Growth Chamber Experiments Research Articles

Potential allelopathic effects of the tropical legume Sesbania virgata on the alien Leucaena leucocephala related to seed carbohydrate metabolism

Allelopathy has been considered a key mechanism to explain the invasiveness of some species. It is well known that invasive plants can affect native plants by producing novel allelochemicals but some exotic plant species may be also sensitive to allelochemicals released by native species, providing a tool to reduce growth and impacts of invasive exotic species. Here, using growth chamber experiments we tested the mutual potential allelopathic effects of Sesbania virgata (a native dominant species) and the alien Leucaena leucocephala seeds. S. virgata was unaffected by seed leachates of L. leucocephala, indicating that, under lab conditions, this legume presents resistance to the phytotoxic compounds produced by seeds of this alien species. In contrast, germination and seedling growth of L. leucocephala were strongly affected by the phytochemicals produced by seeds of S. virgata. A delay in endospermic mobilization of storage carbohydrates (raffinose-family oligosaccharides and galactomannan) was observed in the alien species. These potential allelopathic effects could not be attributed sole to the presence neither of the phytoxic catechin nor of ABA in seed leachates of S. virgata. Our findings indicate that the in vitro behavior of S. virgata is consistent with its aggressiveness in natural environment and suggest sesbanimide as a potential candidate as implicated in the noxious effects of S. virgata on the alien species.

Soil amendments alter plant biomass and soil microbial activity in a semi-desert grassland

We tested the effects of soil biotic disturbance and biochar or woodchip amendments on plant growth, soil microbial biomass and activity, and soil physiochemical parameters in response to disturbance in a semi-desert grassland. In a 78-day growth chamber experiment using six grass species native to the Southwest U.S., we compared the effects of autoclave heatshock, which mimics soil stockpiling in hot drylands, and amendments on plant and microbial biomass, potential extracellular enzyme activity, and soil moisture and nutrient availability. Plant biomass was lowest in woodchip-amended soils, and highest in autoclaved and biochar-amended soils (p < 0.05). Root:shoot ratios were higher in the autoclaved and woodchip-amended soils (p < 0.05). Biochar addition improved soil water-holding capacity resulting in higher dissolved organic carbon (p < 0.001) and nitrogen (p < 0.001). Soil microbial activity and plant biomass were not correlated. Amendment-induced changes in activity could be partially explained by nutrient availability. Neither microbial biomass nor activity recovered to pre-disturbance values. In this study, biochar and woodchip amendment and autoclave-induced changes to moisture and nutrient availability influenced plant biomass allocation and soil microbial activity. Amendments increased carbon, nitrogen, and phosphorus mineralizing enzyme activities with no significant change in microbial biomass, indicating that soil recovery in drylands is a long-term process. Understanding plant-soil feedbacks in drylands is critically important to mitigating climate and anthropogenic-driven changes and retaining or reestablishing native plant communities.

Trait-based model development to support breeding programs. A case study for salt tolerance and rice

Eco-physiological models are increasingly used to analyze G × E × M interactions to support breeding programs via the design of ideotypes for specific contexts. However, available crop models are only partly suitable for this purpose, since they often lack clear relationships between parameters and traits breeders are working on. Taking salt stress tolerance and rice as a case study, we propose a paradigm shift towards the building of ideotyping-specific models explicitly around traits involved in breeding programs. Salt tolerance is a complex trait relying on different physiological processes that can be alternatively selected to improve the overall crop tolerance. We developed a new model explicitly accounting for these traits and we evaluated its performance using data from growth chamber experiments (e.g., R2 ranged from 0.74 to 0.94 for the biomass of different plant organs). Using the model, we were able to show how an increase in the overall tolerance can derive from completely different physiological mechanisms according to soil/water salinity dynamics. The study demonstrated that a trait-based approach can increase the usefulness of mathematical models for supporting breeding programs.

Differential effects of the crustose Diploschistes diacapsis and the squamulose Fulgensia bracteata on the establishment of a Mediterranean grass species

AbstractThe interest of the scientific community in biological soil crusts has grown exponentially over the last decades. One of the scientific research interests is the study of the effect of these crusts on plant establishment. Findings in this topic have been controversial, and some differences were attributed to crust types. Biological soil crusts dominated by lichens are common components of Stipa tenacissima steppes in arid and semi‐arid environments of the southern Mediterranean. In the current study, we conducted growth chamber experiments to investigate the differential effects of two lichen species with continuous crustose thalli (Diploschistes diacapsis) and with squamulose semicontinuous thalli (Fulgensia bracteata) on seed germination, root penetration, shoot emergence and seed viability of the tussock grass species S. tenacissima. Our results showed that under laboratory conditions, two distinct lichen species had significantly different effects on the establishment of S. tenacissima. Our findings clearly demonstrated that D. diacapsis significantly decreased germination, root penetration and shoot emergence of S. tenacissima compared to F. bracteata. This can be related to differences in morphological and physiological characteristics between crustose and squamulose lichens. Overall, we suggest that D. diacapsis and crustose lichens generally can act as natural barrier to the establishment of S. tenacissima.

Germination in Cool‐Season Forage Grasses under a Range of Temperatures

Temperature plays a primary role in seed germination, and germination under a wide range of temperatures can promote early seedling emergence and stand establishment. The objective of this growth chamber experiment was to determine the effects of temperature (5–35°C) on the germination of eight species using 6 annual and 14 perennial cool‐season grass lots. The greatest germination of annual and perennial cool‐season grasses occurred at 20 and 15°C, respectively. Germination of oat (Avena sativa L.) was typically greatest at 10 to 20°C, whereas rye (Secale cereale L.) germination was generally greatest at 5 to 20°C and annual ryegrass (Lolium multiflorum Lam.) germination was greatest at 10 to 30°C. Germination of orchardgrass (Dactylis glomerata L.) and hardinggrass (Phalaris aquatica L.) was greatest at 15 to 20°C. Germination of tall fescue [Schedonorus arundinaceus (Schreb.) Dumort.] varied by type: summer‐active tall fescue had optimal germination between 15 and 25°C but summer‐dormant tall fescue had optimal germination between 10 and 20°C, except for ‘NFTF 1800E–’, which was more sensitive to high temperatures (&gt; 5°C). On the basis of these findings, the seeding rates of rye, oat, orchardgrass, hardinggrass, and tall fescue should be increased by ∼30% if planted in early September rather than late September to account for the reduction in germination; summer‐dormant tall fescue should be planted 2 to 3 wk later (1–15 October) than summer‐active types (early September) in the southern Great Plains.

Drought and simulated deer herbivory reduce growth in Atlantic white cedar seedlings

White-tailed deer are overabundant in much of eastern North America and may be suppressing regeneration in some tree species. Herbivory initially reduces plant biomass, but it has the potential to stimulate compensatory growth. However, stressful environmental conditions may reduce recovery from herbivory. Seedlings of Atlantic white cedar, Chamaecyparis thyoides (L.) Britton, Sterns &amp; Poggenb., are frequently browsed by white-tailed deer and also experience both flooded and dry soils in natural habitats. We conducted a growth chamber experiment to test for a potential interaction between simulated deer browsing and water availability treatments on seedling height, stem diameter, and above- and belowground biomass. Of the 180 study plants, half were subjected to an initial clipping treatment, and all plants were then exposed to high, intermediate, or low water availability for 90 days. We found that, over the short term, seedlings in favorable growing conditions could not compensate for lost biomass after simulated browsing. Drought reduced shoot biomass, and flooded soils reduced root but not shoot growth. Water stress and herbivory did not interact to reduce growth more than the effect of each treatment separately. In general, the effect of water stress on plant size was greater than the effect of simulated herbivory.

Effects of extreme temperatures on the growth and photosynthesis of invasive Bidens alba and its native congener B. biternata

Temperatures are expected to fluctuate widely under climate change but little is known about how extreme temperatures might affect the physiology and performance of invasive compared to native plant species. In this study, we evaluated the effects of high (40/35°C) and low (10/5°C) temperature regimes on the growth and photosynthesis of the invasive Asteraceae species Bidens alba and its native congener B. biternata using a growth chamber experiment. Results showed that invasive B. alba had significantly greater total biomass and relative growth rate, accompanied by higher net photosynthetic rate (Pn), than native B. biternata at both low and high temperature extremes. The reduction in Pn for B. alba was mainly caused by stomatal limitations, but for B. biternata it was caused by non‐stomatal factors, indicating that greater damage to physiological processes may occur in native B. biternata under both low and high temperature stress. Higher cyclic electron flow around photosystem I in invasive B. alba than in native B. biternata under extreme temperatures might alleviate the negative effect of temperature extremes to photosynthetic and thus promote its photosynthetic efficiency. To conclude, the invasive B. alba has both greater cold and heat tolerance than its native congener B. biternata, suggesting that the invader may outperform native species under future extreme temperature conditions.

Seed Treatment Effects on Maize Seedlings Coinfected with Rhizoctonia solani and Pratylenchus penetrans.

The roots of maize seedlings typically are attacked by a complex of organisms that includes fungal pathogens and plant-parasitic nematodes but few studies have examined the effects of these organisms in combination. Rhizoctonia solani can be an important component of the seedling disease complex; like other fungi, its effect on the plant may be influenced by the activity of nematodes such as the root-lesion nematode Pratylenchus penetrans. In this study, we assessed the impact of seed treatments, including fungicide-nematicide combinations, on maize seedlings exposed to R. solani and P. penetrans alone or in combination. In growth-chamber and greenhouse experiments, seed treated with various active ingredient combinations were planted in an autoclaved sand-soil mixture with or without inoculum of R. solani. In some treatments, a suspension of P. penetrans adults and juveniles was added to the sand-soil mixture. In the greenhouse experiments, infection by R. solani caused dramatic reductions in root length, volume, surface area, and numbers of root tips and root forks, whereas P. penetrans infestation alone reduced only shoot fresh weight. Statistical interactions between the effects of the two organisms were not significant, although fungal infestation significantly reduced the numbers of nematodes extracted from roots. Seed treatments significantly improved most root development variables, and the combination that included four fungicides, thiamethoxam, and abamectin was the best treatment for most variables. Results were similar in the growth-chamber experiments, where R. solani caused significant reductions in nearly all shoot and root development measurements, and seed treatment with sedaxane, alone or combined with abamectin, consistently provided the best results. R. solani was more damaging to seedlings than P. penetrans, and the combination of the two organisms did not cause more damage than R. solani alone. Seed-treatment active ingredients that specifically targeted R. solani (sedaxane) and P. penetrans (abamectin) had large positive effects on seedling health, causing significant improvements in root and shoot growth and development compared with untreated seedlings exposed to these pathogens.

Variation in growth and developmental responses to supraoptimal temperatures near latitudinal range limits of gypsy moth Lymantria dispar (L.), an expanding invasive species

AbstractVariation in thermal performance within and between populations provides the potential for adaptive responses to increasing temperatures associated with climate change. Organisms experiencing temperatures above their optimum on a thermal performance curve exhibit rapid declines in function and these supraoptimal temperatures can be a critical physiological component of range limits. The gypsy moth, Lymantria dispar (L.) (Lepidoptera: Erebidae), is one of the best‐documented biological invasions and factors driving its spatial spread are of significant ecological and economic interest. The present study examines gypsy moth sourced from different latitudes across its North American range for sensitivity to high temperature in constant temperature growth chamber experiments. Supraoptimal temperatures result in higher mortality in northern populations compared with populations from the southern range extent (West Virginia and coastal plain of Virginia, U.S.A.). Sublethal effects of high temperature on traits associated with fitness, such as smaller pupal mass, are apparent in northern and West Virginia populations. Overall, the results indicate that populations near the southern limits of the range are less sensitive to high temperatures than northern populations from the established range. However, southern populations are lower performing overall, based on pupal mass and development time, relative to northern populations. This suggests that there may be a trade‐off associated with decreased heat sensitivity in gypsy moth. Understanding how species adapt to thermal limits and possible fitness trade‐offs of heat tolerance represents an important step toward predicting climatically driven changes in species ranges, which is a particularly critical consideration in conservation and invasion ecology.

Short-term Effects of Elevated Temperature and CO2 on Carbon Sequestration in Winter Wheat and Summer Rape

In order to study organic carbon (C) sequestration in different plant parts of different crop species, closed growth chamber experiment was performed with winter wheat (Triticum aestivum L.) and summer rape (Brassica napus L.) in a controlled environment at ambient [21oC/400 ppm] and elevated [25oC/800 ppm] temperature and CO2 conditions. Measurements of organic carbon sequestration were carried out at 28-day period after the treatment. Carbon content was measured with Shimadzu TOC solid sample module SSM-5000A. The results showed that under elevated temperature and CO2 conditions both species of crop have sequestrated the biggest amount of organic carbon in stems (575 mg g-1 in wheat and 545 mg g-1 in rape). Under conditions of [25oC/800 ppm] the amount of organic carbon in leaves of rape increased by 6.9% (p 0.05) between [21oC/400 ppm] and [25oC/800 ppm] in the amount of sequestrated organic carbon in stems and roots of rape. Contrary roots of wheat under conditions of [25oC/800 ppm] have sequestrated significantly bigger (6%, p 0.05) between ambient and elevated temperature and CO2 conditions. Our results suggest that under future elevated temperature and CO2 conditions both investigated crop species will sequester more carbon in their biomass but in different manner.DOI: http://dx.doi.org/10.5755/j01.erem.72.4.16631

Variation in and adaptive plasticity of flower size and drought-coping traits

Although phenotypic plasticity of morphological and physiological traits in response to drought could be adaptive, there have been relatively few tests of plasticity variation or of adaptive plasticity in drought-coping traits across populations with different moisture availabilities. We measured floral size, vegetative size, and physiological traits in four field populations of Leptosiphon androsaceus (Polemoniaceae) that were distributed across a rainfall gradient in California, USA. Measurements were made over 5 years that varied in precipitation. We also conducted a growth chamber experiment in which half-sibs from three populations were divided equally among a well-watered and a drought treatment. We tested for selection on traits in each of the watering treatments, and evaluated whether traits exhibited plasticity. In the field, plant traits exhibited substantial variation across populations and years. Flower size, leaf size, and water-use efficiency (WUE) were generally higher for populations that received greater average rainfall. However, in dry years, we observed a decrease in flower and leaf size, but an increase in WUE across the populations. In the growth chamber experiment, leaf and physiological traits exhibited plasticity, with smaller leaves and higher WUE found in the drought, as compared to the well-watered treatment. Only specific leaf area exhibited differentiation in plasticity among populations. Although there was no observed plasticity in floral size, selection favored smaller flowers in the drought treatment and larger flowers in the well-watered treatment. Our results suggest that moisture availability has led to trait variation in L. androsaceus via a combination of selection and phenotypic plasticity.

N, P, K and S uptake response to various levels of CO2 assimilation and growth rate in lettuce

ABSTRACTUnder conditions of limited nutrient supply, plant nutrient uptake is controlled by the external concentration of the ions. Limited information exists about the whole-plant regulation of nutrient uptake when the supply is adequate. To study the relationship between growth rate and carbon dioxide (CO2) assimilation with nutrient uptake, growth chamber experiments were conducted with temperatures ranging from 10 to 35°C at medium (600 µmol m−2 s−1) and high (1200 µmol m−2 s−1) light intensities. Nutrient solution samples were collected every 24 hours and the concentration of ions was analyzed by Inductively coupled plasma -atomic emission spectroscopy (ICP-AES) and nitrate and ammonium (NO3−/NH4+) conductivity. Leaf photo-synthesis was measured using a closed gas exchange system and the total amount of CO2 assimilated was calculated from dry weight increases. The daily absorption of NO3−, Total nitrogen (N), dihydrogen phosphate (H2PO4−) and potassium (K+) responded linearly to plant growth, while ammonium (NH4+) and sulfate (SO42−) uptake showed a curvilinear response. All the ions studied showed a curvilinear relation with CO2 assimilation.

The role of latitudinal, genetic and temperature variation in the induction of diapause of Papilio glaucus (Lepidoptera: Papilionidae).

A key adaptation in insects for dealing with variable environmental conditions is the ability to diapause. The tiger swallowtail butterflies, Papilio glaucus and P. canadensis are ideal species to explore the genetic causes and population genetic consequences of diapause because divergence in this trait is believed to be a salient factor in maintaining a hybrid zone between these species. Yet little is known about the factors that influence diapause induction in this system. Here we explored how spatial (latitudinal), environmental (temperature) and genetic (hybridization) factors affect diapause induction in this system. Specifically, a series of growth chamber experiments using wild caught individuals from across the eastern United States were performed to: (1) evaluate how critical photoperiod varies with latitude, (2) isolate the stage in which induction occurs, (3) test whether changes in temperature affected rates of diapause induction, and (4) explore how the incidence of diapause is affected in hybrid offspring. We find that induction occurs in the larval stage, is not sensitive to a relatively broad range of temperatures, appears to have a complex genetic basis (i.e., is not simply a dominant trait following a Mendelian inheritance pattern) and that the critical photoperiod increases by 0.4 h with each increasing degree in latitude. This work deepens our understanding of how spatial, environmental and genetic variation influences a key seasonal adaptation (diapause induction) in a well-developed ecological model system and will make possible future studies that explore how climatic variation affects the population dynamics and genetics of this system.

Plant Growth Promotion by Volatile Organic Compounds Produced by Bacillus subtilis SYST2.

Bacterial volatiles play a significant role in promoting plant growth by regulating the synthesis or metabolism of phytohormones. In vitro and growth chamber experiments were conducted to investigate the effect of volatile organic compounds (VOCs) produced by the plant growth promoting rhizobacterium Bacillus subtilis strain SYST2 on hormone regulation and growth promotion in tomato plants. We observed a significant increase in plant biomass under both experimental conditions; we observed an increase in photosynthesis and in the endogenous contents of gibberellin, auxin, and cytokinin, while a decrease in ethylene levels was noted. VOCs emitted by SYST2 were identified through gas chromatography-mass spectrometry analysis. Of 11 VOCs tested in glass jars containing plants in test tubes, only two, albuterol and 1,3-propanediole, were found to promote plant growth. Furthermore, tomato plants showed differential expression of genes involved in auxin (SlIAA1. SlIAA3), gibberellin (GA20ox-1), cytokinin (SlCKX1), expansin (Exp2, Exp9. Exp 18), and ethylene (ACO1) biosynthesis or metabolism in roots and leaves in response to B. subtilis SYST2 VOCs. Our findings suggest that SYST2-derived VOCs promote plant growth by triggering growth hormone activity, and provide new insights into the mechanism of plant growth promotion by bacterial VOCs.

Acclimation to elevated CO2 is improved by low Rubisco and carbohydrate content, and enhanced Rubisco transcripts in the G132 barley mutant

Variations induced by mutagenesis may identify targets for adapting to rising atmospheric CO2 concentrations. G132 is a barley mutant with strong decreases in photosynthesis, Rubisco and carbohydrate content, but increased Rubisco transcripts. We tested the hypothesis that G132 will record a greater stimulation of photosynthesis and dry mass gain than the wild-type (WT) in response to CO2 enrichment. A growth chamber experiment compared the mutant G132 and the WT developed in ambient (∼390μmolmol−1) and elevated (1200μmolmol−1) CO2. Photosynthesis, protein and carbohydrate contents, Rubisco and nitrate reductase activities, leaf morphology, plant dry matter and leaf area were determined. The transcriptome responses of fully expanded leaves of G132 and WT to elevated CO2 were investigated using microarrays and qRT-PCR. Growth in elevated CO2 stimulated photosynthesis more in G132 than in WT, in association with a lack of Rubisco down-regulation and lower carbohydrate levels in G132. Electron transport-related proteins increased in G132 and decreased in WT. Elevated CO2 induced more changes in gene expression in WT than G132. Clustering of the corresponding transcripts showed that the expression of genes involved in carbohydrate synthesis and nitrogen assimilation was enhanced, while that for stress-related genes was repressed in both genotypes, and gene expression for jasmonate metabolism was activated in G132. Elevated CO2 decreased the transcript levels for a greater number of transcription factors in WT than in G132. CO2 enrichment increased leaf area in G132 but not in WT, and induced greater dry mass increases in the mutant. The low Rubisco and carbohydrate content and enhanced Rubisco transcripts recorded in G132 are traits that may improve adaptation to rising CO2.

Atmospheric CO2 concentration effects on rice water use and biomass production.

Numerous studies have addressed effects of rising atmospheric CO2 concentration on rice biomass production and yield but effects on crop water use are less well understood. Irrigated rice evapotranspiration (ET) is composed of floodwater evaporation and canopy transpiration. Crop coefficient Kc (ET over potential ET, or ETo) is crop specific according to FAO, but may decrease as CO2 concentration rises. A sunlit growth chamber experiment was conducted in the Philippines, exposing 1.44-m2 canopies of IR72 rice to four constant CO2 levels (195, 390, 780 and 1560 ppmv). Crop geometry and management emulated field conditions. In two wet (WS) and two dry (DS) seasons, final aboveground dry weight (agdw) was measured. At 390 ppmv [CO2] (current ambient level), agdw averaged 1744 g m-2, similar to field although solar radiation was only 61% of ambient. Reduction to 195 ppmv [CO2] reduced agdw to 56±5% (SE), increase to 780 ppmv increased agdw to 128±8%, and 1560 ppmv increased agdw to 142±5%. In 2013WS, crop ET was measured by weighing the water extracted daily from the chambers by the air conditioners controlling air humidity. Chamber ETo was calculated according to FAO and empirically corrected via observed pan evaporation in chamber vs. field. For 390 ppmv [CO2], Kc was about 1 during crop establishment but increased to about 3 at flowering. 195 ppmv CO2 reduced Kc, 780 ppmv increased it, but at 1560 ppmv it declined. Whole-season crop water use was 564 mm (195 ppmv), 719 mm (390 ppmv), 928 mm (780 ppmv) and 803 mm (1560 ppmv). With increasing [CO2], crop water use efficiency (WUE) gradually increased from 1.59 g kg-1 (195 ppmv) to 2.88 g kg-1 (1560 ppmv). Transpiration efficiency (TE) measured on flag leaves responded more strongly to [CO2] than WUE. Responses of some morphological traits are also reported. In conclusion, increased CO2 promotes biomass more than water use of irrigated rice, causing increased WUE, but it does not help saving water. Comparability with field conditions is discussed. The results will be used to train crop models.

Ultraviolet radiation reduces lichen growth rates

We quantified relative growth rates (RGR) in shade-adapted and melanin-deficient thalli of Cetraria islandica and Lobaria pulmonaria cultivated in short-term growth chamber experiments with and without UV-B radiation. In the first experiment done under optimal PAR (125 μmol m−2 s−1), but high UV-B radiation (1 W m−2), UV-B radiation significantly reduced RGR (P < 0.001). The second experiment with higher PAR, but more natural ratios between wavelength ranges (PAR: 500 μmol m−2 s−1; UV-A: 7 W m−2; UV-B: 0.4 W m−2), caused a reduction in mean RGR in L. pulmonaria to just 45% of rates in experiment 1. Lobaria pulmonaria screened from UV-B radiation had 1.9 and 1.6 times higher RGR than non-screened thalli in experiment 1 and 2, respectively. UV-B radiation significantly induced melanin synthesis in the second experiment only, causing significantly less photoinhibition than in thalli receiving just PAR. This is consistent with PAR-protective roles of melanins. Chlorophylls were not affected by UV-B radiation in any experiment. Because UV-B radiation affected RGR more than pure photobiont responses, the mycobiont is likely the more UV-B-susceptible partner. Apart from reduced RGR, we found little evidence for adverse UV-B effects.

Ecophysiological Responses ofTsuga canadensis(Eastern Hemlock) to Projected Atmospheric CO2and Warming

Abstract Tsuga canadensis (Eastern Hemlock) is a keystone tree species currently experiencing high mortality in southeastern US forests due to Hemlock Woolly Adelgid (Adelges tsugae; HWA) invasion. Investigating the impacts of concurrent climate change on Eastern Hemlock is important given its potential direct effects on this species and possible interactions of climatic factors with the continued spread of HWA. We conducted a 2-factorial experiment in controlled-environment growth chambers to test for the main effects and interactions of projected atmospheric CO2 concentrations and transitional-season warming on gas-exchange traits of Eastern Hemlock saplings collected from a northern Georgia field site. We hypothesized that elevated CO2 would increase photosynthesis and that warming would not influence photosynthesis, given the demonstrated capacity for photosynthetic temperature-acclimation in this species. Saplings in elevated CO2 exhibited ∼30% greater leaf-level photosynthesis (A) and ∼35% greater m...

Increased seedling establishment via enemy release at the upper elevational range limit of sugar maple.

The enemy release hypothesis is frequently invoked to explain invasion by nonnative species, but studies focusing on the influence of enemies on natural plant range expansion due to climate change remain scarce. We combined multiple approaches to study the influence of plant-enemy interactions on the upper elevational range limit of sugar maple (Acer saccharum) in southeastern Québec, Canada, where a previous study had demonstrated intense seed predation just beyond the range limit. Consistent with the hypothesis of release from natural enemies at the range limit, data from both natural patterns of regeneration and from seed and seedling transplant experiments showed higher seedling densities at the range edge than in the core of the species' distribution. A growth chamber experiment manipulating soil origin and temperature indicated that this so-called "happy edge" was not likely caused by temperature (i.e., the possibility that climate warming has made high elevation temperatures optimal for sugar maple) or by abiotic soil factors that vary along the elevational gradient. Finally, an insect-herbivore-exclusion experiment showed that insect herbivory was a major cause of seedling mortality in the core of sugar maple's distribution, whereas seedlings transplanted at or beyond the range edge experienced minimal herbivory (i.e., enemy release). Insect herbivory did not completely explain the high levels of seedling mortality in the core of the species' distribution, suggesting that seedlings at or beyond the range edge may also experience release from pathogens. In sum, while some effects of enemies are magnified beyond range edges (e.g., seed predation), others are dampened at and beyond the range edge (e.g., insect herbivory), such that understanding the net outcome of different biotic interactions within, at and beyond the edge of distribution is critical to predicting species' responses to global change.

CONSIDERING EFFECTS OF TEMPERATURE AND PHOTOPERIOD ON GROWTH AND DEVELOPMENT OF LABLAB PURPUREUS (L.) SWEET IN THE SEARCH OF SHORT-SEASON ACCESSIONS FOR SMALLHOLDER FARMING SYSTEMS

SUMMARYLegumes have gained increased importance in smallholder farming systems of sub-Saharan Africa due to their contribution to household nutrition and health and their ability to grow in low fertility soils. With unpredictable and highly variable rainfall characteristics of the semi-arid areas, short-season grain types are seen as a promising option for drought avoidance. Knowledge of phenological development and, in particular, time to flowering is crucial information needed for estimating the possible production success of new accessions to new environments. The photoperiod-sensitivity of 10 promising short-season Lablab purpureus (L.) Sweet accessions (CPI 525313, CPI 52533, CPI 52535, CPI 52535, CPI 52552, CPI 52554, CPI 60795, CPI 81364, CQ 3620, Q 6880B) were evaluated for their response to varying temperature and daylength regimes in field trials in Limpopo province, South Africa and under controlled conditions in growth chamber experiments in Göttingen, Germany. Photoperiod sensitivity was quantified using the triple-plane rate model of flowering response with time to flowering expressed in thermal time (Tt, °Cd). Additionally, piecewise regression analysis was conducted to estimate the critical photoperiod (Pc) above which time to flowering was delayed significantly. Relatively high variation of time to flowering amongst and within accessions in days after planting (DAP) was observed, ranging from 60 to 120 DAP depending on sowing date or daylength/temperature regime. Furthermore, a clear positive effect of temperature on growth and development of the tested accessions was found and time to flowering expressed as thermal time were consistent for the tested accessions, ranging from 600 to 800 °Cd for daylength &lt;13 h. Only at daylength of ≥13 h and temperatures above 28 °C, development towards flowering was delayed significantly for accessions CPI 52513, CPI 52535, CPI 52554 and CPI 60795 with vegetative growth continuing for &gt;110 DAP. The tested lablab accessions are therefore considered photoperiod insensitive, or weakly photoperiod responsive and are classified as short-day plants (SDP). Since daylength does not exceed 13 h between the latitudes 30 N to 30 S, these lablab accessions are recommended for further testing as short-duration grain legumes.