No abstract

The CO2 concentration in Earth's atmosphere may double during this century. Plant responses to such an increase depend strongly on their nitrogen status, but the reasons have been uncertain. Here, we assessed shoot nitrate assimilation into amino acids via the shift in shoot CO2 and O2 fluxes when plants received nitrate instead of ammonium as a nitrogen source (deltaAQ). Shoot nitrate assimilation became negligible with increasing CO2 in a taxonomically diverse group of eight C3 plant species, was relatively insensitive to CO2 in three C4 species, and showed an intermediate sensitivity in two C3-C4 intermediate species. We then examined the influence of CO2 level and ammonium vs. nitrate nutrition on growth, assessed in terms of changes in fresh mass, of several C3 species and a Crassulacean acid metabolism (CAM) species. Elevated CO2 (720 micromol CO2/mol of all gases present) stimulated growth or had no effect in the five C3 species tested when they received ammonium as a nitrogen source but inhibited growth or had no effect if they received nitrate. Under nitrate, two C3 species grew faster at sub-ambient (approximately 310 micromol/mol) than elevated CO2. A CAM species grew faster at ambient than elevated or sub-ambient CO2 under either ammonium or nitrate nutrition. This study establishes that CO2 enrichment inhibits shoot nitrate assimilation in a wide variety of C3 plants and that this phenomenon can have a profound effect on their growth. This indicates that shoot nitrate assimilation provides an important contribution to the nitrate assimilation of an entire C3 plant. Thus, rising CO2 and its effects on shoot nitrate assimilation may influence the distribution of C3 plant species.

Plant strategies to cope with future droughts may be enhanced by associations between roots and soil microorganisms, including arbuscular mycorrhizal (AM) fungi. But how AM fungi affect crop growth and yield, together with plant physiology and soil carbon (C) dynamics, under water stress in actual field conditions is not well understood. The well-characterized mycorrhizal tomato (Solanum lycopersicum L.) genotype 76R (referred to as MYC+) and the mutant nonmycorrhizal tomato genotype rmc were grown in an organic farm with a deficit irrigation regime and control regime that replaced evapotranspiration. AM increased marketable tomato yields by ~25% in both irrigation regimes but did not affect shoot biomass. In both irrigation regimes, MYC+ plants had higher plant nitrogen (N) and phosphorus (P) concentrations (e.g. 5 and 24% higher N and P concentrations in leaves at fruit set, respectively), 8% higher stomatal conductance (gs), 7% higher photosynthetic rates (Pn), and greater fruit set. Stem water potential and leaf relative water content were similar in both genotypes within each irrigation regime. Three-fold higher rates of root sap exudation in detopped MYC+ plants suggest greater capacity for water uptake through osmotic driven flow, especially in the deficit irrigation regime in which root sap exudation in rmc was nearly absent. Soil with MYC+ plants also had slightly higher soil extractable organic C and microbial biomass C at anthesis but no changes in soil CO2 emissions, although the latter were 23% lower under deficit irrigation. This study provides novel, field-based evidence for how indigenous AM fungi increase crop yield and crop water use efficiency during a season-long deficit irrigation and thus play an important role in coping with increasingly limited water availability in the future.

Root hydraulic redistribution has been shown to occur in numerous plant species under both field and laboratory conditions. To date, such water redistribution has been demonstrated in two fundamental ways, either lifting water from deep edaphic sources to dry surface soils or redistributing water downward (reverse flow) when inverted soil Y Y Y Y s gradients exist. The importance of hydraulic redistribution is not well documented in agricultural ecosystems under field conditions, and would be important because water availability can be temporally and spatially constrained. Herein we report that a North American grapevine hybrid ( Vitis riparia ¥ ¥ ¥ ¥ V. berlandieri cv 420 A) growing in an agricultural ecosystem can redistribute water from a restricted zone of available water under a drip irrigation emitter, laterally across the high resistance pathways of the trunk and into roots and soils on the non-irrigated side. Deuteriumlabelled water was used to demonstrate lateral movement across the vine's trunk and reverse flow into roots. Water redistribution from the zone of available water and into roots distant from the source occurred within a relatively short time frame of 36 h, although overnight deposition into rhizosphere soils around the roots was not detected. Deuterium was eventually detected in rhizosphere soils adjacent to roots on the non-irrigated side after 7 d. Application of identical amounts of water with the same deuterium enrichment level (2%) to soils without grapevine roots showed that physical transport of water through the vapour phase could not account for either downward or transverse movement of the label. These results confirmed that root presence facilitated the transport of label into soils distant from the wetted zone. When deuterium-labelled water was allowed to flow directly into the trunk above the root-trunk interface, reverse flow occurred and lateral movement across the trunk and into roots originating around the collar region did not encounter large disproportionate resistances. Rapid redistribution of water into the entire root system may have important implications for woody perennial cultivars growing where water availability is spatially heterogeneous. Under the predominantly dry soil conditions studied in this investigation, water redistributed into roots may extend root longevity and increase the vines water capacitance during periods of high transpiration demand. These benefits would be enhanced by diminished water loss from roots, and could be equally important to other cited benefits of hydraulic redistribution into soils such as enhancement of nutrient acquisition.

Inorganic N is available to plants from the soil as ammonium MathClass-open(NH4+MathClass-close) and nitrate MathClass-open(NO3-MathClass-close). We studied how wheat grown hydroponically to senescence in controlled environmental chambers is affected by N form (NH4+ vs. NO3−) and CO2 concentration (“subambient,” “ambient,” and “elevated”) in terms of biomass, yield, and nutrient accumulation and partitioning. Wheat supplied with NH4+ as a sole N source had the strongest response to CO2 concentration. Plants exposed to subambient and ambient CO2 concentrations typically had the greatest biomass and nutrient accumulation under both N forms. In general NH4+-supplied plants had higher concentrations of total N, P, K, S, Ca, Zn, Fe, and Cu, while NO3--supplied plants had higher concentrations of Mg, B, Mn, and NO3- - N. NH4+-supplied plants contained amounts of phytate similar to NO3−-supplied plants but had higher bioavailable Zn, which could have consequences for human health. NH4+-supplied plants allocated more nutrients and biomass to aboveground tissues whereas NO3+-supplied plants allocated more nutrients to the roots. The two inorganic nitrogen forms influenced plant growth and nutrient status so distinctly that they should be treated as separate nutrients. Moreover, plant growth and nutrient status varied in a non-linear manner with atmospheric CO2 concentration.