Elevated atmospheric CO2 concentration ameliorates effects of NaCl salinity on photosynthesis and leaf structure of Aster tripolium L.

This study investigated the interaction of NaCl-salinity andelevated atmospheric CO₂ concentration on gas exchange, leafpigment composition, and leaf ultrastructure of the potentialcash crop halophyte Aster tripolium. The plants were irrigatedwith five different salinity levels (0, 25, 50, 75, 100% seawatersalinity) under ambient and elevated (520 ppm) CO₂. Under salineconditions (ambient CO₂) stomatal and mesophyll resistance increased,leading to a significant decrease in photosynthesis and wateruse efficiency (WUE) and to an increase in oxidative stress.The latter was indicated by dilations of the thylakoid membranesand an increase in superoxide dismutase (SOD) activity. Oxidativestress could be counteracted by thicker epidermal cell wallsof the leaves, a thicker cuticle, a reduced chlorophyll content,an increase in the chlorophyll a/b ratio and a transient declineof the photosynthetic efficiency. Elevated CO₂ led to a significantincrease in photosynthesis and WUE. The improved water and energysupply was used to increase the investment in mechanisms reducingwater loss and oxidative stress (thicker cell walls and cuticles,a higher chlorophyll and carotenoid content, higher SOD activity),resulting in more intact thylakoids. As these mechanisms canimprove survival under salinity, A. tripolium seems to be apromising cash crop halophyte which can help in desalinizingand reclaiming degraded land. Key words: Aster tripolium, cash crop halophyte, elevated CO₂, gas exchange, oxidative stress, photosynthesis, salt tolerance, ultrastructure, water use efficiency Received 29 July 2008; Revised 8 October 2008 Accepted 9 October 2008     

Salt tolerance in Aster tripolium L. I. The effect of salinity on growth

Abstract A study of the growth of the maritime halophyte Aster tripolium L. has been carried out over a range of salinity treatments. The regression approach to growth analysis using frequent small harvests has been used to allow ‘continuous’ measurement of growth over a period of 36 d. Salinity was applied with the major ions present in ratios typical of those found in seawater. Growth was inhibited in terms of both dry weight production and leaf expansion at salinity levels equivalent to 0.625 strength sea water (full culture solution 300) and above, with the greatest effect being seen in terms of leaf area. Aster tripolium did not show increased succulence at high salinity, leaf fresh weight to dry weight ratio in fact declined, whilst leaf fresh weight per unit area remained constant. It should be noted that the plants exhibit low growth rates due to the low light intensity used.     

Salt tolerance in Aster tripolium L. II. Ionic regulation

Abstract Measurements of tissue ion contents (Na, K and Cl) were carried out at frequent intervals on plants of Aster tripolium L. grown at a range of salinities for 36 d. Aster tripolium behaved as a typical halophyte showing high levels of inorganic ion accumulation even at low salinities. As salinity increased Na replaced K to a large extent in the shoot but root K was unaffected up to 500 mol m^?3 external NaCl. Shoot (Na + K) concentration on a tissue water basis was maintained constant in all treatments throughout the experiment, whereas shoot (Na + K) on a dry weight basis showed marked fluctuations in some treatments. An increase in (Na + K) per gram dry weight was, however, accompanied by a parallel increase in fresh weight: dry weight (FW : DW) ratio. Transport of (Na + K) to the shoot per unit root weight changed during the experiment in the manner expected, given the observed changes in shoot relative growth rate and FW : DW to result in a constant shoot (Na + K) concentration on a water basis. Chloride was the major balancing anion in the shoot at high salinity, but never accounted for more than 38% of the (Na + K) found in the root tissue. At all salinities (Na + K) salts accounted for the majority of the measured shoot sap osmotic potential. The interactions between salinity, growth, ion transport and osmotic adjustment are discussed.     

Interactive effects of NaCl salinity and elevated atmospheric CO2 concentration on growth,photosynthesis, water relations and chemical composition of the potential cash crop halophyte Aster tripolium L.

This study was aimed at obtaining detailed information about the interaction of NaCl-salinity and elevated atmospheric CO₂ concentration in the potential cash crop halophyte Aster tripolium. Plants were irrigated with 5 different salinity levels (0, 25, 50, 75 and 100% sws) under ambient and elevated (520 ppm) CO₂. Under saline conditions leaf water potential decreased to a value below the one of the nutrient solution. Osmotic adjustment was mainly due to the accumulation of sodium and chloride (includer type). However, the salt was unequally distributed within the plants. K/Na selectivity was high in the lateral roots and low in the petioles, so that these organs served as “salt filters” which prevented an excessive salt accumulation and ion toxicity in the leaf blades and in the main root, the storage organ for organic substances. Despite some signs of ion toxicity and nutrial imbalance, these factors do not seem to be predominantly responsible for the limited salinity tolerance of A. tripolium. In order to maintain a positive water balance the salt treated plants increased stomatal resistance. But at the same time stomata closure led to a significant decrease in photosynthesis and thus in WUE. The impaired assimilation rate contributed to the significant growth depression (50% reduction of the maximum yield between 50% and 75% sws), together with the higher energy consumption needed for various salinity tolerance mechanisms, e.g. for an enhanced synthesis of compatible solutes (proline, carbohydrates) and stress-induced proteins. Elevated atmospheric CO₂ concentration led to a significant increase in photosynthesis and in WUE. The latter indicates, together with a higher water potential, that the water relations of the plants had improved. By reducing stomatal resistance energy gain was maximized. The additional supply with energy-rich organic substances was not employed for producing more biomass but for increasing the investment in salinity tolerance mechanisms, e.g. for an enhanced synthesis of proline, carbohydrates and proteins. These mechanisms led to a higher survival rate under saline conditions, i.e. to an improved salt tolerance. The results of the study indicate that A. tripolium is a promising cash crop halophyte which will probably benefit from rising atmospheric CO₂ concentrations in future.     

Disentangling the photochemical salinity tolerance in Aster tripolium L.: connecting biophysical traits with changes in fatty acid composition

• A profound analysis of A. tripolium photochemical traits under salinity exposure is lacking in the literature, with very few references focusing on its fatty acid profile role in photophysiology.
• To address this, the deep photochemical processes were evaluated by Pulse Amplitude Modulated (PAM) Fluorometry coupled with a discrimination of its leaf fatty acid profile.
• Plants exposed to 125–250 mm NaCl showed higher photochemical light harvesting efficiencies and lower energy dissipation rates. under higher NaCl exposure, there is evident damage of the oxygen evolving complexes (OECs). On the other hand, Reaction Centre (RC) closure net rate and density increased, improving the energy fluxes entering the PS II, in spite of the high amounts of energy dissipated and the loss of PS II antennae connectivity. Energy dissipation was mainly achieved through the auroxanthin pathway. Total fatty acid content displayed a similar trend, being also higher under 125–250 mm NaCl with high levels of omega‐3 and omega‐6 fatty acids. The increase in oleic acid and palmitic acid allows the maintenance of the good functioning of the PS II. Also relevant was the high concentration of chloroplastic C16:1t in the individuals subjected to 125–250 mm NaCl, related with a higher electron transport activity and with the organization of the Light Harvesting Complexes (LHC) and thus reducing the activation of energy dissipation mechanisms.
• All these new insights shed some light not only on the photophysiology of this potential cash‐crop, but also highlight its important saline agriculture applications of this species as forage and potential source of essential fatty acids.

     

Salt tolerance in Aster tripolium L. III. Na and K fluxes in intact seedlings

Abstract Uptake and transport of Na and K was studied using the radioactive tracers ²²Na and ⁴²K in intact Aster tripolium L. seedlings grown at two salinities CS 10 and CS 100, (containing 10mol m^?1 and 100 mol m^?3 Na, respectively, together with other major ions in the proportions found in sea water). At both salinities a much greater proportion of the Na than K taken up by the plant was subsequently transported to the shoot. Most ⁴²K fluxes were reduced by about 40% in CS 100 plants relative to CS 10 except root accumulation which increased. Experiments involving changing the salinity from CS 10 to CS 100 showed that ⁴²K fluxes remained constant for at least 40 h, indicating that competition with Na for uptake sites was not the cause of the reduced flux in CS 100 plants. ²²Na fluxes responded immediately to a change in salinity with all fluxes increasing six-fold when the salinity was raised. When the salinity was lowered, however, root accumulation returned to the level in CS 10 control plants whereas transport to the shoot was inhibited by the previous high salinity treatment, being reduced to only 35% of the rate in CS 10 plants. The time courses of osmotic adjustment and Na accumulation following an increase in salinity were found to be very similar, with sufficient Na being accumulated to account for the observed increase in sap osmotic pressure.     

A Study on Polymorphism in Aster tripolium L. (Sea Aster)

Abstract: Genotypic and environmental variation in Aster tripolium L. was studied in common garden experiments and in transplantation experiments in different saltmarsh sites along the estuarine gradient of the Westerschelde and along the elevation gradient of individual saltmarshes. Analysis of the variation in a number of morphologic characters of the inflorescences indicated that this variation is both environmentally as well as genetically controlled. Morphologic differences between an Aster tripolium population of a brackish marsh and a number of populations of a saline marsh were unchanged whether the plants were growing in their original habitat or in a transplant habitat. The population from the brackish marsh had smaller inflorescences with ray florets and a predominantly biennial life cycle, in contrast with the populations of the saline marsh, which had larger inflorescences usually without or with a few ray florets and showed a tendency to a perennial life cycle. We concluded that the variation between the populations of the saline and the brackish marsh is mainly genetically controlled, while the variation within the saline marsh is mainly environmentally controlled.     

Responses of the stomata of Aster tripolium to calcium and sodium ions in relation to salinity tolerance

In previous work, the stomata of the maritime halophyte Astertripolium L. were shown to close when NaCl concentrations risein the vicinity of the guard cells. Further studies have nowrevealed important effects of calcium on the ionic responsesof the stomata. When the guard cells were presented with KCl,Ca²⁺ suppressed opening in a manner similar to that which hasbecome familiar in other species such as Commelina communisL. However, in the presence of NaCl, Ca²⁺ had the opposite effect,reducing the closing response to NaCl. This pattern of behaviouris discussed in relation to known salt effects on membranes,but the underlying physiological basis remains obscure. A previous study led to the hypothesis that the closing responseof the stomata to Na⁺ ions may make an important contributionto the salinity tolerance of this species. Here we report thatincreasing supplies of Ca²⁺ ions reduce the effect of salinityon stomatal conductance in the whole plant as well as in theisolated epidermis. This finding is consistent with the wellestablished role of calcium in increasing resistance to salinity:in the presence of high calcium the plant can tolerate a greatersalt intake, and hence there is a reduced need for transpirationto be restricted by partial stomatal closure. Key words: Sodium, calcium, Aster tripolium, stomata, salinity tolerance     

Growth and protein profile responses in the halophyte sea aster (Aster tripolium L.) suspension-cultured cells to salinity

We established salt tolerant cell lines, which survived and grew under high salinity conditions with 150 mM (S-150) and 300 mM (S-300) NaCl, to study the effects of salt stress on the proliferation and protein profile of these cells in the halophyte sea aster,Aster tripolium L. These salt-adapted cell lines were produced from leaves and selected by repeated suspension subculture in media containing NaCl every 25 days for five cycles. S-150 cells displayed no inhibition in their growth compared to control cells maintained under non-stressed conditions. S-150 cells exhibited approximately a 15-fold increase in both fresh and dry weight during the 25 days under saline conditions. S-300 cells showed positive growth under severe salt stress, but their dry matter gain was significantly less than that of the S-150 cells, with only a 2.5-fold increase in dry weight. We also detected changes in the protein profile of salt-adapted cells with two specifically induced polypeptides (basic 58.4 and acidic 24.8 kDa) and one enhanced polypeptide (basic 15.1 kDa) in the soluble fraction, and one specifically induced polypeptide (42.0 kDa) in the insoluble fraction.     

Population Variability of Aster tripolium L. of Zhoushan Islands as Related to Habitat

In the identification of the specimens collected from Zhoushan (Chushan) Islands of Zhejiang (Chekiang) Province, the authors have found that the species Aster tripolium L. (Tripolium vulgate Nees) of coastal salt marshes has a rather high amplitude of morphological variation. We, then, made a field investigation in Zhoushan Islands in October of 1980. A mass collection of five different populations of the same species of different ecological sites was carried out. Quantitative measurements of totally eight morphological characters, namely, the height of individual plants, the length and breadth of leaves and ligulate flowers, the number of lower branches, the degree of promineney of tap-roots, and the color of the base of stems, were made. The resulting data of these measurements and calculations were then compared by using methods of E. Anderson's pietorialized scatter diagram and standard deviation in order to bring to light the differentiation pattern of that species occurred in response to different habitats. Samples of soil of different population sites were also collected; their pH values and contents of NaCl were examined. According to the variation pattern of phenotypes as shown in these diagrams and figures, the populations of Aster tripolium of Zhoushan Islands could be roughly divided into the following two types: (1) the tall and long-leaved type of lower marshes with a soil salinity of 0.54%–0.56% of NaCl, including populations Nos. 1 and 2, with plants 49–51 cm. high (mean value), very few or no lower branches, usually dark purplish at the base of stems, and often without prominent tap-roots; (2) the low and short-leaved type of higher marshes and saline seepage areas above the limits of tidal submergence, with a soil salinity of 0.06%–0.17% of NaC1, including populations Nos. 3 and 4, with plants 19–26 cm. high (mean value), a few to manylower branches, and usually greyish white or slightly purplish at the base of stems. It is worth of note that these two different types of phenotypic variation of populations are on the whole corresponding to the two habitat types, which differ from each other chiefly in soil salinity resulted from the different levels of the sites, particularly the distance between the population sites and the salterns, and in the density of plants as well, which could probably be related to the intensity of competition. It is also interesting to note that owing to the site of population No. 5 being subjected to trampling, grazing and moving, the plants are usually mechanically stripped of their terminal buds. This is probably why the members of that population often show a peculiar pseudo- prostrate habit with their lower branches sometimes spreading at right angles to the main stem. However, the pattern of the population differinces obtained is often predetermined by the pattern of sampling. The real pattern of population differentiation of Aster tripolium is more likely clinal instead of abrupt. Without comparative experimental cultivation, it is difficult to decide whether the phenotypic variations mentioned above are genetically fixed or simply environmental modifications. Nevertheless, A. J, Gray et al. (1979) has shown that the large differences exist between populations of Aster tripolium at the different levels on the British salt marshes appear in uniform cultivation and may have a large genetic component. So, it might be expected that both of the factors-genetical and environmental, are likely to play their parts in the population differentiation of the said species. According to the classification of population systems given by V. Grant (1963), the two different types of populations of Aster tripolium of Zhoushan Islands may be considered as two ecological races.     

Aster tripolium L. and Sesuvium portulacastrum L.: two halophytes, two strategies to survive in saline habitats.

Aster tripolium L. (Dollart, Germany) and Sesuvium portulacastrum L. (Dakhla, Morocco) are potential halophytic vegetables, fodder plants, and ornamentals for re-vegetating saline land. To compare their strategies involved in salt tolerance both plants were grown with 0%, 1.5%, and 3% (Aster) or 0%, 2.5%, and 5% (Sesuvium) NaCl in the watering solution. The growth rate was reduced in both species with increasing NaCl concentrations. The quotient of Na(+)/K(+) indicates that Aster accumulates more K(+) in comparison to Na(+) while the reverse is true for Sesuvium. Osmolality of the leaf sap increased with increasing NaCl concentration in both Aster and Sesuvium. Transpiration rate was severely reduced in both Aster (3%) and Sesuvium (5%) plants after 10 d of NaCl watering. The CO(2) assimilation rate decreased in Aster (3%) and Sesuvium (5%) NaCl-treated plants from day 5 to day 10. The most important results from chlorophyll fluorescence measurements were derived from the non-photochemical quenching analysis (NPQ). First, both plants had linearly increasing levels of NPQ with increasing NaCl concentrations. Second, Sesuvium had almost half the NPQ value when compared to Aster under increased soil salinity. In Aster P-ATPase activities were decreased in plants treated with 3% NaCl after three days of treatment, F-ATPase activities increased with increasing NaCl concentrations and no clear changes were measured in V-ATPase activities. In Sesuvium any changes could be observed in the three ATPase activities determined. To conclude, Aster and Sesuvium use different strategies in adaptation to soil salinity.     

Field studies on the variability of populations of Aster tripolium L. in relation to saltmarsh zonation

1. Aster tripolium is a very variable species of which a number of types have been described both on a morphological and ecological level.
2. In permanent plots along the height gradient in the salt marsh it appeared that differences in the A. tripolium subpopulations occur.
3. In the lowest zone of the marsh, dominated by Spartina anglica (Spartinion), the mean life expectancy of individual adult plants is relatively high but it is relatively low for seedlings.
4. In the higher zones of the marsh the mean life expectancy is relatively lower, but for the seedlings it is higher (Puccinellion maritimae).
5. In the Spartina-zone individual A. tripolium plants have about twice the number of shoots as the plants from the other zones.
6. The plants from the Spartina zone produce per individual more generative shoots (absolute) but these have less capitula than elsewhere in the marsh. The number of ripe seeds per head is almost constant everywhere in the marsh.

     

Stomatal responses to sodium ions in Aster tripolium: a new hypothesis to explain salinity regulation in above-ground tissues

A study has been made of the ionic relations of stomata of Aster tripolium L., a maritime halophyte which colonizes coastal saltmarshes. The results obtained allow us to add this species to the growing list for which an involvement of K⁺ transport in stomatal movements has been demonstrated. However, an additional and ecologically important characteristic was found: there was a suppression of stomatal opening by increasing NaCl concentrations. A new hypothesis is offered of the mechanism for controlling salt and water relations in A. tripolium, a species which does not possess glands or other means of excreting salt. It is suggested that when the capacity of the tissues to accumulate salt in cell vacuoles is exceeded, the concentration of Na⁺ ions in the apoplast around the guard cells begins to rise. This causes partial stomatal closure, reduces transpiration and increases water-use-efficiency. Therefore, the flow of salt into the leaves is reduced but growth (and the manufacture of the new photosynthates required to support it) can continue. Aster tripolium can be added to the small list of known species which readily yield isolated epidermis suitable for detailed stomatal studies. Throughout this study, we have compared its stomatal physiology with C. communis, which has been thoroughly investigated in the past.     

Nitrate accumulation and growth of Aster tripolium L. with a continuous and intermittent nitrogen supply*

Abstract The ‘tidal salt marsh’ ecotype of the halophyte Aster tripolium L. was grown in a nutrient solution with either a continuous or an intermittent NO₃^? supply with either Cl^? or SO₄^2? as the alternative anion. With increasing periods of NO₃^? supply per week the rate of the dry weight increment increased. When NO₃^? was supplied for longer than 48 h per week, the dry weight and the organic-N content in the shoots hardly increased, whereas the NO₃^? content in shoots and roots increased further. With alternated supply of a nutrient solution containing NO₃^? with one containing Cl^?, the internal NO₃^? content in the shoot was lower than in shoots grown in solutions in which NO₃^? alternated with SO₄^2?. It is concluded, that NO₃^? does not have a specific function in osmoregulation.     

开放式空气CO2浓度升高与作物/杂草的竞争关系

CO2浓度升高对植物的光合作用、呼吸作用和水分利用等生理过程产生直接影响,进而影响植物的生长繁殖,CO2浓度升高对于具有C3光合途径的植物较具C4光合途径的植物更为有益,由于许多重要的杂草是C4植物,而许多重要的作用是C3植物,CO2浓度升高对杂草/作物的相互关系将有重要影响,本文就全球CO2浓度升高和气候变化对杂草/作物之间竞争关系影响进行综述,同时针对目前研究现状和可持续农业的需要,提出CO2学浓度升高条件下杂草/作物之间竞争关系及未来农田杂草治理方面理论与实践中有待解决的问题。     

Salt induced responses of chloroplast activities in species of differing salt tolerance. Photosynthetic electron transport in Aster tripolium and Pisum sativum

A comparative study was made of the effects of high concentrations of NaCl, KCl and MgCl₂ on two electron transport reactions of thylakoids isolated from a mesophyte, Pisum sativum and a halophyte, Aster tripolium . The rate of photosystem I mediated electron transport from reduced N, N, N', N'-tetramethyl- p -phenylenediamine (TMPD) to methyl viologen was determined polarographically, and photosystem II mediated electron flow from water to 2,6-dichlorophenolindophenol (DCPIP) was monitored spectrophotometrically. The response of photosystem II to increasing in vitro salt concentrations was similar for thylakoids isolated from both A. tripolium and P. sativum , but differences in the response of photosystem I to salinity changes were observed for the two species. Increasing NaCl, KCl and MgCl₂ concentrations produced similar patterns of response of photosystem I activity in P. sativum thylakoids, whilst for A. tripolium KCl induced a completely different response pattern compared to NaCl and MgCl₂. The salinity of the culture medium in which A. tripolium was grown also had an effect on both the absolute in vitro activities of photosystems I and II and their response to changes in salt concentration of the reaction media.     

Elevated atmospheric CO(2) concentration and diurnal cycle induce changes in lipid composition in Arabidopsis thaliana

Few studies regarding the effects of elevated atmospheric CO(2) concentrations on plant lipid metabolism have been carried out. Here, the effects of elevated CO(2) concentration on lipid composition in mature seeds and in leaves during the diurnal cycle of Arabidopsis thaliana were investigated. Plants were grown in controlled climate chambers at elevated (800 ppm) and ambient CO(2) concentrations. Lipids were extracted and characterized using thin layer chromatography (TLC) and gas liquid chromatography. The fatty acid profile of total leaf lipids showed large diurnal variations. However, the elevated CO(2) concentration did not induce any significant differences in the diurnal pattern compared with the ambient concentration. The major chloroplast lipids monogalactosyldiacylglycerol (MGDG) and phosphatidylglycerol (PG) were decreased at elevated CO(2) in favour of phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Elevated CO(2) produced a 25% lower ratio of 16:1trans to 16:0 in PG compared with the ambient concentration. With good nutrient supply, growth at elevated CO(2) did not significantly affect single seed weight, total seed mass, oil yield per seed, or the fatty acid profile of the seeds. This study has shown that elevated CO(2) induced changes in leaf lipid composition in A. thaliana, whereas seed lipids were unaffected.     

Elevated atmospheric partial pressure of CO2 and plant growth

Cotton plants were grown in late spring under full sunlight in glasshouses containing normal ambient partial pressure of CO₂ (32±2Pa) and enriched partial pressure of CO₂ (64±1.5Pa) and at four levels of nitrogen nutrition. Thirty-five days after planting, the total dry weights of high CO₂-grown plants were 2- to 3.5-fold greater than plants grown in normal ambient CO₂ partial pressure. Depending on nitrogen nutrition level, non-structural carbohydrate content (mainly starch) in the leaves of plants grown in normal CO₂ was between 4 and 37% of the total leaf dry weight compared to 39 to 52% in the leaves of high CO₂-grown plants. Specific leaf weight calculated using total dry weight was 1.6- to 2-fold greater than that based on structural dry weight. In high CO₂-grown plants the amount of non-structural carbohydrate translocated from the leaves at night was between 10 and 20% of the level at the end of the photoperiod. This suggests that the plant was unable to utilize all the carbohydrate it assimilated in elevated CO₂ atmosphere. While there was a 1.5-fold enhancement in the rate of CO₂ assimilation in plants grown in 64 Pa CO₂, there was, however, some evidence to suggest that the activities of other metabolic pathways in the plants were not stimulated to the same extent by the enriched CO₂ atmosphere. This resulted in massive accumulation of non-structural carbohydrate, particularly at low level of nitrogen nutrition.Abbreviations A rate of CO₂ assimilation - PPFD photosynthetic photo flux density - NAR net assimilation rate - pCO₂ partial pressure of CO₂ - RGR relative growth rate     

Elevated atmospheric partial pressure of CO2 and plant growth

Summary Cotton and maize plants were grown under full sunlight in glass houses containing normal ambient partial pressure of CO₂ (330±20 bar) and enriched partial pressure of CO₂ (640 ±15 bar) with four levels of nitrogen nutrient. In 40 day old cotton plants grown in high CO₂, there was a 2-fold increase in day weight and a 1.6-fold increase in leaf area compared with plants grown in ambient CO₂. In 30 day old maize plants there was only 20% increase in dry weight in plants grown in 640 bar CO₂ compared with plants grown in 330 bar and no significant increase in leaf area. In both species, at both CO₂ treatments, dry weight and leaf area decreased in similar proportion with decreased nitrogen nutrient.The increase of leaf area in cotton plants at high CO₂ caused a reduction of total nitrogen on a dry weight basis. In cotton assimilation rate increased 1.5 fold when plants were grown with high nitrogen and high CO₂. The increase was less at lower levels of nitrate nutrient. There was a 1.2 fold increase in assimilation rate in maize grown at high CO₂ with high nitrate nutrient.Cotton and maize grown in high CO₂ had a lower assimilation rate in ambient CO₂ compared to plants grown in normal ambient air. This difference was due to the reduction in RuBP carboxylase activity. Water use efficiency was doubled in both cotton and maize plants grown at high CO₂ in all nutrient treatments. However, this increase in water use efficiency was due primarily to reduced transpiration in some treatments and to increased assimilation in others. These data show that plant responses to elevated atmospheric partial pressure of CO₂ depend on complex of partially compensatory processes which are not readily predictable.