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

4. Photosynthesis
4.7 C4 pathway of photosynthesis (Hatch-Slack cycle)

The pathway of СО₂ assimilation discovered by M. Calvin is the primary one. However, There is a large group of plants, comprising more than 500 angiosperm species, in which the primary products of СО₂ fixation and reduction are four-carbon compounds. They are referred to as C4 plants. In 1966, Australian scientists Hatch and Slack

proposed a complete Scheme for the cycle of dark reactions in C4 plants, which became known as the Hatch-Slack cycle.

The C₄ plants include A number of crops predominantly of tropical and subtropical origin—such as corn, millet, sorghum, and sugarcane—as well as many aggressive weeds like bermudagrass, barnyard grass, proso millet, Johnson grass (Aleppo sorghum), and amaranth. As a rule, these are highly productive plants capable of sustained Photosynthesis under significantly elevated temperatures and arid conditions. The leaves of these plants contain Two Types of METABOLISM/14.html">Chloroplasts: regular chloroplasts in the mesophyll Cells and large, granaless chloroplasts lacking Photosystem II in the bundle sheath cells surrounding the vascular bundles.

The C₄ cycle can be divided into two stages: carboxylation, which takes place in the mesophyll cells, and decarboxylation along with carbohydrate synthesis, which occur in the bundle sheath cells. Common to all C₄ plants is that phosphoenolpyruvate (PEP) undergoes carboxylation mediated by PEP carboxylase to form oxaloacetic acid (OAA), which is subsequently reduced to malic acid or aminated to yield aspartic acid.

In the Cytoplasm of mesophyll cells, phosphoenolpyruvate carboxylase attaches СО₂ to phosphoenolpyruvic acid, forming oxaloacetic acid. It is then transported into the chloroplasts, where it is reduced to malic acid with the participation of NADPH. In the presence of ammonium ions, oxaloacetic acid is converted into aspartic acid. Malic and (or) aspartic acids are transferred to the chloroplasts of the bundle sheath cells, where they are decarboxylated to pyruvic acid and СО₂. The СО₂ enters The Calvin Cycle, while pyruvic acid is transported back to the mesophyll cells, where it is converted into phosphoenolpyruvic acid (Fig. 4.6). This mechanism enables plants to photosynthesize even when Stomata are closed due to high temperatures. Furthermore, the products of the Calvin cycle are formed in the chloroplasts of the bundle sheath cells surrounding the vascular bundles. This facilitates the rapid outflow of photoassimilates, thereby enhancing photosynthetic intensity.