The correct answer is option C: delay of senescence by cytokinins.

The Richmond-Long effect refers to the phenomenon where the application of cytokinins delays senescence in plant tissues. Senescence is the process of aging and deterioration in plants, which includes the yellowing of leaves, loss of chlorophyll, and eventually death. Cytokinins are a class of plant hormones that play a crucial role in regulating plant growth and development.

Here is a detailed explanation of the Richmond-Long effect and how cytokinins delay senescence:

1. Introduction to senescence:
Senescence is a natural process that occurs in plants as they age. It involves the programmed breakdown of cellular components, leading to the death of plant tissues. Senescence is characterized by various physiological and biochemical changes, including the degradation of chlorophyll, proteins, and nucleic acids.

2. Role of cytokinins in delaying senescence:
Cytokinins are plant hormones that promote cell division, delay senescence, and regulate various physiological processes in plants. When cytokinin levels are high, they can delay senescence and promote cell division and differentiation.

3. Experimental evidence of the Richmond-Long effect:
The Richmond-Long effect was first observed and described by biologists Richmond and Long in the 1950s. They conducted experiments where they applied cytokinins to detached leaves and observed that the hormone delayed senescence. The leaves treated with cytokinins remained green and functional for a longer period compared to untreated leaves.

4. Mechanism of action:
The exact mechanism by which cytokinins delay senescence is not fully understood. However, several theories have been proposed. One theory suggests that cytokinins delay senescence by inhibiting the breakdown of chlorophyll. Chlorophyll degradation is one of the key processes during senescence, leading to the yellowing of leaves. Cytokinins may prevent the degradation of chlorophyll, thereby maintaining leaf greenness.

5. Practical applications:
The delay of senescence by cytokinins has several practical applications in agriculture and horticulture. By applying cytokinins to crops and flowers, growers can extend the shelf life and quality of harvested produce. This is particularly useful for fruits and vegetables that are highly perishable and prone to senescence-related deterioration.

In conclusion, the Richmond-Long effect refers to the delay of senescence in plant tissues by the application of cytokinins. Cytokinins are plant hormones that can inhibit senescence and promote cell division and differentiation. The exact mechanism of how cytokinins delay senescence is still not fully understood, but it is believed to involve the inhibition of chlorophyll degradation. The Richmond-Long effect has practical applications in agriculture and horticulture, where it can be used to extend the shelf life and quality of harvested produce.