Appearance and accumulation of c(4) carbon pathway enzymes in developing wheat leaves

Soluble protein has been extracted from sections of wheat leaves, from base to tip, and the content of several key enzymes of photosynthetic carbon assimilation in each section has been determined by the protein blot method. In the first leaf, ribulose 1,5-bisphosphate carboxylase (RuBPC) (EC 4.1.1.39) in the basal 0 to 1 centimeter section is about 12% the level in the tip section, whereas phosphoenolpyruvate carboxylase (EC 4.1.1.31) is present in small amounts in the basal section and does not change much in the tip. Pyruvate orthophosphate dikinase (PPDK) (EC 2.7.9.1) first appears in the 4 to 6 centimeter section and increases gradually with development to 10-fold in the tip. Malic enzyme, NADP-dependent (EC 1.3.1.37) also appears in the 4 to 6 centimeter section but remains low to the tip.

Fixation of ¹⁴CO₂ by wheat leaf base sections resulted in 42% of total incorporation into malate and aspartate, indicating β-carboxylation, whereas in the tip section these labeled compounds were only 8% of the total. Although the amount of PPDK in wheat leaves is only 1 to 3% of that in maize leaves, this C₃ PPDK may have a limited role in photosynthesis leading to formation of C₄ compounds. The possibility of a further role, similar to that in C₄ plants, but for intracellular carbon transport in wheat leaves is discussed. The presence of malic dehydrogenase, NADP-specific (EC 1.1.1.82) in wheat leaf chloroplasts was shown, a necessary though not sufficient condition for such a proposed role. Assuming each of the four enzymes associated with C₄ carbon transport were fully active in vivo during photosynthesis, PPDK would still be rate limiting, even in the leaf tip where its activity is maximal. Possible evolutionary and breeding implications are discussed.