Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

4. Photosynthesis
4.10 Effect of Internal and External Factors on Photosynthesis

Photosynthesis takes place in semi-autonomous Organelles—METABOLISM/14.html">Chloroplasts. However, it is largely regulated by plant-internal processes and environmental factors.

Outflow of assimilates. The accumulation of photoassimilates within the chloroplasts and periplastidial space inhibits the Enzymes involved in photosynthesis.

Chlorophyll content. As the cellular chlorophyll content increases, so does The rate of photosynthesis.

Age of the leaf and the plant. Photosynthesis intensity increases during leaf growth, and gradually declines once growth is complete. In many annual plants, photosynthetic activity peaks during the budding and flowering stages before subsequently decreasing.

Light. There is a lower light threshold at which plants initiate photosynthesis. Beyond this, the dependence of photosynthetic rate on light intensity follows a logarithmic curve that eventually reaches a plateau. The slope

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Figure 4.8 - Photorespiration (after V. V. Polovyi)

Figure 4.9 - Dependence of photosynthetic rate on light intensity: 1 - rate of CO₂ emission in the dark (Respiration rate); 2 - photosynthetic compensation point; 3 - state of light saturation

of the light-response curve depends on the Influence of other factors. For instance, in sun-loving plants, the curve plateaus at a significantly higher light intensity than in shade-tolerant plants. The light level at which CO₂ uptake during photosynthesis equals CO₂ emission during respiration is referred to as the compensation point.

The spectral composition of light is also crucial for photosynthesis. Illumination with red light predominantly yields CARBOHYDRATES, whereas blue light promotes the synthesis of amino and organic acids.

Temperature. At low light intensities, photosynthesis proceeds at the same rate at 15 and 25 °C. This is because light-dependent reactions limit the rate of photosynthesis under low light conditions. Under high light intensity, the rate of photosynthesis is limited by dark reactions, and Q₁₀ is approximately equal to 2. For most C3 plants, the optimal temperature ranges from 20-25 ^oC, while for C4 plants it is 25-40 ^oC. Temperatures above the optimum reduce photosynthetic intensity due to chloroplast inactivation and stomatal closure.

CO2 content in the air. Increasing the CO2 concentration from 0.03% to 0.3% enhances the rate of photosynthesis. A further rise in CO₂ concentration up to 1% does not affect photosynthesis, whereas even higher airborne CO₂ levels lead to photosynthetic depression. High CO₂ concentrations are particularly detrimental under high light intensity due to the inhibition of dark reactions. The Effect of carbon dioxide on photosynthesis varies by plant species.

Water supply. Severe water deficit depresses photosynthesis by inducing stomatal closure, which restricts CO₂ diffusion into the leaves, lowers Transpiration, and elevates leaf temperature. Furthermore, dehydration alters enzyme conformation and, consequently, their activity.

Oxygen content in the air averages 21%. Elevated concentrations or the absence of oxygen are detrimental to photosynthesis. Oxygen suppresses The activity of ribulose bisphosphate carboxylase.

Mineral Nutrition. The deficiency of any essential mineral nutrient negatively impacts photosynthesis. Elements such as phosphorus, magnesium, iron, manganese, copper, potassium, and nitrogen are particularly critical. Phosphorylated compounds participate in all Stages of Photosynthesis. Potassium activates phosphorylation processes and regulates stomatal opening. Magnesium is a constituent of chlorophylls and activates carboxylation and NADP reduction reactions. Iron is essential for chlorophyll synthesis. Manganese takes part in the photolysis of water. Copper is a component of plastocyanin. Nitrogen is required for mitochondrial biogenesis and pigment formation.