Explain what determins a tissue to be a sink or a source and

Explain what determins a tissue to be a sink or a source and how to increase sink strength

Solution

Autotrophs such as plants create their own food by the process of photosynthesis, where they utilize sunlight, water and carbon dioxide to produce energy rich sugar molecules and release oxygen. These energy rich sugar molecules can be metabolized and be utilized by either the photosynthetic tissues themselves or it can be transported to the non-photosynthetic tissues for their growth and development or for storage (mainly as sucrose).

Now, based on this, plant tissues can be generalized into 2 categories: Source Tissues and Sink Tissues:

Source Tissues: These are basically the tissues which are photosynthetically active, that is they have the presence of the pigment chlorophyll and are capable of carrying out the process of photosynthesis. They are basically the generators of the energy rich Sugar molecules and are Expoters of the same to the Non-photosynthetic tissues. A mature leaf is the major most site of photosynthesis for almost every plant and is hence the most important source tissue.

Sink Tissues: On the contrary, plant tissues that are photosynthetically inactive, mainly because of the absence of the pigment chlorophyll are not able to carry out the process of photosynthesis and hence are unable to produce energy rich sugar molecules on their own. Hence, these tissues are dependent on the source tissues for the supply of sugar and hence they are importers of the same. Roots, flowers, tubers are examples of some sink tissues.

Sink tissues either utilize the sugar molecules for their growth and development as in roots (called utilization sinks) or they store them as in tubers or fruits (called storage sinks).

Thus, the determination of a tissue to be source or sink can be determined by its ability to carry out the process of photosynthesis and produce sugar molecule. This in turn depends on the presence of the pigment chlorophyll.

Increase of Sink strength:

Sink strength is sought to be increased in order to increase the carbohydrate content of the storage sinks such as in potatoes or to increase the sugar content (sweetness) of fruits such as mangoes and likewise.

Research has proved that expression of heterologous enzymes which were not subject to plant regulatory mechanisms showed to improve the crops and their sink stregth. For example, the expression of mutant AGPases in maize or certain cultivars of potato has shown to increase the sink strength by increasing the seed weight and increasing the starch levels, respectively. Similarly, in tomatoes, the enzyme Sucrose synthase has been shown to be positively correlated with the final fruit size , starch content and tomato fruit relative growth rate and therefore proved that this enzyme contributed to the sink strength.

Thank you!