Effect of growth conditions on carboxylating enzymes of Zea mays plants R. BASSI* and C. PASSERA** *Istituto di Botanica, **Istituto di Chimica Agraria, dell'Universit~ di Padova, Via Gradenigo, 6, 35100 Padova, Italy (Received 2 7 October 1981; in revised form 21 December 1981) Key words: carboxylating enzymes, leaf age, light intensity, C3 -- C4 photosynthesis, Zea Abstract. Phosphoenolpyruvate (PEP) caxboxylase (EC 4.1.1.31) and ribulose-l,5- bisphospate (RuBP) caxboxylase (EC 4.1.1.39) activities in leaves of different maize hybrids grown under field conditions (high light intensity) and in a growth chamber (low light intensity) were determined. Light intensity and leaf age affected PEP earboxy- lase activity, whereas RuBP earboxylase was affected by leaf age only at low light intensity. PEP carboxylase/RuBP earboxylase activity ratio decreased according to light intensity and leaf age. Results demonstrate that Zea mays grown under field con- ditions is a typical C, species in all leaves independently from their position on the stem, whereas it may be a Cs plant when it is grown in a growth chamber at low light intensity. Introduction Higher plants can be generally classified into C3 and C4 plants based on their mode of initial carbon fixation. C3 plants fix C02 primarily through the reductive pentose phosphate pathway, whereas C4 plants use an additional C4 dicarboxylic pathway of photosynthetic carbon assimilation [2, 7]. Plants with a C4 pathway can also be distinguished from C3 plants by other features such as leaf anatomy [13], low C02 compensation point [5], high photosynthesis rate [3], high light saturation [9], high phosphoenol- pyruvate carboxylase/ribulose-1, 5-bisphosphate carboxylate activity ratio [1] and no apparent photorespiration [18]. Zea mays is classified as a C4 photosynthetic plant. However, it has been recently suggested that the distinction between C3 and C4 plants may not be as sharp as generally accepted since in the lowermost leaves of the same maize plant the C3 photosynthetic cycle predominates, whereas in the uppermost ones the C4 pathway [4] predominates. This work was undertaken to verify under which conditions the C3 path- way prevails on the C4 pathway in Zea mays plants. **To whom correspondence should be addressed. Abbreviations: PEP = phosphoenolpyruvate; RuBP = ribulose-1, 5-bisphosphate. 53 Photosynthesis Research 3, 53-58 (1982) 0166-8595/82/010053-6 $00.20/0 © 1982 Martinus Ni/hoff/Dr W. Junk Publishers, The Hague. Printed in the Netherlands