Abstract:     The maintenance of rapid growth under conditions of CO₂ enrichment is directly related to the capacity of new leaves to use or store the additional assimilated carbon (C) and nitrogen (N). Under drought conditions, however, less is known about C and N transport in C₄ plants and the contributions of these processes to new foliar growth. We measured the patterns of C and N accumulation in maize (Zea mays L.) seedlings using 13C and 15N as tracers in CO₂ climate chambers (380 or 750 µmol mol^–1) under a mild drought stress induced with 10% PEG-6000. The drought stress under ambient conditions decreased the biomass production of the maize plants; however, this effect was reduced under elevated CO₂. Compared with the water-stressed maize plants under atmospheric CO₂, the treatment that combined elevated CO₂ with water stress increased the accumulation of biomass, partitioned more C and N to new leaves as well as enhanced the carbon resource in ageing leaves and the carbon pool in new leaves. However, the C counterflow capability of the roots decreased. The elevated CO₂ increased the time needed for newly acquired N to be present in the roots and increased the proportion of new N in the leaves. The maize plants supported the development of new leaves at elevated CO₂ by altering the transport and remobilization of C and N. Under drought conditions, the increased activity of new leaves in relation to the storage of C and N sustained the enhanced growth of these plants under elevated CO₂.

Key words: drought ,  elevated CO₂ ,  allocation ,  carbon ,  nitrogen