#67 Transport of materials from sources to sinks at different seasons

'Source' is the part of a plant where substances are produced (e.g. leaves for sucrose, amino acids) or enter the plant.

'Sink' refers to the part of the plant where the substrate can be stored (e.g. roots or stem for starch).

Examples: 

Sources: 
Leaves - sucrose is produced here.
Root hairs - Nitrates are absorbed here.

Sinks:
Roots/Stems - starch is stored here.
Root tips - amino acids are stored here. 

When a plant is actively photosynthesising and growing, the leaves are generally the major sources of translocated materials. They are constantly producing sucrose, which is carried in the phloem to all other parts of the plant. 

These parts - the sinks - include the roots, the flowers and the fruits:

• The roots may change some of the sucrose to starch and store it. 
• The flowers use the sucrose to make fructose. 
• Later, when the fruits are developing, quite large amounts of sucrose may be used to produce sweet, juicy fruit ready to attract animals. 

But many plants have a time of year when they become dormant. During this stage, they wait out harsh conditions in a state of reduced activity. 

Dormant plants do not photosynthesise, but survive on their stored starch, oils and other materials. When the seasons change, they begin to grow again. Now the stored materials are converted to sucrose and transported to the growing region. 

For example, potato plants are not able to survive the cold frost of winter. 

• During the summer, the leaves photosynthesise and send sucrose down into underground stems. Here, swellings called tubers develop. The cells in the root tubers change the sucrose to starch and store it. 

•  In winter, the leaves die. Nothing is left of the potato plant above ground - just the stem tubers beneath the soil.  

• In spring, they begin to grow new shoots and leaves. The starch in the tubers is changed back to the sucrose, and transported in the phloem to the growing stems and leaves. This will continue until the leaves are above ground and photosynthesise.  

So in summer, the leaves are sources and the growing stem tubers are sinks. In spring, the stem tubers are sources and the growing leaves are sinks. 

Conclusion: 

Phloem can transfer sucrose in either direction - up or down the plant. This isn't true for the transport of water in the xylem vessels. That can only go upwards, because transpiration always happens at the leaf surface, and it is this that provides the 'pull' to draw water up the plant.