Carbon Dioxide Exchange Rates Research Articles

Carbon Dioxide Exchange Rate and Chlorophyll Content of Turfgrasses Treated with Flurprimidol or Mefluidide

Plant growth regulators (PGRs) are being used increasingly for high maintenance turf production. A greenhouse and growth chamber study was conducted to determine the effect of two PGRs, mefluidide and flurprimidol, on the carbon dioxide exchange rate, chlorophyll content, and specific leaf weight of annual bluegrass and creeping bentgrass. Rate-response and time course studies were conducted. In the rate-response study, increasing flurprimidol rates caused a linear decrease in the carbon dioxide exchange rate (CER). Mefluidide had no effect on the CER in the rate-response experiment but did cause a significant drop in the CER at 4 and 8 days after treatment (DAT) in the time course study. Both PGRs increased chlorophyll content at 16 DAT in the time course study. In the rate-response study, chlorophyll content displayed a quadratic response to an increasing mefluidide rate. The 0.28 kg ha−1 rate of mefluidide increased the chlorophyll content to 46 μg cm−2 from 21.5 μg cm−2 for control plants. Both PGRs increased specific leaf weight, although for flurprimidol the effect was significant only at the 0.1 level of probability. PGRs decreased the CER and increased the chlorophyll content and specific leaf weight for both species tested. The physiologic effects measured were short lived, and so the physiologic significance of these changes is difficult to determine without further research.

Physiological and morphological responses of red alder and sitka alder to flooding

Red alder (Alnus rubra Bong.) and sitka alder (A. viridis ssp. sinuata [Regel] Löve & Löve) are nitrogen‐fixing woody species that grow sympatrically along the Pacific coast of North America. Red alder is found in poorly drained lowlands, as well as in soils of moist upland slopes, whereas sitka alder generally colonizes well‐drained soils. To identify factors that contribute to flood tolerance, we conducted greenhouse experiments subjecting both species to a 20‐day flood and 10‐day recovery and red alder to a 50‐day flood and 20‐day recovery. We determined the effect of this stress on nitrogenase activity, root and nodule alcohol dehydrogenase (ADH) activity, lenticel and adventitious root development, relative growth rate (RGR), and leaf gas exchange. After 24 h of flooding, nitrogenase activity could not be detected in either species. Limited nitrogenase activity did return in red alder at the end of a 10‐day recovery following the 20‐day flood, but sitka alder showed no recovery of nitrogenase activity. After 50 days of continuous flooding, red alder nitrogenase activity returned to pretreatment levels. Red alder root and nodule ADH activity was more than twice that of sitka alder under flooded conditions. Sitka alder showed extensive root mortality and leaf abscission over the same 20‐day flooding period. Flooded red alder exhibited an initial decline in root RGR, but recovered between days 10 and 20 with the formation of adventitious roots. Furthermore, initiation of adventitious roots in red alder coincided with an increase in stomatal conductance without a similar recovery of carbon dioxide exchange rate. Sitka alder formed few adventitious roots, lost much of its root and leaf biomass, and showed no restoration of growth during flooding or recovery. Different responses of red and sitka alder to flooding serve as a partial explanation for the different patterns of distribution of these species and suggest some adaptations of red alder that permit flood tolerance.

Seasonal variation of carbon dioxide exchange rates above and below a boreal jack pine forest

Fluxes of carbon dioxide were measured above and below a boreal jack pine stand with the eddy covariance method. Our goal was to examine the seasonal variation of the net carbon dioxide exchange between the ecosystem and the atmosphere and to understand the relative contributions of the vegetation and rhizosphere to these fluxes. Over the course of the growing season the jack pine stand was a small, net sink of carbon dioxide—47 g C m −2 were assimilated during a 115 day period between spring (day 144) and autumn (day 259). On a daily basis, the magnitude of the net uptake of CO 2 by the forest was relatively small, less than 4 g C m −1 day −1. We also observed many days when carbon dioxide was lost from the forest stand (up to 2 g C m −2 day −1). Day to day sums of net carbon dioxide exchange exhibited little seasonality, despite the seasonality experienced by controlling environmental and biological variables. The aseasonality of net canopy CO 2 exchange occurred because temporal variations associated with canopy photosynthesis and plant and microbial respiration offset one another over the growing season. Canopy photosynthesis rates of the jack pine stand were evaluated using CO 2 flux measurements made above and below the canopy. Daily totals of canopy photosynthesis were a linear function of available solar energy and exhibited a quadratic dependence on temperature, with optimum photosynthesis occurring between 15 and 25°C. We also observed that the net daily uptake of carbon dioxide was independent of soil moisture in the upper 0.15 m of the soil profile, despite the wide range of soil moisture that was experienced. Finally, the net gain of carbon by a jack pine canopy came with a relatively large respiratory expense. On average, 68% of the carbon gained by photosynthesis was consumed by plant respiration, over the course of a day.

Alleviation of salt stress by arbuscular‐mycorrhizal Glomus species in Lactuca sativa plants

Improved salt tolerance of mycorrhizal plants is commonly attributed to their better mineral nutrition, particularly phosphorus. However, the effect of arbuscular‐mycorrhizal (AM) fungi on salt tolerance may not be limited to this mechanism. We investigated the possibility that non‐nutritional effects of AM fungi, based on proline accumulation or increased photosynthesis and related parameters, can influence the tolerance of lettuce (Lactuca sativa L.) to salinity. Three levels of salt (3, 4 and 5 g NaCl kg‐1 dry soil) were applied and plants were maintained under these conditions for 7 weeks. The salt‐treated AM plants produced greater root and shoot dry weights than unfertilized or P‐fertilized non‐AM controls. With increasing salinity, both shoot and root dry weights were reduced, but this decrease was greater in uninoculated plants. In particular, shoot dry weight was not reduced in G. fasciculatum‐colonized plants as a consequence of salt, whereas in uninoculated plants it was reduced by about 35% at the highest salt level. Proline accumulation was considerably lower for P‐amended non‐AM and for AM plants except for G. mosseae‐colonized plants than was the case for unamended plants. Transpiration, carbon dioxide exchange rate (CER), stomatal conductance and water use efficiency (WUE) were higher in mycorrhizal plants. At 5 g NaCl kg‐1, both photosynthesis and WUE increased by more than 100% in mycorrhizal treatment relative to uninoculated plants. The contents of phosphorus of P‐fertilized non‐AM plants was similar to or higher than those of G. mosseae‐ and G. fasciculatum‐colonized plants. Plants colonized by G. deserticola had the highest P‐content regardless of salt level. Hence, the effect of G. mosseae and G. fasciculatum on salt tolerance in this experiment could not be attributed to a difference in the P content. The mechanisms by which these two fungi alleviated salt stress appeared to be based on physiological processes (increased CER, transpiration, stomatal conductance and WUE) rather than on nutrient uptake (N or P).

An experimental set-up to study carbon, water, and nitrate uptake rates by hydroponically grown plants.

The experimental system described allows concomitant hourly measurements of CO2, H2O, and NO3 uptake rates by plants grown hydroponically in a greenhouse. Plants are enclosed in an airtight chamber through which air flows at a controlled speed. Carbon dioxide exchange and transpiration rates are determined from respective differences of concentrations of CO2 and water vapor of the air at the system inlet and outlet. This set-up is based on the "open-system" principle with improvements made on existing systems. For instance, propeller anemometers are used to monitor air flow rates in the chamber. From their signal it is possible to continuously adjust air speed to changing environmental conditions and plant activity. The air temperature inside the system therefore never rises above that outside. Water and NO3 uptake rates are calculated at time intervals from changes in the volume and the NO3 concentration of the nutrient solution in contact with the roots. The precise measurement of the volume of solution is achieved using a balance which has a higher precision than any liquid level sensors. Nitrate concentration is determined in the laboratory from aliquots of solution sampled at time intervals. A number of test runs are reported which validate the measurements and confirm undisturbed conditions within the system. Results of typical diurnal changes in CO2, H2O, and NO3 uptake rates by fruiting tomato plants are also presented.

Role of Calyx Lobes in Gas Exchange and Development of Persimmon Fruit

The role of calyx lobes in CO2 exchange in persimmon (Diospyros kaki Thunb.) fruit was investigated during fruit growth and development. Carbon dioxide exchange rates of attached and detached fruit were measured in light and dark conditions in the field after calyx lobes were removed. Calyx lobes were removed at fruit growth stages I and III, which were defined as the period when fruit diameter increases >0.3 mm·d-1 (Zheng et al., 1990). Removing calyx lobes at stage I significantly inhibited fruit growth, while removing them at stage III had no effect on growth. Two weeks after calyx lobes were removed at stage I, CO2 exchange decreased 80% in light and dark conditions compared with the control fruit. The rapid decreases of CO2 exchange rate by calyx lobe removal at stage I were obvious if expressed per fruit or on a fresh weight basis. In contrast, treatment at stage III had no effect on CO2 exchange rate of fruit and no effect on fruit growth. However, when the calyx lobe scars were sealed with Vaseline soon after calyx lobe removal at stage III, an immediate decline in CO2 exchange rate in the dark occurred with simultaneous inhibition of the final swell in fruit growth. A possible relationship between fruit growth potential and gas-exchange capacity is discussed.

Net Carbon Dioxide Exchange Rates of Raspberries at Varying Irradiances, Carbon Dioxide Concentrations, and Air and Soil Temperatures

The influence of irradiance, CO2, and temperature on whole-plant net C exchange rate (NCER) of micropropagated raspberries (Rubus idaeus L. cv. `Heritage') was examined in 1994. Irradiances >1000 μmol–m–2–s–1 PAR were required for light saturation, and net photosynthesis (Pn) greatly increased under CO2 enrichment (up to 2000 μl–liter–1) and was optimum at 17C. Temperature effects were separated in another experiment using varying air and soil temperatures (15, 20, 25, 30, and 35C) under saturated light and ambient CO2 levels (350 μl–liter–1). Both air and soil temperature influenced net Pn, with maximum rates occurring at an air/soil temperature of 17/25C and each contributing 71.2% and 26.7%, respectively, to the total variation explained by a polynomial model (R2 = 0.96). Dark respiration and root respiration rates also increased significantly with elevated air and soil temperatures. Therefore, results from this study indicate that maximum net Pn occurred at an air/soil temperature of 17/25C and that irradiance, CO2 levels, and shoot and root temperatures are all important factors in examining NCER in raspberries.

Diurnal Carbon Dioxide Exchange Rates of Greenhouse Roses under Artificial Light as Compared with Daylight Conditions in Summer

Abstract Carbon dioxide exchange rates (CER) of greenhouse roses (cut flowers) were measured under daylight conditions in a greenhouse in July, and under artificial light only (300 μmol m−2 s−1 PPFD in 18 h day−1) at two CO2 concentrations (350 and 700 μmol mol−1). The daily CER varied considerably from day to day owing to the large variation in solar radiation. Light saturation of CER seemed not to be reached even on clear days, and a light dose (PAP = number of photosynthetic active photons) produced by variable light over one week in summer gave the same total CER as a similar PAP produced by a constant PPFD. CER at constant PPFD increased rapidly during the first two hours of the photoperiod, followed by a slight increase during the subsequent hours, before CER slightly decreased towards the end of the photoperiod. Raising the CO2 concentration significantly increased CER during the entire photoperiod, and by 32% as a mean for the whole photoperiod. Elevated CO2 decreased the night respiration of the ...

Seasonal response of groundnut genotypes to arbuscular mycorrhiza — Bradyrhizobium inoculation

Nine cultivars of groundnut ( Arachis hypogaea L.) earlier shown to respond to arbuscular mycorrhiza (AM) in a soil poor in available N or P were studied in the same soil over three subsequent seasons for their response to AM (M) and Bradyrhizobium (R) singly or in combination (M + R). The responses were measured for six traits, namely, the percentage of root length colonized by AM (AMC), Root P, Biomass (BM), carbon dioxide exchange rate (CER), leaf nitrogen and pod yield. Differences in four treatments, namely control, M alone, R alone and (M + R) were statistically evaluated for those traits across the cultivars and seasons. High improvements over control were consistently recorded under (M + R) in experimental plots of 75 m 2, particularly for yield though responses were generally genotype and season-dependent. A basis for explaining improved yield sustenance by dual inoculation with arbuscular mycorrhiza and Bradyrhizobium was provided by the significant correlation between AMC and root P and of yield with them in addition to the desirable correlations between CER, BM and Root P. The results provide ample reason for integrating AM schedules into breeding and agronomic approaches in programmes on stabilizing groundnut yields.

Photosynthetic parameters in glyphosate‐treated sugarbeet (Beta vulgaris L.)

SummaryVariable fluorescence and carbon dioxide exchange rate (CER) were measured in intact leaves of glyphosate‐treated sugarbeet plants, to investigate rapid effects on photosynthesis. In one experiment, a significant effect on variable fluorescence and CER was found within 8 h after herbicide application. The effect on stomatal conductance was not significant. In a second experiment, the effects of light or dark conditions during herbicide application and 24 h later were evaluated. Under light conditions glyphosate significantly reduced variable fluorescence after 4 h. However under initial conditions of 24 h darkness, followed by alternating 8 h light/8 h dark, the effect was not significant until 44 h after treatment, by which time the plants had received several hours of light. The eflect on variable fluorescence 8 h after herbidde application correlated positively with plant survival 7 days later.

Response of wheat cultivars to different soil nitrogen and moisture regimes: III. Leaf water content, conductance, and photosynthesis1

Abstract Two greenhouse studies were conducted with contrasting patterns of soil water availability and nitrogen (N) supply to extend our knowledge of the combined effect of these environmental factors on cultivar response of wheat (Triticum aestivum L.). Several parameters of water relations and photosynthesis were measured. Results showed that at the beginning of stem elongation and before imposing water stress, the low N treatment decreased leaf conductance and carbon dioxide (CO2) exchange rate (CER). This effect was more significant in the case of the cultivar Nesma than the other cultivars. Leaf conductance was a much better indicator of CER than chlorophyll content. Overall, water stress decreased leaf conductance. Under mild stress, Nesma and Merchouch 8 behaved similarly, but Nesma was more sensitive to water stress. Increased N decreased CER under water stress but tended to have no effect on chlorophyll concentration. Under well watered conditions, N application helped the plants maintain greate...

Photosynthetic and Respiratory Characteristics of Peanut Cultivars Adapted to Varying Night Temperatures1

Abstract Relatively mild night temperatures can reduce leaf carbon dioxide exchange rates (CER) and dry matter (DM) accumulation in peanut (Arachis hypogaea L.). To investigate differences among cultivars in response to long-term exposure to a range of night temperatures, three peanut cultivars (OAC Ruby, Chico, and Early Bunch) with known differences in chilling sensitivity were grown in controlled-environment cabinets at the University of Guelph, Ontario. Effects of long-term exposure to night temperatures from 9 to 20 C were assessed in terms of leaflet and whole plant CER, DM accumulation, and phenological development. Effects of night temperature on rate of phenological development and DM accumulation were consistent with differences in accumulation of degree-days. Cultivars did not differ in daytime leaf CER response to all night temperatures except 9 C, at which CER for OAC Ruby was higher than for Early Bunch or Chico. CER in the 9 C treatment was 92% of the CER at 20 C for OAC Ruby and 80% for Early Bunch and Chico. Continuous exposure to night temperatures of 10C reduced CER sensitivity to low daytime temperature in OAC Ruby, but Early Bunch was unaffected. Specific respiration rates were higher for plants of OAC Ruby than Early Bunch in the 10 C treatment, with indications that these differences were due to increased maintenance requirements. The ability of OAC Ruby to adapt to cool-night conditions may have a significant impact on crop performance in cooler environments.

Crop Net Carbon Dioxide Exchange Rate and Radiation Use Efficiency in Soybean

AbstractPlant growth is directly related to the fraction of the C fixed by photosynthesis that is converted into phytomass. We report here the results of a study aiming at the estimation of the net CO2 exchange rates (CNCER) of a soybean crop over the growing season. Hourly values of CNCER were estimated based on eddy correlation measurements of CO2 fluxes above the canopy, on chamber measurements of CO2 fluxes at the soil surface, and on the assumption that root respiration can be computed as a fraction of plant daily photosynthesis. Mean daytime CNCER varied between 0.70 and 0.83 mg m−2s−1 for a leaf area index (LAI) greater than 1, with a maximum hourly value of 1.48 mg m−2s−1. Absolute mean nighttime CNCER values were usually <20% of daytime values. Hourly CNCER varied nonlinearly with intercepted photosynthetically active radiation (IPAR). The accuracy of the CNCER estimates was evaluated by comparing the radiation use efficiency of the soybean crop calculated using CNCER estimates with radiation use efficiency calculated using phytomass accumulation. Agreement between the two approaches was within 10% for LAI>2, but calculated CNCER overestimated radiation use efficiency based on phytomass accumulation for smaller values of LAI.

Effect of maleic hydrazide on photosynthesis, carbohydrate metabolism, and senescence of burley tobacco (Nicotiana tabacum L.)

Maleic hydrazide (MH), a systematic agent used for control of axillary growth (suckers) in tobacco production, has been shown to alter cured-leaf quality and increase yield compared to manual sucker control. The objective of this study was to determine how MH, applied at rates ranging from zero (manual sucker control) to four times the usual commercial rate of 3.36 kg ha−1, influenced the carbon dioxide exchange rate (CER), carbohydrate metabolism, and senescence of upper-canopy leaves. Leaves treated with various MH rates had residue levels ranging from zero to over 600 μg g−1. Leaf area was decreased and specific leaf weight was increased by increasing MH rates. Oven-dried leaf weight, however, was similar for all treatments. CER was markedly decreased 2 weeks after applying MH rates above 1.68 kg ha−1. Chlorophyll concentration was also decreased by MH application, but not as rapidly or to the same extent as observed for CER. Sucrose and starch levels were significantly increased within 2 weeks after applying MH. As with CER, MH effects on sucrose and starch accumulation were maximum at 3.36 kg ha−1 MH with no further change at higher rates. Activities of sucrose-phosphate synthase (SPS) and adenosine diphosphate glucose (ADPG) pyrophosphorylase were not affected by any MH treatment. The results indicated that the increased sucrose and starch accumulation was due to decreased translocation of assimilate and not to increased photosynthesis. Because MH did not markedly alter total plant dry matter accumulation, MH-induced decreases in CER and chlorophyll were possibly compensated for by leaves lower in the canopy. This conclusion was supported by the lack of effect of MH on starch and sucrose accumulation and chlorophyll concentration of leaves in the middle of the canopy. While MH did inhibit photosynthesis of upper-canopy leaves, the results indicated that, in general, plant senescence was not markedly affected.

Effect of dinitroaniline herbicides on rhizobia, nodulation and N2(C2H2) fixation of four groundnut cultivars

SummaryField, greenhouse and laboratory investigations were conducted to determine the effect of four dinitroaniline herbicides on rhizobia, nodulation and nitrogen fixation of four groundnut cultivars. Benefin, dinitramine and nitralin used at recommended levels decreased nodule dry weight, nitrogenase activity and total nitrogen of groundnut tops and pod yield in three cultivars Kadiri 71‐1, Kadiri‐2, ICGS‐11 and not for a fourth cultivar, Kadiri‐3 of groundnut (Arachis hypogaea L.), but fluchloralin used at the recommended level increased the nodulation rate, nitrogenase activity and total nitrogen of groundnut tops and pod yield compared to untreated plants. Studies were conducted in vitro to determine the relative toxicity of the herbicides on four Rhizobium strains isolated from the nodules of four cultivars of groundnut. It was found that various strains of rhizobia differ in their sensitivity to different rates of the herbicides tested. Carbon dioxide exchange rate (CER) of all the cultivars which received herbicide treatment was measured at different time intervals to determine the relationship between photosynthesis and inhibition of nodulation. The lack of adverse effect on the CER of four cultivars when treated at recommended concentrations indicated that nitrogen fixation was affected in cultivars Kadiri 71‐1, Kadiri‐2 and ICGS‐11 due to inhibition of nodulation.

Variations in Leaf Gas Exchange Traits of Saccharum Including Feral Sugarcane, Saccharum spontaneum L.

The effects of photosynthetic photon flux density (PPFD) and temperature on leaf gas exchange rates of ten lines of Saccharum spontaneum L. collected from the tropics to temperate regions in North-West Pacific zones, S. sinense Roxb. cv. Yomitanzan, native sugarcane in South-West Islands, and Saccharum hybrid cv. NCo310 were investigated. The relationship of gas exchange traits and rates was examined. Significant line- and species -differences (P<0.05) were found in the carbon dioxide exchange rate (CER) at high PPFD (1700 μM photon m-2s-1), leaf stomatal conductance at low PPFD (180 μM photon m-2s-1), contents of total nitrogen, soluble protein, and chlorophyll, PEP carboxylase and malic enzyme activity, and specific leaf area (SLA). CER at high PPFD (P<0.05) of Tainan, S. spontaneum was significantly higher than that of Saccharum hybrid cv. NCo 310, while CERs of JW2, JW32, and JW41 of S. Spontaneum were significantly higher than that of S. sinense Roxb. cv. Yomitanzan. CER of Tainan exhibited the highest value in a range of 20 to 35°C. Consequently, this suggests that S. spontaneum such as Tainan, is a useful germplasm source in sugarcane breeding. CER of Saccharum correlated significantly with leaf stomatal conductance (r=0.496, P<0.01), the malic enzyme activity of Mn type (r=0.838, P<0.01) and Mg type (r=0.547, P<0.01), and the chlorophyll content (r=0.466, P<0.01).

Genetic variation in pigeon pea (Cajanus cajan (L.) Mill sp.) in response to successive cycles of water stress

Four pigeon pea [Cajanus cajan (L.) Mill sp.] cultivars were studied during two cycles of development of water stress and recovery. During these two cycles the genetic variation in vegetative development, leaf water potential, relative water content, photosynthesis, and stomatal conductance was followed. Plants were grown in a greenhouse and irrigated every seventh day. On two occasions (30 days and 80 days from sowing) water was withheld for 13 days in one group of plants and 16 days in another. Control plants were irrigated as usual. The four cultivars differed in their response to drought, with ICPL 215 being the most tolerant cultivar. In the plants exposed to the 13-day water stress, the first stress cycle resulted in preconditioning of plants such that higher values for carbon dioxide exchange rate and relative water content were observed in the second stress cycle. The longer stress period (16 days) resulted in some damage to the photosynthetic capacity and in a slow recovery rate in both cycles. During the second drought cycle the plants retained a water content above the critical value, possibly due to osmotic adjustment. It seems that water status parameters, especially relative water content, may serve as indicators of drought tolerance in pigeon pea varieties and may be useful in breeding programs for cultivation of pigeon pea under semiarid conditions.

A data base of crop nutrient use, water use, and carbon dioxide exchange in a 2O square meter growth chamber: I. Wheat as a case study.

A data set is given describing the daily nutrient uptake, gas exchange, environmental conditions, and carbon (C), and nutrient partitioning at harvest for the entire canopy and root system of a wheat crop (Triticum aestivum, cv. Yecora Rojo). The data were obtained from a 20 m2 stand of wheat plants grown from planting to maturity in a closed, controlled environment, and include daily nutrient uptake [macronutrients, nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S); and micronutrients, iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), and molybdenum (Mo)], canopy carbon dioxide (CO2) exchange rates, and transpiration. Environmental factors such as relative humidity, air temperature, nutrient solution temperature, pH and electrical conductivity, and photoperiod were controlled in the chamber to specific set points. A detailed description of biomass yield for each of the 64 plant growth trays comprising the 20 m2 of growth area is also provided, and includes dry weights of grain, straw, chaff, and roots, along with the concentration of nutrients in different plant tissues and the percent carbohydrate, fat, and protein. To our knowledge, this information represents one of the most extensive data sets available for a canopy of wheat grown from seed to maturity under controlled environmental and nutritional conditions, and thus may provide useful information for model development and validation. A methods section is included to qualify any assumptions that might be required for the use of the data in plant growth models, along with a daily event calendar indicating when adjustments in set points and occasional equipment or sensor failures occurred.

C3 AND C4 PLANT RESPONSES TO PHOSPHORUS-LIMITING ENVIRONMENTS

The anatomical and biochemical characteristics of C4 plants may make them more efficient than C3 plants in nutrient deficient environments. A greenhouse pot study was conducted in the summer of 1992 to compare the responses of six C3, four C4 and two C3-C4 intermediate species to three levels of phosphorus nutrition: adequate, deficient, and severely deficient. Severely deficient P nutrition reduced shoot weight by 74% and 85% on average in the C3 and C4 species, respectively. C3 plants appear to be more efficient than the C4 in producing shoot biomass under stressful P conditions and two genera having C3, C4 and C3-C4 intermediate species exhibited a trend in the order of C4(least efficient) C3-C4 intermediate, C3 (most efficient). P nutrition was more significant than species or plant type in determining both the concentration of P in plant tissues and the carbon dioxide exchange rate (CER). A more detailed study is underway to compare biomass partitioning and P allocation schemes of these C3 and C4 and C3-C4 intermediate type plants.

Effects of root hypoxia and a low P supply on relative growth, carbon dioxide exchange rates and carbon partitioning in Pinus serotina seedlings

The present study was conducted to determine how 10 weeks of root hypoxia and a low P supply altered relative growth, and carbon acquisition and partitioning in a moderately flood tolerant pine. Pond pine (Pinus serotina Michx.) seedlings were grown in continuously flowing solution culture at 5 or 100 μM P, under aerobic or hypoxic solution conditions. Staggered harvests were used to ascertain changes in biomass allocation and relative growth over time. Carbon dioxide exchange rates (CER) were determined by infrared gas analysis, and needles were analyzed for inorganic P (Pi), sucrose, reducing sugars and starch. Although aeration treatment had no significant effect on shoot dry weight or shoot ontogeny, root dry weight of hypoxic seedlings was significantly lower than that of aerobic seedliings after 8 weeks, regardless of the P treatment. Mean relative growth rates (RGR) of roots in the high P treatment initially decreased under hypoxia, but recovered by the sixth week with the production of adventitious roots. Two weeks of hypoxic growth conditions decreased CER and stomatal conductances of seedlings in the high P treatment by more than 30% relative to their aerobic counterparts. Stomatal closure was not accompanied by a decrease in intracellular CO2, but was accompanied by an increase in starch accumulation. Recoveries of CER, stomatal conductance and carbohydrate metabolism coincided with the recovery of root growth. Low P growth conditions did not significantly affect shoot or root dry weight until the sixth week of treatment. However, differences in seedling RGR, particularly needle RGR, were discernable during the first 2 weeks. Low P treatment effects on CER paralleled changes in needle RGR, with needle RGR more affected than CER. After 6 weeks, CER of aerobically grown seedlings in the low P treatment were only 15% lower than CER of seedlings in the high P treatment, despite a 31% and 75% reduction in needle RGR and Pi concentrations, respectively. Increased starch concentrations of recently expanded needles at this time were probably a result of diminished growth. The inhibitory effect of a low P supply on shoot growth, more specifically on needle expansion and emergence of new fascicular needles, probably limited net carbon fixed per plant more than any direct effect of low P on CER per se.