How delaying post-silking senescence in lower leaves of maize plants increases carbon and nitrogen accumulation and grain yield

Planting maize at high densities leads to early leaf senescence, and the resulting reduction in the number of lower leaves affects the plant's root function and lowers its grain yield. However, the nature of the process by which lower leaf senescence affects biomass accumulation and grain yield formation in maize is not clear. This study aimed to shed light on how these factors are related by investigating the effects of the plant growth regulator 6-benzyladenine (6-BA) on the senescence of lower leaves of maize plants. In two maize cultivars planted at densities of 67,500 (low density, LD) and 90,000 (high density, HD) plants ha−1, plants treated with 6-BA maintained a high green leaf area index (LAI) longer than control (CK) plants, enabling them to maintain a higher photosynthetic rate for a longer period of time and produce more biomass before reaching physiological maturity. Spraying the lower leaves of maize plants with a 6-BA solution increased the distribution of 13C-photosynthates to their roots, lower leaves and bracts, a result that can be ascribed to a decreased retention of 13C-photosynthates in the stem and grain. In both seasons of the experiment, maize plants treated with 6-BA accumulated more N in grain and maintained a higher N content in roots and leaves, especially in lower leaves, than CK. Increased C assimilation in the lower leaves may explain why N uptake in plants subjected to the 6-BA treatment exceeded that in CK plants and why both photosynthesis rate and dry matter accumulation were maintained throughout grain filling. Our results suggest that a suitable distribution of C and N in leaves post-silking may maintain plant root function, increase N use efficiency, maximize the duration of high LAI, and increase grain yield.