Assumptions of Plastochron Index: Evaluation With Soya Bean Under Field Drought Conditions

Erickson and Michelini (1957) derived the plastochron index(PI) and a term sometimes referred to as the plastochron ratio(PR), as quantitative expressions of the vegetative developmentof plants. With the stable plant growth in environmental chambersand glasshouses, the assumptions used to derive these termshave been validated. However, more recently these expressionsare being used to characterize growth under the unstable conditionsresulting from the imposition of stress. This study examinesthe validity of the assumptions used to derive PI and PR forfield-grown soya beans [Glycine max (L.) Merrill] subjectedto drought stress. Under stress conditions, the assumptionswere not satisfied. In fact, observing change in PR appearedto be a good method for detecting drought stress in these plants.An alternate method for calculating PI based on a single, youngleaf was developed. This alternate method appeared to be a moresensitive indicator of changes in leaf emergence rate underunstable conditions. Plastochron index, plastochron ratio, Glycine max (L.), soya bean, drought, leaf growth     

Proline Accumulation and Relative Water Content in Soya Bean (Glycine max) Varieties under Water Stress

The effects of water stress on proline accumulation and relativewater content of four soya bean varieties were studied. Twolocal strains, Kalitur and T-49, showed a smaller increase inproline content and greater reduction in relative water contentthan did JS-2 or Bragg under the same conditions of stress. Glycine max, proline accumulation, relative water content, water stress     

Water Potential and Component Potentials in Expanded and Unexpanded Leaves of Two Xeric Grasses

Water (ψ), osmotic (ψ_s+ψ_m) and pressure (ψ_p) potentials were measured in three leaf regions of Agropyron dasystachyum and A. smithii grown in the field. Spanner-type thermocouple psychrometers were used to measure ψ and (ψ_m+ψ_m). Absolute water content (AWC) was measured gravimetrically. The ψ and ψ_p were slightly lower in the emerging leaf blade (EBI) than in the last fully emerged leaf blade (FEBI); (ψ_s+ψ_m) and AWC were similar in the two regions. A gradient as large as 0.7 MPa was observed between the EBI and the base of the same emerging leaf (EBs); the latter included the meristematic regions. Although (ψ_s+ψ_m) and ψ_p were generally higher in the EBs, the gradients diminished as the level of stress increased in the shoot. Under moderate water stress the ψ_p of the EBs remained constant relative to the ψ_p in the exposed blades. The large ψ gradient within the growing leaf could have resulted from high resistance imposed by poor vascular development in the intercalary meristem. Ability to maintain a relatively large ψ gradient may be of general significance in buffering the growing region of xeric grass leaves from extreme, short-term fluctuations in water stress that occur in exposed leaf blades.     

Changes in the Water Balance and Photosynthesis of Onion, Bean and Cotton Plants under Saline Conditions

The osmotic concentration (osmotic potential) of onion leaf sap did not adjust to chloride salinity, and consequently water potential, turgor, stomatal aperture and transpiration were reduced. Although osmotic concentration of bean and cotton leaf sap did adjust to a saline root medium and turgor was no less in the salinized plants than in the controls, stomata of the salinized plants remained only partly open and transpiration was reduced. Net photosynthesis of onion plants was reduced by salinity (this effect being much enhanced in a hot dry atmosphere) but it could be rapidly raised to the level of the controls by inducing elevated leaf turgor. Stomatal closure was initially responsible for most of the ～30 % reduction in photosynthesis of salinized beans. This was due to interference with CO₂ diffusion and could be overcome by raising the CO₂ concentration in the air. At a later stage of growth, salinity affected the light reaction of bean photosynthesis, and elevation of the air CO₂ had little effect. Closure of stomata of salinized cotton plants had only a relatively small effect on net photosynthesis. Light intensity and CO₂ concentration experiments showed that salinity was reducing the photosynthesis of cotton leaves mainly by affecting the light reaction of photosynthesis. It is concluded that chloride salinity does affect the water balance and rate of photosynthesis of plants and that the nature and degree of the effects will depend upon climatic conditions and may be very different between plant species and in the same species at different periods of growth.     

Photoinhibition of Photosynthesis in Vitis berlandieri and Vitis rupestris Leaves under Field Conditions

Photoinhibition of photosynthesis was investigated in Vitis berlandieri and Vitis rupestris leaves under field conditions at different sampling time in a day. The degree of photoinhibition was determined by means of the ratio of variable to maximum chlorophyll fluorescence (F_v/F_m) and photosynthetic electron transport measurements. When the photochemical efficiency of PS2, F_v/F_m, markedly declined, F₀ increased significantly in leaves of V. berlandieri, while F₀ did not increase in V. rupestris leaves. Isolated thylakoids of leaves of V. berlandieri showed significant inhibition of whole chain and PS2 activities at midday. A smaller inhibition was observed for V. rupestris. Later, the leaves reached maximum PS2 efficiencies similar to those observed early in the morning during sampling at evening. The artificial exogenous electron donor Mn²⁺ failed to restore PS2 activity in both species, while DPC and NH₂OH significantly restored PS2 activity in V. rupestris midday leaf samples. Quantification of the PS2 reaction centre protein D1 and 33 kDa protein of water splitting complex following midday exposure of leaves showed pronounced differences between V. berlandieri and V. rupestris leaves. The marked loss of PS2 activity noticed in midday samples was mainly due to the marked loss of D1 protein in V. berlandieri while in V. rupestris it was the 33 kDa protein.     

水分胁迫下荔枝叶片过氧化物酶和IAA氧化酶活性的变化

以适应山地栽培的抗旱性较强的东刘1号和适应河边栽培的抗旱性较弱的陈紫2年生荔枝（Litchi chinensis Sonn.)实生苗为试验材料,研究了水分胁迫下叶片细胞胞质,与（细胞）壁以离子键结合和壁以共价键结合的过氧化的酶（POD）和IAA氧化酶活性的变化。结果表明：在叶片中POD主要是以与壁以离子键结合的POD存在,占总活性的51．15％－52．15％,其次是细胞胞质POD,占44．20％－44．74％,与壁以共价键结合的POD活性最低,仅占3．44％－3．65％。与POD不同,IAA氧化酶绝大多数存在于细胞胞质中,占总活性的88．93％－89．29％,其次是少量的与壁以离子键结合的IAA氧化酶,占7．32％－7．63％,与壁以共价键结合的IAA氧化酶活性最低,仅占3．39％－3．44％;2个品种间差异不明显。水分胁迫下,叶片细胞胞质以及与壁以离子键和壁以共价键结合的POD和IAA氧化酶（比）活性均上升,抗旱笥较强的品种上升的幅度均大于抗旱性较弱的品种。     

Rate of Increase in Size and Dry Weight of Individual Pods of Field Grown Soya Bean Plants

The dry weight of the whole fruit, the pod wall and an enclosedseed of randomly harvested soya beans is estimated from theexternal dimensions of the attached pod. The relations betweendimensions and dry weight are independent of cultivar and growthcondition and can be used on pods from 1 cm in length untilthe seeds reach their maximum fresh weight. Dimensions of tagged pods of three cultivars of field grownsoya beans differing in time to reach maturity were measuredevery 2–3 days from initial pod elongation until maturation.Dry weights for each pod were estimated from the dimensions,and the dry weight accumulation with time was fitted to thelogistic function to find the growth rate that best characterizedthe data for each pod. The final weight, the specific growthrate and the maximum growth rate of the whole fruit, the podwall and a single seed were subjected to analysis of variance. The most significant difference between pods of these cultivarswas the specific growth rate of individual seeds, which decreasedwith increasing maturity group. There were no differences ingrowth of the pod wall. However, most of the variation was betweenindividual pods within a cultivar, where the rate of dry weightaccumulation of the whole fruit, governed largely by the seedgrowth rate times the number of seeds, was highly correlatedwith the earlier growth of the pod wall. This suggests thatthe growth of individual whole fruit was determined early inpod development and was slightly influenced by factors appliedduring the period of rapid seed growth. Glycine max (L.) Merrill, Soya bean, seed growth analysis, specific growth rate     

Far-Red Light Reflection from Green Leaves and Effects on Phytochrome-Mediated Assimilate Partitioning under Field Conditions

The influence of plant spacing and row orientation on spectral distribution of light received by growing soybean (Gylcine max [L.] Merr.) plants was measured under field conditions. Light absorption, reflection and transmission of individual leaves showed that most of the blue and red was absorbed while most of the far-red was either reflected or transmitted. Plants growing in the field received different ratios of far-red relative to red, depending on nearness and/or orientation of other vegetation. Plants grown in close-spaced rows, or high population densities, received higher far-red/red ratios than did those grown in wide rows, or sparse populations. Heliotropic movements of the leaves also contributed to the far-red reflection patterns associated with row orientation. Under field conditions, differences in far-red/red ratios associated with nearness of competing vegetation became more pronounced with low solar angle near the end of the day. Plants exposed to far-red for 5 minutes at the end of each day in controlled environments, and those grown in close-spaced rows in the field, developed longer internodes and fewer branches. Red, far-red photoreversibility in the controlled environment study indicated involvement of phytochrome. Dry matter partitioning among plant components in the field was related to far-red/red light ratio received during growth and development.     

Diurnal Changes in Endogenous Abscisic Acid in Leaves of Pearl Millet (Pennisetum americanum (L. ) Leeke) under Field Conditions

Diurnal variation in leaf abscisic acid (ABA) content was investigatedin pearl millet (Pennisetum americanum (L. ) Leeke) growingin the field in the semi-arid tropics and subjected to varyingdegrees of water stress. There was a two- to three-fold change in ABA content duringthe photoperiod in three groups of ‘severely’ stressedplants of the genotype BJ 104. Maximum ABA occurred mid-morning(1030 h). ABA levels then declined to a minimum at 1500 h. Changesin ABA content of ‘moderately’ stressed and fullyirrigated plants were smaller, but still significant. Though,when averaged over the day, levels of ABA of the five groupswere positively related to the degree of water stress, relationshipsbetween ABA concentration and total water () or turgor (_p) potentialsvaried considerably with time of sampling. Within groups, changesin ABA contents during the day were not always accounted forby changes in or _p. Temporal changes in leaf ABA content similar to those foundin BJ 104, and largely unrelated to , were observed in the genotypesSerere 39 and B282 in a subsequent year. Leaf ABA content of droughted plants (BJ 104) did not declineappreciably overnight despite a marked increase in . However,a large reduction in ABA content with increase in did occurfollowing heavy rainfall. Diurnal changes in stomatal conductance (g₁) of BJ 104 couldnot be simply accounted for by temporal changes in total leafABA content, even when allowance was made for effects of irradianceand other environmental variables on g₁. It is suggested thatthe sensitivity of stomata to ABA, or accessibility of the hormoneto the stomatal complex, changes during the day.     

Water Stress and Leaf Growth of Field Beans (Vicia faba L.) in the Field: Water Potentials and Laminar Expansion

Field beans were grown in three different irrigation treatments.The growth of each leaf was followed and estimates made of plantwater potential at dawn and of plant water potential, solutepotential, and pressure potential in the afternoon. The growthin area of the leaves against time was fitted with a logisticcurve from which the parameters of leaf growth were defined.The parameters were the area at which a leaf unfolds, the meangrowth rate, and the duration of growth, which combine to givethe final leaf size. Water stress reduced the final leaf sizeby reducing both the area at unfolding and the mean growth rate.The duration of growth was not consistently altered. Final leafsize was closely correlated with the plant water potential inthe afternoon and apparently more with pressure than solutepotential. Vicia faba, field bean, water stress, leaf area, leaf growth, water potential     

Changes in the Threshold of Generation of Action Potentials by Spinal Motoneurons under Conditions of Their Natural Activation

We estimated the peculiarities of changes in the threshold of generation of action potentials (APs) by spinal motoneurons (MNs) related to increases in the frequency of discharges evoked by adequate somatosensory stimulation. In experiments on decerebrated cats, we intracellularly recorded the membrane potential (MP) from MNs of the mm. gastrocnemius + soleus. Natural activation of MNs was provided by controlled stretching of the homonymous muscles; AP thresholds were measured using the first derivative of changes in the MP and statistical limits of variations of this derivative. In different cases, the threshold could either increase or decrease with a rise in the frequency; moreover, oppositely directed changes in the threshold could be observed in some MNs within different phases of stretch-evoked activities of these cells. Dissimilarities of the dependences of the thresholds on the AP frequency were clearly manifested in the case of short-lasting increases in the excitability of the MNs, which were usually observed at the beginning of microelectrode recording. We conclude that, under conditions of natural activation of spinal MNs, the pattern of modulation of the AP threshold upon different intensities of excitation is not strictly determined.     

Fat Metabolism in Higher Plants: LVII. A Comparison of Fatty Acid-Synthesizing Enzymes in Chloroplasts Isolated from Mature and Immature Leaves of Spinach

Chloroplasts isolated from immature leaves of spinach (Spinacia oleracea) differ in enzyme levels from those isolated from mature leaves. On a chlorophyll basis, immature chloroplast preparations had 5- to 6-fold higher capacity to synthesize fatty acids from 2-¹⁴C-acetate compared to plastids isolated from mature leaves. This difference was correlated with higher activities for the enzymes, acetyl coenzyme A synthetase, malonyl coenzyme A synthetase, acetyl coenzyme A carboxylase, and oleyl coenzyme A transferase in plastid pressates obtained from immature leaves. Disrupted chloroplast preparations from both mature and immature leaves retained the ability to incorporate 2-¹⁴C-acetate into fatty acids in a pattern similar to that by isolated chloroplasts. 2-¹⁴C-Acetate, 2-¹⁴C-acetyl coenzyme A, 2-¹⁴C-malonate, and 1,3-¹⁴C malonyl coenzyme A were readily incorporated into a number of fatty acids. Moreover, the synthesis of oleate by chloroplast pressates from these substrates was strongly inhibited by KCN, flavin adenine mononucleotides and dinucleotides, and anaerobic conditions, while linolenic acid synthesis was unaffected by these compounds.     

Chloroplast DNA in Mature and Senescing Leaves: A Reappraisal

The fate of plastid DNA (ptDNA) during leaf development has become a matter of contention. Reports on little change in ptDNA copy number per cell contrast with claims of complete or nearly complete DNA loss already in mature leaves. We employed high-resolution fluorescence microscopy, transmission electron microscopy, semithin sectioning of leaf tissue, and real-time quantitative PCR to study structural and quantitative aspects of ptDNA during leaf development in four higher plant species (Arabidopsis thaliana, sugar beet [Beta vulgaris], tobacco [Nicotiana tabacum], and maize [Zea mays]) for which controversial findings have been reported. Our data demonstrate the retention of substantial amounts of ptDNA in mesophyll cells until leaf necrosis. In ageing and senescent leaves of Arabidopsis, tobacco, and maize, ptDNA amounts remain largely unchanged and nucleoids visible, in spite of marked structural changes during chloroplast-to-gerontoplast transition. This excludes the possibility that ptDNA degradation triggers senescence. In senescent sugar beet leaves, reduction of ptDNA per cell to ∼30% was observed reflecting primarily a decrease in plastid number per cell rather than a decline in DNA per organelle, as reported previously. Our findings are at variance with reports claiming loss of ptDNA at or after leaf maturation.In vascular plants, copy numbers of plastid genomes (plastomes) frequently range from <100 per cell in meristematic cells to several thousand per cell in fully developed diploid leaf parenchyma cells. Microscopy studies have shown that the multicopy organelle genomes are usually condensed in more or less distinct DNA regions (nucleoids) within the organelle matrix or stroma.During development, the ratio of nuclear to organelle genomes appears to be relatively stringently regulated (Herrmann and Possingham, 1980; Rauwolf et al., 2010). Disregarding greatly varying absolute values (summarized in Rauwolf et al., 2010; Liere and Börner, 2013), there is little dispute that the number of plastid genomes and nucleoids per organelle and cell increase during early leaf development in higher plants (Kowallik and Herrmann, 1972; Selldén and Leech, 1981; Baumgartner et al., 1989; Fujie et al., 1994; Li et al., 2006; Rauwolf et al., 2010). This increase is usually accompanied by an increase in both size and number of plastids per cell (Butterfass, 1979). By contrast, data about plastid DNA (ptDNA) amounts in chloroplasts and cells of mature, ageing, and senescent tissue differ and are highly controversial. Basically two patterns have been described: the maintenance of more or less constant amounts of ptDNA per cell and/or organelle (Li et al., 2006; Zoschke et al., 2007; Rauwolf et al., 2010; Udy et al., 2012) or a significant decrease in copy number brought about by either continued organelle and cell division without ptDNA replication (Lamppa and Bendich, 1979; Scott and Possingham, 1980; Tymms et al., 1983) or by ptDNA degradation (Baumgartner et al., 1989; Sodmergen et al., 1991). In a series of communications, Bendich and coworkers recently reported that ptDNA levels decline drastically before leaf maturation in several plant species. In Arabidopsis thaliana and maize (Zea mays), ptDNA levels were reported to decrease early and precipitously as leaves mature. It was concluded that, in fully expanded leaves, most chloroplasts contain no or only insignificant amounts of DNA long before the onset of leaf senescence (Oldenburg and Bendich, 2004; Rowan et al., 2004; Oldenburg et al., 2006; Shaver et al., 2006; Rowan et al., 2009). Retention of ptDNA was proposed to be dispensable after the photosynthetic machinery was established in that the plastome-encoded photosynthesis genes were no longer needed in adult leaves. Degradation or even entire loss of ptDNA was considered as an event during plastid and leaf development, common to all plants (Rowan et al., 2009). ptDNA degradation was also suggested to act as a signal inducing senescence (Sodmergen et al., 1991).A priori, there is no reason why different ptDNA patterns should not occur, and there is indeed evidence that organelle DNA can behave differently in different materials, both quantitatively and structurally (e.g., Selldén and Leech, 1981; Baumgartner et al., 1989). However, since contradictory data were reported for the same species that were grown under comparable, if not identical, conditions (Rowan et al., 2004, 2009; Li et al., 2006; Oldenburg et al., 2006; Shaver et al., 2006; Zoschke et al., 2007; Evans et al., 2010; Udy et al., 2012), it is apparent that some of them must reflect methodological insufficiencies of the experimental approaches employed.From a physiological point of view, the existence of DNA-deficient plastids in photosynthetically competent tissue seems unlikely. For instance, due to its susceptibility to photooxidative damage, the D1 protein (PsbA), a plastome-encoded core subunit of photosystem II, must be replaced continuously by a complex repair system to maintain photosynthesis (Prasil et al., 1992). This replacement requires de novo synthesis of the short-lived D1. There are no data available supporting an extreme mRNA stability, protein stability, or for another compensating biochemistry, preserving organelle functions for weeks or even months. The maximum mRNA half-life reported for psbA is in the range of 40 h (Kim et al., 1993).Resolving this controversy is of considerable scientific interest, both from a theoretical and an applied perspective. We therefore analyzed the fate of ptDNA in mature, ageing, and senescent leaves of four commonly studied higher plant species (Arabidopsis, sugar beet [Beta vulgaris], tobacco [Nicotiana tabacum], and maize; Figure 1) for which conflicting data have been reported. Four complementary methods were used for assessing the presence of ptDNA as well as its quantitative and morphological changes during leaf development: an improved 4′,6-diamidino-2-phenylindole (DAPI)–based fluorescence microscopy approach including deconvolution of fluorescence images, electron microscopy, semithin sectioning across leaf laminas, and real-time quantitative PCR (see Methods).Open in a separate windowFigure 1.Developmental Leaf Series of Sugar Beet, Tobacco, and Arabidopsis.(A) Sugar beet leaves, developmental stages II to VI (left to right; see text). Inset: leaf stages y1 and y3. Arrows indicate necrotic areas. Bar = 5 cm.(B) Tobacco leaves, developmental stages II and IV to VI. Inset: leaf stages y1 and y2. Bar = 5 cm; bar in inset = 1 cm.(C) Arabidopsis plants (left) from which leaves of developmental stages I to VI were taken. Bar = 4 cm.Figure 2, Supplemental Methods, and Supplemental Data Sets 1 to 4 present representative micrographs of developmental series of DAPI-stained chloroplasts in leaf spongy parenchyma cells of late ontogenetic stages from sugar beet, Arabidopsis, tobacco, and maize displaying clearly discernible nucleoid patterns. Figures 1A to 1C document some of the leaves from which samples were taken. Mesophyll cells of juvenile leaves investigated in our previous work (Li et al., 2006; Zoschke et al., 2007; Rauwolf et al., 2010) were included for comparison (Supplemental Data Sets 1 to 4, panels 1 to 37, 84 to 94, 112 to 117, and 123 to 128). The staining specificity of the fluorochrome was confirmed enzymatically. Treatment with DNase, but not DNase-free RNase or Proteinase K, either before or after staining with the fluorochrome, abolished the fluorescence but did not significantly affect chloroplast structure (compare with Rauwolf et al., 2010; see Methods).Open in a separate windowFigure 2.DAPI-DNA Fluorescence of Mature, Senescent, and Prenecrotic Leaf Mesophyll Cells or Cell Segments.Representative DAPI-stained squashed mesophyll cells of sugar beet ([A] to [C]), Arabidopsis ([D] to [F]), tobacco ([G] and [H]), and maize ([I] and [J]) leaflets or leaves (cell detail in [C], [E], [F], and [H]) of the developmental stages III/IV (I), IV ([A] and [D]), V ([B], [E], and [G]), and VI ([C], [F], [H], and [J]). Note that (E) represents a cell fragment of Supplemental Data Set 2, panel 102. Bar = 5 μm in (A), also for (B) to (J).     

Carbon Exchange Rate of Intact Individual Soya Bean Pods. 1. Response to Step Changes in Light and Temperature

Carbon exchange rates (CER) of individual intact field-grownsoya bean [Glycine max (L.) Merr.] pods were measured continuouslywith a mobile gas analysis laboratory. Conditions in pod chamberssimulated those experienced normally by pods except for experimentalmodification of incident radiation or pod temperature. Undernormal conditions, CER (where positive CER represents CO₂ evolution)fluctuated diurnally with a morning rise followed by a slowafternoon and evening decline which was similar among pods whichwere measured simultaneously. The frequency of measurementspermitted detection of rapid CER responses to step changes inlight and pod temperature. CER rapidly decreased and increasedwhen the chamber was alternately exposed to full sunlight andcomplete darkness, respectively. CER responded similarly tosteps up [from ambient to elevated (+ 10°C) temperature]and steps down (from elevated to ambient temperature), respectively.Thus, a temperature sensitive process which regulated pod CERwas located within the pod. CER ranged from less than 0·1to more than 1·2 mg CO₂ h^–1 pod^–1 over theperiod of rapid dry-matter accumulation. Glycine max (L.) Merr., Soya bean, carbon exchange rate, light, temperature     

田间小麦叶片光合效率日变化与光合“午睡”的关系

小麦灌浆初期叶片(旗叶)晴天中午光合速率下降(“午睡”)伴随了气孔导度、胞间CO_2浓度下降,而气孔限制值中午升高,进一步证实气孑L中午关闭是光合“午睡”的一个重要原因。叶片光合效率的中午下降并非都伴随着光合“午睡”现象。当两者同时发生时,胞间CO_2浓度降低,而光合速率与气孔导度、胞间CO_2浓度之间的相关性高于光合速率与光合效率之间的相关性。这些事实表明。即使光合效率中午下降是光合“午睡”的部分原因,但较之气孔中午关闭只是一个次要原因。     

水分胁迫下花生叶片AhNCED1蛋白表达与气孔开度的变化

研究了水分胁迫下不同花生抗旱品种叶片气孔开度和相对含水量变化,分析TAhNCEDI基因和AhNCED1蛋白进行表达情况,发现水分胁迫下,叶片相对含水量下降,叶片气孔开度降低,叶片AhNCED1基因和AhNCED1蛋白表达增强。抗旱品种较之敏旱品种在响应水分胁迫初期时（1h）AhNCED1基因和AhNCED1蛋白表达较强,叶片气孔开度下降较快,引发气孔关闭,其叶片相对含水量较高,保水能力较强。ABA合成抑制剂naproxen处理后,叶片AhNCED1基因和AhNCED1蛋白的表达减弱,气孔开度快速增加,水分胁迫下花生叶片AhNCED1蛋白表达可能影响气孔开闭。     

A Dynamic Growth Model of Vegetative Soya Bean Plants: Model Structure and Behaviour under Varying Root Temperature and Nitrogen Concentration

A differential equation model of vegetative growth of the soyabean plant (Glycine max (L.) Merrill cv. ‘Ransom’)was developed to account for plant growth in a phytotron systemunder variation of root temperature and nitrogen concentrationin nutrient solution. The model was tested by comparing modeloutputs with data from four different experiments. Model predictionsagreed fairly well with measured plant performance over a widerange of root temperatures and over a range of nitrogen concentrationsin nutrient solution between 0.5 and 10.0 mmol in the phytotron environment. Sensitivity analyses revealedthat the model was most sensitive to changes in parameters relatingto carbohydrate concentration in the plant and nitrogen uptakerate. Key words: Glycine max (L.) Merrill, dry matter, nitrogen uptake, partitioning, photosynthesis, respiration, sensitivity analysis     

Importance of Aerodynamic Resistance to Water Use Efficiency in Three Conifers under Field Conditions

The quantitative importance of aerodynamic resistance to H₂O vapor and CO₂ exchange was determined for shoots from saplings of three conifers (Abies lasiocarpa [Hook] Nutt., Pinus contorta Dougl., Juniperus communis L.) under natural conditions in the field. A combination of relatively low stomatal resistances (<300 seconds per centimeter) and low wind speeds (<30 centimeters per second) led to substantial contributions of the aerodynamic resistance (R_wv^a) to water use efficiency (WUE = photosynthesis/transpiration) for all three species. For A. lasiocarpa, transpiration was calculated to be 44% less and photosynthesis 17% less due to the presence of R_wv^a, which led to a predicted increase in WUE of 57% compared to the calculated WUE when R_wv^a was assumed negligible. Similar increases in WUE were computed for P. contorta (48%) with somewhat smaller values for J. communis (34%). These results are discussed in terms of the estimated importance of R_wv^a on water and photosynthetic relations of plants that have relatively low stomatal resistances and grow in microhabitats with low winds.     

Measured and Calculated Transpiration in Trifolium repens under Different Water Potentials

The relationship between transpiration measured gravimetrically,a generalized transpiration equation, and the ratio VPD/r_leafwas investigated in Trifolium repens plants subjected to varyingwater potentials. Dawn leaf water potential was measured witha pressure chamber, leaf diffusion resistance with a diffusionporometer, leaf temperatures with a thermistor, and relativehumidity with an aspirated psychrometer. During drought transpirationrates determined by both methods were quite similar particularlyat the lowest water potentials. After rewatering calculatedrates were somewhat higher than measured ones. It is concludedthat transpiration calculated by the indirect method is a usefuland reasonable estimate of transpiration for single plants undervarying water potentials.