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PubMed2000

Chlorophyll fluorescence and CO(2) assimilation of black spruce seedlings following frost in different temperature and light conditions.

Effects of artificial frosts on light-saturated photosynthesis (A(max)) and ground, maximal and variable fluorescence variables (F(o), F(m), and F(v) and F(v)/F(m)) were monitored on 1-year-old foliage of black spruce seedlings (Picea mariana (Mill.) BSP) grown at high (25 degrees C), moderate (15 degrees C) and low (5 degrees C) temperatures and moderate (240 &mgr;mol m(-2) s(-1)) and low (80 &mgr;mol m(-2) s(-1)) irradiances. Photoinhibition of 1-year-old foliage was greater in seedlings grown in moderate light than in seedlings grown in low light. Photoinhibition increased with decreasing growth chamber temperature at both irradiances. Most changes in F(v)/F(m) were caused by changes in F(v). Exposure to -4 degrees C decreased both F(v)/F(m) and A(max) compared with control values. The effect of the -4 degrees C frost treatment was greater in seedlings grown in low light than in seedlings grown in moderate light, probably because seedlings grown in moderate light were already partially photoinhibited before the frost treatment. Following -4 degrees C treatment, neither F(v)/F(m) nor A(max) recovered in seedlings grown in low light. Light-saturated photosynthesis decreased with decreasing growth chamber temperature. Light-saturated photosynthesis was more sensitive to the -3 and -4 degrees C frost treatments in seedlings grown at 25 degrees C than in seedlings grown at 15 and 5 degrees C. The A(max) of seedlings grown at 15 degrees C was sensitive only to the -4 degrees C frost treatment, whereas A(max) of seedlings grown at 5 degrees C was not sensitive to any of the frost treatments. Recovery of A(max) following frost took longer in seedlings grown at high temperatures than in seedlings grown at low temperatures. For seedlings grown at the same temperature but under different irradiances, both A(max) and F(v)/F(m) reflected damage to the photosynthetic system following a moderate frost. However, for seedlings grown at the same irradiance but different temperatures, A(max) provided a more sensitive indicator of frost damage to the photosynthetic system than F(v)/F(m) ratio.

Journal Article↗

PubMed2003

Responses by coleoptiles of intact rice seedlings to anoxia: k(+) net uptake from the external solution and translocation from the caryopses.

This study evaluated the effects of anoxia on K(+) uptake and translocation in 3-4-d-old, intact, rice seedlings (Oryza sativa L. cv. Calrose). Rates of net K(+) uptake from the medium over 24 h by coleoptiles of anoxic seedlings were inhibited by 83-91 %, when compared with rates in aerated seedlings. Similar uptake rates, and degree of inhibition due to anoxia, were found for Rb(+) when supplied over 1.5-2 h, starting 22 h after imposing anoxia. The Rb(+) uptake indicated that intact coleoptiles take up ions directly from the external solution. Monovalent cation (K(+) and Rb(+)) net uptake from the solution was inhibited by anoxia to the same degree for the coleoptiles of intact seedlings and for coleoptiles excised, 'aged', and supplied with exogenous glucose. Transport of endogenous K(+) from caryopses to coleoptiles was inhibited less by anoxia than net K(+) uptake from the solution, the inhibition being 55 % rather than 87 %. Despite these inhibitions, osmotic pressures of sap (pi(sap)) expressed from coleoptiles of seedlings exposed to 48 h of anoxia, with or without exogenous K(+), were 0.66 +/- 0.03 MPa; however, the contributions of K(+) to pi(sap) were 23 and 16 %, respectively. After 24 h of anoxia, the K(+) concentrations in the basal 10 mm of the coleoptiles of seedlings with or without exogenous K(+), were similar to those in aerated seedlings with exogenous K(+). In contrast, K(+) concentrations had decreased in aerated seedlings without exogenous K(+), presumably due to 'dilution' by growth; fresh weight gains of the coleoptile being 3.6- to 4.7-fold greater in aerated than in anoxic seedlings. Deposition rates of K(+) along the axes of the coleoptiles were calculated for the anoxic seedlings only, for which we assessed the elongation zone to be only the basal 4 mm. K(+) deposition in the basal 6 mm was similar for seedlings with or without exogenous K(+), at 0.6-0.87 micro mol g(-1) f. wt h(-1). Deposition rates in zones above 6 mm from the base were greater for seedlings with, than without, exogenous K(+); the latter were sometimes negative. We conclude that for the coleoptiles of rice seedlings, anoxia inhibits net K(+) uptake from the external solution to a much larger extent than K(+) translocation from the caryopses. Furthermore, K(+) concentrations in the elongation zone of the coleoptiles of anoxic seedlings were maintained to a remarkable degree, contributing to maintenance of pi(sap) in cells of these elongating tissues.

Biological Transport↗

PubMed2006

Phylodiversity-dependent seedling mortality, size structure, and disease in a Bornean rain forest.

Density-dependent models that partition neighbors into conspecifics and heterospecifics ignore the great variation in effect of heterospecifics on focal plants. Both evolutionary theory and empirical results suggest that the negative effect of other plants on a focal plant should be higher for closely related neighbors than for less related neighbors. Using community-wide seedling mortality data from a forest where density dependence has previously been found, we searched for significant phylogenetic neighborhood effects (the "phylodiversity" neighborhood) on seedling (<50 cm tall) survival at various spatial scales. Logistic regression models were used, with 19-mo survival of individual seedlings as the response. We found a significant positive effect of nearest taxon phylodiversity on seedling survival at the 36-m2 scale and the 4-m2 scale, indicating that seedling survival is enhanced by being in a neighborhood where heterospecifics are not closely related. At all scales there was a strong negative effect of conspecific seedling density on focal survival, and at small scales there was also an effect of heterospecific density, indicating generalized competition. We place these results (for seedling dynamics over a relatively short period of time) in the context of changes in phylodiversity between different size classes of plants in the same forest, which integrate the effects of dynamics of all size classes over long time periods. At the 36-m2 scale, there was an increase in nearest taxon phylodiversity (i.e., a decrease in phylogenetic clustering) from the seedlings (<50 cm tall) to the poles (1-5 cm diameter), consistent with the positive effect of local phylodiversity on seedling survival. In contrast, there was a marked decrease in average phylodiversity from seedlings to saplings at the same scale. The trees in the 1600 m2 surrounding the seedling plots had much lower phylodiversity than either the seedlings or saplings. Taken together, these results suggest that (1) over short time and spatial scales, local seedling phylodiversity has a positive effect on seedling survival, possibly via interaction with pathogens (which we discuss in detail), but (2) over longer time periods and larger spatial scales the effect of abiotic-related mortality results in habitat filtering for phylogenetically conserved traits.

Biodiversity↗

PubMed1997

Geographical variation in water relations, hydraulic architecture and terpene composition of Aleppo pine seedlings from Italian provinces.

Ecotypic variations in leaf conductance, soil-to-leaf hydraulic conductance, components of tissue water potential, hydraulic architecture parameters and xylem embolism were examined in greenhouse-grown two-year-old Aleppo pine (Pinus halepensis Mill.) seedlings from six origins representing the geographic range of the species in Italy. Cortical resin composition of the seedlings was also determined. Measurements were made on well-watered seedlings and on seedlings subjected to recurring severe drought. Drought-stressed seedlings had lower mean leaf conductances, transpiration rates and soil-to-leaf hydraulic conductances than well-watered seedlings. They also exhibited more negative osmotic potentials, higher relative water deficit at incipient plasmolysis, but a similar maximum modulus of elasticity. Drought-stressed seedlings showed a higher degree of xylem embolism, a lower Huber value, lower leaf specific conductivity and lower specific conductivity than well-watered seedlings. Drought-stressed seedlings of provenances from more xeric habitats (Tremiti, Porto Pino and Mottola) had greater leaf conductances, transpiration rates and soil-to-leaf hydraulic conductances than drought-stressed seedlings of provenances from more mesic habitats (Imperia, Otricoli and Vico del Gargano). They also showed higher osmotic adjustment and a lower degree of xylem embolism. Among provenances, there were no significant differences in hydraulic architecture parameters in response to the drought treatment; however, Tremiti and Porto Pino seedlings displayed smaller drought-induced reductions in specific conductivity and leaf specific conductivity, respectively, than seedlings from other provenances. These differences suggest that seedlings from xeric provenances, especially Tremiti, have greater resistance to desiccation than seedlings from mesic provenances. No clear association was found between terpene variability and the other traits investigated, although terpene composition was related to the geographical distribution of the provenances. We conclude that the drought-tolerance responses of Tremiti make it a more suitable provenance than the others for establishment on sites prone to severe soil water deficits.

Journal Article↗

PubMed1994

The Response of Maize Seedlings of Different Ages to Hypoxic and Anoxic Stress (Changes in Induction of Adh1 mRNA, ADH Activity, and Survival of Anoxia).

Previously we showed that there is only a transient induction of alcohol dehydrogenase 1 (Adh1) transcripts and only a small induction of alcohol dehydrogenase (ADH) enzyme activity in root tips of maize (Zea mays L.) seedlings subjected to strict anaerobiosis without prior acclimation by exposure to low O2 (D.L. Andrews, B.G. Cobb, J.R. Johnson, M.C. Drew [1993] Plant Physiol 101: 403-414). Acclimation of root tips of seedlings by low O2 before anoxia appeared to be necessary for full induction of ADH. Here we have examined the effect of seedling age on changes in the protein content, induction of Adh1 transcripts, and ADH enzyme activity in 5-mm root tips, root axes, and shoots of maize (cv TX5855). Their ability to survive anoxia was also recorded. Some seedlings were sparged with 4% O2 for 6 or 18 h (a hypoxic pretreatment) followed by anoxia (sparged with N2) for up to 48 h. Other seedlings were not acclimated before anoxia. In general, younger seedlings had higher initial (aerobic) levels of total protein, Adh1 transcripts, and ADH activity than did seedlings that were 2 d older. For younger seedlings, anoxia alone induced Adh1 transcripts, which reached a peak within 6 to 12 h, whereas ADH activity increased throughout the 48-h treatment. For older seedlings, anoxia caused only a small, transient induction of Adh1 transcripts or ADH activity. For seedlings of either age, hypoxia induced Adh1 transcripts and ADH activity, both of which were increased further by subsequent anoxia in the younger seedlings but to a lesser extent in the older seedlings. Despite differences in ADH activity, roots of seedlings of either age showed a similar resistance to anoxia. Thus, acclimation of maize seedlings to survive anoxia does not appear to be related to induction of high levels of ADH activity.

Journal Article↗

PubMed2006

A rapid and robust method of identifying transformed Arabidopsis thaliana seedlings following floral dip transformation.

BACKGROUND: The floral dip method of transformation by immersion of inflorescences in a suspension of Agrobacterium is the method of choice for Arabidopsis transformation. The presence of a marker, usually antibiotic- or herbicide-resistance, allows identification of transformed seedlings from untransformed seedlings. Seedling selection is a lengthy process which does not always lead to easily identifiable transformants. Selection for kanamycin-, phosphinothricin- and hygromycin B-resistance commonly takes 7-10 d and high seedling density and fungal contamination may result in failure to recover transformants. RESULTS: A method for identifying transformed seedlings in as little as 3.25 d has been developed. Arabidopsis T1 seeds obtained after floral dip transformation are plated on 1% agar containing MS medium and kanamycin, phosphinothricin or hygromycin B, as appropriate. After a 2-d stratification period, seeds are subjected to a regime of 4-6 h light, 48 h dark and 24 h light (3.25 d). Kanamycin-resistant and phosphinothricin-resistant seedlings are easily distinguished from non-resistant seedlings by green expanded cotyledons whereas non-resistant seedlings have pale unexpanded cotyledons. Seedlings grown on hygromycin B differ from those grown on kanamycin and phosphinothricin as both resistant and non-resistant seedlings are green. However, hygromycin B-resistant seedlings are easily identified as they have long hypocotyls (0.8-1.0 cm) whereas non-resistant seedlings have short hypocotyls (0.2-0.4 cm). CONCLUSION: The method presented here is an improvement on current selection methods as it allows quicker identification of transformed seedlings: transformed seedlings are easily discernable from non-transformants in as little as 3.25 d in comparison to the 7-10 d required for selection using current protocols.

Journal Article↗

PubMed2006

The role of genetic and chemical variation of Pinus sylvestris seedlings in influencing slug herbivory.

This study investigated the genetic and chemical basis of resistance of Pinus sylvestris seedlings to herbivory by a generalist mollusc, Arion ater. Using feeding trials with captive animals, we examined selective herbivory by A. ater of young P. sylvestris seedlings of different genotypes and correlated preferences with seedling monoterpene levels. We also investigated the feeding responses of A. ater to artificial diets laced with two monoterpenes, Delta(3)-carene and alpha-pinene. Logistic regression indicated that two factors were the best predictors of whether seedlings in the trial would be consumed. Individual slug variation (replicates) was the most significant factor in the model; however, alpha-pinene concentration (also representing beta-pinene, Delta(3)-carene and total monoterpenes due to multicollinearity) of needles was also a significant factor. While A. ater did not select seedlings on the basis of family, seedlings not eaten were significantly higher in levels of alpha-pinene compared to seedlings that were consumed. We also demonstrated significant genetic variation in alpha-pinene concentration of seedlings between different families of P. sylvestris. Nitrogen and three morphological seedling characteristics (stem length, needle length and stem diameter) also showed significant genetic variation between P. sylvestris families. Artificial diets laced with high (5 mg g(-1) dry matter) quantities of either Delta(3)-carene or alpha-pinene, were eaten significantly less than control diets with no added monoterpenes, supporting the results of the seedling feeding trial. This study demonstrates that A. ater selectively feed on P. sylvestris seedlings and that this selection is based, in part, on the monoterpene concentration of seedlings. These results, coupled with significant genetic variation in alpha-pinene concentration of seedlings and evidence that slug herbivory is detrimental to P. sylvestris fitness, are discussed as possible evidence for A. ater as a selective force on the evolution of defensive chemistry in P. sylvestris.

Animals↗

PubMed2003

Growth and nutrient uptake of ectomycorrhizal Pinus sylvestris seedlings in a natural substrate treated with elevated Al concentrations.

Models of the effects of elevated concentrations of aluminum (Al) on growth and nutrient uptake of forest trees frequently ignore the effects of mycorrhizal fungi. In this study, we present novel data indicating that ectomycorrhizal mycelia may prevent leaching of base cations and Al. Mycorrhizal and non-mycorrhizal Pinus sylvestris L. seedlings were grown in sand obtained from the B-horizon of a local forest. In Experiment 1, non-mycorrhizal seedlings and seedlings inoculated with Hebeloma cf. longicaudum (Pers.: Fr.) Kumm. ss. Lange or Laccaria bicolor (Maire) Orton were provided with nutrient solution containing 2.5 mM Al. Aluminum did not affect growth of non-mycorrhizal seedlings or seedlings inoculated with L. bicolor. Seedlings colonized by H. cf. longicaudum had the highest biomass production of all seedlings grown without added Al, but the fungus did not tolerate Al. Shoots of seedlings colonized by L. bicolor had the lowest nitrogen (N) concentrations but the highest phosphorus (P) concentrations of all seedlings. The treatments had small but significant effects on shoot and root Al concentrations. In Experiment 2, inoculation with L. bicolor was factorially combined with the addition of a complete nutrient solution, or a solution lacking the base cations K, Ca and Mg, and solutions containing 0 or 0.74 mM Al. Seedling growth decreased in response to 0.74 mM Al, but the effect was significant only for non-mycorrhizal seedlings. Mycorrhizal seedlings generally had higher P concentrations than non-mycorrhizal seedlings. Aluminum reduced P uptake in non-mycorrhizal plants but had no effect on P uptake in mycorrhizal plants. Mycorrhizal colonization increased the pH of the soil solution by about 0.5 units and addition of Al decreased the pH by the same amount. We conclude that the presence of ectomycorrhizal mycelia decreased leaching of base cations and Al from the soil.

Agaricales↗

PubMed2005

Refugial forests of the southern Appalachians: photosynthesis and survival in current-year Abies fraseri seedlings.

Fraser fir (Abies fraseri (Pursh) Poiret) is an endemic, high-elevation conifer confined to six relict mountaintop communities in the southern Appalachian Mountains, USA. High adult mortality has occurred over the past 50 years, possibly the result of an introduced insect (Adelges piceae Ratzeburg), air pollution, or both. Knowledge of the mechanisms of and limitations to seedling establishment may allow reestablisment and perpetuation of this unique community type, notwithstanding global climate change. We monitored seedling emergence and mortality in relation to photosynthetic performance and water relations in microsites differing in canopy openness (sunlight exposure) over the summer of 2004. Abundance of cotyledonous seedlings in early summer was 2.3 times greater (849 versus 366 seedlings m(-2)) in microsites with lower sky exposure (greater canopy closure) than in microsites with greater sky exposure (greater canopy openness). In contrast, late-season abundance and survival were greater in areas beneath more open canopies than in areas beneath less open canopies (3.3 times and 11.7 times greater, respectively). However, newly emerged seedling survival in a completely open site (no overhead canopy) was zero, despite an initial density of 124 seedlings m(-2). Seedling water status was similar in open- and closed-canopy sites (-0.52 and -0.74 MPa, respectively). Photosynthetic carbon gain was higher in newly emerged seedlings at open canopy than at closed canopy sites, especially during early morning. Based on photosynthetic light response curves and measured sunlight regimes, seedlings in open canopy sites were estimated to assimilate 3.3-4.5 times more carbon than seedlings at closed sites. Reductions in carbon gain of closed-site seedlings, primarily a result of limited sunlight, corresponded to substantial increases in seedling mortality (98 versus 79% in open canopy sites). Thus, sunlight exposure, which reflects overstory canopy structure, appears to be an important factor influencing newly emerged seedling survival and distribution.

Abies↗

PubMed2004

Lipid utilization, gluconeogenesis, and seedling growth in Arabidopsis mutants lacking the glyoxylate cycle enzyme malate synthase.

The aim of this research was to test the role of the glyoxylate cycle enzyme malate synthase (MLS) in lipid utilization, gluconeogenesis, and seedling growth in Arabidopsis. We hypothesized that in the absence of MLS, succinate produced by isocitrate lyase (ICL) could still feed into the tricarboxylic acid cycle, whereas glyoxylate could be converted to sugars using enzymes of the photorespiratory pathway. To test this hypothesis we isolated knock-out mls mutants and studied their growth and metabolism in comparison to wild type and icl mutant seedlings. In contrast to icl seedlings, which grow slowly and are unable to convert lipid into sugars (Eastmond, P. J., Germain, V., Lange, P. R., Bryce, J. H., Smith, S. M. & Graham, I. A. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 5669-5674), mls seedlings grow faster, use their lipid more rapidly, and are better able to establish as plantlets. Transcriptome and metabolome analyses show that icl seedlings exhibit many features characteristic of carbohydrate starvation, whereas mls seedlings differ relatively little from wild type. In the light mls seedlings generate more sugars than icl seedlings, and when fed with [14C]acetate, 14C-labeling of sugars is three times greater than in icl seedlings and more than half that in wild type seedlings. The mls seedlings also accumulate more glycine and serine than icl or wild type seedlings, consistent with a diversion of glyoxylate into these intermediates of the photorespiratory pathway. We conclude that, in contrast to bacteria and fungi in which MLS is essential for gluconeogenesis from acetate or fatty acids, MLS is partially dispensable for lipid utilization and gluconeogenesis in Arabidopsis seedlings.

Arabidopsis↗

PubMed1992

Absolute and relative growth of Douglas-fir seedlings of different sizes.

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings of a single seed source were grown in a bare-root nursery for two years to produce five different stock types by varying spacing and transplanting treatments. They were then planted in the forest together with one-year-old container seedlings of the same seed source, which constituted a sixth treatment. Stem volume mean relative growth rate (R(v)) was low immediately after planting for all stock types except the container seedlings, and increased over the following 7.6 years. An 8-week greenhouse test of the six stock types showed that dry weight mean relative growth rate (R(w)) generally decreased with seedling dry weight, but this effect was less obvious after planting, because only the three smaller stock types showed a decrease in R(w) with size. In another experiment, bare-root Douglas-fir seedlings were grown at five different spacings in a nursery for two years, and seedlings from each spacing treatment were sorted into large or small by height. The resulting 10 treatments were planted in the forest and stem volumes determined over 6.7 years. The linear effect of nursery spacing on stem volume was no longer evident 3.6 years after planting, but large seedlings had greater stem volume than the small seedlings throughout the 6.7 years of the study. There was no indication that R(v) declined with time, but small seedlings had higher R(v) than large seedlings. Relative growth rates of two-year-old Douglas-fir were depressed for a year or two after planting, but then remained relatively constant, or increased during the ensuing 5 years. Relative growth rates of smaller seedlings were greater than those of larger seedlings so that relative biomass differences decreased with time, and the time advantage (the time necessary for small seedlings to reach the present biomass of large stock) of larger stock decreased. Absolute size differences increased with time.

Journal Article↗

PubMed1992

Root growth and water use efficiency of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and lodgepole pine (Pinus contorta Dougl.) seedlings.

One-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and lodgepole pine (Pinus contorta Dougl.) seedlings were grown for 17 weeks in 100-cm deep, 7.8-liter containers. Two Douglas-fir provenances, one from a wet and one from a dry site in coastal British Columbia, and two lodgepole pine provenances, one from a wet and one from a dry site in interior British Columbia, were grown in wet (522% water content) or dry (318% water content) peat/vermiculite soil in a factorial design. Each container was sealed so that water loss occurred only through the seedling. Five harvests were made at three to five week intervals and water use, dry matter increment, root length and root weight were determined at each harvest. Stomatal conductance and shoot water potentials were measured during the last 12 weeks of the experiment. Lodgepole pine seedlings had greater dry matter production, water use, stomatal conductance and new root length than Douglas-fir seedlings. New root weight of lodgepole pine seedlings exceeded that of Douglas-fir seedlings during the last five weeks of the experiment, and specific root length (root length per unit root weight) of new roots was higher for lodgepole pine seedlings throughout the experiment. Douglas-fir seedlings showed higher water use efficiency (WUE) than lodgepole pine seedlings, and both species showed higher WUE in the dry soil treatment. Douglas-fir seedlings had lower water potentials and higher water uptake rates per unit of new root length than lodgepole pine seedlings, although water uptake rates per unit of root dry weight showed little difference between species. Soil water treatment influenced specific root length of new roots, water uptake per unit of new root length, and WUE in Douglas-fir seedlings more than in lodgepole pine seedlings.

Journal Article↗

PubMed1993

Free amino acids and protein in Scots pine seedlings cultivated at different nutrient availabilities.

Scots pine (Pinus sylvestris L.) seedlings of a provenance from northern Sweden were cultivated hydroponically for 7 weeks in a climate chamber. The nutrient solution contained either 2.5 (low-N) or 50 (high-N) mg N l(-1) with other essential elements added in a fixed optimal proportion to the nitrogen. After 5 and 7 weeks, the seedlings were analyzed for growth, total nitrogen and other essential nutrients, protein and free amino acids. Low-N seedlings grew more slowly and had higher root/shoot ratios than high-N seedlings. With respect to total nitrogen, the effect of the lower nutrient supply was mainly on the nitrogen content of the whole plant and the allocation of nitrogen among tissues, not on tissue nitrogen concentration. This was also the case for potassium, phosphorus, calcium and magnesium. The proportions by weight among these macronutrients in the whole seedlings were similar in both nutrient regimes. The proportion and concentration of sulfur were significantly lower in low-N seedlings than in high-N seedlings, because of a lower net uptake of sulfur than of other macronutrients. The shoot, needles and stem of low-N seedlings had higher concentrations of free amino acids and lower concentrations of protein than the shoot, needles and stem of high-N seedlings. Arginine dominated the pool of free amino acids in the low-N seedlings, whereas glutamine predominated in the high-N seedlings. We conclude that Scots pine seedlings accumulated soluble nitrogen as arginine when net protein synthesis was limited by factors other than nitrogen availability. Nutritional imbalance, as revealed by growth characteristics and a suboptimal proportion and concentration of sulfur in the seedlings, probably affected synthesis of S-amino acids, resulting in the diversion of assimilated nitrogen to arginine instead of protein.

Journal Article↗

PubMed1997

Carry-over effects of ozone on root growth and carbohydrate concentrations of ponderosa pine seedlings.

Ozone exposure decreases belowground carbon allocation and root growth of plants; however, the extent to which these effects persist and the cumulative impact of ozone stress on plant growth are poorly understood. To evaluate the potential for plant compensation, we followed the progression of ozone effects, with particular emphasis on the development of new roots. Ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings were exposed to ozone for 2 years. Following removal of the seedlings from ozone, root growth was assessed to characterize the carry-over effects on new root production, and carbohydrate concentrations were measured to determine if allocation strategies differed among ozone treatments. Four months after removal from ozone, dormant seedlings had significantly lower starch concentrations in stems, coarse roots and fine roots than control seedlings. Following root flushing, starch concentrations in all seedlings decreased, with ozone-treated seedlings containing significantly less starch, sucrose, fructose, glucose and total monosaccharides than control seedlings. There was some evidence that stem starch was mobilized to compensate partially for the lower concentrations of root starch in ozone-treated seedlings; however, there was significantly less new root production in seedlings previously exposed to ozone for 2 years than in control seedlings. Early senescence of older needle age classes, perhaps resulting in inadequate available photosynthate, may be responsible for the reduction in new root production during the year following exposure to ozone. Stored carbohydrate reserves, which were depleted in seedlings previously exposed to ozone, were insufficient to compensate for the ozone-induced reduction in canopy photosynthate. We conclude that there are carry-over effects of ozone exposure on ponderosa pine seedlings, including an enhanced potential for seedling susceptibility to other stresses even in respite years when ozone concentrations are low.

Journal Article↗

PubMed1999

Ectomycorrhizas and cadmium toxicity in Norway spruce seedlings.

We studied the effects of ectomycorrhizal colonization by Laccaria bicolor (Maire) Orton S238 and Paxillus involutus (Batsch) Fr. 533 on cadmium (Cd) toxicity in Norway spruce seedlings (Picea abies (L.) Karst.). Both mycorrhizal and nonmycorrhizal seedlings were exposed to 0 (control), 0.5 or 5 &mgr;M CdSO(4) for 9 weeks in a sand culture system with frequent addition of nutrient solutions. In pure culture, P. involutus and L. bicolor showed similar Cd tolerance. However, in symbiosis, the Cd treatments decreased colonization by L. bicolor but not by P. involutus. Paxillus involutus ameliorated the negative effects of 0.5 &mgr;M Cd on shoot and root growth and chlorophyll content of old needles, whereas L. bicolor did not. Mycorrhizal colonization did not affect Cd concentrations of old needles and roots of seedlings. Despite differences between the ectomycorrhizal fungi in colonization and ability to alleviate Cd toxicity of seedlings, both species reduced Cd concentrations of young needles to a similar degree compared with nonmycorrhizal seedlings. However, in the 0.5 &mgr;M Cd treatment, the Cd content of needles of seedlings colonized by P. involutus was increased, whereas the Cd content of needles of seedlings colonized by L. bicolor was similar to that of needles of nonmycorrhizal seedings. When the amount of Cd translocated to needles was expressed on a root length basis to account for differences in the size of the root systems, the amount of Cd translocated to the needles was similar in seedlings mycorrhizal with P. involutus and in nonmycorrhizal seedlings. All mycorrhizal seedlings were similarly affected by 5 &mgr;M Cd, indicating that the amelioration efficiency of ectomycorrhizal fungi is dependent on the metal concentration to which the roots are exposed. Concentrations of P, K, Ca, Mg and Mn were decreased by 5 &mgr;M Cd to a similar extent in both nonmycorrhizal and mycorrhizal seedlings. In contrast to L. bicolor, P. involutus increased P uptake and altered patterns of root branching. We conclude that mycorrhizas alleviate Cd-induced reductions in growth of Picea abies seedlings. Although the two mycorrhizal fungi examined differed in their ability to alleviate Cd toxicity, these differences were not related to differences in Cd uptake or translocation to the shoot of the mycorrhizal seedlings. We suggest that amelioration of Cd toxicity by P. involutus may be a result of improved P nutrition or changes in root morphology, or both.

Journal Article↗

PubMed1994

Differential Gene Expression in Chilling-Acclimated Maize Seedlings and Evidence for the Involvement of Abscisic Acid in Chilling Tolerance.

An acclimation phenomenon was characterized in seedlings of chilling-sensitive maize (Zea mays L.) inbred G50 (Pioneer). Seedlings were germinated at 27[deg]C for 3 d and then exposed to chilling treatments of 4, 5, or 6[deg]C for 2, 4, 7, or 10 d in darkness. Damage symptoms in the more severe treatments included a waterlogged appearance and a discoloration of the tissue. The symptoms were most obvious in the mesocotyl. After a 10-d grow-out period in the greenhouse, moderately damaged seedlings exhibited chlorotic areas, an occasional disruption in leaf expansion, and a constriction of the mesocotyl. Growth and survival were improved by first exposing seedlings to a 14[deg]C acclimation treatment for 3 d before applying the chilling treatment. After chilling at 5[deg]C for 7 d, 79% of the acclimated seedlings survived, whereas only 22% of the nonacclimated seedlings survived. Differences in gene expression between acclimated and control seedlings were investigated using subtraction and differential screening techniques. Transcripts corresponding to three genes, car333, car30, and car757 (chilling acclimation responsive), were present in higher levels in seedlings after acclimation. Sequence analysis identified car333 as cat3, which encodes maize mitochondrial catalase isozyme 3. Characterization of these three clones revealed that all corresponding transcripts were elevated in acclimated seedlings in a manner that depended on the organ, i.e. coleoptile, mesocotyl, or root. Although transcripts were elevated in all three organs in response to acclimation, car30 was most abundant in the coleoptile and root, whereas cat3 and car757 were most abundant in the coleoptile and mesocotyl. Catalase activity followed the same general trend as cat3 transcript levels. Exogenous treatment with abscisic acid (ABA) resulted in an improvement in growth and survival of nonacclimated, chilled seedlings. Inhibition of ABA biosynthesis with fluridone abolished acclimation-induced chilling tolerance, and exogenous application of ABA to fluridone-treated seedlings restored chilling tolerance. Exogenous ABA treatment also resulted in increases in cat3, car30, and car757 transcript levels and catalase activity in the same organ-specific manner as in acclimated seedlings. These results indicate that ABA synthesis is essential for chilling tolerance. However, measurement of ABA levels in mesocotyls during acclimation and chilling revealed only a marginal increase during acclimation and a dramatic increase during chilling, regardless of whether or not seedlings were acclimated. Thus, although ABA may be required for chilling tolerance, we have no conclusive evidence that the acclimation process is mediated by ABA.

Journal Article↗

PubMed2005

Comparison of seedling recruitment under arborescent palms in two Neotropical forests.

Certain overlying strata in forests may disproportionately reduce seedling density and species richness. For eight arborescent palm species, we quantified the relative restriction of seedling recruitment under individual palms versus non-palm sites and extended to the landscape scale by quantifying the total area covered by arborescent palms at Barro Colorado Island (BCI), Panama and La Selva Biological Station, Costa Rica. We also examined whether differences among palm species in restricting seedling recruitment were associated with differences in crown architecture, litter depth, and light availability. Woody seedlings had lower mean density/m2 and mean number of species/m2 under individual palms than at non-palm sites for all four palm species at BCI, but for none at La Selva. Estimated species richness for woody seedlings, derived via rarefaction, was lower under palm than non-palm microsites at both BCI and La Selva, but not for non-woody seedlings. Differences in seedling density corresponded to some key architectural characters that differed among the palm species. Light availability was lower under palm than non-palm microsites at both BCI and La Selva, but only estimated species richness of woody seedlings at BCI was strongly correlated with % canopy openness. The coverage of arborescent palms was much lower at BCI than La Selva. Therefore, at BCI, the relative restriction of woody seedling recruitment under individual palms does not accumulate greatly at the landscape scale. At La Selva, for woody seedlings, only estimated species richness was relatively limited under palms, and non-woody seedlings had relatively lower mean density/m2 and mean number of species/m2 under only one palm species. Therefore, the relative restriction of seedling recruitment by arborescent palms at La Selva is limited at both individual and landscape scales.

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PubMed2005

Arbuscular mycorrhizal fungi in the tree seedlings of two Australian rain forests: occurrence, colonization, and relationships with plant performance.

The roots of rain forest plants are frequently colonized by arbuscular mycorrhizal fungi (AMF) that can promote plant growth in the nutrient poor soils characteristic of these forests. However, recent studies suggest that both the occurrence of AMF on rain forest plants and the dependence of rain forest plants on AMF can be highly variable. We examined the occurrence and levels of AMF colonization of some common seedling species in a tropical and a subtropical rain forest site in Queensland, Australia. We also used a long-term database to compare the growth and mortality rates of seedling species that rarely formed AMF with those that regularly formed AMF. In both forests, more than one-third of the seedling species rarely formed AMF associations, while 40% of species consistently formed AMF in the tropical site compared to 27% in the subtropical site. Consistent patterns of AMF occurrence were observed among plant families at the two sites. Variation among seedling species in AMF occurrence or colonization was not associated with differences in seed mass among species, variation in seedling size and putative age within a species, or lack of AMF inoculum in the soil. Comparisons of four seedling species growing both in the shaded understory and in small canopy gaps revealed an increase in AMF colonization in two of the four species in gaps, suggesting that light limitation partially explains the low occurrence of AMF. Seedling survival was significantly positively associated with seed biomass but not with AMF colonization. Furthermore, seedling species that regularly formed AMF and those that did not had similar rates of growth and survival, suggesting that mycorrhizal and nonmycorrhizal strategies were equivalent in these forests. Furthermore, the high numbers of seedlings that lacked AMF and the overall low rate of seedling growth (the average seedling required 6 years to double its height) suggest that most seedlings did not receive significant indirect benefits from AMF through connection to canopy trees via a common mycorrhizal network.

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