Compensation Point And Potato Tuber Yield

We probably all know that photosynthesis is the process, using energy from sunlight, whereby plants manufacture their own food (as sugars) from carbon dioxide and water. However, plants are continuously using up this sugar in respiration, whereby energy is released to fuel various life processes.

SIGNIFICANCE OF COMPENSATION POINT
If more sugar is produced than is used in respiration, the surplus can be used in various syntheses (e.g. growth), or stored (usually as starch). However, if less is produced, then the plant cannot survive without taking drastic measures to curtail respiration, such as jettisoning leaves or entering dormancy. The point where both photosynthesis and respiration proceed at the same rate (the point where the graphs intersect - see diagram) is called the 'compensation point'. As you will see, this compensation point is highly significant both to potato tuber yield, and to the growing of all plants.


The compensation point is not static but varies according to light intensity and temperature. This diagram, taken from W.H. Muller(1979) - who cites the work of H.G Lundegardh - shows approximately the relative rates of photosynthesis and respiration in potato leaves during ten minute exposures to both shade (1/25 sunlight) and full sunlight. As you can see, as temperature and/or light levels vary, respiration and photosynthesis react in slightly different ways, causing the compensation point to also vary.

HOW IT AFFECTS GROWING TECHNIQUES
Since, as growers, we are interested in producing optimal results, we can use the graph to determine the optimum temperature for this. For example, in full sunlight and with no other limiting factor, 20 deg C should produce the greatest surplus of sugars (almost 13 milligrams, the difference between the respiration and photosynthesis curves). If temperatures rise too high (above 40 deg C), then the compensation point is passed and death will ensue.

But there is more to it than this. Because only photosynthesis is dependent upon daylight, a plant will require a far higher rate of photosynthesis than respiration, since enough sugar must be produced to sustain 24 hour respiration. Muller states that the rate of photosynthesis is normally 8 to 12 times higher than respiration during daylight hours.

Furthermore, Muller also cites Irish potatoes kept continuously at different temperatures. Large tubers were produced at 20 deg C, whereas no tubers were formed on plants kept at 29 deg C. Clearly this is highly significant. It goes to show that we need to watch temperatures, especially in glasshouses in the summer months.


Reference: Muller, W.H.(1979) Botany - A Functional Approach (Lond: Collier Macmillan Publishers; and New York: Macmillan Publishing Inc.)