(1) In tropical plants like maize, sugar cane, Jowar, Amaranthus, Atriplex, etc, there exists an alternative pathway of photosynthesis. This pathway is called HSK pathway of photosynthesis named after its discoverers, viz., Hatch, Slack and Kortschak.
(2) HSK pathway occurs in low concentration of `CO_(2)` as low as 10 ppm (parts per million) in the atmosphere.
(3) In HSK pathway, the initial product is a 4-carbon compound, i.e., oxaloacetic acid (oxalocetate). Therefore, this pathway of photosynthesis is also known as `C_(4)` pathway.
(4) The various reactions that occur during HSK pathway are completed in two parts at two different sites which are as follows :
(i) Reactions in mesophyll chloroplasts :
(1) Initially, the atmospheric carbon dioxide which enters the mesophyll chloroplasts is accepted by PEPA (phosphoenol pyruvic acid) a 3-carbon compound to form a 4-C compound called oxaloacetic acid in the presence of water and PEP carboxylase enzyme as shown below :
`underset((C_(3)))(PEPA)+CO_(2)+H_(2)O overset("PEP carboxylase")to underset((C_(4)))(OA A+H_(3)PO_(4))`
(2) Oxaloacetic acid is reduced to malic acid (malate) by the enzyme malate dehydrogenase in the presence of `NADPH_(2)` or changed aspartic acid by amination in the presence of `NADPH_(2)` and the enzyme transminase as shown below :
`OA A+NADPH_(2) overset("Malate dehydrogenase")tounderset(("Malate"))("Malic acid")+NADP`
`OA A+NADPH_(2)+NH_(3) overset("Transaminase")to underset(("Malate"))("Aspartic acid")+NADP+H_(2)O`
(ii) Reactions in bundle sheath chloroplasts :
(1) Malic or aspartic acid is then transported to the bundle sheath chloroplasts where it undergoes decarboxylation to form pyruvic acid (pyruvate) with the release of `CO_(2)` as shown below :
`underset((4C))("Malic acid") +NADP to underset(("Pyruvate"))("Pyruvic acid") +underset((3C))(CO_(2)+)NADPH_(2)`
(2) `CO_(2)` released in the above reaction is accepted by RuBP (second carbon acceptor). It enters the Calvin cycle and eventually forms glucose which is stored as starch in the bundle sheath chloroplasts.
(3) Pyruvic acid which is formed due to decarboxylation of malic acid is transported to mesophyll cells where it undergoes phosphorylation to regenerate PEPA as shown below :
`underset((3C))("Pyruvic")+ATP overset("Phosphorylation") to underset((3C)+iP)( PEPA +ADP )`
(4) From the above account it is abivous that in `C_(4)` plants, `CO_(4)` is fixed twice in two different cells during the daytime.
`CO_(2)` fixation in `C_(4) ` plants can occur at very low concentrantions of `CO_(2)` i.e., as low as 1-2 parts per million. `C_(4)` plants can carry on photosynthesis enven if the light intensity is high, temperature is high and the amount of available water is less.
