Describe: (a) sources as the parts of plants that release sucrose or amino acids (b) sinks as the parts of plants that use or store sucrose or amino acids

In plants, the concepts of sources and sinks are crucial for understanding how nutrients, particularly sucrose and amino acids, are produced, transported, and utilized. Here’s a detailed description of both:

(a) Sources

Definition: Sources are the parts of the plant that produce and release sucrose or amino acids into the phloem for transport to other parts of the plant.

• Photosynthetic Tissues: The primary sources of sucrose are the leaves, where photosynthesis occurs. During this process, chloroplasts in mesophyll cells convert carbon dioxide and water into glucose, which is then converted into sucrose for transport.
• Storage Organs: Other sources can include storage organs like roots and tubers (e.g., potatoes), which store carbohydrates and can release sugars when needed. Additionally, young green stems may also act as sources when they are photosynthetically active.
• Amino Acid Production: Amino acids are synthesized mainly in mature leaves and roots. The leaves produce amino acids during photosynthesis, while roots can absorb nitrogen from the soil to synthesize them. These amino acids are then loaded into the phloem for transport to various sinks.

(b) Sinks

Definition: Sinks are the parts of the plant that utilize or store sucrose or amino acids received from the sources.

• Growing Tissues: Sinks typically include actively growing tissues such as young leaves, flowers, fruits, and seeds. These tissues require energy and building blocks for growth and development, making them critical sinks for sucrose and amino acids.
• Storage Organs: In addition to growing tissues, storage organs like roots, tubers, and seeds act as sinks by storing excess nutrients for later use. For example, during seed development, amino acids are taken up from the phloem by embryos for protein synthesis.
• Metabolic Activity: Sinks utilize sucrose for energy through cellular respiration or convert it into starch or other carbohydrates for storage. Amino acids are used to synthesize proteins necessary for growth and metabolic functions.