Light Absorption and Competition in Mixed Communities of Hordeum Leporinum-Euphorbia Scordifolia

The effect of Euphorbia scordifolia and Hordeum leporinum competition on leaf area development, radiant energy absorption, and dry matter production was evaluated in a field experiment. Profile measurements (0-0.3, 0.3-0.6, 0.6-0.9, and >0.9 m above ground) of absorbed photosynthetically active radiation (APAR) and leaf area index (LAI) by species were taken at four densities of E. scordifolia (0, 1, 4, and 12 plants per m²). APAR calculated for H. leporinum in mixed communities was 79, 77, and 49 % of the APAR in H. leporinum and LAI was reduced to 81, 65, and 37 %. LAI of H. leporinum was concentrated in the 0.3-0.6 m layer, while the taller E. scordifolia plants had the greatest LAI above 0.6 m. By absorbing radiant energy in the upper canopy, E. scordifolia reduced APAR penetrating to H. leporinum. Measurements of net photosynthetic and transpiration rates, leaf temperature, and stomatal conductance confirmed the importance of competition for PAR for plant growth and metabolism. This revised version was published online in June 2006 with corrections to the Cover Date.     

On the Mechanism of Resistance to Paraquat in Hordeum glaucum and H. leporinum: Delayed Inhibition of Photosynthetic O(2) Evolution after Paraquat Application

The mechanism of resistance to paraquat was investigated in biotypes of Hordeum glaucum Steud. and H. leporinum Link. with high levels of resistance. Inhibition of photosynthetic O₂ evolution after herbicide application was used to monitor the presence of paraquat at the active site. Inhibition of photosynthetic O₂ evolution after paraquat application was delayed in both resistant biotypes compared with the susceptible biotypes; however, this differential was more pronounced in the case of H. glaucum than in H. leporinum. Similar results could be obtained with the related herbicide diquat. Examination of the concentration dependence of paraquat-induced inhibition of O₂ evolution showed that the resistant H. glaucum biotype was less affected by herbicide compared with the susceptible biotype 3 h after treatment at most rates. The resistant H. leporinum biotype, in contrast, was as inhibited as the susceptible biotype except at the higher rates. In all cases photosynthetic O₂ evolution was dramatically inhibited 24 h after treatment. Measurement of the amount of paraquat transported to the young tissue of these plants 24 h after treatment showed 57% and 53% reductions in the amount of herbicide transported in the case of the resistant H. glaucum and H. leporinum biotypes, respectively, compared with the susceptible biotypes. This was associated with 62% and 66% decreases in photosynthetic O₂ evolution of young leaves in the susceptible H. glaucum and H. leporinum biotypes, respectively, a 39% decrease in activity for the resistant H. leporinum biotype, but no change in the resistant H. glaucum biotype. Photosynthetic O₂ evolution of leaf slices from resistant H. glaucum was not as inhibited by paraquat compared with the susceptible biotype; however, those of resistant and susceptible biotypes of H. leporinum were equally inhibited by paraquat. Paraquat resistance in these two biotypes appears to be a consequence of reduced movement of the herbicide in the resistant plants; however, the mechanism involved is not the same in H. glaucum as in H. leporinum.     

The mechanism of resistance to paraquat is strongly temperature dependent in resistant Hordeum leporinum Link and H. glaucum Steud.

Paraquat-resistant biotypes of the closely-related weed species Hordeum leporinum Link and H. glaucum Steud. are highly resistant to paraquat when grown during the normal winter growing season. However, when grown and treated with paraquat in summer, these biotypes are markedly less resistant to paraquat. This reduced resistance to paraquat in summer is primarily a result of increased temperature following herbicide treatment. The mechanism governing this decrease in resistance at high temperature was examined in H. leporinum. No differences were observed between susceptible and resistant biotypes in the interaction of paraquat with isolated thylakoids when assayed at 15, 25, or 35 °C. About 98 and 65% of applied paraquat was absorbed through the leaf cuticle of both biotypes at 15 and 30 °C, respectively. Following application to leaves, more herbicide was translocated in a basipetal direction in the susceptible biotype compared to the resistant biotype at 15 °C. However, at 30 °C more paraquat was translocated in a basipetal direction in the resistant biotype. Photosynthetic activity of young leaf tissue from within the leaf sheath which had not been directly exposed to paraquat was measured 24 h after treatment of plants with para. quat. This activity was inhibited in the susceptible biotype when plants were maintained at either 15 °C or 30 °C after treatment. In contrast, photosynthetic activity of such tissue of the resistant biotype was not inhibited when plants were maintained at 15 °C after treatment, but was inhibited at 30 °C. The mechanism of resistance in this biotype of H. leporinum correlates with decreased translocation of paraquat and decreased penetration to the active site. This mechanism is temperature sensitive and breaks down at higher temperatures.We are grateful to Zeneca Agrochemicals, Jealotts Hill, Berkshire, UK who provided [¹⁴C]paraquat. E.P. was supported through a Ph.D. scholarship from the Australian International Development Assistance Bureau and C.P. was the recipient of an Australian Research Council Postdoctoral Fellowship.     

Negative effects of habitat degradation and fragmentation on the declining grassland plant Trifolium montanum

Changes in land use have resulted in a strong decline in the plant diversity of nutrient-poor grasslands, but little is known about the combined effects of habitat degradation and fragmentation on populations of individual species. We studied these effects on stage structure, recruitment, reproduction and offspring fitness in populations of the declining perennial grassland plant Trifolium montanum in central Germany. Density and survival probability of juvenile plants decreased with light competition, measured as leaf area index (LAI) above T. montanum plants, resulting in aged populations with few juvenile plants at unmanaged sites with higher LAI. Reproduction of T. montanum was not related to LAI, but increased strongly with local density, suggesting pollinator limitation in fragmented populations with a low density of flowering plants. In the common garden, the survival of sown offspring increased with mean seed size, whereas seed production of offspring decreased with isolation, and in strong contrast to previous studies, also decreased with size and density of the population of origin. This could be due to increased inbreeding because of pollination between closely related neighbouring plants in dense and large populations. Our results indicate that both habitat degradation and fragmentation have negative effects on populations of T. montanum, but affect different phases of the life cycle. In the short term, the effects of habitat degradation are more important than those of fragmentation, and populations of T. montanum are primarily threatened by an increase in light competition in unmanaged sites, which rapidly affects the dynamics of the populations. The observed opposite effects of habitat fragmentation on reproduction and offspring fitness indicate that the effects of population size, density and isolation on plant fitness and population viability may be complex.     

Insufficient nitrogen supply from symbiotic fixation reduces seasonal crop growth and nitrogen mobilization to seed in highly productive soybean crops

Nitrogen (N) supply can limit the yields of soybean [Glycine max (L.) Merr.] in highly productive environments. To explore the physiological mechanisms underlying this limitation, seasonal changes in N dynamics, aboveground dry matter (ADM) accumulation, leaf area index (LAI) and fraction of absorbed radiation (fAPAR) were compared in crops relying only on biological N₂ fixation and available soil N (zero-N treatment) versus crops receiving N fertilizer (full-N treatment). Experiments were conducted in seven high-yield environments without water limitation, where crops received optimal management. In the zero-N treatment, biological N₂ fixation was not sufficient to meet the N demand of the growing crop from early in the season up to beginning of seed filling. As a result, crop LAI, growth, N accumulation, radiation-use efficiency and fAPAR were consistently higher in the full-N than in the zero-N treatment, leading to improved seed set and yield. Similarly, plants in the full-N treatment had heavier seeds with higher N concentration because of greater N mobilization from vegetative organs to seeds. Future yield gains in high-yield soybean production systems will require an increase in biological N₂ fixation, greater supply of N from soil or fertilizer, or alleviation of the trade-off between these two sources of N in order to meet the plant demand.     

Salicylic acid promotes plant growth and salt-related gene expression in <Emphasis Type="Italic">Dianthus superbus</Emphasis> L. (Caryophyllaceae) grown under different salt stress conditions

Salt stress is a critical factor that affects the growth and development of plants. Salicylic acid (SA) is an important signal molecule that mitigates the negative effects of salt stress on plants. To elucidate salt tolerance in large pink Dianthus superbus L. (Caryophyllaceae) and the regulatory mechanism of exogenous SA on D. superbus under different salt stresses, we conducted a pot experiment to evaluate leaf biomass, leaf anatomy, soluble protein and sugar content, and the relative expression of salt-induced genes in D. superbus under 0.3, 0.6, and 0.9% NaCl conditions with and without 0.5 mM SA. The result showed that exposure of D. superbus to salt stress lead to a decrease in leaf growth, soluble protein and sugar content, and mesophyll thickness, together with an increase in the expression of MYB and P5CS genes. Foliar application of SA effectively increased leaf biomass, soluble protein and sugar content, and upregulated the expression of MYB and P5CS in the D. superbus, which facilitated in the acclimation of D. superbus to moderate salt stress. However, when the plants were grown under severe salt stress (0.9% NaCl), no significant difference in plant physiological responses and relevant gene expression between plants with and without SA was observed. The findings of this study suggest that exogenous SA can effectively counteract the adverse effects of moderate salt stress on D. superbus growth and development.     

Leaf dynamics of a deciduous forest canopy: no response to elevated CO<Subscript>2</Subscript>

Leaf area index (LAI) and its seasonal dynamics are key determinants of terrestrial productivity and, therefore, of the response of ecosystems to a rising atmospheric CO₂ concentration. Despite the central importance of LAI, there is very little evidence from which to assess how forest LAI will respond to increasing [CO₂]. We assessed LAI and related leaf indices of a closed-canopy deciduous forest for 4 years in 25-m-diameter plots that were exposed to ambient or elevated CO₂ (542 ppm) in a free-air CO₂ enrichment (FACE) experiment. LAI of this Liquidambar styraciflua (sweetgum) stand was about 6 and was relatively constant year-to-year, including the 2 years prior to the onset of CO₂ treatment. LAI throughout the 1999–2002 growing seasons was assessed through a combination of data on photosynthetically active radiation (PAR) transmittance, mass of litter collected in traps, and leaf mass per unit area (LMA). There was no effect of [CO₂] on any expression of leaf area, including peak LAI, average LAI, or leaf area duration. Canopy mass and LMA, however, were significantly increased by CO₂ enrichment. The hypothesized connection between light compensation point (LCP) and LAI was rejected because LCP was reduced by [CO₂] enrichment only in leaves under full sun, but not in shaded leaves. Data on PAR interception also permitted calculation of absorbed PAR (APAR) and light use efficiency (LUE), which are key parameters connecting satellite assessments of terrestrial productivity with ecosystem models of future productivity. There was no effect of [CO₂] on APAR, and the observed increase in net primary productivity in elevated [CO₂] was ascribed to an increase in LUE, which ranged from 1.4 to 2.4 g MJ^–1. The current evidence seems convincing that LAI of non-expanding forest stands will not be different in a future CO₂-enriched atmosphere and that increases in LUE and productivity in elevated [CO₂] are driven primarily by functional responses rather than by structural changes. Ecosystem or regional models that incorporate feedbacks on resource use through LAI should not assume that LAI will increase with CO₂ enrichment of the atmosphere.     

Water relations and growth responses of Uniola paniculata (sea oats) to soil moisture and water-table depth

Summary This study examined the water relations and growth responses of Uniola paniculata (sea oats) to (1) three watering regimes and (2) four controlled water-table depths. Uniola paniculata is frequently the dominant foredune grass along much of the southeastern Atlantic and Gulf coasts of the United States, but its distribution is limited in Louisiana. Throughout most of its range, U. paniculata tends to dominate and be well adapted to the most exposed areas of the dune where soil moisture is low. Dune elevations in Louisiana, however, rarely exceed 2 m, and as a result the depth to the water table is generally shallow. We hypothesized that if U. paniculata grows very near the water-table, as it may in Louisiana, it will display signs of water-logging stress. This study demonstrated that excessive soil moisture resulting from inundation or shallow water-table depth has a greater negative effect on plant growth than do low soil moisture conditions. Uniola paniculata's initial response to either drought or inundation was a reduction of leaf (stomatal) conductance and a concomitant decrease in leaf elongation. However, plants could recover from drought-induced leaf xylem pressures of less than-3.3 MPa, but prolonged inundation killed the plants. Waterlogging stress (manifested in significantly reduced leaf stomatal conductances and reduced biomass production) was observed in plants grown at 0.3 m above the water table. This stress was relieved, however, at an elevation of 0.9 m above the water table. As the elevation was increased from 0.9 to 2.7 m, there were no signs of drought stress nor a stimulation in growth due to lower soil moisture. We concluded that although U. paniculata's moisture-conserving traits adapt it well to the dune environment, this species can grow very well at an elevation of only 0.9 m above the water table. Field measurements of water-table depth in three Louisiana populations averaged about 1.3 m. Therefore, the observed limited distribution of U. paniculata along the Louisiana coast apparently cannot be explained by water-logging stress induced by the low dune elevations and the corresponding shallow water-table depth.     

Acclimation of CO(2) Assimilation in Cotton Leaves to Water Stress and Salinity

Cotton (Gossypium hirsutum L. cv Acala SJ2) plants were exposed to three levels of osmotic or matric potentials. The first was obtained by salt and the latter by withholding irrigation water. Plants were acclimated to the two stress types by reducing the rate of stress development by a factor of 4 to 7. CO₂ assimilation was then determined on acclimated and nonacclimated plants. The decrease of CO₂ assimilation in salinity-exposed plants was significantly less in acclimated as compared with nonacclimated plants. Such a difference was not found under water stress at ambient CO₂ partial pressure. The slopes of net CO₂ assimilation versus intercellular CO₂ partial pressure, for the initial linear portion of this relationship, were increased in plants acclimated to salinity of −0.3 and −0.6 megapascal but not in nonacclimated plants. In plants acclimated to water stress, this change in slopes was not significant. Leaf osmotic potential was reduced much more in acclimated than in nonacclimated plants, resulting in turgor maintenance even at −0.9 megapascal. In nonacclimated plants, turgor pressure reached zero at approximately −0.5 megapascal. The accumulation of Cl⁻ and Na⁺ in the salinity-acclimated plants fully accounted for the decrease in leaf osmotic potential. The rise in concentration of organic solutes comprised only 5% of the total increase in solutes in salinity-acclimated and 10 to 20% in water-stress-acclimated plants. This acclimation was interpreted in light of the higher protein content per unit leaf area and the enhanced ribulose bisphosphate carboxylase activity. At saturating CO₂ partial pressure, the declined inhibition in CO₂ assimilation of stress-acclimated plants was found for both salinity and water stress.     

Bulb Development in Onion (Allium cepa L.) III. Effects of the Size of Adjacent Plants, Shading by Neutral and Leaf Filters, Irrigation and Nitrogen Regime and the Relationship between the Red: Far-red Spectral Ratio in the Canopy and Leaf Area Index

Field experiments were done to investigate why onion crops witha high leaf area index (LAI) initiate bulb scales and maturesooner than those with a low LAI. When small plants were growneither mixed with large plants or in pure stands, those in themixed stands initiated bulb scales earlier than those in purestands. The timing of bulb development therefore depended onthe size of neighbouring plants and the LAI of the stand andwas not simply determined by the size or age of individual plants.Shading plots with neutral filters which caused no spectralchanges slightly accelerated bulb development compared to unshadedplots but shading by a canopy of climbing pea and bean leaveshad a larger effect. Lack of irrigation advanced bulb maturitybut in the highly irrigated treatments plots of high plant densitystill initiated bulb scales and matured before those of lowdensity. Quantitative relationships between the change in R : FR andeither LAI or total radiation interception were determined foronion leaf canopies. It is suggested that the decrease in thered to far-red spectral ratio (R : FR) as LAI increases maybe the cause of the earlier bulb scale initiation and maturitythat occurs as LAI increases. Onion, Allium cepa (L.), spacing, competition, leaf area index, red: far red, irrigation, nitrogen, shading, bulbing     

Growth Rate, Photosynthesis and Respiration in Relation to Leaf Area Index

This work examined three possible explanations of growth rateresponses to leaf area index (LAI) in which growth rate perunit of ground area (crop growth rate, CGR) increased to a plateaurather than decreasing above an optimum LAI at which all lightwas intercepted. Single leaf photosynthetic measurements, andwhole plant 24 h photosynthesis and respiration measurementswere made for isolated plants and plants in stands using Amaranlhushybridus, Chenopodium album, and two cultivars of Glycine maxgrown at 500 and 1000 µimol m^–2 S^–1 photosyntheticphoton flux density at 25 °C. CGR, relative growth rate(RGR), and LAI were determined from 24 h carbon dioxide exchangeand leaf area and biomass measurements. Respiration increasedrelative to photosynthesis with crowding in A. hybridus andthere was an optimum LAI for CGR. In contrast, the ratio ofrespiration to photosynthesis was constant across plant arrangementin the other species and they had a plateau response of CGRto LAI. Neither increased leaf photosynthetic capacity at highLAI nor a large change in biomass compared to the change inLAI could account for the plateau responses. It was calculatedthat maintenance respiration per unit of biomass decreased withdecreasing RGR in C. album and G. max, but not A. hybridus,and accounted for the plateau response of CGR to LAI. Sincesimilar decreases in maintenance respiration per biomass atlow RGR have been reported for several other species, a constantratio of respiration to photosynthesis may occur in more speciesthan constant maintenance respiration per unit of biomass. Amaranlhus hybridus L., Chenopodium album L., Glycine max L Merr, soybean, photosynthesis, respiration, growth, leaf area index     

Osmotic Adjustment in Leaves of VA Mycorrhizal and Nonmycorrhizal Rose Plants in Response to Drought Stress

Osmotic adjustment in Rosa hybrida L. cv Samantha was characterized by the pressure-volume approach in drought-acclimated and unacclimated plants brought to the same level of drought strain, as assayed by stomatal closure. Plants were colonized by either of the vesicular-arbuscular mycorrhizal fungi Glomus deserticola Trappe, Bloss and Menge or G. intraradices Schenck and Smith, or were nonmycorrhizal. Both the acclimation and the mycorrhizal treatments decreased the osmotic potential (Ψ_π) of leaves at full turgor and at the turgor loss point, with a corresponding increase in pressure potential at full turgor. Mycorrhizae enabled plants to maintain leaf turgor and conductance at greater tissue water deficits, and lower leaf and soil water potentials, when compared with nonmycorrhizal plants. As indicated by the Ψ_π at the turgor loss point, the active Ψ_π depression which attended mycorrhizal colonization alone was 0.4 to 0.6 megapascals, and mycorrhizal colonization and acclimation in concert 0.6 to 0.9 megapascals, relative to unacclimated controls without mycorrhizae. Colonization levels and sporulation were higher in plants subjected to acclimation. In unacclimated hosts, leaf water potential, water saturation deficit, and soil water potential at a particular level of drought strain were affected most by G. intraradices. G. deserticola had the greater effect after drought preconditioning.     

Inheritance of bipyridyl herbicide resistance in Arctotheca calendula and Hordeum leporinum

The mode of inheritance of resistance to bipyridyl herbicides in bipyridyl-resistant biotypes of Arctotheca calendula and of Hordeum leporinum was investigated. F₁ plants from reciprocal crosses between diquat-resistant and -susceptible plants of A. calendula showed an intermediate response to diquat application that was nuclearly inherited. Treatment of F₂ plants with 100 g ai ha^-1 of diquat or 800 g ai ha^-1 of paraquat killed all homozygous-susceptible plants, caused severe injury to heterozygous plants but only slight or no injury to homozygous-resistant plants. Back crosses of F₁ to susceptible plants exhibited intermediate and susceptible phenotypes. The observed segregation ratios in F₂ and test-cross populations fitted predicted segregation ratios, 1:2:1 (R:I:S) and 1:1 (I:S) respectively, showing that bipyridyl resistance is conferred by a single incompletely-dominant gene. Biotypes of paraquat-resistant and -susceptible H. leporinum were crossed reciprocally. F₁ plants from reciprocal crosses showed an intermediate response to paraquat application. The F₂ progeny showed segregation ratios that fitted the predicted segregation ratio of 1:2:1 (R:I:S) forinheritance of resistance being governed by a single partially-dominant gene.     

Gender-related traits in the dioecious shrub Empetrum rubrum in two plant communities in the Magellanic steppe

Following the theory on costs of reproduction, sexually dimorphic plants may exhibit several trade-offs in energy and resources that can determine gender dimorphism in morphological or physiological traits, especially during the reproductive period.In this study we assess whether the sexes of the dioecious species Empetrum rubrum differ in morphological and ecophysiological traits related to water economy and photochemical efficiency and whether these differences change in nearby populations with contrasting plant communities.We conducted physiological, morphological, sex ratio, and cover measurements in E. rubrum plants in the Magellanic steppe, North-Eastern part of Tierra del Fuego (Argentina), from two types of heathlands with differing community composition.We found differences between sites in soil pH and wind speed at the canopy level. E. rubrum plants exhibited lower photosynthetic height and higher LAI (leaf area index), lower RWC (relative water content) and higher water-use efficiency (lower Δ¹³C) in the heathland with harsher environmental conditions. Gender dimorphism in the physiological response was patent for photochemical efficiency and water use (RWC and Δ¹³C discrimination), with males showing a more conservative strategy in relation to females. Accordingly, male-biased sex ratio in the stress-prone community suggested a better performance of male plants under stressful environmental conditions. The integrated analysis of all variables (photochemical efficiency, RWC, leaf dry matter content (LDMC), pigments, and Δ¹³C) indicated an interaction between gender and heathland community effects in the physiological response. We suggest that female plants may exhibit compensatory mechanisms to face their higher reproductive costs.     

The effect of small-scale environmental changes on disease incidence and severity in a natural plant-pathogen interaction

Summary The incidence and severity of Rhynchosporium secalis infections were assessed in a large population of Hordeum leporinum. Transects were set out in four directions from five trees to determine the effect of shading. Under the tree canopy 60.3% of H. leporinum plants were infected while only 11.2% were infected away from the canopy. Disease severity, on those plants which were infected, was higher under the canopy (mean 12.4% and 13.0% leaf area diseased for the flag and first leaves, respectively) than away from the canopy (means of 7.8% and 5.0% for the flag and first leaves respectively). Plants under the tree canopy contained on average 23% more nitrogen, raising the possibility that the susceptibility of the host changed in response to nitrogen levels. However, the observed pattern is also consistent with the hypothesis that shade-associated changes in the environment enhanced the ability of the pathogen to infect and develop on the host. The data clearly demonstrate the importance of small-scale environmental factors on natural host-pathogen interactions. These environmental factors may cause differential selection for disease resistance within a host population, which may ultimately lead to the formation of sub-populations with differing levels of resistance.     

Development and structure of foliage in wheat stands of different density

The effect of sowing density (21, 85, 340 and 3400 plants per 1 m on the leaf area index (LAI), vertical arrangement of developing foliage and dry matter production was investigated in stands of wheat (Triticum aestivum L. cv. Niva).

1. 1.
   With increasing sowing density.     
   
   

A comparison of the water relations characteristics of Helianthus annuus and Helianthus petiolaris when subjected to water deficits

The effect of water deficits on the water relations and stomatal responses of Helianthus annuus and Helianthus petiolaris were compared in plants growing in the glasshouse under controlled conditions. Unirrigated plants of both genotypes were subjected to two different stress rates in which predawn leaf water potentials declined steadily at either 0.15 MPa day^?1 or 0.50 MPa day^?1. In both genotypes water stress induced a gradual and similar decrease in leaf conductance from 1.6 to 0.3 cm s^?1 as water potential decreased from-0.5 to-2.0 MPa. The relationship between leaf conductance and leaf water potential was not affected by the rate of stress development. Development of predawn leaf water potentials of-1.3 MPa had no significant effect on the relative water content at zero turgor, the apoplastic water content or the volumetric elastic modulus of whole leaves in either species, but decreased the osmotic potential at full turgor and zero turgor by 0.22 MPa and decreased the turgid weight: dry weight ratio from 10.6 to 8.4 in H. annuus, but not in H. petiolaris. In H. annuus leaves expanded during stress development, changes in the osmotic potential at full turgor induced by water deficits did not disappear on rewatering.     

Ribosomal DNA repeat unit polymorphism in 25 Hordeum species

Summary Tandemly repeated DNA sequences containing structural genes encoding ribosomal RNA (rDNA) were investigated in 25 species of Hordeum using the wheat rDNA probe pTA71. The rDNA repeat unit lengths were shown to vary between 8.5 and 10.7 kb. The number of length classes (1–3) per accession generally corresponded to the number of nucleolar organizing regions (NORs). Intraspecific variation was found in H. parodii, H. spontaneum and H. leporinum, but not in H. bulbosum. Restriction analysis showed that the positions of EcoRI, SacI and certain BamHI cleavage sites in the rRNA structural genes were highly conserved, and that repeat unit length variation was generally attributable to the intergenic spacer region. Five rDNA BamHI restriction site maps corresponded to the following groups of species: Map A — H. murinum, H. glaucum, H. leporinum, H. bulbosum, H. marinum, H. geniculatum; Map B — H. leporinum; Map C — H. vulgare, H. spontaneum, H. agriocrithon; Map D — H. chilense, H. bogdanii; and Map E — remaining 14 Hordeum species. The repeat unit of H. bulbosum differed from all other species by the presence of a HindIII site. The closer relationship of H. bulbosum to H. leporinum, H. murinum and H. glaucum than to H. vulgare was indicated by their BamHI restriction maps.Contribution No. 1169, Plant Research Centre     

Limitations on photosynthesis of competing individuals in stands and the consequences for canopy structure

The canopy structure of a stand of vegetation is determined by the growth patterns of the individual plants within the stand and the competitive interactions among them. We analyzed the carbon gain of individuals in two dense monospecific stands of Xanthium canadense and evaluated the consequences for intra-specific competition and whole-stand canopy structure. The stands differed in productivity, and this was associated with differences in nitrogen availability. Canopy structure, aboveground mass, and nitrogen contents per unit leaf area (N_area) were determined for individuals, and leaf photosynthesis was measured as a function of N_area. These data were used to calculate the daily carbon gain of individuals. Within stands, photosynthesis per unit aboveground mass (P_mass) of individual plants increased with plant height, despite the lower leaf area ratios of taller plants. The differences in P_mass between the tallest most dominant and shortest most subordinate plants were greater in the high-nitrogen than in the low-nitrogen stand. This indicated that competition was asymmetric and that this asymmetry increased with nitrogen availability. In the high-nitrogen stand, taller plants had a higher P_mass than shorter ones, because they captured more light per unit mass and because they had higher photosynthesis per unit of absorbed light. Conversely, in the low-nitrogen stand, the differences in P_mass between plants of different heights resulted only from differences in their light capture per unit mass. Sensitivity analyses revealed that an increase in N_area, keeping leaf area of plants constant, increased whole-plant carbon gain for the taller more dominant plants but reduced carbon gain in the shorter more subordinate ones, which implies that the N_area values of shorter plants were greater than the optimal values for maximum photosynthesis. On the other hand, the carbon gain of all individual plants, keeping their total canopy N constant, was positively related to an increase in their individual leaf area. At the same time, however, increasing the leaf area for all plants simultaneously reduced the carbon gain of the whole stand. This result shows that the optimal leaf area index (LAI), which maximizes photosynthesis of a stand, is not evolutionarily stable because at this LAI, any individual can increase its carbon gain by increasing its leaf area.     

Effects of elevated CO2 on leaf area dynamics in nodulating and non-nodulating soybean stands

Aims

The effects of elevated CO₂ on leaf area index (LAI) vary among studies. We hypothesized that the interactive effects of CO₂ and nitrogen on leaf area loss have important roles in LAI regulation.

Methods

We studied the leaf area production and loss using nodulating soybean and its non-nodulating isogenic line in CO₂-controlled greenhouse systems.

Results

Leaf area production increased with elevated CO₂ levels in the nodulating soybean stand and to a lesser extent in the non-nodulating line. Elevated CO₂ levels accelerated leaf area loss only in nodulating plants. Consequently, both plants exhibited a similar stimulation of peak LAI with CO₂ elevation. The accelerated leaf loss in nodulating plants may have been caused by newly produced leaves shading the lower leaves. The nodulating plants acquired N throughout the growth phase, whereas non-nodulating plants did not acquire N after flowering due to the depletion of soil N. N retranslocation to new organs and subsequent leaf loss were faster in non-nodulating plants compared with nodulating plants, irrespective of the CO₂ levels.

Conclusion

LAI regulation in soybean involved various factors, such as light availability within the canopy, N acquisition and N demands in new organs. These effects varied among the growth stages and CO₂ levels.