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.

Plants maintain water balance by varying hydraulic properties, and plasma membrane intrinsic proteins (PIPs) may be involved in this process.  Leaf xylem and root hydraulic conductivity and the mRNA contents of four highly expressed ZmPIP genes (ZmPIP1;1, ZmPIP1;2, ZmPIP2;2, and ZmPIP2;5) in maize (Zea mays) seedlings were investigated.  Under well-watered conditions, leaf hydraulic conductivity (K_leaf) varied diurnally and was correlated with whole-plant hydraulic conductivity.  Similar diurnal rhythms of leaf transpiration rate (E), K_leaf and root hydraulic conductivity (K_root) in well-watered plants are important for maintaining whole-plant water balance.  After 2 h of osmotic stress treatment induced by 10% polyethylene glycol 6000, the K_root of stressed plants decreased but K_leaf increased, compared with well-watered plants.  The mRNA contents of four ZmPIPs were significantly up-regulated in the leaves of stressed plants, especially for ZmPIP1;2.  Meanwhile, ZmPIP2;5 was significantly down-regulated in the roots of stressed plants.  After 4 h of osmotic stress treatment, the E and leaf xylem water potentials of stressed plants unexpectedly increased.  The increase in K_leaf and a partial recovery of K_root may have contributed to this process.  The mRNA content of ZmPIP1;2 but not of the other three genes was up-regulated in roots at this time.  In summary, the mRNA contents of these four ZmPIPs associated with K_leaf and K_root change in maize seedlings during short-term osmotic stress, especially for ZmPIP1;2 and ZmPIP2;5, which may help to further reveal the hydraulic resistance adjustment role of ZmPIPs.  

Mung bean (Vigna radiata L.) has the potential to establish symbiosis with rhizobia, and symbiotic association of soil micro flora may facilitate the photosynthesis and plant growth response to elevated [CO2]. Mung bean was grown at either ambient CO2 400 μmol mol–1 or [CO2] ((550±17) μmol mol–1) under free air carbon dioxide enrichment (FACE) experimental facility in North China. Elevated [CO2] increased net photosynthetic rate (Pn), water use efficiency (WUE) and the non-photochemical quenching (NPQ) of upper most fully-expanded leaves, but decreased stomatal conductance (Gs), intrinsic efficiency of PSII (Fv´/Fm´), quantum yield of PSII (ΦPSII) and proportion of open PSII reaction centers (qP). At elevated [CO2], the decrease of Fv´/Fm´, ΦPSII, qP at the bloom stage were smaller than that at the pod stage. On the other hand, Pn was increased at elevated [CO2] by 18.7 and 7.4% at full bloom (R2) and pod maturity stages (R4), respectively. From these findings, we concluded that as a legume despite greater nutrient supply to the carbon assimilation at elevated [CO2], photosynthetic capacity of mung bean was still suppressed under elevated [CO2] particularly at pod maturity stage but plant biomass and yield was increased by 11.6 and 14.2%, respectively. Further, these findings suggest that even under higher nutrient acquisition systems such as legumes, nutrient assimilation does not match carbon assimilation under elevated [CO2] and leads photosynthesis down-regulation to elevated [CO2].

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 (Vpmax) 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 (Vmax) 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.