The maintenance of rapid growth under conditions of CO_2 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 C_4 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 13 C and 15 N as tracers in CO_2 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 CO_2. Compared with the water-stressed maize plants under atmospheric CO_2, the treatment that combined elevated CO_2 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 CO_2 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 CO_2 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 CO_2.
Global environmental change affects plant physiological and ecosystem processes.The interaction of elevated CO2,drought and nitrogen(N) deficiency result in complex responses of C4 species photosynthetic process that challenge our current understanding.An experiment of maize(Zea mays L.) involving CO2 concentrations(380 or 750 μmol mol-1,climate chamber),osmotic stresses(10% PEG-6000,-0.32 MPa) and nitrogen constraints(N deficiency treated since the 144th drought hour) was carried out to investigate its photosynthesis capacity and leaf nitrogen use efficiency.Elevated CO2 could alleviate drought-induced photosynthetic limitation through increasing capacity of PEPC carboxylation(V pmax) and decreasing stomatal limitations(SL).The N deficiency exacerbated drought-induced photosynthesis limitations in ambient CO2.Elevated CO2 partially alleviated the limitation induced by drought and N deficiency through improving the capacity of Rubisco carboxylation(V max) and decreasing SL.Plants with N deficiency transported more N to their leaves at elevated CO2,leading to a high photosynthetic nitrogen-use efficiency but low whole-plant nitrogen-use efficiency.The stress mitigation by elevated CO2 under N deficiency conditions was not enough to improving plant N use efficiency and biomass accumulation.The study demonstrated that elevated CO2 could alleviate drought-induced photosynthesis limitation,but the alleviation varied with N supplies.
