Benzyladenine effects on the development of the photosynthetic apparatus in Zea mays: Studies on photosynthetic activity, enzymes and (etio)chloroplast ultrastructure

Abstract

Primary leaf segments from 8‐day‐old dark‐grown, and from 4‐ and 8‐day‐old light‐grown seedlings of Zea mays L. cv. Fronica, were treated with 10‐bM benzyladenine (BA) in the dark for 14 h. The segments were then studied after an exposure to light for 14 h. Photosynthetic activity (O2 evolution and CO2 fixation) and chlorophyll accumulation were stimulated by BA in dark‐grown leaf segments with etioplastids in the earliest stage of development. In these segments BA stimulated the activities of ribulose‐1,5‐bisphosphate carboxylase (EC 4.1.1.39), phosphoenolpyruvate carboxylase (EC 4.1.1.31), NADP+‐malic enzyme (EC 1.1.1.40) and pyruvate, orthophosphate dikinase (EC 2.7.9.1). In segments taken from 4‐ and 8‐day light‐grown seedlings, BA did not enhance the photosynthetic activity nor the chlorophyll accumulation. The activity of the enzymes mentioned above, was significantly enhanced by the BA‐treatment.BA mainly affected grana stacking in mesophyll cell chloroplasts in primary leaf segments taken from 3‐ to 5‐day light‐grown seedlings. Stroma thylakoid development was stimulated only in leaf segments from 3‐day‐old plants. At the same time BA accelerated grana loss in chloroplasts of bundle sheath cells, a typical phenomenon of development in such chloroplasts. Stroma thylakoid length in these chloroplasts increased by a BA treatment in segments from 3‐ and 4‐day light‐grown plants. A significantly higher number of chloroplasts was only observed with segments taken from 8‐day light‐grown seedlings and treated with BA.The etiochloroplast number in segments taken from 8‐day etiolated plants was significantly higher in BA‐treated segments after 26 h illumination. In etiochloroplasts from both mesophyll and bundle sheath cells, BA enhanced grana stacking after illumination for 4 h or more, whereas stroma membrane length was significantly higher only after 26 h light.It is concluded that the effects of BA depend on the developmental stage. BA accelerates the development of mesophyll and bundle sheath cell (etio)chloroplasts, but does not affect the ultrastructure of mature chloroplasts.