The leaf photosynthesis and nitrogen (N) translocation in three large-spike lines and control cultivar (Xi’nong 979) of winter wheat (Triticum aestivum L.) were studied in 2010–2011 and 2011–2012. The objectives of this study were to investigate the differences in the physiological characteristics of large-spike lines and control cultivar and identify the limiting factors that play a role in improving the yield of breeding materials. The average yield, grain number per spike, kernel weight per spike, and 1 000-kernel weight of the large-spike lines were 16.0, 26.8, 42.6, and 15.4%, respectively, significantly higher than those of control. The average photosynthetic rates (Pn) were not significant between the large-spike lines and control cultivar during the active growth period. The average PSII maximum energy conversion efficiency (Fv/Fm), PSII actual quantum efficiency (ФPSII), photochemical quenching coefficient (qP), PSII reaction center activity (Fv´/Fm´) and water-use efficiency (WUE) of the large-spike lines were 1.0, 5.1, 3.6, 0.8, and 43.4%, respectively, higher than those of the control during the active growth stages. The N distribution proportions in different tissues were ranked in the order of grains>culms+sheathes>rachis+ glumes>flag leaves>penultimate leaves>remain leaves. This study suggested that utilization of the large-spike wheat might be a promising approach to obtain higher grain yield in Northwest China.

    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.

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.

Grassland plays an important role in the global carbon cycle and climate regulation. However, there are still large uncertainties in grassland carbon pool and also its role in global carbon cycle due to the lack of measured grassland biomass at regional scale or global scale with a unified survey method, particular for below-ground biomass. The present study, based on a total of 44 grassland sampling plots with 220 quadrats across Ningxia, investigated the characteristics of above-ground biomass (AGB), below-ground biomass (BGB), litter biomass (LB), total biomass (TB) and root:shoot ratios (R:S) for six predominantly grassland types, and their relationships with climatic factors. AGB, BGB, LB and TB varied markedly across different grassland types, the median value ranging from 28.2-692.6 g m-2 forAGB, 130.4-2036.6 g m-2 for BGB, 9.2-82.3 g m-2 for LB, and 168.0-2 681.3 g m-2 for TB. R:S showed less variation with median values from 3.2 to 5.3 (excluding marshy meadow). The different grassland types showed similar patterns of biomass allocation, with more than 70% BGB for all types. There is evidence of strong positive effects associated with mean annual precipitation (MAP) and negative effects associated with mean annual temperature (MAT) on AGB, BGB, and LB, although both factors have the opposite effect on R:S.