The Responses of Net Photosysthesis to Light and Temperature in Spartina townsendii(sensu lato), a C4 Species from a Cool Temperate Climate

Carbon dioxide and water vapour exchanges for single attachedleaves of the temperate C₄ grass Spartina townsendii were measuredunder controlled environment conditions in an open gas-exchangesystem. The responses of net photosynthesis, stomatal resistance,and residual resistance to leaf temperature and photon fluxdensity are described. The light and temperature responses ofnet photosynthesis in S. townsendii are compared to informationon these responses in both temperate C₃ grasses and sub-tropicalC₄ grasses. Adaptation of photosynthesis in this C₄ speciesto a cool temperate climate is indicated both by the light andtemperature responses of net photo-synthesis. Unlike the C₄grasses examined previously, significant rates of net photosynthesiscan be detected at leaf temperatures below 10?C. Rates of netphotosynthesis equal or exceed those reported for temperateC₃ grasses at all of the temperature (5–40?C) and photonflax density (13–2500µmol m^–2 s^–1) conditionsexamined. Maximum rates of net photosynthesis in S. townsendiiare almost double those reported for C₃ herbage grasses. Unliketemperate C₃ grasses, the major limitation to net photosynthesisat low leaf temperatures (10?C and below) is the stomatal resistance,showing that the low residual resistance characteristic of C₄species is maintained in S. townsendii even at low leaf temperatures.     

Influences of Water Status, Temperature, and Root Age on Daily Patterns of Root Respiration for Two Cactus Species

Daily patterns of root respiration measured as CO₂, efflux werestudied at various soil water potentials, temperatures, androot ages for individual, attached roots of the barrel cactusFerocactus acanthodes and the platyopuntia Opuntia ficus-indica.The daily patterns of root respiration for both establishedroots and rain roots followed the daily patterns of root temperature.Root respiration increased when root temperature was raisedfrom 5 °C to 50 °C for F. acanthodes and from 5 °Cto 55 °C for O. ficus-indica; at 60 °C root respirationdecreased 50° from the maximum for F. acanthodes and decreased25° for O. ficus-indica. Root respiration per unit d. wtdecreased with root age for both species, especially for rainroots. Root respiration rates for rain roots were reduced tozero at a soil water potential (     

Effects of Temperature and Water Potential on Germination, Radicle Elongation and Emergence of Mungbean

Controlled environment experiments were performed to determinethe effects of temperature and water potential on germination,radicle elongation and emergence of mungbean (Vigna radiata(L.) Wilczek cv. IPB-M79-17-79). The effects of a range of constant temperatures (15–45°C) and water potentials (0 to –2.2 MPa) on germinationand radicle elongation rates were studied using an osmoticumtechnique, in which seeds were held against a semi-permeablemembrane sac containing a polyethylene glycol solution. Linearrelationships were established between median germination time(Gt₅₀) and water potential at different temperatures, and betweenreciprocal Gt₅₀ (germination rate) and temperature at differentwater potentials. Germination occurred at potentials as lowas –2.2 MPa at favourable temperatures (30–40 °C),but was fastest at 40 °C when water was not limiting, withan estimated base temperature (T_b) of about 10 °C. Subsequentradicle elongation, however, was restricted to a slightly narrowertemperature range and was fastest at 35 °C. The conceptof thermal time was used to develop an equation to model thecombined effects of water potential and temperature on germination.Predictions made using this model were compared with the actualgermination obtained in a related series of experiments in columnsof soil. Some differences observed suggested the additionalimportance of the seed/soil/water contact zone in influencingseed germination in soil. Seedling emergence appeared to reflectfurther the radicle elongation results by occurring within anarrower range of temperatures and water potentials than germination.Emergence had an estimated T_b of 12.6 °C and was fastestat 35 °C. A soil matric potential of not less than about–0.5 MPa at sowing was required to obtain 50% or moreseedling emergence. Key words: Germination, temperature, water potential     

Physiology and Growth of Wheat Across a Subambient Carbon Dioxide Gradient

Two cultivars of wheat (Triticum aestivum L.), 'Yaqui 54' and'Seri M82', were grown along a gradient of daytime carbon dioxideconcentrations ([CO₂]) from near 350-200 µmol CO₂ mol^-1air in a 38 m long controlled environment chamber. Carbon dioxidefluxes and evapotranspiration were measured for stands (plantsand soil) in five consecutive 7·6-m lengths of the chamberto determined potential effects of the glacial/interglacialincrease in atmospheric [CO₂] on C₃ plants. Growth rates andleaf areas of individual plants and net assimilation per unitleaf area and daily (24-h) net CO₂ accumulation of wheat standsrose with increasing [CO₂]. Daytime net assimilation (P_D, mmolCO₂ m^-2 soil surface area) and water use efficiency of wheatstands increased and the daily total of photosynthetic photonflux density required by stands for positive CO₂ accumulation(light compensation point) declined at higher [CO₂]. Nighttimerespiration (R_N, mmol CO₂ m^-2 soil surface) of wheat, measuredat 369-397 µmol mol^-1 CO₂, apparently was not alteredby growth at different daytime [CO₂], but R_N /P_D of stands declinedlinearly as daytime [CO₂] and P_D increased. The responses ofwheat to [CO₂], if representative of other C₃ species, suggestthat the 75-100% increase in [CO₂] since glaciation and the30% increase since 1800 reduced the minimum light and waterrequirements for growth and increased the productivity of C₃plants.Copyright 1993, 1999 Academic Press Atmospheric carbon dioxide, carbon accumulation, evapotranspiration, light compensation point, net assimilation, respiration, Triticum aestivum, water use efficiency, wheat     

The Carbon Economy of Rubus chamaemorus L. I. Photosynthesis

Studies on the photosynthetic activity of Rubus chamaemorusL. in controlled environment conditions are reported. Theseshow that material collected from Moor House National NatureReserve, England has a photosynthetic light saturation pointof 100 J m^–2s^–1 (380–720 nm) and a temperatureoptimum for photosynthesis between 10 and 15 °C. A markeddecline in net CO₂ uptake is evident at temperatures in excessof 18 ° C; this persists for some time after return to anoptimum temperature regime. Leaves show rapid responses to changesin both light intensity and temperature, and show no evidenceof an endogenous rhythm in photosynthetic rate.     

Effects of Light and CO2 on Net Photosynthesis Rates of Stands of Aubergine and Amaranthus

Net photosynthetic rates per unit ground area for plant standsof Solanum melongena L. var. esculentum (aubergine) and Amaranthuscaudatus L. var. edulis (grain amaranth) were measured over10 min intervals in an airtight, glass, controlled-environmentcabinet for a range of light flux densities provided by thediurnal variation in daylight. Light response curves for photosynthesisof stands, grown at ambient CO₂ concentration, were definedat 400, 800 and 1200 vpm CO₂. Light compensation points for these stands were around 20-30J m^-2 s^-1 and decreased slightly at higher CO₂ concentrations.For aubergine, a C₃ species, the short-term effects of CO₂ enrichmentwere to increase the initial slope as well as the asymptoteof the light response curve, reducing light saturation at moderateto high light flux densities; but for amaranthus, a C₄ species,saturation was less apparent and CO₂ enrichment scarcely increasedphotosynthesis except at light flux densities above 150 J m^-2s^-1. The canopies intercepted 93-98% of incident light. The efficiencyof utilization of intercepted light in photosynthesis (µgCO₂ J^-1) increased from zero at the light compensation pointto a maximum at an optimum light flux density of about 100 Jm^-2 s^-1 (the optimum rose a little with CO₂ enrichment) anddecreased slightly with further increase in light. Maximum utilizationefficiencies at 400 vpm CO₂ were 8-9 µg CO₂ J^-1. Enrichmentto 1200 vpm did not affect the peak utilization efficiency ofthe C₄ amaranthus, but increased that aubergine to 12·2µg CO₂ J^-1 (equivalent to some 14% when using the heatof combustion of plant dry matter to convert to the dimensionlessform). This is among the highest recorded efficiencies of lightutilization for stands, and relates to the exceptionally favourableenvironment, with optimal control of CO₂ concentration, humidity,temperature, water supply and mineral nutrition.Copyright 1993,1999 Academic Press Amaranthus caudatus L. var. edulis, Solanum melongena L. var. esculentum, canopy photosynthesis, CO₂ enrichment, light interception, light utilization, photosynthetic efficiency     

Ecophysiological Mechanisms used by Aster kantoensis, an Endangered Species, to Withstand High Light and Heat Stresses of its Gravelly Floodplain Habitat

Aster kantoensis Kitam., an endangered plant species of thefamily Compositae, is a local endemic to the gravelly floodplainsof a few rivers in central Japan. The successful growth of A.kantoensis is mainly restricted to sparsely vegetated siteswhere, due to lack of continuous vegetation, high radiant energyinput results in stressful conditions with excessive light andheat. To reveal the ecophysiological characteristics which enablethe species to cope with such environmental stresses, we measuredleaf temperature, shoot architecture and photosynthetic andtranspirational responses together with the microclimate ofthe natural habitat. Even under sunny summer conditions, theleaf temperature of A. kantoensis was much lower (35–39°C)than the soil surface temperature (max. 60°C). The relationshipbetween leaf position (height from the ground) and leaf temperatureshowed that the caulescent rosette form of A. kantoensis helpsavoid leaf overheating. Moreover, in situ gas exchange measurementsrevealed that the high transpirational capacity (as high as10 mmol H₂O m^-2s^-1) was effective in controlling leaf temperature,as long as the soil water supply was not severely limited. Sinceit has effective mechanisms to avoid the multiple stresses indigenousto its gravelly floodplain habitat, A. kantoensis can maintaina high photosynthetic rate (up to 30 µmol CO₂m^-2s^-1) withoutany midday depression under sunny summer conditions. Copyright2000 Annals of Botany Company Aster kantoensis Kitam., gravelly floodplain, high light stress, leaf temperature, photosynthesis, shoot architecture, transpiration     

Gas Exchange and Carbohydrate and Nitrogen Concentrations in Leaves of Pascopyrum smithii (C3) and Bouteloua gracilis (C4) at Different Carbon Dioxide Concentrations and Temperatures

Pascopyrum smithii (C₃) andBouteloua gracilis (C₄) are importantforage grasses native to the Colorado shortgrass steppe. Thisstudy investigated photosynthetic responses of these grassesto long-term CO₂enrichment and temperature in relation to leafnonstructural carbohydrate (TNC) and [N]. Glasshouse-grown seedlingswere transferred to growth chambers and grown for 49 d at twoCO₂concentrations (380 and 750 µmol mol^-1) at 20 and 35°C, and two additional temperatures (25 and 30 °C) at750 µmol mol^-1CO₂. Leaf CO₂exchange rate (CER) was measuredat a plant's respective growth temperature and at two CO₂concentrationsof approx. 380 and 700 µmol mol^-1. Long-term CO₂enrichmentstimulated CER in both species, although the response was greaterin the C₃,P. smithii . Doubling the [CO₂] from 380 to 750 µmolmol^-1stimulated CER ofP. smithii slightly more in plants grownand measured at 30 °C compared to plants grown at 20, 25or 35 °C. CO₂-enriched plants sometimes exhibited lowerCER when compared to ambient-grown controls measured at thesame [CO₂], indicating photosynthetic acclimation to CO₂growthregime. InP. smithii , such reductions in CER were associatedwith increases in TNC and specific leaf mass, reductions inleaf [N] and, in one instance, a reduction in leaf conductancecompared to controls. InB. gracilis , photosynthetic acclimationwas observed more often, but significant changes in leaf metabolitelevels from growth at different [CO₂] were generally less evident.Temperatures considered optimal for growth (C₃: 20 °C; C₄:35 °C) sometimes led to CO₂-induced accumulations of TNCin both species, with starch accumulating in the leaves of bothspecies, and fructans accumulating only inP. smithii. Photosynthesisof both species is likely to be enhanced in future CO₂-enrichedand warmer environments, although responses will sometimes beattenuated by acclimation. Acclimation; blue grama (Bouteloua gracilis (H.B.K.) Lag ex Steud.); leaf nitrogen concentration; nonstructural carbohydrates; photosynthesis; western wheatgrass (Pascopyrum smithii (Rydb.) Love)     

Environmental Influences on Carbon Recycling in a Terrestrial CAM Bromeliad, Bromelia humilis Jacq.

The effects of night-time temperature, leaf-to-air vapour pressuredeficit (VPD) and water stress on CO₂ recycling in Bromeliahumilis Jacq. grown under two light and nitrogen regimes wereinvestigated. At night-time temperatures above 30°C, integratednet dark CO₂ uptake was severely reduced and CO₂ for malatesynthesis was mainly derived from dark respiration. At 35°C,up to 84% of the CO₂ liberated by dark respiration was refixedinto malic acid. Below 30 °C only nitrogen deficient plantsshowed significant recycling. No significant differences wereobserved between high and low light grown plants in CO₂ recycling.A doubling of leaf-to-air VPD from 7-46 Pa kPa^–1 to 15.49Pa kPa^–1 resulted in a 2- to 20-fold decrease in leafconductance and about 50 to 65% reduction in integrated darkCO₂ uptake. However, about twice as much CO₂ was recycled atthe higher VPD as in the lower. Ten days of water stress resultedin 80 to 100% recycling of respiratory CO₂. Under high VPD andwater stress treatments, the amount of water potentially savedthrough recycling of CO₂ reached 2- to 6-fold of the actualtranspiration. In general, nitrogen deficient plants had higherper cent recycling of respiratory CO₂ in response to high night-timetemperature, increased VPD or water stress. The results emphasizethe ecological relevance of carbon recycling in CAM plants. Key words: Bromelia humilis, CAM, PPFD, dark respiration, temperature, VPD, water stress     

Photosynthesis and photorespiration in the genus Oryza

Photosynthetic gas exchange has been surveyed in 22 of the 23species currently placed in the genus Oryza and constitutingthe wild relatives of cultivated rice. Unimproved, wild germplasmof a number of species showed light-saturated assimilation ratesin atmospheric air at least as great as cultivars and elitebreeding material of the cultigen O. satlva. One of these specieswas O. australiensis, different accessions of which were significantly(P=0.001) superior in assimilation rate to the 0. satlva genotypestested, including representatives of the Indica, Japonica andJavanica subgroups. Amongst species, assimilation rate was correlatedpositively with light saturation and with carboxylation efficiency.The wild species fell into two distinct groupings accordingto whether they originated from sun or shade habitats, withthe higher assimilation rates being associated with the sunspecies. Assimilation rates were also higher in diploids thanin tetraploids and this was associated with the fact that allsun species are diploids and all tetraploids are shade species.The carbon dioxide compensation concentrations ranged from 28to 43µmol mol^–1 with the two lowest values (28 and32µmol mol^–1) coming from accessions of O. rufipogon.The mean value for the absolute quantum yield of photosynthesismeas ured on attached leaves was 0.060. There was a large rangein the activity of phosphoenolpyruvate (PEP) carboxylase witha number of species having rates several times those of C₃ species.Some species with the highest assimilation rates were assessedfor photorespiratory losses and these were generally around30% and similar to O. satlva cultivars. However, a range ofO. rufipogon accessions had photorespiration rates significantly(P=0.01) lower than the O. sativa genotypes tested. No speciesin the genus possessed C₄ photosynthetic metabolism though somedid overlap with compensation concentrations and phosphoenolpyruvatecarboxylase activities reported for C₃–C₄ intermediatespecies. The potential value of wild relatives to the improvementof cultivated rice is discussed. Key words: Oiyza, photorespiration, photosynthesis, rice, wild rice     

Variations with Age in Net Assimilation Rate and other Growth Attributes of Sugar-beet, Potato, and Barley in a Controlled Environment: With two Figures in the Text

Sugar-beet, potato, and barley plants were grown in a controlledenvironment, for periods of up to 10 weeks from sowing, witha light intensity of 1,8oo f.c. (4·9 cal./cm.²/hr.) anda temperature of 20° C. during the 18-hour photoperiod and15° C. during the dark period, to test whether net assimilationrate varied with age and differed between the three species. Net assimilation rate of all species based on leaf area (E_A)fell approximately linearly with time. During 5 weeks E_A ofsugar-beet decreased by only about 20 per cent. and E_A of potatodecreased by 50 per cent. E_A of barley remained approximatelyconstant for 4 weeks after sowing and was halved during thesubsequent 4 weeks. The average value of E_A for all times wasgreatest for sugarbeet and least for barley. Net assimilation rates based on leaf weight (E_W) and leaf N(E_N) decreased at about 15 per cent. of the initial value perweek for all species; this was similar to the mean rate of decreaseof E_A of potato and barley, but greater than that of E_A of sugar-beet.Mean values of E_W or E_N for potato and barley were similar andless than for sugar-beet. Relative growth rate (R_W), relative leaf growth-rate (R_A), andleaf-area ratio (F) fell with time at similar rates for allspecies. Average values of R_W decreased and of F increased inthe order sugar-beet, potato, barley. R_A was greatest for potatoand least for barley.     

Cold-Tolerant Crop Species Have Greater Temperature Homeostasis of Leaf Respiration and Photosynthesis Than Cold-Sensitive Species

Some plant species show constant rates of respiration and photosynthesismeasured at their respective growth temperatures (temperaturehomeostasis), whereas others do not. However, it is unclearwhat species show such temperature homeostasis and what factorsaffect the temperature homeostasis. To analyze the inherentability of plants to acclimate respiration and photosynthesisto different growth temperatures, we examined 11 herbace-ouscrops with different cold tolerance. Leaf respiration (R_area)and photosynthetic rate (P_area) under high light at 360 µll^–1 CO₂ concentrations were measured in plants grown at15 and 30°C. Cold-tolerant species showed a greater extentof temperature homeostasis of both R_area and P_area than cold-sensitivespecies. The underlying mechanisms which caused differencesin the extent of temperature homeostasis were examined. Theextent of temperature homeostasis of P_area was not determinedby differences in leaf mass and nitrogen content per leaf area,but by differences in photosynthetic nitrogen use efficiency(PNUE). Moreover, differences in PNUE were due to differencesin the maximum catalytic rate of Rubisco, Rubisco contents andamounts of nitrogen invested in Rubisco. These findings indicatedthat the temperature homeostasis of photosynthesis was regulatedby various parameters. On the other hand, the extent of temperaturehomeostasis of R_area was unrelated to the maximum activity ofthe respiratory enzyme (NAD-malic enzyme). The R_area/P_area ratiowas maintained irrespective of the growth temperatures in allthe species, suggesting that the extent of temperature homeostasisof R_area interacted with the photosynthetic rate and/or thehomeostasis of photosynthesis.     

Carbon Metabolism in Seagrasses: II. PATTERNS OF PHOTOS YNTHETIC CO2 INCORPORATION

Patterns of initial photosynthetic CO₂ incorporation were determinedfor some seagrasses and were related to activities of primarycarbon fixing enzymes, carbonic anhydrase activities, and ¹³Cvalues. According to the incorporation patterns, Cymodocea nodosa wasa C4 species while Thalassia hemprichli and Thalassodendronciliatum were C₃ plants. Halophila stipulacea showed an unusualincorporation pattern which could be viewed as intermediatebetween typical C₃ and C₄ pathways. The activity ratios of ribulose-l,5-bisphosphate carboxylase (RUBPcase) to phosphoenolpyruvatecarboxylase (PEPcase) were about 3 for Thalassodendron ciliatumand 1 for Cymodocea nodosa and Halophila stipulacea. The lattervalue, which is intermediate to ratios found in terrestrialC₃ and C₄ plants, may correlate with the incorporation patternsfound for Halophila stipulacea. Since the C₄ seagrass lackedthe Kranz anatomy, it may, in addition, point to a flexibleincorporation potential for these plants. The high ¹³C values found in these and other seagrasses didnot correlate with their photosynthetic pathways as in terrestrialplants. This discrepancy is probably due to a ‘closedsystem’ type of photosynthesis in which CO₂ is efficientlyutilized. The C₃ species which utilize CO₂ enzymatically must convertexogenous HCO-₃ to CO₂ internally. Even though carbonic anhydraseactivities were very low, conversion rates seemed to be sufficientfor high rates of photosynthesis. Since enzymatic fixation ratesapproached photosynthetic rates even at CO₂ saturation, thelimitation for these seagrasses to express their high photosyntheticpotential is most probably the HCO^–₃ uptake system.     

Resolution of Net Dark Fixation of Carbon Dioxide into Its Respiration and Gross Fixation Components in Bryophyllum daigremontianum

Dark respiration rate increased with temperature between 10and 24°C (Q₁₀ =2.3–2.7). The rate of gross dark CO₂fixation (GDF) was affected by temperature, but irregularly.Cumulative GDF was not affected by temperature in this range.Cumulative respiration increased from 17 per cent of cumulativeGDF at 10°C, to 72 per cent at 24°C and was thus responsiblefor the 65 per cent drop in net dark fixation between thesetwo temperatures. and respiration rates were functions of the light intensityin the preceding light period. The function for cumulativeGDFwas of the saturation form, maximum accumulation being obtainedat 12 mW cm^–2. It is concluded that both GDF and respirationrates depend on levels of substrates formed during the lightperiod. However, the rate of GDF did not appear to be directlyrelated to the rate of respiration.     

Evidence for Essential Maintenance Respiration of Leaves of Xanthium Strumarium at High Temperature

Mesophyll resistance to photosynthetic carboxylation (r'_m) wasused as a criterion for leaf integrity. It was measured, at25 °C, in the light, before and after periods of high temperature(3 h at 38 °C) in the dark. During the high temperatureperiods, respiration (R_D) of attached leaves of Xanthium strumariumwas suppressed from 27%-36% by either low [O₂] (1.04% or 0.21%v.v.) or high [CO₂] (840 µl 1^–1) in the ambientair. Neither treatment affected rates of R_D or photo-respirationduring the second period at 25 °C. There was no significant increase of r'_m when R_D was not suppressedduring the high temperature treatment. When R_D was suppressedat high temperatures, r'_m increased from about 3s cm^–1before, to about 26 s cm^–1 after the high temperaturetreatment. The increase depended upon the degree of suppression. It is concluded that increased R_D at high temperature in Xanthiumleaves is partly the result of an increase of energy demandingmaintenance. The subsequent rate of carbon dioxide fixationis reduced when this increase of maintenance-induced respirationis inhibited.     

The Effect of Temperature and Light on Gas Exchange and Acid Accumulation in the C3-CAM Plant Clusia minor L

Gas exchange and organic acid accumulation of the C₃-CAM intermediateClusia minor L. were investigated in response to various day/nighttemperatures and two light regimes (low and high PAR). For bothlight levels equal day/night temperatures between 20°C and30°C caused a typical C₃ gas exchange pattern with all CO₂uptake occurring during daylight hours. A day/ night temperatureof 15°C caused a negative CO₂ balance over a 24 h periodfor low-PAR-grown plants while high-PAR-grown plants showeda CAM gas exchange pattern with most CO₂ uptake taking placeduring the dark period. However, there was always a considerablenight-time accumulation of malic acid which increased when thenight-time temperature was lowered and had its maximum (54 mmolm^–2) at day/night temperature of 30/15°C. A significantamount of malic acid accumulation (23 mmol m^–2) in low-PAR-grownplants was observed only at 30/15°C. Recycling of respiratoryCO₂ in terms of malic acid accumulation reached between 2·0and 21·5 mmol m_–2 for high-PAR-grown plants whilethere was no significant recycling for low-PAR-grown plants.Both low and high-PAR-grown plants showed considerable night-timeaccumulation of citric acid. Indeed under several temperatureregimes low-PAR-grown plants showed day/night changes in citricacid levels whereas malic acid levels remained approximatelyconstant or slightly decreased. It is hypothesized that lowand high-PAR-grown plants have different requirements for citrate.In high-PAR-grown plants, the breakdown of citrate preventsphotoinhibition by increasing internal CO₂ levels, whereas inlow-PAR-grown plants the night-time accumulation of citric acidmay function as an energy and carbon saving mechanism. Key words: C. minor, C₃, CAM, citric acid, light intensity     

Frequencies, Microclimate and Root Properties for Three Codominant Perennials in the Northwestern Sonoran Desert on North-vs.South-facing Slopes

At a site in the northwestern Sonoran Desert the percent groundcover for the C₃subshrubEncelia farinosawas eight-times higheron more arid 20° south-facing slopes than on 20° north-facingslopes at 820 m elevation, and was six-times higher on north-facingslopes at a 300-m-lower elevation, also the more arid condition.The ground cover of the C₄bunchgrassPleuraphis rigidadecreasedover 50% from 20° north-facing slopes to the more arid conditionsof a 36° north-facing slope, a 20° south-facing slopeand a 20° north-facing slope at a 300-m-lower elevation.The CAM leaf succulentAgave desertialso had greater ground coverfor the 20° north-facing slopes at 820 m compared with 520m. For these three codominants that averaged 58% of the totalground cover, the key for the relative frequency ofE. farinosawasapparently its greater root growth on the warmer slopes duringthe winter. The key for the other two species was most likelysoil water availability, especially during the seedling stageforA. deserti. The wetter soil conditions on 20° north-facingslopes at 820 m apparently led to individual plants ofP. rigidathatwere twice as large as on south-facing slopes. Thus root propertiesmay exert the primary influence on relative plant frequencyin this desert ecosystem for which soil temperature and wateravailability are crucial.Copyright 1997 Annals of Botany Company Agave deserti; Encelia farinosa; Pleuraphis rigida; rooting patterns; soil temperature; Sonoran Desert; water availability     

Variation in Photorespiration in Lolium

The rate of photorespiration in several grass species was shownto be highly variable and dependent on the species, genotype,and conditions under which the plants were grown. Photorespiration,measured as oxygen uptake, was negligible in Cenchrus ciliarisand Paspalum dilatatum but significant in Lolium spp. and Festucaarundinacea. There were significant differences in the rateof photorespiration among plants within a Lolium populationof diverse origin and these differences were independent ofthe conditions under which the plants were grown. Among thetemperate grasses there was a significant correlation betweenphotorespiration and the CO₂-compensation concentration andboth parameters were very low in P. dilatatum. Plants grownin day/night temperatures of 15/10 °C compared with 25/20°C had faster rates of dark respiration but slower ratesof light respiration when measured at the same temperature.Photorespiration was faster than dark respiration although differencesin respiration among plants in the light were not shown in thedark.     

A comparison of net and gross rates of oxygen production as a function of light intensity in some natural plankton populations and in a Synechococcus culture

In vitrorates of gross and net oxygen production were measuredas a function of light intensity in some plankton communitiescollected from Bedford Basin, Nova Scotia, and in a monoclonalculture of Synechococcus. The rate of gross oxygen productionwas measured by a technique in which the stable oxygen isotope,¹⁸O, serves as a photosynthetic tracer Net oxygen productionwas measured by automated Winkler technique. The rate of communityrespiration in the light was then determined by the differencebetween gross and net rates of oxygen production. In the naturalpopulations examined, neither gross nor net oxygen productionrates were significantly inhibited at the highest light intensitymeasured (500–800 µE m^–2 s^–1) In a samplein which the dark respiration rate was small relative to themaximal rate of production [P_max;sensu Platt et al (1980) JMar. Res., 38, 687–701] the rates of ‘light’respiration were 3 times greater. In two other communities,with high rates of dark respiration relative to P_maxthe ratesof ‘light’ respiration were closer to rates of darkrespiration. In the Synechococcus clone, both gross and netoxygen production rates were inhibited at high light intensities.Rates of ‘light’ respiration were found to varyas a function of light intensity. The greatest rates of respirationwere measured in samples incubated at light intensities thatwere just saturating (100 µE m^–2 s^–1). Therates of ¹⁴C production were also measured as a function oflight intensity The photosynthetic quotients, based on ¹⁴C productionrates and gross oxygen production rates, average 1 9     

Effects of Prolonged Darkness on the Sensitivity of Leaf Respiration to Carbon Dioxide Concentration in C3and C4Species

Predicting responses of plant and global carbon balance to theincreasing concentration of carbon dioxide in the atmosphererequires an understanding of the response of plant respirationto carbon dioxide concentration ([CO₂]). Direct effects of thecarbon dioxide concentration at which rates of respiration ofplant tissue are measured are quite variable and their effectsremain controversial. One possible source of variation in responsivenessis the energy status of the tissue, which could influence thecontrol coefficients of enzymes, such as cytochrome-c oxidase,whose activity is sensitive to [CO₂]. In this study we comparedresponses of respiration rate to [CO₂] over the range of 60to 1000 µmol mol^-1in fully expanded leaves of four C₃andfour C₄herbaceous species. Responses were measured near themiddle of the normal 10 h dark period, and also after another24 h of darkness. On average, rates of respiration were reducedabout 70% by the prolonged dark period, and leaf dry mass perunit area decreased about 30%. In all species studied, the relativedecrease in respiration rate with increasing [CO₂] was largerafter prolonged darkness. In the C₃species, rates measured at1000 µmol mol^-1CO₂averaged 0.89 of those measured at 60µmol mol^-1in the middle of the normal dark period, and0.70-times when measured after prolonged darkness. In the C₄species,rates measured at 1000 µmol mol^-1CO₂averaged 0.79 of thoseat 60 µmol mol^-1CO₂in the middle of the normal dark period,and 0.51-times when measured after prolonged darkness. In threeof the C₃species and one of the C₄species, the decrease in theabsolute respiration rate between 60 and 1000 µmol mol^-1CO₂wasessentially the same in the middle of the normal night periodand after prolonged darkness. In the other species, the decreasein the absolute rate of respiration with increase in [CO₂] wassubstantially less after prolonged darkness than in the middleof the normal night period. These results indicated that increasingthe [CO₂] at the time of measurement decreased respiration inall species examined, and that this effect was relatively largerin tissues in which the respiration rate was substrate-limited.The larger relative effect of [CO₂] on respiration in tissuesafter prolonged darkness is evidence against a controlling roleof cytochrome-c oxidase in the direct effects of [CO₂] on respiration.Copyright 2001 Annals of Botany Company Carbon dioxide, respiration, Abutilon theophrasti(L.), Amaranthus retroflexus(L.),Amaranthus hypochondriacus (L.), Datura stramonium(L.), Helianthus annuus(L.), Solanum melongena(L.), Sorghum bicolor(L. Moench), Zea mays