BIOLOGY Volume 1 - A Guide to General Biology - 2004

7. AUTOTROPHIC NUTRITION

7.8. Factors affecting photosynthesis

7.8.2. Graphs of photosynthetic intensity

The primary external Factors affecting the rate of Photosynthesis are light intensity, carbon dioxide concentration, and Temperature. If changes in any of these factors are plotted on the horizontal axis, the curves representing the dependence of photosynthetic rate on these factors take the form shown in Fig. 7.19. Initially, as the value of a limiting factor increases, There is a linear increase in The rate of photosynthesis. Subsequently, as another factor or factors become limiting, the reaction rate levels off and stabilizes.

In the Structure/133.html">Discussion that follows, we will assume that only the factor under consideration is changing, while all other factors remain at their optimal values.

Light intensity

At low light levels, the rate of photosynthesis increases in direct proportion to the increase in incident light (see Fig. 7.19). Gradually, under the Influence of other factors, the rate of photosynthesis plateaus. Light intensity on a clear summer day is approximately 100,000 lux (10,000 FOOT-candles), whereas normal photosynthetic processes require a light intensity of only 10,000 lux. Therefore, for most plants (except shade-adapted species), light is not the primary limiting factor in photosynthesis. Very high light intensities can lead to chlorophyll bleaching and a slowdown of photosynthetic reactions. However, plants that constantly grow under such conditions are usually well adapted to them; for example, their leaves may be covered with a thick cuticle or densely covered in trichomes.

Carbon dioxide concentration

Carbon dioxide is used in the dark reactions to produce sugars. Under normal conditions, carbon dioxide is the principal limiting factor in photosynthesis. The atmosphere contains between 0.03% and 0.04% carbon dioxide. Increasing its concentration in the air (see the results of experiment 3, Fig. 7.20) can increase the rate of photosynthesis. An optimal concentration of 0.5% can be maintained for short periods, but prolonged exposure to such levels becomes harmful to the plant. Therefore, a carbon dioxide concentration of approximately 0.1% is considered most favorable. Certain greenhouse crops, such as tomatoes, are cultivated precisely in a carbon dioxide-enriched atmosphere. Currently, there is great interest in plants capable of efficiently removing carbon dioxide from the atmosphere while producing increased yields. Such plants, known as C4 plants, are discussed in section 7.9.

Temperature

The dark reactions, and to some extent the light-dependent reactions, are enzyme-controlled; consequently, ambient temperature is of great importance. For temperate plants, the most favorable temperature is approximately 25 °C. For every 10 °C rise in temperature, the reaction rate doubles (up to 35 °C), although other data indicate that plants develop better at 25 °C.

7.11. Why does the rate of photosynthesis decrease at higher temperatures?

Chlorophyll concentration

Chlorophyll concentration in itself is not a factor that limits photosynthesis. However, the underlying causes of a drop in chlorophyll levels can be significant: diseases (powdery mildew, rust, viral infections), micronutrient deficiencies (section 7.10.1), or normal senescence processes. When a leaf turns yellow, it is said to have become chlorotic, and The process of yellowing is called chlorosis. Chlorotic patches are often a symptom of disease or mineral deficiency. Certain elements, such as iron, magnesium, and nitrogen (the latter two being direct constituents of the chlorophyll molecule), are essential for chlorophyll synthesis and are therefore particularly important. In addition, plants require potassium. Another cause of chlorosis is a lack of light, as light is required for The final stage of chlorophyll synthesis.

Specific inhibitors

If photosynthesis is suppressed, the plant will inevitably die. This principle has been the basis for The Development of various herbicides, such as DCMU (dichlorophenyldimethylurea). This compound short-circuits the non-cyclic electron flow in METABOLISM/14.html">Chloroplasts, thereby inhibiting the light reactions. DCMU has played a crucial role in The Study of the light-dependent reactions of photosynthesis.

Two other factors have a major impact on crop growth and are of more general significance for Plant Growth and photosynthesis: Water availability and environmental pollution.

Water

Water serves as a primary raw material for photosynthesis. However, because water affects a vast number of cellular processes, it is impossible to isolate and evaluate its direct effect on photosynthesis alone. Nevertheless, by studying The amount of organic matter synthesized in water-stressed plants, it is evident that temporary wilting leads to a sharp drop in yield. Even when plants show no visible changes, a minor water deficit can result in a significant decrease in yield. The reasons for this are complex and not fully understood. One clear factor is stomatal closure during wilting, which prevents carbon dioxide from entering for photosynthesis. Furthermore, it has been shown that water deficiency leads to the accumulation of Abscisic acid, a growth inhibitor, in the leaves of certain plants.

Environmental pollution

Certain industrial gases, such as ozone and sulfur dioxide, are highly toxic to plant leaves even in small quantities, although the exact mechanisms are not yet fully established. For instance, cereal crops in polluted areas lose up to 15% of their biomass, particularly during dry summers. Lichens have been found to be extremely sensitive to sulfur dioxide. Soot clogs Stomata and reduces the transparency of the leaf epidermis.

7.12. Consider when, under natural conditions,

a) light intensity and

b) temperature may act as limiting factors for photosynthetic processes.