Plants distribute their energy resources strategically. Sources like mature leaves produce sugars, while sinks like growing roots and fruits consume them. This balance determines how plants grow and develop.

Sink strength influences which parts get more resources. Strong sinks like fruits compete for limited sugars. This allocation changes as plants grow and respond to their environment, shaping their overall structure and function.

Source-Sink Dynamics

Source and Sink Tissues

• Source tissues are net exporters of carbon assimilates
  • Mature leaves are the primary source tissues in plants
  • Carry out photosynthesis to produce sugars and other organic compounds
  • Export excess assimilates to sink tissues via the phloem
• Sink tissues are net importers of carbon assimilates
  • Include growing roots, shoots, developing fruits, and storage organs (tubers, bulbs)
  • Depend on imported assimilates from source tissues to sustain growth and development
  • Compete for available assimilates based on their sink strength

Sink Strength and Competitive Sinks

• Sink strength refers to the ability of a sink tissue to attract and import assimilates
  • Determined by the size, metabolic activity, and developmental stage of the sink
  • Sinks with higher sink strength are more competitive and receive a larger share of assimilates
  • Examples of strong sinks include rapidly growing fruits, tubers, and meristems
• Competitive sinks vie for limited assimilates exported by source tissues
  • Allocation of assimilates among sinks is influenced by their relative sink strengths
  • Changes in sink strength during plant development can alter assimilate partitioning patterns
  • Environmental factors (temperature, water availability) can also affect sink strength and competition

Assimilate Allocation

Assimilate Partitioning and Phloem Allocation

• Assimilate partitioning refers to the distribution of carbon assimilates among different plant organs
  • Determines the relative growth and development of shoots, roots, fruits, and storage organs
  • Regulated by source-sink relationships, plant hormones, and environmental factors
  • Partitioning patterns can change during different stages of plant growth and development
• Phloem allocation involves the long-distance transport of assimilates from source to sink tissues
  • Assimilates are loaded into the phloem in source tissues and unloaded in sink tissues
  • Phloem transport is driven by a pressure gradient generated by active loading and unloading processes
  • Phloem sap contains sugars, amino acids, hormones, and other organic compounds

Carbon Partitioning

• Carbon partitioning refers to the distribution of carbon among different metabolic pathways and compounds
  • Assimilated carbon can be used for growth, respiration, storage, or defense
  • Carbon partitioning is influenced by the developmental stage, environmental conditions, and genetic factors
  • Example: During fruit development, a larger proportion of carbon is allocated to sugar synthesis and storage
  • Carbon partitioning can also be affected by abiotic stresses (drought, nutrient deficiency) and biotic stresses (herbivory, pathogen infection)

Types of Sinks

Metabolic Sinks

• Metabolic sinks are tissues or organs that actively metabolize imported assimilates
  • Include actively growing tissues such as shoot and root meristems, expanding leaves, and developing flowers
  • Assimilates are used for cell division, cell expansion, and synthesis of structural and functional compounds
  • Metabolic sinks have a high demand for energy and carbon skeletons to sustain their growth and development
• Examples of metabolic sinks:
  • Apical meristems in shoots and roots
  • Expanding leaves and petals
  • Developing embryos in seeds

Storage Sinks

• Storage sinks are tissues or organs that accumulate and store assimilates for future use
  • Include specialized storage organs such as tubers (potatoes), bulbs (onions), and tap roots (carrots)
  • Assimilates are converted into storage compounds such as starch, fructans, or lipids
  • Storage sinks act as reservoirs of carbon and energy for plant growth, reproduction, and stress responses
• Examples of storage sinks:
  • Potato tubers accumulating starch
  • Onion bulbs storing fructans
  • Soybean seeds accumulating proteins and lipids