Problem 196 - Objective Biology for NEET

Question

Assertion: Cytokinin delays the leaf senescence. Reason: Cytokinin promotes nutrient mobilization.

Step-by-Step Solution

Answer

The assertion is correct and the reason is valid. Cytokinins do delay leaf senescence by promoting nutrient mobilization within the plant, transferring nutrients from older to younger leaves to prolong their vitality.

Step 1: Understand the Role of Cytokinin

Cytokinins are a class of plant growth substances that promote cell division, or cytokinesis, in plant roots and shoots. They are involved in numerous plant growth and development processes.

Step 2: Connecting Cytokinin and Leaf Senescence

Cytokinins are known to delay the process of leaf senescence. This is largely because they promote nutrient mobilization, which aids in sustaining the health and vitality of the leaves.

Step 3: Understand Nutrient Mobilization

Nutrient mobilization refers to the transfer of nutrients from older leaves (sources) to the younger, developing ones (sinks). This process ensures optimal use of nutrients within the plant body and supports plant growth.

Step 4: Formulating the Final Statement

Given the role of cytokinins and the process of nutrient mobilization, it can be understood that cytokinins do delay leaf senescence by promoting nutrient mobilization. Hence, the assertion is correct and the reason provides appropriate justification for the same.

Key Concepts

Cytokinin

Cytokinin is a crucial plant hormone that plays a vital role in promoting cell division within plant roots and shoots. By encouraging cytokinesis, or the process in which cells divide, cytokinins are paramount in various growth and development functions. They assist in producing new plant cells and tissues, which are essential for maintaining robust plant growth. Additionally, cytokinins are active participants in the regulation of developmental processes such as leaf expansion and the maturation of chloroplasts. They significantly impact the timing and occurrence of certain physiological responses, making them indispensable for plant vitality.

Leaf Senescence

Leaf senescence is a natural phase in the life cycle of a plant where leaves age and eventually die. This process involves a series of complex cellular changes, leading to the yellowing of leaves, nutrient reallocation, and the recycling of essential nutrients. Senescence serves a functional purpose by redirecting nutrients from older leaves to other parts of the plant, such as young leaves, flowers, or seeds, which need these resources for growth and development. Cytokinin helps delay this senescence process, thereby extending the life and functionality of leaves. By maintaining healthy leaves longer, plants can continue photosynthesis and nutrient accumulation, which is advantageous for overall plant health.

Nutrient Mobilization

Nutrient mobilization is a critical process in plants that involves the transfer of nutrients from older, senescing leaves to growing parts of the plant. During this process, nutrients like nitrogen, phosphorus, and others are mobilized from sources (older leaves) to sinks (younger leaves and developing parts). This transfer is essential for sustaining new shoot growth, flowering, and seed development. Cytokinins play a significant role in enhancing nutrient mobilization by maintaining the nutrient-rich state of the leaves and ensuring nutrients are efficiently utilized and transported within the plant. This action helps to prolong leaf vitality and delay senescence, facilitating a more efficient use of resources.

Plant Growth Substances

Plant growth substances encompass a wide array of hormones like cytokinins, auxins, gibberellins, ethylene, and abscisic acid, each orchestrating various aspects of growth and development in plants. These substances may promote or inhibit growth, trigger flowering, affect fruit ripening, or modulate leaf abscission. For example, auxins promote root initiation, while gibberellins can stimulate stem elongation and seed germination. Cytokinins specifically are best known for promoting cell division and delaying leaf senescence through nutrient mobilization. Collectively, these hormones interact dynamically to ensure plant adaptation, survival, and reproduction, thus enhancing their ability to thrive in various environmental conditions.