(i)Explain,with the aid of examples,what is meant by the term limiting factor in photosynthesis.?

Question

(ii)suggest how a gardener could use a greenhouse to produce more favouarable conditions for photosynthese.

Answer 1

Ans (i)- Limiting factor is any factor beyond which the rate of photosynthesis can not be increased, until the factor is dealed with.The maximum rate of photosynthesis will be constrained by a limiting factor. This factor will prevent the rate of photosynthesis from rising above a certain level even if other conditions needed for photosynthesis are improved. This limiting factor will control the maximum possible rate of the photosynthetic reaction.

For instance, increasing the temperature from 10ºC to 20ºC could double the rate of photosynthesis as the plant's enzymes will be closer to their optimum working temperature. As the temperature is increased, molecules in the cells will be moving at a faster rate due to kinetic theory. If the temperature is raised above a certain level, the rate of photosynthesis will drop as the plant's enzymes are denatured. They will therefore be more likely to join onto the enzymes and react.

The amount of water available to the plant will affect the rate of photosynthesis. If the plant does not have enough water, the plant's stomata will shut and the plant will be deprived of CO². It is difficult in normal lab conditions to prove that water directly affects photosynthesis unless a heavy isotope is used to trace the path of water.

Chlorophyll is needed for photosynthesis. This can be proved by studying a variegated leaf. It is however very difficult to study how different levels of chlorophyll in the plant will affect it's photosynthesis rate. This is because in a variegated leaf the cells either contain chlorophyll or they don't.

Carbon dioxide concentration will directly affect the rate of photosynthesis as it is used in the photosynthesis reaction. It is also easy to change the amount of carbon dioxide that the plant receives.

Light is also directly used in the photosynthesis reaction and is easy to change in normal lab conditions. Thus it can also be a limiting factor.

Ans (ii) Hydroponically grown plants are raised in greenhouses under carefully monitored and controlled conditions. Gravel is normally used as the medium to support the plant's roots within the hydroponics greenhouse. Carefully mixed nutrients are then periodically fed to the crops in liquid form - a method called sub-irrigation culture. Once seedlings are planted almost all of this work is done by automation. In these hydroponic greenhouses sensors within the gravel are used to determine when the plants need more nutrition and turn on the pumps, giving the plants the just the right amount of solution. The greenhouse allows the farmer to provide the plants with just the right amount of heat and light.

These hydroponically grown plants are fed the exact nutrients needed for rapid growth and volume production. Some hydroponic gardeners use special tanks that are made from big drums which are cut in half. The bottom of each half is brazed with brass tubing. A hose hose is clamped to one end of the tube. The other end is attached to a similar tube which is brazed into a small can.

Inside the tanks the can is painted with an asphalt based paint in order to prevent metal surfaces from rusting. The tanks are filled with small sized gravel pellets.

The hydroponic gardening process starts with 'planting the tanks' by first starting seedlings in carefully prepared paper cups full of vermiculate to allow the hydroponic solution to enter. After the plants are started, the entire cup should be placed in the gravel. The plant's roots are irrigated from below as the fluid flows down the hose and into the gravel. The system is set up to ensure that the crops are moist but are never flooded with solution for more than a few seconds.

The advantages of hydroponic gardening in a greenhouse are numerous. When crops are grown hydroponically, crop yields are increased dramatically over conventional agriculture. For example acre of land will yield five to the tons of tomatoes. The same amount of space will yield 60 to 300 tons when grown hydroponically. A lettuce crop grown in a hydroponic greenhouse will yield an increase of 12,000 pounds over conventionally grown lettuce.

Hydroponic gardening provides other advantages besides increased crop yield. The hydroponic greenhouse environment extends the growing season and doesn't require heavy labor. And of course the obvious -there's no need for greenhouse soil!