Fruit ripening is a complex, genetically programmed process that involves changes in color, flavor, aroma, and texture. This process is regulated by plant hormones, one of which is a simple gaseous compound.
Step 1: Understanding the Question:
The question asks to identify the gas that functions as a plant hormone to trigger and promote the ripening of fruit.
Step 2: Detailed Explanation:
Let's evaluate the role of each gas in the context of fruit biology:
(A) Oxygen (O$_2$): Oxygen is essential for aerobic respiration, the process by which the fruit generates energy. While respiration is active during ripening, oxygen itself is not the hormonal trigger.
(B) Carbon dioxide (CO$_2$): Carbon dioxide is a product of respiration. In fact, high levels of CO$_2$ are often used in controlled atmosphere storage to inhibit the ripening process and extend the shelf life of fruits.
(C) Ethylene (C$_2$H$_4$): Ethylene is a gaseous plant hormone that plays a central role in initiating and coordinating many aspects of ripening, especially in climacteric fruits (like bananas, apples, and tomatoes). It triggers the breakdown of starches into sugars, the softening of cell walls, and the development of color and aroma.
(D) Nitrogen (N$_2$): Nitrogen is a largely inert gas that makes up about 78% of the atmosphere. It has no direct hormonal role in ripening and is often used in modified atmosphere packaging to displace oxygen and slow down spoilage.
Thus, ethylene is the specific gas that acts as a ripening hormone.
Step 3: Final Answer:
Ethylene is the gas that acts as a natural ripening hormone in fruits.