Climacteric (botany)

Climacteric refers to a phase in the fruit maturation process where there is a significant increase in the rate of respiration followed by the natural process of ethylene production, leading to ripening. This phase is crucial for the development of flavor, color, and texture in many fruits. Climacteric fruits include apples, bananas, tomatoes, and avocados, among others. In contrast, non-climacteric fruits, such as grapes, citrus, and strawberries, do not experience this surge in respiration and ripen without the influence of ethylene after harvest.

Overview

The climacteric phase is characterized by a peak in respiration, known as the climacteric peak, which is closely associated with increased ethylene production, a plant hormone that plays a key role in the ripening process. Ethylene acts as a signaling molecule, triggering various biochemical pathways that lead to changes in fruit texture (softening), color (degreening), and the development of flavor and aroma compounds.

Physiology

The onset of the climacteric phase varies among different fruit species and is influenced by both genetic and environmental factors. The increase in ethylene production can be autocatalytic, meaning that the ethylene produced by the fruit can stimulate its own production, leading to a rapid increase in ethylene levels within the fruit.

Ethylene Production

Ethylene is synthesized from the amino acid methionine through a series of enzymatic reactions. The key enzyme in this pathway is ACC synthase, which converts S-adenosylmethionine (SAM) to 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene. The expression of ACC synthase is upregulated during the climacteric rise, leading to increased ethylene production.

Respiration Rate

The climacteric rise in respiration is a hallmark of climacteric fruit ripening. This increase in respiration is an energy-consuming process, associated with the breakdown of stored carbohydrates to produce energy for the ripening process. The increased respiration rate is closely linked to ethylene production and action.

Climacteric Fruits

Climacteric fruits exhibit a wide range of physiological and biochemical changes during ripening, which are initiated and coordinated by ethylene. These changes include:

• Softening of the fruit flesh, due to the breakdown of cell wall components
• Changes in color, such as the degradation of chlorophyll (green pigment) and the synthesis of carotenoids (yellow, orange, or red pigments)
• Development of flavor and aroma compounds
• Conversion of starches to sugars, leading to an increase in sweetness

Non-Climacteric Fruits

In contrast to climacteric fruits, non-climacteric fruits do not show a significant increase in respiration or ethylene production after harvest. Their ripening is not regulated by ethylene, and they must be harvested when fully ripe or near ripeness for optimal quality.

Significance in Agriculture

Understanding the climacteric process is essential for the post-harvest management of fruits. Manipulation of ethylene levels, through either the application of ethylene or ethylene inhibitors, can control the ripening process, extending the shelf life of climacteric fruits. This has significant implications for the transportation, storage, and marketing of fresh fruits.

See Also

• Ethylene
• Fruit ripening
• Post-harvest losses (fruits and vegetables)
• Respiration (physiology)