An Optimum Wind Speed for Plant Growth

Observations on the growth-rate of stands of young plants ofBrassica napus in a wind tunnel show that relative growth-raterises with wind speed at low wind speeds but falls again whenwind speed is further increased. A wind speed of 0.3m./sec.caused optimal growth in these experiments. Changes in relativegrowth-rate were small.     

DEPOSITION OF AIR-BORNE LYCOPODIUM SPORES ON CYLINDERS

For the study of deposition of air-borne particles on plant and trap surfaces a small wind tunnel has been constructed giving turbulent or streamline flow up to about 10 m./sec. The efficiency with which cylinders of from 0.018 to 2.0cm. diameter, coated with glycerine jelly, catch Lycopodium spores at wind speeds over the range 1–10 m./sec. has been measured experimentally with a Cascade Impactor, operated iso-kinetically, as standard. Efficiency has been found to increase as wind speed increases, and as cylinder diameter decreases. Similar effects have been observed in the field. Efficiencies observed are lower than predicted by Sell and Glauert, but agree well with those predicted by Langmuir and Blodgett, except with the narrowest cylinders. The standard vertical sticky traps used in routine trappings of fungus spores, pollen, and crop-protectant sprays and dusts have a low trapping efficiency.     

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.     

The Effect of Artificial Wind on the Growth-rate of Plants in Water Culture: With Plate and three Figures in the Text

Young plants of Brassica napus (rape), Hordeum vulgare, andPisum sativum growing in water culture have been exposed tofour continuous wind speeds for from 4 to 5 days. The plantswere exposed to natural daylight, humidity, and temperaturein a wind tunnel in which only air movement was controlled.The wind speeds found among the plants in the four sectionsof this tunnel were approximately 0·3, 0·7, 1·7,and 4·0 m./sec. The results showed no significant changein relative growth-rate or net assinilation rate with wind speed. Previously published results, obtained with plants in soil orsand culture, have differed from those in the present experimentsin showing a fall in the amount of growth as wind speed increased.It is suggested that the reduction in growth found in theseprevious experiments was caused by partial drying out of theplants, whilst in the present experiments water-supply to theroots was abundant and this effect was considerably reduced.     

Trade Winds Reduce Growth and Influence Gas Exchange Patterns in Papaya Seedlings

Four experiments were conducted to determine the effect of tradewinds in Guam, USA, on growth and gas exchange of three papaya(Carica papaya L.) cultivars. ‘Known You 1’, ‘Sunrise’,and ‘Tainung 2’ papaya seedlings at two differentstages of development were exposed to 0, 36 or 100% ambientwind. Wind exposure reduced stem height and leaf or stem dryweight in most cases, but had little influence on root growth.Net CO₂assimilation (A_CO2) at midday was lower for seedlingsexposed to wind than for those protected from wind. Dark respirationof exposed seedlings increased as much as 120% above that ofthe protected seedlings during the night. Wind exposure decreasedwhole plant evapotranspiration by up to 36% during the photoperiod,but increased evapotranspiration by 58–87% during thenocturnal period. Responses to wind exposure were similar amongcultivars, except that growth of ‘Tainung 2’ seedlingswas less affected by wind than that of the other cultivars.Seedlings that were exposed to the various wind treatments fromgermination onwards were less influenced by wind exposure thanwere seedlings that were grown in a protected nursery beforebeing exposed to the various wind treatments. These data indicatethat: (1) ambient trade winds in Guam are strong enough to decreasethe growth of papaya seedlings; (2) plant age influences theresponse; (3) stem and leaf growth are more influenced thanroot growth; and (4) decreasedA_CO2 and increased dark respirationmay be partly responsible for growth reduction. Copyright 2001Annals of Botany Company Carica papaya, gas exchange, wind     

The Development of Fast-Start Performance in Fishes: Escape Kinematics of the Chinook Salmon (Oncorhynchus tshawytscha)

C-starts are high acceleration swimming movements critical forpredator avoidance by fishes. Since larval fishes are particularlyvulnerable to predation, C-start behavior is likely to be especiallyimportant during early life history stages. This paper examinesthe developmental changes in C-start performance with kinematicdata on immature chinook salmon (Oncorhynchus tshawytscha) (eleuthroembryostage, sensu Balon, 1975). The scaling of C-start kinematicsof immature fishes differs from that of adults. Adult C-startdurations increase with increasing body length while C-startdurations of immature fishes decrease (e.g., adult stage 1 duration[sec] = 0.0019.length [L] [cm] $ 0.026 [R² = 0.77] [Webb, 1978];eleuthroembryos stage 1 duration [sec] = –0.026L [cm]$ 0.100 [R² = 0.81]). Distance traveled during stage 2 alsodiffers between adult and immature fishes. Adult distance traveledscales directly with length (distance [cm] = 0.38L^1.01 [cm],R² = 0.96 [Webb, 1978]) while chinook eleuthroembryo distancetraveled is positively allometric with length (distance [cm]=0.37L¹³¹ [cm], R² = 0.83). There are similarities in the developmentof C-starts and burst swimming. For example, mean velocity scalessimilarly between the two locomotor modes (For burst swimming:U_mean [cm/sec] = 8.1 ± 1.1L [cm] $ 4.89 [R² = 0.86] [Webband Corolla, 1981]. For C-start stage 2: U_mean [cm/sec] = 10.96L[cm] - 14.09 [R² = 0.70]). This study demonstrates that C-startescape performance improves during early post-hatching development.Comparisons of immature chinook salmon fast-starts with dataon larval burst swimming and on adult C-starts suggest thatchanges specific to developing fish affect the scaling of kinematicparameters.     

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     

Humidity Pretreatment Affects the Responses of Stomata and CO2 Assimilation to Vapor Pressure Difference in C3 and C4 Plants

Experiments were carried out to investigate the long-term influenceof humidity on the short-term responses of stomata and CO₂ assimilationto vapor pressure difference in Oryza sativa (rice, C₃ species)and Panicum maximum (green panic, C₄ species). Plants were grownfor four weeks in growth chambers set at 35% and 85% relativehumidity at 25C air temperature, 38+2 Pa CO₂ partial pressureand 1,700µmol m^-2s^-1 photon flux density. Soil was saturatedwith water in both humidity treatments. Low humidity pretreatmentscaused low leaf conductance and low rates of transpiration andCO₂ assimilation in O. sativa, but small changes in stomatalresponses to humidity and in CO₂ assimilation were found inP. maximum. From the short-term gas exchange experiments, itwas noted that the responsiveness of leaf conductance to vaporpressure difference were affected by humidity pretreatmentsin O. sativa, whereas unaffected in P. maximum. In O. sativameasurements of CO₂ assimilation as a function of internal CO₂partial pressure (A-Ci curve) indicated that low humidity pretreatmentsreduced the CO₂ assimilation at high internal CO₂ partial pressure,but the initial slope of the A-Ci curve was unaffected. Furthermore,plant characteristics such as total dry weight and leaf areaof plants subjected to low umidity were lower than plants subjectedto high humidity. The reductions in O. sativa, however, werelarger than in P. maximum. Stomatal frequency from low humiditygrown plant was higher than that from high humidity grown plantsin both species although there is no significant difference.The data indicated that if the short term inhibition of netCO₂ assimilation at a high vapor pressure difference was imposedduring vegetative growth, the photosynthetic biochemistry andthe resultant plant growth were largely depressed in O. sativa,a C₃ species. (Received May 26, 1992; Accepted November 2, 1992)     

Are Mistletoes Shade Plants? CO2Assimilation and Chlorophyll Fluorescence of Temperate Mistletoes and their Hosts

Mistletoes usually have slower rates of photosynthesis thantheir hosts. This study examines CO₂assimilation, chlorophyllfluorescence and the chlorophyll content of temperate host–parasitepairs (nine hosts parasitized by Ileostylus micranthus and Carpodetusserratus parasitized by Tupeia antarctica). The hosts of I.micranthus had higher mean annual CO₂assimilation (3.59 ±0.41 µmol m^-2 s^-1) than I. micranthus(2.42 ± 0.20µmol m^-2 s^-1), and C. serratus(2.41 ± 0.43 µmolm^-2 s^-1) showed higher CO₂assimilation than T. antarctica(0.67± 0.64 µmol m^-2 s^-1). Hosts saturated at significantlyhigher electron transport rates (ETR) and light levels thanmistletoes. The positive relationship between CO₂assimilationand electron transport suggests that the lower CO₂assimilationrates in mistletoes are a consequence of lower electron transportrates. When photosynthetic rates, ETR and chlorophyll a /b ratioswere adjusted for photosynthetically active radiation, hostsdid not have significantly higher CO₂assimilation (3.21 ±0.37 µmol m^-2 s^-1) than mistletoes (2.54 ± 0.41µmol m^-2 s^-1), but still had significantly higher ETRand chlorophyll a / b ratios. The electron transport rates,saturating light and chlorophyll a / b ratios of sun leavesfrom mistletoes were similar to host shade leaves. These responsesindicate that in comparison with their hosts, mistletoe leaveshave the photosynthetic characteristics of the leaves of shadeplants. Copyright 2000 Annals of Botany Company CO₂assimilation, photosynthetic active radiation (PAR), chlorophyll fluorescence, electron transport rate (ETR), photochemical quenching (q_p), non-photochemical quenching (q_n), sun and shade leaves, chlorophyll content, Ileostylus micranthus, Tupeia antarctica, New Zealand     

The Responses of Net Photosynthesis to Vapour Pressure Deficit and CO2 Concentration in Spartina townsendii (sensu lato), a C4 Species from a Cool Temperate Climate

As the initial part of a detailed study of photosynthetic CO₂assimilation in the temperate C₄ grass, Spartina townsendii,the responses of net photosynthesis to the leaf-air vapour pressuredeficit (VPD) and to CO₂ concentration are examined. Water vapourand CO₂ exchange for single attached leaves were measured undercontrolled-environment conditions in an open gas-exchange system.The responses of net photosynthesis, stomatal resistance (r_s),and residual resistance (r_r) to vapour pressure deficit(VPD)and CO₂ concentration under a range of light and temperatureconditions are reported. Net photosynthesis was insensitiveto increase in the VPD up to 1.0 kPa, but beyond this valuenet photosynthesis decreased with further increase in VPD asa result of an increase in r_s. The residual resistance was notaffected by VPD under any of the conditions examined. Net photosynthesisresponded linearly to increase in the CO₂ concentration in theexternal air (C_a up to the normal atmospheric concentrationwhere there was a sharp change in the response, net photosynthesisbeing independent of any further increase in C_a. Differencesbetween the response curves observed here and in other studiesare discussed and the possible reasons for these differencesare considered.     

Influence of Wind Speed on the Sensitivity of Ryegrass to SO2

The sensitivity of S23 Lolium perenne L. to 11 parts 10^–8SO₂ was investigated at two different wind speeds. At the higherwind speed of 25 m min^–8, SO₂ caused significant reductionsin leaf area, root/shoot ratio, and all dry weight fractionsmeasured. At the lower wind speed of 10 m min^–1 growthreductions were not found. The differences in sensitivity ofplants to SO₂ at different wind speeds is discussed in relationto boundary layer resistances of leaves. It is concluded thatthe sensitivity of a plant to a particular pollutant shouldno longer be measured in terms of only the concentration andlength of exposure.     

Nitrogen Use Efficiency in Growth of Polygonum cuspidatum Sieb. et Zucc

The growth of Polygonum cuspidatum in sand culture was analysedunder varying nutrient conditions. Nitrogen availability influencednitrogen uptake of plants through the uptake rate per unit rootweight rather than the amount of root. In turn, the differentamounts of nitrogen taken up affected plant growth through theireffects on the rate of leaf expansion. Net assimilation rate (NAR) increased with nitrogen contentper unit leaf area (C), but further increase in leaf nitrogencaused diminishing returns of NAR Optimal nitrogen content perunit leaf area (C_opt) to maximize dry-matter production of aleaf could be determined by drawing a tangent from the onginto a curvilinear relation between NAR and C. This optimal contentdivides a nitrogen-limiting range (C < C_opt) from a carbon-limitingone (C> C_opt) along the axis of nitrogen content. Under nitrogenlimitation, efficiency of nitrogen use in dry-matter productioncould increase if the plant had a larger carbon sink. This givesa qualitative explanation to reduced shoot-to-root ratio underlimited availability of nitrogen. Polygonum cuspidatum Sieb. et Zucc, Japanese knotweed, carbon sink, growth analysis, leaf nitrogen, net assimilation rate, nitrogen use efficiency     

Response of Photosynthesis, Stomatal Conductance and Water Use Efficiency to Elevated CO2 and Nutrient Supply in Acclimated Seedlings of Phaseolus vulgaris L.

Plants of Phaseolus vulgaris L were grown from seed in open-topgrowth chambers at present day (350 µmol mol^–1)and double the present day (700 µmol mol^–1) atmosphericCO₂ concentration with either low (L, without additional nutrientsolution) or relatively high (H, with additional nutrient solution)nutrient supply Measurements of assimilation rate, stomatalconductance and water use efficiency were started 17 d aftersowing on each fully expanded, primary leaf of three plantsper treatment Measurements were made in external CO₂ concentrations(C₂) of 200, 350, 450, 550 and 700 µmol mol^–1 andrelated to both C_a and to C₁, the mean intercellular space CO₂concentration Fully adjusted, steady state measurements weremade after approx 2 h equilibration at each CO₂ concentration The rate of CO₂ assimilation by leaves increased and stomatalconductance decreased similarly over the range of C_a or C₁ inall four CO₂ and nutrient supply treatments but both assimilationrate and stomatal conductance were higher in the high nutrientsupply treatment than in the low nutrient treatment The relationbetween assimilation rate or stomatal conductance and C₁ wasnot significantly different amongst plants grown in present-dayor elevated CO₂ concentration in either nutrient supply treatment,i e there was no evidence of down regulation of photosynthesisor stomatal response Increase in CO₂ concentration from 350to 700 µmol mol^–1 doubled water use efficiency ofindividual leaves in the high nutrient supply treatment andtripled water use efficiency in the low nutrient supply treatment The results support the hypothesis that acclimation phenomenaresult from unbalanced growth that occurs after the seed reservesare exhausted, when the supply of resources becomes growth limiting CO₂ enrichment, Phaseolus vulgaris L., net CO₂ assimilation rate, stomatal conductance, water use efficiency     

Skylark optimal flight speeds for flying nowhere and somewhere

Flight is energetically very costly. For birds the mechanicalpower in relation to airspeed is characterized by a U-shapedfunction. From this function we can derive optimal flight speedsassociated with minimum power (V_mp), minimum cost of transport(V_mr) and minimum overall time of migration (V_mt). Since flightis energetically so costly, aerial displays and song flightcan potentially serve as signals reliably indicating the individualquality or resource potential of the signaler. In order to maximizethe amount of song flight produced, we expect V_mp during songflight, while during migration we rather expect V_mr or V_mv Wecompared flight speeds of skylarks (Alauda arvensis) duringsong flight and migration flight, respectively. In this speciespredicted V_mp = 5.5 m/s, V_mr = 10.5 m/s, and V_mt = 12.1 m/s.The preferred airspeed during song flight did not differ significantlyfrom the predicted V_mp, while airspeed during migration wassignificantly higher than V_mr and Vmp indicating that flightspeed is a flexible trait that birds adjust to different situations.Why the skylarks speed up so much on migration is still unclear,but it may be that due to the shape of the predicted power curve,variation in cost of transport at high speeds is relativelysmall.     

An Analysis of Growth of Oil Palm Seedlings in Full Daylight and in Shade

Growth of seedling oil palms (Elaeis guineensis), in full daylightand under three levels of shade, was studied using growth analysistechniques. In full day-light, net assimilation rates (E_A) betweeno.15 and 0.32 g./dm.²/week were obtained associated with lowleaf-area ratios (F) giving relative growth-rates (R_W) rangingfromI I.8 to 3.2 per cent, per day. There were no indicationsof seasonal differences within the small range of values found. The plants take about 90 days to adapt to shade conditions becausethe mean plastochron is 24 days, and shading effects are beststudied on plants grown since germination under the shade treatments.Very different pictures of response to shade were obtained usingplants grown initially in full light followed by 90 days' shadebefore sampling compared with plants grown under shade sincegermination. In the latter, except at the lowest light levelused, I I.I per cent, of full day-light, there was very littleeffect of light on E_A or R_w, although the F values decreasedas light increased. Extrapolation of the F values to the extinctionpoint gave values similar to those obtained in another experimenton the effect of a number of shade levels on F. The physiological and ecological implications of these findings,particularly the low growth-rates and shade tolerance, are discussed.     

Fluorescence induction in chloroplasts isolated from the green alga Bryopsis maxima III. A fluorescence transient indicating proton gradient across the thylakoid membrane

The yield of chlorophyll a fluorescence in dark-adapted intactchloroplasts isolated from the green alga, Bryopsis maxima,showed, after the first wave of the fluorescence induction wasover, a peak labelled M₁ at about 10th sec of illumination.The time to reach M₁ during continuous illumination inverselydepended upon exciting light intensity. The appearance of thepeak M₁ was accelerated by the addition of methyl viologen aselectron acceptor and delayed in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea.KCN had no effect on the peak M₁ at a concentration where photosyntheticoxygen evolution was completely suppressed. Thus, the peak M₁appears to be related to electron transport but not to the carbonreducing cycle. Carbonylcyanide m-chlorophenylhydrazone, NH₄Cl and methylaminediminished or eliminated M₁. On the other hand, an enhancementof the fluorescence yield at M₁ was observed in the presenceof energy transfer inhibitors. Valinomycin plus KCl also increasedheight of the peak M₁. However, the combined addition of valinomycinand dinitrophenol resulted in the complete elimination of thepeak M₁. These results indicate that the fluorescence peak M₁occurring at about 10 sec of illumination is linked to a protongradient across the thylakoid membrane. (Received July 7, 1977; )     

Growth Analysis of Solution Culture-grown Winter Rye, Wheat and Triticale at Different Relative Rates of Nitrogen Supply

At low nitrogen (N) supply, it is well known that rye has ahigher biomass production than wheat. This study investigateswhether these species differences can be explained by differencesin dry matter and nitrogen partitioning, specific leaf area,specific root length and net assimilation rate, which determineboth N acquisition and carbon assimilation during vegetativegrowth. Winter rye (Secale cereale L.), wheat (Triticum aestivumL.) and triticale (X Triticosecale) were grown in solution cultureat relative addition rates (R_N) of nitrate-N supply rangingfrom 0.03–0.18 d^-1and at non-limiting N supply under controlledconditions. The relative growth rate (R_W) was closely equalto R_Nin the range 0.03–0.15 d^-1. The maximalR_W at non-limitingnitrate nutrition was approx. 0.18 d^-1. The biomass allocationto the roots showed a considerable plasticity but did not differbetween species. There were no interspecific differences ineither net assimilation rate or specific leaf area. Higher accumulationof N in the plant, despite the same relative growth rate atnon-limiting N supplies, suggests that rye has a greater abilityto accumulate reserves of nitrogen. Rye had a higher specificroot length over a wide range of sub-optimal N rates than wheat,especially at extreme N deficiency (R_N=0.03–0.06 d^-1).Triticale had a similar specific root length as that of wheatbut had the ability to accumulate N to the same amount as ryeunder conditions of free N access. It is concluded that thebetter adaptation of rye to low N availability compared to wheatis related to higher specific root length in rye. Additionally,the greater ability to accumulate nitrogen under conditionsof free N access for rye and triticale compared to wheat maybe useful for subsequent N utilization during plant growth.In general, species differences are explained by growth componentsresponsible for nitrogen acquisition rather than carbon assimilation.Copyright 1999 Annals of Botany Company Growth analysis, nitrogen, nitrogen productivity, partitioning, specific root length, Secale cereale L.,Triticum aestivum L., X Triticosecale, winter rye, winter wheat, winter triticale.     

Responses of CO2 assimilation to changes in irradiance: laboratory and field data and a model for beans (Phaseolus vulgaris L.)

The responses of net CO₂ assimilation to sudden changes in irradiancewere studied in Phaseolus vulgaris L. in the laboratory andthe field. For irradiance changes between 50 µmol m^–2s^–1 to 350 µmol m^–2 s^–1 in the laboratory,assimilation rate increased with half-times of 2.7 and 4.1 minin well-watered and water-stressed plants, respectively. Ina field experiment with a change in irradiance from 400 to 1200µmol m^–2 s^–1 the response was faster (half-time=c.1.2 min). In all cases when irradiance was returned to a lowvalue, assimilation declined rapidly with a half-time of approximately1 min, which approached the time resolution of the gas-exchangesystem. The corresponding changes in stomatal conductance in responseto both increasing and decreasing irradiance were much slowerthan the assimilation responses, indicating that biochemicalprocesses, rather than CO₂ supply, primarily determined theactual rate of assimilation in these experiments. The conceptof stomatal limitation to photosynthesis is discussed in relationto these results. A simple model for assimilation in a fluctuating light environmentis proposed that depends on a steadystate light response curve,an ‘induction lag’ on increasing irradiance, andan induction-state memory. The likely importance of taking accountof such induction lags in natural canopy microclimates is considered. Key words: Models, Phaseolus vulgaris, photosynthetic induction, CO₂ assimilation, stomatal limitation, sunflecks, water stress     

The Effect of Segment Length on Conductance Measurements in Lonicera fragrantissima

The effect of stem segment length on conductance measurementswas determined by repeatedly shortening the transport distancein isolated stem segments of Lonicera fragrantissima. If vesselends limit water flow, shorter segment lengths could resultin higher readings of K_h (hydraulic conductance per unit pressuregradient). The mean K_h in the successively shortened segmentsshowed no statistically significant change when shortened from20 to 2 cm, even though maximum vessel lengths ranged from 6to 14 cm. This suggests vessel ends may be less limiting towater flow than are vessel lumens. There was a statisticallysignificant drop in K_h (19%, s.e.=0·29%) when segmentswere shortened from 2 to 1 cm, regardless of the position ofthe segment within the branch. The K_h remained constant withapplied pressure in 2 cm segments while it was more variablewith applied pressure in 1 cm segments. Based on the paint-infusionmethod mean vessel lengths were from 0·6 to 1·2cm. The consistently lower K_h in 1 cm segments might be dueto an end effect in segments with a high percentage of cut openvessels. Within a branch, distal and proximal stem segmentshad lower K_h values than segments from intermediate positions,but nodes had no measurable impact on K_h. Longer segments allowfor more accurate and consistent measurements of K_h since theyintegrate localized variation in K_h while minimizing the segmentend effect. Key words: Vessel length, vessel end, hydraulic conductance, paint-infusion method, Lonicera fragrantissima     

The Effect of Vapour Pressure Deficit on Carbon Isotope Discrimination in the Desert Shrub Larrea tridentata(Creosote Bush)

Larrea tridentata (creosote bush) seedlings were subjected tothree regimens of atmospheric humidity in a growth chamber experiment.Relative humidity was varied to achieve daytime vapour pressuredeficits (VPD) during growth of 29, 48 and 77 kPa. Photosyntheticgas exchange, carbon isotope composition and biomass productionwere measured after 8–10 weeks of treatment. Whereas stomatalconductance (g) declined linearly with increasing ambient VPD,CO₂ assimilation rate (A) was not measurably affected by changesin ambient VPD. This resulted in a decrease in intrinsic wateruse efficiency (ratio of CO₂ assimilation to stomatal conductance;A/_g) with increasing VPD. Leaf carbon isotope discrimination(A) was negatively correlated (r² = 088) with A/g ratios. Carbonisotope discrimination also correlated positively with ratiosof internal (C₁) to ambient (c_a) CO₂ levels determined by gasexchange measurements (c₁/c). The ratio of c₁ to c_a was lowerat higher VPD levels. Leaf biomass decreased with increasingambient VPD and correlated positively with. Root to leaf biomassratio increased at higher VPD levels and correlated negativelywith. Key words: Larrea tridentata, vapour pressure deficit, carbon isotope discrimination, intrinsic water-use efficiency