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Seasonal Changes and Genetic Variability for DNA Concentration, and Cellular Contents of Soluble Protein, Chlorophyll, Ribulose Bisphosphate Carboxylase, and Pyruvate,Pi Dikinase Activity in Maize Leaves¹

Photosynthetic enzymes and resistances to CO₂ diffusion are likely to limit photosynthetic rates. In order to improve these traits, by classical breeding or genetic engineering, usable genetic variability must be demonstrated, and the effects of the environment should be evaluated. Seasonal changes and genetic variability were determined for pyruvate,Pi dikinase (PPDK) activity, soluble protein, chlorophyll, and DNA for several inbreds of maize (Zea mays L.) grown for 2 years in the field. Ribulose-l,5-bisphosphate carboxylase (RuBPCase) activity and quantity (determined immunologically) were measured in 1982. Whereas flow cytometric determinations of the amount of DNA per nucleus revealed less than 15% variability among these inbreds, the DNA concentration in the leaf blades varied up to 50%. These findings suggest that cellular volume of these inbreds varied as much as 50%. It was therefore reasoned that these traits should be expressed on a relative per-cell basis (i.e., per mg DNA basis) to make meaningful comparisons of the inbreds.DNA concentration in the youngest fully expanded leaf increased at a slow, but relatively constant rate during both seasons. The PPDK activity and RuBPCase concentration were highest around silking, and declined thereafter, about 3 weeks before concentrations of soluble protein and chlorophyll began decreasing. Significant line x time interactions were ascribed to differences in maturity of the inbreds. Significant genetic variability was observed for PPDK activity. From 15 to 18% of the soluble protein was comprised of RuBPCase. The amount of RuBPCase per cell varied up to 50% among inbreds. Inbreds with larger cells (e.g. Mo17 and B73) were generally higher in soluble protein and chlorophyll as compared to inbreds with smaller cells. This conclusion would not have been revealed if the data had been expressed on a basis other than DNA. The significant genetic variability observed for PPDK, RuBPCase, and cell volume provides a basis for further investigations on limitations to photosynthetic rates by these three traits.

Key Words: Zea mays L. • C₄ plants • Photosynthesis • Cell volume • Flow cytometry

² Graduate research assistant and professor of plant physiology, respectively, Dep. of Agronomy, Univ. of Wisconsin, Madison, WI 53706. Present addresses: Dr. R. Maag Ltd., CH-8157 Dielsdorf, Switzerland (G.R.B.); and Head, Dep. of Agronomy, Univ. of Illinois, Urbana, IL 61801 (L.E.S.).

Received for publication June 25, 1984.

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