematics and the composition of uncountable phytochemicals.
As previously reported in the post “ Teaching Botany: the Taylor Swift method applied to plants “, students generally find plant science less attractive than other biological sciences such as animal biology and biomedicine. Moreover, professionals dedicated to plant biology education also face challenges related to plant awareness disparity – a term that describes our inability to notice plants in the environment and recognise their relevance for ecosystems and societies.
To make botany classes more engaging, an interesting approach could be to look at plants with another glance. In this post, we explore eye-catching strategies to study pigments hidden in the cells of common food plants.
DISCOVERING THE MAGIC OF FOOD CROPS (behind their nutritional value)
According to estimates of the United Nations, botanical species account for more than 80% of the human diet. Indeed, food crops are essential to our nutrition as they represent the fundamental sources of primary metabolites – carbohydrates from cereals, proteins from pulses, and fats from a bunch of oil crops – as well as a broad range of micronutrients that support the correct functioning of human bodies.
Although global crop production has increased in the last decades, almost 60% of the world population still suffer malnutrition related to microelements such as vitamins and other secondary metabolites – that humans are unable to produce but are vital for preventing chronic pathologies, boosting the immune system and reducing the risk of cardiovascular disease. For example, diverse horticultural species enhance our diets with powerful antioxidants – biologically active molecules that fight against oxidative stress by neutralizing and scavenging toxic Reactive Oxygen Species (ROS).
DELVING DEEP INTO FOOD CROPS PIGMENTS: a rainbow of colours
Among the seven colours of the spectrum—red, orange, yellow, green, blue, indigo, and violet—humans primarily perceive the green of plants. This is because chlorophylls, the photosynthetic pigments responsible for green hues, dominate as the most abundant in the plant kingdom. Chlorophylls accumulate in chloroplasts, the intracellular organelles found in actively photosynthetic organs like leaves. As leaves become old, the degradation of chlorophylls unmasks the presence of carotenoids, accessory pigments that create a vivid yellow to red colouring.
In addition to senescent leaves, also mature fruits of several botanical species change colour from green to yellow-orange-red due to the accumulation of carotenoids (i.e., xanthophylls and carotenes, the precursors of Vitamin A) in specific organelles called chromoplasts. Instead, cells of other ripening fruits that acquire red to blue colours accumulate other pigments called flavonoids in the vacuole – an organelle that not only serves as a repository for reserve and waste but is also essential for maintaining osmotic pressure between the cell and the external environment.
To unlock the stunning hidden world of important plant pigments (chlorophylls, carotenoids and flavonoids) in practical experience (Figure 1), you just have to use a simple optical microscope to observe freshly prepared slices of common food crops like basil (Ocimum basilicum, Lamiaceae family), pepper (Capsicum annuum, Solanaceae family), onion (Allium cepa, Amaryllidaceae family) and chicory (Cichorium intybus, Asteraceae family).
Figure 1. Unveiling the Colours of Food Crops, Hands-On. Plant material: young leaves of basil (or other aromatic plants), unripe and ripe pepper fruits, bulb of red onion, and head of red chicory (radicchio rosso). Lab Material: Petri dishes, blades and tweezers, glass slides and coverslips, bottle with water. Lab Equipment: optical microscope.
SEEING CHLOROPHYLLS: leaves of aromatic plants
Plants produce their own food through photosynthesis (a complex process that occurs through interconnected reactions) by using sunlight. It all begins within the membranes of the chloroplast, where clusters of chlorophylls and accessory pigments work together to capture solar energy.
A closer look at leaves, the major photosynthetic organs, also reveal the presence of specific cell types constituting the protective layers (upper and lower epidermis) and pores (stomata) that modulate the interaction between the plant and the environment. For more information, see the Botanical pill “The Leaf: Birth, Life & Death of a Photosynthetic Organ”.
TIP 1: Since chlorophylls are soluble in alcohol, they can be extracted from green tissues using ethanol. Experiment: place one leaf of mint (or parsley) in a first test tube with water and another one in a second test tube with 95% ethanol. Check the difference after a while!
SEEING CAROTENOIDS: fruit and root crops
In pepper, green fruits are the immature version of ripe fruits: the chloroplasts have not yet differentiated into chromoplasts and still contain chlorophylls. When mature, fruits accumulate red xanthophylls (e.g., capsanthin) or yellow xanthophylls (e.g., violaxanthin), depending on variations of the carotenoid biosynthetic pathway. In these organs, pigments accumulate in the mesocarp – the tissue produced from the transformation of the ovary wall following double fertilization.
TIP 2: Since carotenoids are fat-soluble pigments, they can be extracted from orange tissues using lipids. Experiment: place some slices of carrot in a first test tube with water and other ones in a second test tube with vegetable oil. Check the difference after a while!
SEEING FLAVONOIDS: horticultural species
To date, plant scientists have identified almost 6000 flavonoids, divided into different groups. Some of them (e.g., flavones) are colourless while others (e.g., anthocyanins) give red to blue pigmentation to different organs. These molecules play key functions in photoprotection mechanisms in plants, and are appreciated for their antioxidant and anti-inflammatory properties in human nutrition.
Among others, Anthocyanins usually accumulate in red-to-blue flowers and fruits (e.g., berries), where they facilitate the attraction of animals that mediate pollination and seed dispersal. But these pigments can also be found in other organs of anthocyanin-rich varieties of edible species, such as red onion bulbs or Trevisan Radicchio leaves.
TIP 3: Anthocyanins are water-soluble pigments thatchange colour depending on the pH. Experiment: place water extracts of red-to-blue berries in two test tubes and change the pH by adding a few drops of HCl or a few drops of KOH. Check the difference after a while!
CREDITS
Pictures were taken at the University of Urbino during practices with 2nd year students of the course “Crop Biology”, University degree in Sciences of human nutrition.
The post Teaching Botany: a deep dive into plant pigments in crops appeared first on Botany One.
