Photosynthesis

Photosynthesis is the process by which sugars are manufactured:

Carbon dioxide (from leaf stomata) + water (from roots) + light energy (sun) --> carbohydrates (sugars)

If any ingredient (carbon dioxide, water or light energy) are in limited supply then sugar production will be restricted.  Hence photosynthesis will be restricted by the following:

1)  Poor air exchange and/or circulation (see sections on “Humidity” and “Pruning”) will cause carbon dioxide levels to be restricted.

2)  Light supply can be restricted by the presence of old or overcrowded foliage (see section on “Pruning”).

3)  System design is critical to ensure roots receive sufficient water. 

Most of the heat from sunlight is used to warm up the leaf for transpiration.  Only ~3% is used for photosynthesis. successful

Respiration

Respiration is the process of converting sugars (that are produced during photosynthesis) into adenosine triphosphate (ATP).  ATP is the energy source that is used by plants for their various processes.  Respiration occurs in 'mitochondria' (a component of all plant cells) and these are very concentrated in the areas of the most active growth.

ATP provides energy for the following:

1)  Growth in buds, shoots, root hairs and root tips.

2)  Nutrient uptake.

3)  Movement of sugars into the phloem for distribution to other areas of the plant.

Respiration occurs as follows:

Simple sugars (produced by photosynthesis) +   oxygen (from roots)  --->  ATP (energy) + carbon dioxide + water + heat

Hence, using the above equation you can see that respiration rates decrease if oxygen availability is limited.  The rate of oxygen uptake can be hindered by several factors including:

1)  Insufficient new root growth - new roots (life span is only a few days) are the main supply route for oxygen (along with nutrients and water), therefore if new root growth is restricted oxygen supplies will be restricted.

2)  Insufficient oxygen in the nutrient/water – caused by inadequate aeration of nutrient/water (see section on “Aeration of nutrient”).

Photorespiration: This occurs when plants are subjected to excessive heat and light which typically occurs when the temperature exceeds 35°C.  In this process plants burn up sugars without converting them into ATP.  This typically causes plant growth to slow or even cease depending upon how much ATP is wasted.
 

Transpiration

Transpiration is the process of water evaporating from a plant. This loss of water by evaporation effectively draws nutrients and water from the soil and then causes their distribution to all plant cells.

Transpiration mostly occurs through the leaves. As the leaf warms during the day, water is converted into vapour which then diffuses into the atmosphere via the leafs’ stomata.  Most of the heat from sunlight is used to warm up the leaf for transpiration – less than 5% is used for photosynthesis.

Most of the water absorbed by plants is transpired. The remainder is used as a reagent in photosynthesis, in keeping cells turgid (or hydrated), and in enabling various chemical reactions to occur.

Inadequate transpiration will cause poor distribution of nutrients.  In cucumbers and tomatoes this results in symptoms such as 'blossom-end rot' which is due to a calcium deficiency at the plant's extremities.  Factors restricting transpiration are:

1)  High humidity (see section on “Humidity”).

2)  Low air temperature and light levels (see section on “Temperature”).