The Glycine-Glomus-Bradyhizobium symbiosis. VIII. Phosphorususe efficiency of CO2 and N2 fixation in mycorrhizal soybean

Soybean [ Glycine max (L.) Merr. cv. Hobbit] plants nodulated by Bradyhizobium japonicum strain USDA 110 were grown in pot cultures in severely P- and N-deficient soil and either colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or fertilized with a high (HP) or low (LP) level of KH₂PO₄ (0.6 or 0.3 m M , respectively), After 7 weeks of growth, nodule and chloroplast activities (C₂H₂ reduction and CO₂ exchange rate) were determined. Photosynthetic P-use efficiency of CO₂ fixation was significantly higher in VAM than in HP plants, while that of nitrogenase activity was lower. The LP plants were intermediate in both respects. The ratio of nodule to chloroplast activity [mol C₂H₂ reduced (mol CO₂ fixed)⁻¹] was highest in HP and lowest in VAM plants. Root colonization by the VAM fungus significantly increased nodule number and dry weight and reduced nodule specific mass and activity in comparison to HP plants. In spite of lower nodule activity, VAM plants were significantly larger and had higher N concentrations than the HP plants. The results suggest nonnutritional. VAM-elicited and host-mediated effects on the symbiotic functions of the legume association.     

Glycine-Glomus-Bradyrhizobium Symbiosis : X. Relationships between Leaf Gas Exchange and Plant and Soil Water Status in Nodulated, Mycorrhizal Soybean under Drought Stress

Soybean (Glycine max [L.] Merr.) plants were colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe (VAM plants) or fertilized with KH₂PO₄ (nonVAM plants) and grown for 50 days under controlled conditions. Plants were harvested over a 4-day period during which the soil was permitted to dry slowly. The harvest was terminated when leaf gas exchange was no longer measurable due to drought stress. Significantly different effects in shoot water content, but not in shoot water potential, were found in VAM and nonVAM plants in response to drought stress. Leaf conductances of the two treatments showed similar response patterns to changes in soil water and shoot water potential but were significantly different in magnitude and trend relative to shoot water content. The relationships between transpiration, CO₂ exchange and water-use efficiency (WUE) were the same in VAM and nonVAM plants in response to decreasing soil water and shoot water potential. As a function of shoot water content, however, WUE showed different response patterns in VAM and nonVAM plants.     

The Glycine-Glomus-Bradyrhizobium symbiosis. IX. Nutritional, morphological and physiological responses of nodulated soybean to geographic isolates of the mycorrhizal fungus Glomus mosseae.

The objective of the work was to determine differences in plant response to geographic isolates of a vesicular-arbuscular mycorrhizal (VAM) fungus, and to demonstrate the need for such determinations in the selection of desirable host-endophyte combinations for practical applications. Soybean ( Glycine max (L.) Merr.) plants were inoculated with Bradyrhizobium japonicum and isolates of the VAM-fungal morphospecies Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe, collected from an arid (AR), semiarid (SA) or mesic (ME) area. Inoculum potentials of the VAM-fungal isolates were determined and the inocula equalized, achieving the same level of root colonization (41%, P >0.05) at harvest (50 days). Plants of the three VAM treatments (AR, SA and ME) were evaluated against von VAM controls. Significant differences in plant response to colonization were found in dry mass, leaf K, N and P concentrations, and in root/shoot, nodule/root, root length/leaf area and root length/root mass ratios. The differences were most pronounced and consistent between the AR and all other treatments. Photosynthesis and nodule activity were higher ( P <0.05) in all VAM treatments, but only the AR plants had higher ( P <0.05) photosynthetic water-use efficiency than the controls. Nodule activity, evaluated by H₂ evolution and C₂H₂ reduction, differed significantly between treatments. The results are discussed in terms of nutritional and non-nutritional effects of VAM colonization on the development and physiology of the tripartite soybean association in the light of intraspecific variability within the fungal endophyte.     

Defoliation effects on mycorrhizal colonization, nitrogen fixation and photosynthesis in the Glycine-Glomus-Rhizobium symbiosis

Soybean [ Glycine max (L.) Merr. cv. Wells] plants grown in a greenhouse were inoculated with Rhizobium japonicum strain 61A118 and the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thaxt. sensu Gerd.) Gerd. & Trappe. Plants were defoliated (26, 48 and 66%) throughout the growth period and evaluated for VAM colonization, N₂, fixation and photosynthesis at harvest (six weeks). Photosynthate stress as a result of defoliation affected nodulation and nodule activity most severely. Colonization of the roots by the VAM fungus was little affected in comparison, and the intensity of colonization increased with increasing stress. The CO₂-exchange rate decreased less with defoliation than did leaf mass, and photosynthetic efficiency increased with the severity of defoliation. The increase in photosynthetic efficiency was significantly correlated with increases in leaf P (r = 0.91) and N (r = 0.97) concentrations. The results suggest that the VAM fungus should not be regarded as a simple P source and C sink in the tripartite legume association. Threeway source/sink relationships (VAM-P, Rhizobium-N, and host leaf-C) are discussed.     

Parasitic and mutualistic associations between a mycorrhizal fungus and soybean: The effect of phosphorus on host plant-endophyte interactions

Soybean [ Glycine max (L.) Merr. cv. Kent] plants were colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum (Thaxt. sensu Gerd.) Gerd. and Trappe in pot cultures using an inert medium and a nutrient solution. Phosphorus was provided initially as 0, 25,50, 100 or 200 mg hydroxyapatite [HAP, Ca₁₀(PO₄)₆(OH)₂] per pot. Under the low (0 mg HAP) and high (100 and 200 mg HAP) P regimes, VAM plants showed 20, 25 and 38% growth retardation, respectively, relative to non-colonized controls. At 50 mg HAP, VAM plant growth was significantly enhanced (14%). Dry weight and P content of both VAM and control plants increased with increased P availability throughout the HAP gradient. Intraradical VAM fungal biomass increased linearly with increasing P availability. Extraradical VAM fungal biomass was smaller than the intraradical component of the fungus at the lowest and highest levels of P addition in the growth medium. The ratio of extra- to intraradical mycelium, a suggested index of VAM fungal effectiveness, was greatest for the 50 mg HAP treatment, coinciding with growth enhancement of the host plant. This enhanced growth of the host at an intermediate P level was apparently a result of increased P uptake by the endophyte.     

Effects of drought on host and endophyte development in mycorrhizal soybeans in relation to water use and phosphate uptake

Bethlenfalvay, G. J., Brown, M. S., Ames, R. N. and Thomas, R. S. 1988. Effects of drought on host and endophyte development in mycorrhizal soybeans in relation to water use and phosphate uptake. - Physiol. Plant. 72: 565–571.
Soybean [ Glycine max (L.) Merr.] plants were grown in pot cultures and inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or provided with P fertilizer (non-VAM plants). After an initial growth period (21 days), plants were exposed to cycles of severe, moderate or no drought stress over a subsequent 28-day period by rewatering at soil water potentials of -1.0, -0.3 or -0.05 MPa. Dry weights of VAM plants were greater at severe stress and smaller at no stress than those of non-VAM plants. Phosphorus fertilization was applied to produce VAM and non-VAM plants of the same size at moderate stress. Root and leaf P concentrations were higher in non-VAM plants at all stress levels. All plants were stressed to permanent wilting prior to harvest. VAM plants had lower soil moisture content at harvest than non-VAM plants. Colonization of roots by G. mosseae did not vary with stress, but the biomass and length of the extraradical mycelium was greater in severely stressed than in non-stressed plants. Growth enhancement of VAM plants relative to P-fertilized non-VAM plants under severe stress was attributed to increased uptake of water as well as to more efficient P uptake. The ability of VAM plants to deplete soil water to a greater extent than non-VAM plants suggests lower permanent wilting potentials for the former.     

Nutrient transfer between the root zones of soybean and maize plants connected by a common mycorrhizal mycelium

The objective of the study was to determine whether nutrient fluxes mediated by hyphae of vesicular-arbuscular mycorrhizal (VAM) fungi between the root zones of grass and legume plants differ with the legume's mode of N nutrition. The plants, nodulating or nonnodulating isolines of soybean [ Glycine max (L.) Merr.], were grown in association with a dwarf maize ( Zea mays L.) cultivar in containers which interposed a 6-cm-wide root-free soil bridge between legume and grass container compartments. The bridge was delimited by screens (44 μm) which permitted the passage of hyphae, but not of roots and minimized non VAM interactions between the plants. All plants were colonized by the VAM fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe. The effects of N input to N-sufficient soybean plants through N₂-fixation or N-fertilization on associated maize-plant growth and nutrition were compared to those of an N-deficient (nonnodulating, unfertilized) soybean control. Maize, when associated with the N-fertilized soybean, increased 19% in biomass, 67% in N content and 77% in leaf N concentration relative to the maize plants of the N-deficient association. When maize was grown with nodulated soybean, maize N content increased by 22%, biomass did not change, but P content declined by 16%. Spore production by the VAM fungus was greatest in the soils of both plants of the N-fertilized treatment. The patterns of N and P distribution, as well as those of the other essential elements, indicated that association with the N-fertilized soybean plants was more advantageous to maize than was association with the N₂-fixing ones.     

Glycine-Glomus-Rhizobium Symbiosis: V. Effects of Mycorrhiza on Nodule Activity and Transpiration in Soybeans under Drought Stress

Soybean (Glycine max [L.] Merr.) plants were nodulated (Bradyrhizobium japonicum) and either inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe or left uncolonized. All plants were grown unstressed for 21 days initially. After this period, some VAM and non-VAM plants were exposed to four 8-day drought cycles while others were kept well watered. Drought cycles were terminated by rewatering when soil moisture potentials reached −1.2 megapascal. Nodule development and activity, transpiration, leaf conductance, leaf and root parameters including fresh and dry weight, and N and P nutrition of VAM plants and of non-VAM, P-fed plants grown under the same controlled conditions were compared. All parameters, except N content, were greater in VAM plants than in P-fed, non-VAM plants when under stress. The opposite was generally true in the unstressed comparisons. Transpiration and leaf conductance were significantly greater in stressed VAM than in non-VAM plants during the first half of the final stress cycle. Values for both VAM and non-VAM plants decreased linearly with time during the cycle and converged at a high level of stress (−1.2 megapascal). Effects of VAM fungi on the consequences of drought stress relative to P nutrition and leaf gas exchange are discussed in the light of these findings and those reported in the literature.     

Drought resistance of mycorrhizal pepper plants independent of leaf P concentration - response in gas exchange and water relations

Pepper ( Capsicum annuum L.) plants with and without the VA-mycorrhizal fungus Ghmus deserticola Trappe. Bloss and Menge (VAM and NVAM. respectively), were drought acclimated by four drought cycles (DA) or kept well watered (NDA). All plants were then subjected to an additional drought followed by a 3-day irrigation recovery period. Measurements of water relations, gas exchange and carbohydrates were made at selected intervals throughout the drought cycles and recovery. To equalize growth and avoid higher P in VAM plants. NVAM plants received higher P fertilization. Consequently, similar transpirational surface and shoot mass were achieved in all treatments, but NVAM had a higher tissue P concentration than VAM plants. Plants that were either VAM or DA, but especially the VAM-DA plants, tended to be high in net photosynthetic flux (A), A per unit of tissue P concentration (A/P), stomatal conductance (g) or leaf turgor (Ψ_p) during high environmental stress or recovery from stress. During this time, NVAM-NDA plants had low A. A/P and leaf chlorophyll, but high soluble carbohydrate concentrations in their leaves. All VAM and DA plants had some osmotic adjustment compared to the NVAM-NDA plants, but VAM-DA plants had the most. Osmotic adjustment was not due to accumulation of soluble carbohydrate. The high turgor, A and g in the VAM-DA plants during and following environmental stress indicated superior drought resistance of these plants; however, osmotic adjustment was only apparent during recovery and cannot account for the observed drought resistance during environmental stress. Drought resistance of VAM-DA plants was not attributable to high leaf P concentration or confounded by differences in plant transpirational surface.     

Effect of enhanced atmospheric CO2 on mycorrhizal colonization by Glomus mosseae in Plantago lanceolata and Trifolium repens

Plantago lanceolata L. and Trifolium repens L. were grown for 16 wk in ambient (360 μmol mol⁻¹) and elevated (610 μmol mol⁻¹) atmospheric CO₂. Plants were inoculated with the arbuscular mycorrhizal (AM) fungus Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe and given a phosphorus supply in the form of bonemeal, which would not be immediately available to the plants. Seven sequential harvests were taken to determine whether the effect of elevated CO₂ on mycorrhizal colonization was independent of the effect of CO₂ on plant growth. Plant growth analysis showed that both species grew faster in elevated CO₂ and that P. lanceolata had increased carbon allocation towards the roots. Elevated CO₂ did not affect the percentage of root length colonized (RLC); although total colonized root length was greater, when plant size was taken into account this effect disappeared. This finding was also true for root length colonized by arbuscules. No CO₂ effect was found on hyphal density (colonization intensity) in roots. The P content of plants was increased at elevated CO₂, although both shoot and root tissue P concentration were unchanged. This was again as a result of bigger plants at elevated CO₂. Phosphorus inflow was unaffected by CO₂ concentrations. It is concluded that there is no direct permanent effect of elevated CO₂ on mycorrhizal functioning, as internal mycorrhizal development and the mycorrhizal P uptake mechanism are unaffected. The importance of sequential harvests in experiments is discussed. The direction for future research is highlighted, especially in relation to C storage in the soil.     

Interactions between elevated CO2 concentration, nitrogen and water: effects on growth and water use of six perennial plant species

Two experiments are described in which plants of six species were grown for one full season in greenhouse compartments with 350 or 560 μ mol mol^–1 CO₂. In the first experiment two levels of nitrogen supply were applied to study the interaction between CO₂ and nitrogen. In the second experiment two levels of water supply were added to the experimental set-up to investigate the three-way interaction between CO₂, nitrogen and water. Biomass and biomass distribution were determined at harvests, while water use and soil moisture were monitored throughout the experiments. In both experiments a positive effect of CO₂ on growth was found at high nitrogen concentrations but not at low nitrogen concentrations. However, plants used much less water in the presence of low nitrogen concentrations. Drought stress increased the relative effect of elevated CO₂ on growth. Available soil moisture was used more slowly at high CO₂ during drought or at high nitrogen concentrations, while at low nitrogen concentrations decreased water use resulted in an increase in soil moisture. The response to the treatments was similar in all the species used. Although potentially faster growing species appeared to respond better to high CO₂ when supplied with a high level of nitrogen, inherently slow-growing species were more successful at low nitrogen concentrations.     

Preinoculation of lettuce and onion with VA mycorrhizal fungi reduces deleterious effects of soil salinity

The hypothesis that inoculation of transplants with vesicular-arbuscular mycorrhizal (VAM) fungi before planting into saline soils alleviates salt effects on growth and yield was tested on lettuce (Lactuca sativa L.) and onion (Allium cepa L.). A second hypothesis was that fungi isolated from saline soil are more effective in counteracting salt effects than those from nonsaline soil. VAM fungi from high- and low-salt soils were trap-cultured, their propagules quantified and adjusted to a like number, and added to a pasteurized soil mix in which seedlings were grown for 3–4 weeks. Once the seedlings were colonized by VAM fungi, they were transplanted into salinized (NaCl) soil. Preinoculated lettuce transplants grown for 11 weeks in the saline soils had greater shoot mass compared with nonVAM plants at all salt levels [2 (control), 4, 8 and 12 dS m^–1] tested. Leaves of VAM lettuce at the highest salt level were significantly greener (more chlorophyll) than those of the nonVAM lettuce. NonVAM onions were stunted due to P deficiency in the soil, but inoculation with VAM fungi alleviated P deficiency and salinity effects; VAM onions were significantly larger at all salt levels than nonVAM onions. In a separate experiment, addition of P to salinized soil reduced the salt stress effect on nonVAM onions but to a lesser extent than by VAM inoculation. VAM fungi from the saline soil were not more effective in reducing growth inhibition by salt than those from the nonsaline site. Colonization of roots and length of soil hyphae produced by the VAM fungi decreased with increasing soil salt concentration. Results indicate that preinoculation of transplants with VAM fungi can help alleviate deleterious effects of saline soils on crop yield.     

The influence of the mycorrhiza Glomus etunicatum on drought acclimation in safflower and wheat

A study was done to determine the effects of vesicular‐arbuscular mycorrhizal (VAM) colonization on drought acclimation of host plants. Safflower ( Carthamus tinctorius L. cv. S555) and wheat ( Triticum aestivum L. cv. Anza) were grown under environmentally controlled conditions with or without the VAM fungus, Glomus etunicatum Becker and Gerd., and were either acclimated (by pre‐exposing plants to a 10–11 day drought period) or unacclimated to drought. Plants from all treatments were then exposed to drought for 9 days, and plant water status and root water uptake were measured. To minimize interactions between drought and P uptake, growth periods were adjusted so that acclimated and unacclimated plants were similar in size when measurements were made. When wheat was acclimated to drought, osmotic adjustment occurred (leaf solute potentials of leaf tissue rehydrated to full turgor were approximately 0.5 MPa lower in acclimated than unacclimated plants); in safflower, osmotic adjustment was minimal when plants were acclimated. Consequently, acclimated wheat plants were able to tolerate drought better than unacclimated plants, and maintained higher leaf water potentials and relative water contents as soil water was depleted. For both safflower and wheat, acclimated plants had higher water use efficiency, and therefore produced more biomass when water availability was limited, than unacclimated plants. However, mycorrhizal colonization did not affect osmotic adjustment, plant water status, water use efficiency or water uptake in either plant species, and therefore had no effect on drought acclimation or resistance.     

Occurrence and activity of hydrogenase in symbiotic Frankia from field-collected Alnus incana

Occurrence and activity of the hydrogen uptake enzyme were studied in root nodule homogenates made from plants of Alnus incana (L.) Moench collected from field sites in the northern part of Sweden. Nitrogenase (EC 1.7.99.2) activity (estimated by acetylene reduction) and hydrogen evolution were studied in excised nodules. All Frankia sources showed acetylene reduction activity, and possessed a hydrogen uptake system. Hydrogen uptake in nodule homogenates from the Frankia sources measured at 23.8 μM H₂ ranged from 0.04 to 5.0 μmol H₂ (g fresh weight nodule)⁻¹ h⁻¹. The H₂ uptake capacity of nodule homogenates from one of the Frankia sources was almost 8 times higher than the hydrogen evolution from nitrogenase, both expressed on a nodule fresh weight basis. Frankia sources from field sites 6 and 11 showed K_m for H₂ of 13.0 and 23.6 μM H₂, respectively. This indicates similarities in the hydrogen uptake enzymes in the two Frankia sources. It is concluded that hydrogen uptake is a common characteristic in Frankia.     

Oxygen-induced recovery from short-term nitrate inhibition of N2 fixation in white clover plants from spaced and dense stands

Nitrogenase (N₂ase; EC 1.18.6.1) activity (H₂ evolution) and root respiration (CO₂ evolution) were measured under either N₂:O₂ or Ar:O₂ gas mixtures in intact nodulated roots from white clover ( Trifolium repens L.) plants grown either as spaced or as dense stands. The short-term nitrate (5 m M ) inhibition of N₂-fixation was promoted by competition for light between clover shoots, which reduced CO₂ net assimilation rate. Oxygen-diffusion permeability of the nodule declined during nitrate treatment but after nitrate removal from the liquid medium its recovery parallelled that of nitrogenase activity. Rhizosphere pO₂ was increased from 20 to 80 kPa under N₂:O₂. A simple mono-exponential model, fitted to the nodule permeability response to pO₂, indicated NO⁻₃ induced changes in minimum and maximum nodule O₂-diffusion permeability. Peak H₂ production rates at 80 kPa O₂ and in Ar:O₂ were close to the pre-decline rates at 20 kPa O₂. At the end of the nitrate treatment, this O₂-induced recovery in nitrogenase activity reached 71 and 82%; for clover plants from spaced and dense stands, respectively. The respective roles of oxygen diffusion and phloem supply for the short-term inhibition of nitrogenase activity in nitrate-treated clovers are discussed.     

Exchange of oxygen and its role in energy dissipation during drought stress in tomato plants

To elucidate how excess light energy is dissipated during water deficit, net photosynthesis (P_N), stomatal conductance (g_s), intercellular CO₂ concentration (c_i) and Chl a fluorescence were investigated in control and drought-stressed tomato plants ( Lycopersicon esculentum ). Gross O₂ evolution (E_o) and gross O₂ uptake (U_o) were determined by a mass spectrometric ¹⁶O/¹⁸O₂ isotope technique. Under drought stress P_N, g_s, c_i and U_o decline. While photochemical fluorescence quenching decreases under water deficit, non-photochemical quenching rises. The maximal efficiency of PSII measured in the dark is not affected by drought; however, in the light, E_o decreases under water deficit. The ratio P_N/E_o falls under stress while the ratio U_o/E_o increases. We conclude that tomato plants follow a double strategy to avoid photodamage under drought stress conditions: (1) a substantial portion of light energy is emitted as heat and PSII activity is downregulated. This results in a decrease in E_o as well as P_N and U_o. Despite reduced charge separation at PSII, the decline of CO₂ assimilation because of lowered stomatal conductance and metabolic changes results in the need of degrading excessive photosynthetic electrons. (2) Oxygen is used as an alternative electron acceptor in photorespiration or Mehler reaction and U_o rises relative to E_o.     

The effects of soil and atmospheric drought on photosynthesis and stomatal control of gas exchange in three coniferous species

The responses of steady-state CO₂ assimilation rate (A), transpiration rate (E), and stomatal conductance (g_s) to changes in leaf-to-air vapour pressure difference (δW) on one hand and to increasing soil drought on the other hand were examined in 2-year-old seedlings of Pseudotsuga menziesii, Pseudotsuga macrocarpa and Cedrus atlantica. Analysing the data through A vs intercellular CO₂ molar fraction (c_i) graphs, we could determine stomatal and mesophyll contributions to changes in A as δW or soil drought were increased. Increasing soil drought affected g_s and mesophyll photosynthesis independently, since clearly distinct predawn leaf water potential (ψ_p) regions appeared in which either stomatal or mesophyll effects prevailed for explaining the changes in A. The two Pseudotsuga species exhibited a large ψ_P range (between ca -0.8 and -1.5 to -1.9 MPa) in which only stomata were responsible for the decrease in A. A dramatic decline in mesophyll photosynthesis was noticed starting from values as high as -1.2 MPa ( C. atlantica ), -1.5 MPa ( P. macrocarpa ) and -1.9 MPa ( P. menziesii ). Increasing ΔW at high soil water content led to a sharp decline in A primarily due to an alteration of mesophyll photosynthesis. Stomatal conductance for CO₂ diffusion was affected in a lesser extent and in close correlation with the changes in mesophyll photosynthesis, which could suggest the existence of a functional linkage between mesophyll photosynthesis and stomata. Surprisingly, the drought resistant P. macrocarpa exhibited the least conservative water use efficiency in response to the two types of drought. In this species drought adaptation seems to be mainly due to its high root growth and soil prospection ability.     

Nodulation and nitrogenase activity in nitrogen-fixing woody plants stimulated by CO2 enrichment of the atmosphere

The responses of three species of nitrogen-fixing trees to CO₂ enrichment of the atmosphere were investigated under nutrient-poor conditions. Seedlings of the legume, Robinia pseudoacacia L. and the actinorhizal species, Alnus glutinosa (L.) Gaertn. and Elaeagnus angustifolia L. were grown in an infertile forest soil in controlled-environment chambers with atmospheric CO₂ concentrations of 350 μl ⁻¹ (ambient) or 700 μl ⁻¹. In R. pseudoacacia and A. glutinosa , total nitrogenase (N₂ reduction) activity per plant, assayed by the acetylene reduction method, was significantly higher in elevated CO₂, because the plants were larger and had more nodule mass than did plants in ambient CO₂. The specific nitrogenase activity of the nodules, however, was not consistently or significantly affected by CO₂ enrichment. Substantial increases in plant growth occurred with CO₂ enrichment despite probable nitrogen and phosphorus deficiencies. These results support the premises that nutrient limitations will not preclude growth responses of woody plants to elevated CO₂ and that stimulation of symbiotic activity by CO₂ enrichment of the atmosphere could increase nutrient availability in infertile habitats.     

Persistence of photosynthetic components and photochemical efficiency in ears of water-stressed wheat (Triticum aestivum)

Ear photosynthesis may be an important source of C for grain growth in water-stressed plants of cereals. The main objectives of this work were to determine the stability of the photosynthetic apparatus and the photochemical efficiency of ears in plants subjected to post-anthesis drought. Plants of wheat ( Triticum aestivum L. cv. Granero INTA) were grown in pots under a rain shelter and subjected to water stress (soil water potential around −0.6 to −0.8 MPa) starting 4  days after anthesis. Post-anthesis drought substantially accelerated the loss of chlorophyll, Rubisco and the light-harvesting complex of photosystem II (LHCII) in the flag leaf, but the degradation of these photosynthetic components was much less affected by water deficit in awns and ear bracts. Quantum yield of PSII (Φ_PSII) decreased in leaves of water-stressed plants. In contrast, ear bracts had a higher Φ_PSII than leaves, and Φ_PSII of ear bracts did not decrease at all in response to drought. Removing the grains immediately before fluorescence measurements (less than 30 min) slightly reduced Φ_PSII, indicating that CO₂ supplied by grain respiration may contribute to the high photochemical efficiency of ears in droughted plants. However, other factors may be involved in maintaining high Φ_PSII, since even in the absence of grains Φ_PSII remained much higher in ear bracts than in the flag leaf. The relative stability of ear photosynthetic components and their relatively high photochemical efficiency may help to maintain ear photosynthesis during the grain filling period in droughted plants.     

Photorespiration in C4 grasses remains slow under drought conditions

The CO₂-concentrating mechanism present in C₄ plants decreases the oxygenase activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and, consequently, photorespiratory rates in air. Under drought conditions, the intercellular CO₂ concentration may decrease and cause photorespiration to increase. The C₄ grasses Paspalum dilatatum Poiret, Cynodon dactylon (L.) Pers. and Zoysia japonica Steudel were grown in soil and drought was imposed by ceasing to provide water. Net CO₂ assimilation ( A ) and stomatal conductance to water vapour decreased with leaf dehydration. Decreased carbon and increased oxygen isotope composition were also observed under drought. The response of A to CO₂ suggested that the compensation point was zero in all species irrespective of the extent of drought stress. A slight decrease of A as O₂ concentration increased above 10% provided evidence for slow photorespiratory gas exchanges. Analysis of amino acids contained in the leaves, particularly the decrease of glycine after 30 s in darkness, supported the presence of slow photorespiration rates, but these were slightly faster in Cynodon dactylon than in Paspalum dilatatum and Zoysia japonica . Although the contents of glycine and serine increased with dehydration and mechanistic modelling of C₄ photosynthesis suggested slightly increased photorespiration rates in proportion to photosynthesis, the results provide evidence that photorespiration remained slow under drought conditions.