Question

Yellowish colour of autumn foliage is due to the presence of a type of xanthophyll pigment called as (a) lutein (b) lycopene (c) fucoxanthin (d) zeaxanthin.

Short answer

The yellowish colour of autumn foliage is due to the presence of zeaxanthin, which is option (d).

Step-by-step solution

Understanding the Question

The exercise asks to identify the specific type of xanthophyll pigment that is responsible for giving autumn foliage its yellowish color. Xanthophylls are a class of carotenoids that act as accessory pigments in plants, absorbing light in the range of green to yellow spectrum.

Identifying the Correct Xanthophyll Pigment

Among the options given, we need to find which one is known for giving leaves their yellowish hue in the fall. This requires knowledge of plant pigments and their roles in leaf coloration.

Selecting the Correct Answer

The xanthophyll pigment that gives autumn foliage its yellowish color is known as zeaxanthin, which is option (d).

Key concepts

Xanthophyll Pigment

When you wander through a forest in the fall, you're treated to a tapestry of colors, with leaves displaying hues from bright yellow to deep red. This phenomenon occurs due to changes in plant pigments. Specifically, the yellowish color in autumn foliage is attributed to xanthophyll pigments. These pigments belong to a larger family of compounds known as carotenoids, which also include famously orange carotenes—think carrots.

Xanthophylls are responsible for capturing light during photosynthesis. They absorb blue light, which is why leaves typically appear green throughout the growing season combined with the abundant chlorophyll. However, as the days shorten and temperatures drop, chlorophyll breaks down, unmasking the once hidden xanthophylls. This change reveals the stunning fall colors we associate with autumn foliage.

Importantly, xanthophyll pigments also protect plant tissues from the harmful effects of excess light, acting as a type of sunscreen by dispersing excess energy that could damage chlorophyll or interact with oxygen to create reactive molecules.

Plant Pigments

Plant pigments are essential for life on Earth. They are the molecules responsible for absorbing sunlight and converting it into chemical energy through the process of photosynthesis. The major classes of plant pigments include chlorophylls, carotenoids (which xanthophylls are a part), and anthocyanins. Each serves an important role and absorbs light in different parts of the spectrum.

Chlorophylls are the most widespread plant pigments and absorb light primarily in the blue and red wavelengths but reflect green, hence the green color of many plants. Carotenoids, such as xanthophylls and carotenes, absorb in the blue wavelengths and reflect yellow, orange, and red light. Finally, anthocyanins, which become prominent in some plants during autumn, absorb green light and give plants red, purple, and blue hues.

Understanding how these pigments interact and change during different seasons helps us appreciate the dynamic beauty of plant life and the intricate processes that sustain it. As temperatures cool and daylight shortens, the dominant chlorophyll pigments degrade, allowing the carotenoids and sometimes anthocyanins to become more apparent, leading to the vibrant display of autumn leaves.

Zeaxanthin

Among the xanthophyll pigments, one particular type has been highlighted for its role in the fall colors — zeaxanthin. As autumn approaches, and the chlorophyll in leaves breaks down, zeaxanthin becomes more visible, lending a bright yellow color to leaves before they fall off the trees. This pigment is not just for show, however; it has a vital function within the leaf cells. Zeaxanthin helps to protect the plant by playing a role in the process known as non-photochemical quenching, where it helps to dissipate excess light energy as heat.

Zeaxanthin, along with other carotenoids like lutein, is also found within the human retina where it is thought to play a protective role against the harmful effects of light. Including foods high in these pigments in your diet, such as kale, spinach, and egg yolks, can contribute to eye health. In the context of the autumn foliage question from the textbook, zeaxanthin is indeed the pigment responsible for the yellow color in leaves as option (d) suggested.