How is nitrogen predominantly characterized in terms of its mobility in plants?

Nitrogen is predominantly characterized as being highly mobile within plants, significantly influencing older leaves first. This mobility is primarily due to nitrogen's essential role in various physiological processes, particularly in protein synthesis and plant metabolism. As nitrogen is a key component of amino acids, nucleic acids, and chlorophyll, plants can translocate nitrogen from older tissues to new growth when there is a deficiency in supply.

Older leaves, which are considered to be the nutrient reserves for the plant, mobilize their nitrogen content to support the growth of newer tissues. This translocation helps the plant maintain overall growth and health even when the external nitrogen supply is low. Therefore, the characteristic of nitrogen being highly mobile in influencing older leaves aligns with the plant's survival strategy during nutrient stress.

Other options, while mentioning mobility or affect on various leaves, do not accurately reflect the specific behavior of nitrogen in plants. For instance, stating that nitrogen affects newer leaves first would imply a different physiological mechanism that does not represent how plants utilize nitrogen during times of nutrient limitation.