Which process is directly affected by signaling molecules in plants under stress?

Signaling molecules play a crucial role in how plants respond to stress, such as drought, pathogen attack, or other environmental challenges. When a plant experiences stress, signaling molecules like jasmonic acid, salicylic acid, and ethylene are produced, which initiate a series of physiological and biochemical responses to defend against the stressor.

The activation of defense mechanisms encompasses various actions, including the production of secondary metabolites, strengthening of cell walls, and triggering the expression of specific defense-related genes. This response not only enhances the plant's ability to cope with adverse conditions but also helps to fortify its defenses against potential threats.

In contrast, other options may not directly relate to the immediate response to stress. For example, while the growth of new leaves and carbohydrate production are essential functions of a plant, they may be downregulated during stress rather than activated. The absorption of sunlight, primarily through photosynthesis, is also essential for plant health but is not a direct response to stress signals; in fact, stress conditions often reduce photosynthetic activity. Thus, the most pertinent process directly influenced by signaling molecules during stress is the activation of defense mechanisms.