Role of cytokinin in enhanced productivity of maize supplied with NH₄⁺ and NO₃⁻

Supplying both N forms (NH₄⁺+NO₃⁻) to the maize (Zea mays L.) plant can optimize productivity by enhancing reproductive development. However, the physiological factors responsible for this enhancement have not been elucidated, and may include the supply of cytokinin, a growth-regulating substance. Therefore, field and gravel hydroponic studies were conducted to examine the effect of N form (NH₄⁺+NO₃⁻ versus predominantly NO₃⁻) and exogenous cytokinin treatment (six foliar applications of 22 μM 6-benzylaminopurine (BAP) during vegetative growth versus untreated) on productivity and yield of maize. For untreated plants, NH₄⁺+NO₃⁻ nutrition increased grain yield by 11% and whole shoot N content by 6% compared with predominantly NO₃⁻. Cytokinin application to NO₃⁻-grown field plants increased grain yield to that of NH₄⁺+NO₃⁻-grown plants, which was the result of enhanced dry matter partitioning to the grain and decreased kernel abortion. Likewise, hydroponically grown maize supplied with NH₄⁺+NO₃⁻ doubled anthesis earshoot weight, and enhanced the partitioning of dry matter to the shoot. NH₄⁺+NO₃⁻ nutrition also increased earshoot N content by 200%, and whole shoot N accumulation by 25%. During vegetative growth, NH₄⁺+NO₃⁻ plants had higher concentrations of endogenous cytokinins zeatin and zeatin riboside in root tips than NO₃⁻-grown plants. Based on these data, we suggest that the enhanced earshoot and grain production of plants supplied with NH₄⁺+NO₃⁻ may be partly associated with an increased endogenous cytokinin supply.