PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

5. PHOTOSYNTHESIS

Ecology of Photosynthesis

The ecology of Photosynthesis refers to the dependence of photosynthetic productivity on environmental factors. The impact of these factors and plant adaptation to them are of critical importance for crop production.

IMPACT OF ENVIRONMENTAL Factors

Light

Leaves absorb 80-85% of photosynthetically active radiation (PAR) and approximately 55% of total solar radiation. Photosynthetically active radiation (PAR) is light with a wavelength of 400-700 nm. About 1.5-2% of the absorbed energy is utilized in photosynthesis.

The dependence of the photosynthetic rate on light intensity follows a logarithmic curve. A direct correlation between the process rate and energy input is observed only at low light intensities. In many heliophytes (sun-loving plants), maximum photosynthetic intensity occurs at an illumination level equal to half of full sunlight. Under these conditions, light saturation is reached. Further increases in illumination do not enhance photosynthesis. Shade-tolerant plants utilize low light intensities more efficiently. In these plants, photosynthetic light saturation is achieved at 1,000-2,000 lux, whereas in sun-loving woody plants, it occurs at 10,000-40,000 lux. C₃ plants exhibit lower photosynthetic light saturation compared to C₄ plants.

In the light saturation range, the photosynthetic rate is significantly higher than the Respiration rate. The illumination level at which CO₂ uptake during photosynthesis is balanced by CO₂ release during respiration is known as the light compensation point.

In addition to intensity, the spectral quality of light is also crucial for photosynthesis. The highest photosynthetic rate occurs under red light. This light is always present in direct solar radiation, and when the sun is low, red light energy predominates, which is optimal for exciting chlorophyll molecules. Blue light carries significantly more energy, while infrared light is insufficient.

Plants grown under red and blue light differ significantly in the composition of their photosynthetic products. Blue light promotes The formation of a large amount of non-carbohydrate compounds (Amino Acids, organic acids, etc.). However, the photosynthetic rate increases significantly when a small amount of blue light is added to red light.

Carbon Dioxide Content

CO₂ is the primary substrate for photosynthesis, and its content determines the process intensity. The atmospheric concentration of CO₂ is 0.03%. At this concentration, the photosynthetic rate is only 50% of the maximum, which is achieved when CO₂ levels in the air rise to 0.3%.

Evidently, the photosynthetic process evolved under higher CO₂ concentrations. This allows for CO₂ enrichment of C₃ plants in greenhouses to increase crop yields. Such enrichment is ineffective for C₄ plants because they possess a specialized mechanism for concentrating CO₂.

CO₂ enters the leaves through the Stomata. Although the area of the stomata accounts for only 1-2% of the leaf surface, The rate of CO₂ diffusion is high because it is proportional to the circumference of the pores.

Temperature

Primary photophysical processes of photosynthesis (energy absorption and migration) are temperature-independent. Processes of photosynthetic phosphorylation are highly sensitive to temperature. For enzymatic carbon reduction reactions, Q₁₀ = 2-3.

The overall dependence of photosynthesis on temperature is represented by a unimodal curve. This curve features three cardinal temperature points: the minimum at which photosynthesis begins, the optimum, and the maximum.

The lower temperature limit for photosynthesis in plants from northern latitudes ranges from -15 to -1 °C, while in tropical plants, it is +4 to +8 °C. In temperate plants, maximum photosynthetic intensity is reached between 20 and 25 °C, while further temperature increases lead to process inhibition (especially above 40 °C). Some desert plants are capable of photosynthesis at 58 °C.

Temperature limits can be shifted through prior hardening and plant adaptation to temperature gradients. Carboxylation reactions, sucrose and starch synthesis, and sucrose transport from leaves are the most sensitive to temperature changes.

Water Regime

Water directly participates in photosynthesis as an oxidation substrate and a source of oxygen. The degree of leaf Hydration determines stomatal opening and CO₂ uptake. Stomata close during full water saturation and in arid periods. Prolonged drought inhibits all photosynthetic processes. Maximum photosynthesis is observed under a slight water deficit (5-20%).

Mineral Nutrition

For the photosynthetic apparatus to function normally, the plant must be supplied with a full spectrum of macro- and micronutrients. Aerial and ROOT nutrition systems are closely interrelated. Mineral elements are integral Components of the photosynthetic apparatus, which defines their role.

Mg is a component of chlorophyll and participates in The activity of Enzymes involved in ATP and NADP synthesis, as well as carboxylation;

Fe is essential for The Biosynthesis of chlorophylls, Cytochromes, and ferredoxin;

Mn and Cl are required for the photooxidation of water;

Cu is a component of plastocyanin;

N deficiency affects the formation of pigment systems and the activity of RuBP carboxylase and Other Enzymes;

insufficient P levels disrupt light and, in particular, dark reactions of photosynthesis and severely inhibit growth processes;

a decrease in K content leads to the disruption of all photosynthetic processes, stomatal function, and water balance.

Oxygen

Photosynthesis typically occurs under aerobic conditions with an atmospheric O₂ concentration of 21%. The ambient oxygen level usually exceeds the optimal concentration for photosynthesis. High O₂ concentrations (23-30%) inhibit photosynthesis and stimulate Photorespiration, thereby reducing RuBP carboxylase activity. The complete absence of oxygen is unfavorable for photosynthesis.

Diurnal and seasonal rhythms of photosynthesis

Photosynthetic intensity increases with sunrise, reaching its peak between 9:00 and 12:00. On overcast, mild days, this rate remains steady. In hot weather, as leaf temperature rises and water content decreases, the rate of photosynthesis drops. Often, a secondary increase in intensity is observed around 16:00-17:00. Photosynthetic activity declines after 22:00 with The Setting of the sun.

Seasonal changes depend on the plant's climatic zone. In deserts, all fluctuations are determined by the specifics of ontogenesis. In ephemerals, maximum photosynthetic activity is observed in late March to early April, coinciding with the onset of fruiting. In plants that complete their vegetation cycle in early summer, peak photosynthesis occurs during this period. For those with longer growing seasons, the seasonal maximum occurs just before the onset of drought. In arctic plants, photosynthetic intensity is lower at the beginning and end of the growing season, when frosts are more frequent.

Plant productivity

Maximum yields can be achieved by creating the following optimal conditions:

1. increasing leaf surface area in crops;

2. extending the duration of active photosynthetic apparatus function throughout the day and the growing season (supported by proper agricultural practices and mineral fertilizers);

3. high photosynthetic productivity and maximum daily dry matter accumulation;

4. maximum translocation of photosynthetic products to economically important Organs.

Achieving high yields requires selective breeding and genetic research aimed at increasing the intensity of photosynthesis, the rate of assimilate translocation, and the net photosynthetic productivity.