Source–Sink Relations and Yield Formation

A leaf supplies a living crop, not only the harvest

A tomato crop needs carbon for new leaves, roots, stems, maintenance and developing fruits. An illuminated mature leaf can export photosynthetic products, but its output is not reserved entirely for the product the grower hopes to harvest.

A source exports assimilated material; a sink imports it for use or storage. Mature leaves are common sources, while developing fruits, seeds and growing tissues are common sinks. These are functional roles at a particular time rather than permanent labels attached to every organ.

The transport article describes phloem connections, and the photosynthesis article explains carbon gain and respiration. Here we ask how those processes connect to yield formation.

An organ can change its role

A young expanding leaf can import material before becoming a substantial exporter. A storage organ can accumulate reserves and later supply new growth. The plant’s developmental stage therefore changes which parts provide or demand material.

A sink’s activity depends on more than its size. Development, transport connections and its capacity to use or store assimilates matter. A large structure is not automatically the strongest importing destination in every condition.

Phloem allocation is not a universal nearest-sink rule or an equal division among organs. The diagram is an illustrative account of specified deliveries, not a model that predicts where a crop will send its next unit of carbon.

Yield can face supply limits and destination limits

Source limitation means that the available supply constrains the development being considered. Light, functional leaf area, water status and other conditions can influence supply. Sink limitation means the capacity or activity of the importing destinations constrains the outcome.

These descriptions depend on the stage and conditions. A crop can lose potential grain or fruit sites before later filling begins. Additional assimilate supply may not recreate those lost reproductive structures.

Adding more fruit sites, removing leaves or changing nutrition alters interacting processes. Such changes can affect photosynthesis, transport, development and competition among organs. A useful response needs crop-specific evidence rather than a fixed source-to-yield conversion.

A delivery account is not a biomass-growth account

Suppose a hypothetical source exports assimilates containing 10 g of carbon over a stated interval. A specified account records deliveries of 6 g C to developing fruits, 3 g C to roots and 1 g C to other sinks. The deliveries add to 10 g C.

The 6 g C delivered to fruits is not automatically 6 g of fruit mass or 6 g C retained in new fruit tissue. Imported material can be respired or used in several processes, and fresh fruit also contains water and other constituents. Separate delivery, respiration, storage and harvested composition.

All three delivery terms use elemental carbon mass and the same interval. If additional destinations or transport-storage changes matter, include them in the boundary rather than silently forcing a complete allocation. Detailed whole-system resource accounts remain in Food Systems.

Harvested output is one part of total production

Harvest index is commonly expressed as harvested economic yield divided by a defined total biomass on a consistent mass basis. The crop, organs included, timing and moisture basis must be stated. Definitions can differ, especially when underground organs or perennial systems are involved.

In a hypothetical cereal example, grain dry mass is 4 kg and total above-ground dry mass including grain is 10 kg at the same assessment. The defined above-ground harvest index is 0.40. The denominator already includes the grain; adding it again would change the meaning.

This ratio alone does not establish nutritional output, soil protection, resource efficiency or long-term performance. A crop with another product or biomass boundary requires a clearly defined comparison rather than an automatic ranking.

Connect the mechanism with useful observations

Observe functional canopy condition, reproductive stage, set or retained organs, and the quality of the intended harvest. A fruit count and fruit mass answer different questions; dry mass and fresh mass do too.

A change in yield after a management action can arise through more than one pathway. Record what changed and what remained comparable. If the action affects both source supply and sink number, a single final yield does not isolate either mechanism.

Source–sink reasoning helps explain why good leaf growth need not guarantee a large harvested product. It also connects crop development to pruning, thinning, nutrition and environmental management while leaving detailed operating recommendations in their appropriate topics.

Worked examples

Check your understanding

1. Is a young leaf always a source?

   Show answer and reasoning

   No.

   An expanding leaf can initially import assimilates, then later export them.

2. Does 6 g C delivered to fruit mean 6 g of fresh fruit was produced?

   Show answer and reasoning

   No.

   Carbon delivery, retained biomass and fresh mass have different meanings and units.

3. Can additional photosynthetic supply always restore reproductive sites lost earlier?

   Show answer and reasoning

   No.

   Developmental loss and subsequent filling capacity are different constraints.

Key ideas

• Sources and sinks are time-dependent functions.
• Both supply and destination capacity can limit an outcome.
• Assimilate delivery is not the same as retained yield.
• State organ, interval, moisture and biomass boundaries in every comparison.