Plastids are a distinctive feature of plant cells. They synthesise and store food in algal and plant cells. All plastids begin as proplastids and mature into a variety of specialised mature plastids depending on the function of the cell. Mature plastids can even transition between different forms.

Plant cells have three distinct kinds of plastids:

1. Chloroplast: This organelle contains chlorophyll and carotenoids and is responsible for photosynthesis.
2. Chromoplast: Carotene and xanthophylls are found in the chromoplast. They colour flowers and fruits and aid in pollination and seed dissemination.
3. Leucoplast: They are colourless and store a variety of food ingredients, such as amyloplasts, proteinoplasts, or aleuroplasts, and elaioplasts.

Chromoplast

These are pigment-producing and pigment-storing plastids. Carotene and xanthophylls are found in them. Chromoplasts are the pigments that give leaves, fruits, flowers, and vegetables their hues. They produce hues other than green. These are found in flowers and ripe fruits. Additionally, chromoplasts aid in the pollination and dissemination of seeds.

Characteristics of Chromoplasts

• Chromoplasts are carotenoids-containing plastids.
• They lack chlorophyll yet produce a variety of other colours.
• Carotenoid pigments are responsible for the yellow, orange, and red hues found in fruits, flowers, old leaves, and roots, among other things.
• Chromoplasts can arise from chloroplasts that are green. Chlorophyll and thylakoid membranes degrade and carotenoids accumulate, for example, during fruit ripening.
• During fruit ripening, the change of chloroplasts to chromoplasts is quite visible. Carotenoids are synthesised in large quantities. As a result, chlorophyll and thylakoids are oxidised.
• While plastid DNA stays unaltered after transformation, ribosomes and rRNA are lost.
• Certain chromoplasts are capable of reverting to chloroplasts, for example, the chromoplasts of carrot root and citrus fruit, pumpkin, and cucumber. They lose their carotene pigment and acquire chlorophyll and a thylakoid system.
• Gibberellin and nitrates induce redifferentiation to chloroplasts.
• In plants such as pepper and tomato, chromoplast formation is irreversible.
• Chromoplasts are generated from proplastids or leucoplasts in various plants such as papaya, carrot, mango, and watermelon.
• Light, temperature, and nutrients all play a role in chromoplast production.
• They include plastoglobules and carotenoid-lipoprotein substructures that are responsible for storing particular carotenoid pigments and lipoprotein fibrils.
• Carotenoids are classified as carotene and xanthophylls. Carotene is an orange pigment, while xanthophyll is a yellow pigment.
• Xanthophylls, such as fucoxanthin, lutein, and others, contain oxygen.
• Carotenoids, such as carotene and lycopene, are composed entirely of carbon and hydrogen.

Structure and classification

Chromoplasts can be separated and categorised into four distinct varieties using a light microscope. The first kind is formed of granulated proteic stroma. The second is made up of protein crystals and granules of amorphous pigment. The third kind is made up of crystals of protein and pigment. The fourth form is a chromoplast composed entirely of crystals. An electron microscope provides more details, allowing the identification of substructures such as globules, crystals, membranes, fibrils, and tubules. The substructures of chromoplasts are absent from the mature plastid from which they separated.