Effect of water potential on seed germination

The response of seed germination to substrate water potential was determined for several plant species of the arctic tundra. Seeds were collected from Cape Thompson and Eagle Summit, Alaska and germinated on dialysis membranes over water solutions of polyethylene glycol with osmotic potentials of 0 to −6 bars. Germination did not occur with potentials below −3 bars, except for three fellfield species. Germination was delayed at lower osmotic potentials. Because the response of most species was similar, substrate water potential is probably not a factor affecting the establishment of most tundra plant species from seeds.     

Seed weight patterns of Acacia tortilis from seven seed provenances across Botswana

Laboratory‐based seed storage systems have been developed as an alternative to in situ conservation for indigenous woody plant species. However, interactions between seed quality and environmental variables must be known for each species prior to seed collection, storage and sowing to ensure effective conservation. This study investigated Acacia tortilis seed weight/quality patterns across seven Botswana seed provenances in relation to: soil nutrient status, altitude; latitude; slope angle; % grass cover; height and density of other woody plants nearby. The higher rainfall and relatively eutrophic seed provenances of north‐western Botswana (Chobe, Okavango and Makgadikgadi) were associated with large A. tortilis species and seeds, as well as higher densities of woody plants. Spatial variation in seed weights of A. tortilis was principally a function of rainfall and soil organic carbon. Although more work is required to establish the relationship between seed weight and germination rates for A. tortilis, this research suggests that seed collection should focus on sites with high levels of rainfall and soil organic carbon.     

Photoinhibition of white clover seed germination at low water potential

Photosensitivity of germination of white clover ( Trifolium repens L. cv. Podkowa) seeds was studied under water deficit (low water potential) conditions at 25°C. The seeds showed negative photoblastism, which was most pronounced at -0.03 MPa polyethylene glycol solution. Inhibition was observed at two different wavelength bands with maxima at 660 nm (R) and around 730 nm (FR). Red light acted identically to white light (maximum inhibition ca 50%). The effect of far-red illumination was less inhibitory (20–30%). The photoresponse required long illuminations (3 h exposures); saturation level was at 0.1 W m⁻², independently of the light quality. White clover seed germination showed no reversibility of the effects of R and FR light. Prolonged illumination with R and FR increased the inhibition, and intermittent illumination had a higher effect than a continuous one. It was concluded that the photoinhibition of germination of seeds of Trifolium repens involves a reaction dependent on the rate of phytochrome interconversion, a property that is characteristic for the high irradiance reaction.     

Technique for ecological studies of seed germination in relation to soil water potential

Summary Seeds were germinated in soils of known matric potential (ψ_m) achieved by adding the requisite amount of water to air-dry soil and mixing for several days. The quantity of water was derived from calibration curves of water content against ψ_m by use of pressureplate extraction equipment. Soils were transferred to plastic Petri dishes in which seeds were sown. Variations of the technique permitted germination counting through the transparent dish lid, or by opening the dish and either resealing or discarding the replicate dish. Measurements of ethylene and carbon dioxide in the soil atmosphere suggest that neither gas accumulated to a level which could interfere with interpretation of results. Some species showed sensitivity of germination to water potential which was correlated with the relative wetness of their habitats.     

Modeling seed germination in Melisa officinalis L. in response to temperature and water potential

Seed germination is greatly influenced by both temperature (T) and water potential (ψ) and these factors largely determine germination rate (GR) in the field. Quantitative information about T and ψ effects on seed germination in lemon balm (Melisa officinalis L.) is scarce. The main objective of this study was to quantify seed germination responses of lemon balm to T and ψ, and to determine cardinal temperatures in a laboratory experiment. A segmented model was used to describe the effects of ψ (i.e., T) on GR and other germination parameters. The segmented model estimates were 7.2 °C for base (T _b), 28.9 °C for optimum (T _o), 40.1 °C for ceiling temperature (T _c) and 1.64 physiological days (f _o) (equivalent to a GR_max of 0.610 d^?1 and a thermal time of 35.6 °C days) to reach 50 % maximum germination in the control (0 MPa) treatment (R ² = 0.99, RMSE = 0.005 day^?1). The inherent maximum rate of germination (days) was calculated by the [GR_max = 1/f _o] model. ψ affected cardinal temperatures. From 0 to ?0.76 MPa, when ψ increased, T _b was a constant 7.2 °C to ?0.38 MPa and increased linearly to 20.1 °C as ψ decreased. T _o and f _o increased linearly from 28.9 to 30 °C, and from 1.64 to 5.4 day^?1, respectively as ψ decreased. However, there was no signification difference in T _o as ψ decreased nor did T _c decrease from 40.1 to 35 °C as ψ decreased. T _b, T _c and GR_max were the sole parameters affected by ψ and could be used to characterize differences between ψ treatments with respect to GR at various Ts. Therefore, the segmented model and its parameters can be used in lemon balm germination simulation models.     

Growth features of Acacia tortilis and Acacia xanthophloea seedlings and their response to cyclic soil drought stress

Seedlings of Acacia tortilis (Forsk) Hyne and Acacia xanthophloea Benth. were raised under controlled glasshouse conditions. Control plants were watered daily while other treatments involved withholding water for 2, 4 and 6 days with 1‐day rehydration to container capacity. Compared to A. tortilis, A. xanthophloea seedlings showed higher leaf area, relative growth rates and total dry weight production under adequate water supply conditions. However, with increased water stress, A. xanthophloea seedlings could not alter their pattern of carbon allocation, retaining their root : shoot (r : s) ratio of about 0.5. By comparison, A. tortilis seedlings shifted carbon allocation to the roots, leading to a r : s ratio of 1.5 in water‐stressed seedlings, compared to 0.5 in the control plants. The ability of A. tortilis to reallocate carbon to the roots away from the shoots and to actually increase root growth compared to A. xanthophloea was a dehydration postponement strategy that may be important in species survival during drought.     

Seed viability and germination success of Acacia tortilis along land‐use and aridity gradients in the Eastern Sahara

Our study focuses on the keystone species Acacia tortilis and is the first to investigate the effect of domestic ungulates and aridity on seed viability and germination over an extensive part of the Eastern Sahara. Bruchids infest its seeds and reduce their viability and germination, but ingestion by ruminant herbivores diminishes infestation levels and enhances/promotes seed viability and germination. The degree of these effects seems to be correlated with animal body mass. Significantly reduced numbers of wild ruminant ungulates have increased the potential importance of domestic animals and pastoral nomadism for the functionality of arid North African and Middle Eastern ecosystems. We sampled seeds (16,543) from A. tortilis in eight areas in three regions with different aridity and land use. We tested the effect of geography and sampling context on seed infestation using random effects logistic regressions. We did a randomized and balanced germination experiment including 1193 seeds, treated with different manure. Germination time and rates across geography, sampling context, and infestation status were analyzed using time‐to‐event analyses, Kaplan–Meier curves and proportional hazards Cox regressions. Bruchid infestation is very high (80%), and the effects of context are significant. Neither partial infestation nor adding manure had a positive effect on germination. There is a strong indication that intact, uningested seeds from acacia populations in the extremely arid Western Desert germinate more slowly and have a higher fraction of hard seeds than in the Eastern Desert and the Red Sea Hills. For ingested seeds in the pastoralist areas we find that intact seeds from goat dung germinate significantly better than those from camel dung. This is contrary to the expected body‐mass effect. There is no effect of site or variation in tribal management.     

Quantitative models characterizing seed germination responses to abscisic Acid and osmoticum

Mathematical models were developed to characterize the physiological bases of the responses of tomato (Lycopersicon esculentum Mill. cv T5) seed germination to water potential (ψ) and abscisic acid (ABA). Using probit analysis, three parameters were derived that can describe the germination time courses of a seed population at different ψ or ABA levels. For the response of seed germination to reduced ψ, these parameters are the mean base water potential (¯ψ_b, MPa), the standard deviation of the base water potential among seeds in the population (σ_ψb, MPa), and the “hydrotime constant” (θ_H, MPa·h). For the response to ABA, they are the log of the mean base ABA concentration ([unk]ABA_b, m), the standard deviation of the base ABA concentration among seeds in the population (σ_ABAb, log[m]), and the “ABA-time constant” (θ_ABA, log[m]·h). The values of ¯ψ_b and [unk]ABA_b provide quantitative estimates of the mean sensitivity of germination rate to ψ or ABA, whereas σ^ψ_b and σ_ABAb account for the variation in sensitivity among seeds in the population. The time constants, θ_H and θ_ABA, indicate the extent to which germination rate will be affected by a given change in ψ or ABA. Using only these parameters, germination time courses can be predicted with reasonable accuracy at any medium ψ according to the equation probit(g) = [ψ - (θ_H/t_g) - ¯ψ_b]/σ_ψb, or at any ABA concentration according to the equation probit(g) = [log[ABA] - (θ_ABA/t_g) - log[[unk]ABA_b]]/σ_ABAb, where t_g is the time to radicle emergence of percentage g, and ABA is the ABA concentration (m) in the incubation solution. In the presence of both ABA and reduced ψ, the same parameters can be used to predict seed germination time courses based upon strictly additive effects of ψ and ABA in delaying the time of radicle emergence. Further analysis indicates that ABA and ψ can act both independently and interactively to influence physiological processes preparatory for radicle growth, such as the accumulation of osmotic solutes in the embryo. The models provide quantitative values for the sensitivity of germination to ABA or ψ, allow evaluation of independent and interactive effects of the two factors, and have implications for understanding how ABA and ψ may regulate growth and development.     

Physical and chemical response of juvenile Acacia tortilis trees to browsing. Experimental evidence

1. Changes in nutritional value and accessibility of leaves following browsing are important in the dynamics of plant–herbivore interactions because they influence the fitness of the plant attacked and the future utilization of it by the herbivore.
2. Hand pruning of Acacia tortilis , a spinescent tree common in savanna ecosystems of eastern Africa, resulted in higher biomass of spines and new shoots in pruned trees than in unpruned controls.
3. Pruned trees allocated a higher proportion of shoot biomass to spines than unpruned ones, whereas the proportion of leaf biomass in new shoots was slightly reduced. Because increases in spine biomass and density following pruning are coupled with an increase in shoot production, it is concluded that higher production of spines is an inducible response of Acacia tortilis to pruning.
4. No significant changes in the concentration of total phenolics, condensed tannins or leaf nitrogen were induced by pruning.
5. Irrespective of treatment, high foliar concentrations of nitrogen were correlated with an increase in twig production for a given leaf biomass and a reduction in the concentration of secondary substances in leaves. This relation may lead to a conflict between foraging efficiency and nutrition for browsers of A. tortilis.     

Phytochromes differentially regulate seed germination responses to light quality and temperature cues during seed maturation

The ratio of red to far-red light (R : FR) experienced by seeds during maturation affects germination, but the genetic regulation of this effect is poorly understood. In Arabidopsis thaliana , responses to R : FR are governed by five phytochrome photoreceptors, PHYA–PHYE . PHYA , PHYB and PHYE mediate germination, but their roles in germination response to the seed maturation environment are largely unknown. Seeds of A. thaliana phytochrome mutants and natural accessions were matured in a factorial combination of cold (16 °C) and warm (24 °C) temperatures and high (R : FR = 1) and low (R : FR = 0.6) R : FR environments, resembling sunlight and foliar shade, respectively. Germination was observed in resulting seeds. All five phytochromes mediated germination responses to seed maturation temperature and/or R : FR environment. PHYA suppressed germination in seeds matured under cold temperature, and PHYB promoted germination under the same conditions. PHYD and PHYE promoted germination of seeds matured under warm temperature, but this effect diminished when seeds matured under reduced R : FR. The A. thaliana natural accessions exhibited interesting variation in germination responses to the experimental conditions. Our results suggest that the role of individual PHY loci in regulating plant responses to R : FR varies depending on temperature and provide novel insights into the genetic basis of maternal effects.     

Drought-induced adaptive changes in the seedling anatomy of Acacia ehrenbergiana and Acacia tortilis subsp. raddiana

This study, conducted on Acacia species (Acacia ehrenbergiana Hayne and Acacia tortilis subsp. raddiana (Savi) Brenan) to determine their adaptive capacity to tolerate drought and suitability for reforestation, revealed that leaf water potential (ψ) decreased in both the species with increase in drought intensity. With increase in the intensity of drought, vessel diameter increased in A. ehrenbergiana, causing a significant decline in vessel frequency mm^?2 of the transverse wood surface, while it decreased in A. tortilis, leading to a crowded vessel population. Vessel-wall thickness, in conjunction with inter-vessel pit membrane thickness, showed a positive correlation with drought stress in both the species. Ray dimensions generally decreased in A. ehrenbergiana but increased in A. tortilis under increasing degree of drought. The transverse fiber-wall area decreased in A. ehrenbergiana, thus lowering the density (r = 0.996) and enhancing the vulnerability of wood (r = ?0.979) under the drought stress, but increased in A. tortilis, causing a high density (r = ?0.979) and low vulnerability of wood (r = 0.869), under the same set of conditions. Correlation of wood density with vulnerability index was stronger in A. ehrenbergiana (r = ?0.993) than in A. tortilis (r = ?0.753). Diameters and thickness of inter-vessel pit membrane were linearly correlated with increasing intensity of drought in both the species, but its area fraction per vessel segment increased due to water stress in A. ehrenbergiana and decreased in A. tortilis. This study indicated that, on the whole, A. tortilis has a greater capacity to tolerate the harshness of drought than A. ehrenbergiana.     

Phase-sequence of redroot pigweed seed germination responses to ethylene and other stimuli

Phase-sequence studies showed that light, ethylene, and high temperature each enhanced germination of redroot pigweed (Amaranthus retroflexus L.) seeds when given during the first 24 hours of seed imbibition. Responses were maximal during the first 12 hours. After 48 hours all three stimuli given together caused 75% germination but each alone was ineffective. The main influence of water potential on seed germination occurred at about 24 hours, but the influence of CO₂ extended into the second and third days. Germination was reduced by water stress (−4 bars) or CO₂-free air, but ethylene reversed the reduction even when administered after several days incubation. This suggested that environmental and hormonal factors affected redroot pigweed seeds at two distinct stages in the sequence of germination events.     

Life cycle of the coleopter Bruchidius raddianae and the seed predation of the Acacia tortilis Subsp. raddiana in Tunisia

Seeds of Acacia tortilis (Forsskal) Hayne ssp. raddiana (Save) Brenan, a savannah tree of great potential for forestry and an excellent browse for game and livestock, is heavily infested by the bruchid beetle Bruchidius raddianae Anton & Delobel. The development from egg to beetle, and the various development stages were investigated using Scanning Electron Microscopy (SEM). The adult B. raddianae lays eggs on the green pod in the autumn. The first instar larva hatches from five to seven weeks and develops outside the host seed. From the second instar onwards development took place inside the host seed. Pupation takes about three weeks, late in the summer. The beetle of B. raddianae is univoltine and the newly emerged adult makes an exit hole to leave the seed. During the development from first instar lava to imago, all embryonic tissue is destroyed. This results in a weakness of its soil seed stock, which reduces the possibilities of natural regeneration of the species.     

Hydrothermal time analysis of tomato seed germination responses to priming treatments

Controlled hydration of seeds followed by drying (seed priming) is used to break dormancy, speed germination, and improve uniformity of radicle emergence. To date, empirical trials are used to predict optimal priming conditions for a given seed lot. Since priming is based upon seed water relations, it was hypothesized that the sensitivity of germination to reduced water potential before priming might be mechanistically related to, and therefore predictive of, priming responsiveness. Analyses of germination of 13 tomato (Lycopersicon esculentum Mill.) seed lots at two temperatures (15C and 20C) and three water potentials (0, -0.28 and -0.43 MPa) showed that seed lot germination responses could be quantitatively characterized by parameters derived from thermal time, hydrotime, and hydrothermal time models (R²0.73-0.99). Six of the seed lots were primed at two temperatures (15°C and 20°C) and three water potentials (-1.0, -1.5 and -2.0 MPa) for various durations, dried, and their subsequent germination rates analysed according to hydropriming time and hydrothermal priming time models. The responses of germination rates to priming were characterized by hydropriming time (HP) and hydrothermal priming time (HTP) constants and the minimum water potential (min) and temperature (Tmin) for achieving a priming effect. The values of min and Tmin varied relatively little among tomato seed lots, and the generalized values of min=2.39 MPa and Tmin=9.10°C accounted for 74% (15°C), 57% (20°C), and 62% (across both temperatures) of the increase in germination rates following priming. Nonetheless, while the hydrothermal time models described germination patterns both before and after priming, there was relatively little predictive relationship between them.     

The composition and properties of gum exudates from subspecies of Acacia tortilis

Gum specimens from Acacia tortilis ssp. spirocarpa, ssp. raddiana var. pubescens (two specimens) and ssp. heteracantha (three specimens) have been analysed. The results are of chemotaxonomic interest. Although the gum from ssp. raddiana var. pubescens appears to be intermediate between those from ssp. spirocarpa and ssp. heteracantha in terms of some of the analytical parameters, the overall impression is that ssp. raddiana is more similar chemically to ssp. spirocarpa than to ssp. heterocantha. The latter yields a viscous, proteinaceous polysaccharide that differs from those from both ssp. spirocarpa and ssp. raddiana by having a much higher ratio of arabinose to galactose, higher nitrogen and methoxyl contents, and much higher intrinsic viscosity and molecular weight; preliminary experiments have shown this gum to consist of two components.