Fig. 1.

Sink and source relationship for nitrogen in plants. Developing organs require nitrogen, which is supplied either from nitrogen uptake in roots (source 1) or retranslocation from old leaves (source 2). When the plant is growing at high irradiance and high nutrient availability, nitrogen is supplied from the two sources. Retranslocation from old leaves is accelerated when nutrient supply is limited, and lowered when irradiance is low.