Jean-Pierre Simon

Effects of fire regime on the serotiny level of jack pine

1 Serotiny, the capacity to retain seed in the plant canopy, has evolved in many species under the selective pressure of fires. The effect of disturbance type (lethal or nonlethal fire), time-since-fire and different fire regimes on the serotiny of jack pine (Pinus banksiana), was evaluated in populations from two adjacent landscapes in the southern part of the Canadian boreal forest. The island landscape (Lake Duparquet) has a complex fire regime of small fires of variable intensity, whereas the adjacent mainland has a fire regime characterized by large intense fires. 2 Twenty-four jack pine populations (11 island and 13 mainland) on xeric sites were sampled for the degree of serotiny of trees. Fire history and age structure were reconstructed for each population using the fire scar method. For each tree, recruitment was categorized as after a lethal fire, after a nonlethal fire or in the absence of fire. 3 Likelihood chi-square tests were used to investigate the variation in serotiny at individual, population and landscape levels. 4 At the individual level, the results support our prediction that the occurrence of lethal fires favours trees with high serotiny while low serotiny trees are favoured by other types of disturbances. 5 At the population level, the frequency of low serotiny trees increases with time since stand initiation, as a result of higher establishment opportunities after disturbances other than lethal fires. The proportion of low serotiny trees also increases with the occurrence of nonlethal fires. 6 Significant differences were found between the two landscapes. On the mainland, serotinous trees were more abundant, whereas on the islands where nonlethal fires were recorded, low serotiny trees were more frequent. These results support the hypothesis that fire imposes differential selective pressures on serotiny in jack pine.

Effect of low temperature on the photosynthetic metabolism of the C4 grass Echinochloa crus-galli

SummaryCO2 curves of photosynthesis and activities of the four C4 enzymes and Ribulose bisphosphate carboxylase (RUBPc) were compared in two populations of the C4 grass Echinochloa crus-galli from contrasting thermal environments (Québec and Mississippi). Analyses were conducted both before and after 14 h of chilling at 7°C under high light conditions. This comparison provides the opportunity to assess which steps of the C4 pathway are more susceptible to become limiting at low temperatures. Both populations maintained, after chilling, a pattern of CO2 fixation typical of C4 plants with photosynthesis saturating at low external CO2 concentrations. However, the chilling treatment led to reductions in carbon uptake and in the activities of the C4 enzymes. RUBPc activity was not significantly affected by chilling. Reductions in photosynthesis and in C4 enzyme activities following the chilling treatment were significantly larger for plants of the Mississippi population. The enzyme data suggest that two steps of the C4 pathway, NADP+-malate dehydrogenase and pyruvate Pi dikinase, are likely to be associated with the reduction of CO2 uptake in C4 plants under cool conditions. When the experiment was replicated under enriched atmospheric CO2 (675 μl l-1 CO2), similar differences were observed between the two populations. CO2 enrichment resulted in an increase of activity for phospho-enol-pyruvate carboxylase and NADP+-malate dehydrogenase while activities of phospho-enol-pyruvate carboxylase and NADP+-malic enzyme were less reduced following chilling. Such an interaction was not observed for gas exchange parameters but net photosynthesis was lower when plants were grown under enriched CO2.

Adaptation and acclimation of higher plants at the enzyme level: Latitudinal variations of thermal properties of NAD malate dehydrogenase in Lathyrus japonicus Willd. (Leguminosae)

SummaryThermal properties of NAD malate dehydrogenase were investigated in 8 clonal populations of Lathyrus japonicus Willd. collected over a 16° latitudinal gradient in eastern North America. Clones from warmer summer sites had higher activation energies (Ea) and enhanced thermostability for the enzyme under three contrasting growth temperature regimes. A strong component of acclimation was associated with Ea, but not with thermostability. Activity levels of the enzyme were higher in warm-adapted genotypes but cold-acclimated plants had similar low levels of specific activity. The clones were differentiated by ordination in a pattern related to climatic origin.

Thermal adaptation and acclimation of higher plants at the enzyme level: kinetic properties of NAD malate dehydrogenase and glutamate oxaloacetate transaminase in two genotypes of Arabidopsis thaliana (Brassicaceae)

SummaryKinetic properties of NAD malate dehydrogenase (MDH) and glutamate oxaloacetate transaminase (GOT) were analyzed in two genotypes of Arabidopsis thaliana collected in two sites of contrasting climates. Plants from each genotype were acclimated under controlled conditions at four different thermoperiods: 5–10° C, 7–15° C, 15–25° C and 25–28° C. Apparent energy of activation for MDH of the cold adapted genotype were significantly lower at low temperatures of acclimation, while for GOT, significant differences were found but no clear patterns emerge from the data. No differences of significance between the two genotypes were observed for apparent Km and Kcat of both enzymes. For MDH, apparent Kms for oxaloacetic acid increased as a positive function of assay temperature but for GOT, Kms for α-oxoglutaric acid did not vary significantly over the 10–35° C assay temperature range. Kcat values for both enzymes increased about 2 fold for every 10° C raise in assay temperature. Concentrations of both enzymes significantly increased in plants of both genotypes acclimated to the coldest thermoperiod. The concentration of GOT was signficantly higher in plants of the cold adapted genotype acclimated to 5–10° C and 7–15° C. Results suggest that MDH and GOT from the cold adapted genotype are more efficient in the modulation of catalysis at low temperatures, while the opposite is found for plants of the warm-adapted genotype through enhanced thermostability of the mitochondrial fraction of MDH.

A Numerical Taxonomic Study of Selected Citrus Species (Rutaceae) Based on Biochemical Characters

Nine taxa of cultivated Citrus were studied by cluster analysis and principal coordi- nate analysis using 86 biochemical characters reported in the literature. Two main affinity groups were obtained. The first clusters C. medica, C. aurantifolia, and C. limon; the second includes C. reticulata, C. sinensis, C. paradisi, C. aurantium, C. jambhiri, and C. grandis, the last species being, however, isolated from the main grouping. Our results show taxonomic congruence with those obtained by Barrett and Rhodes and based solely on morphological data.

Adaptation and acclimation of higher plants at the enzyme level: temperature-dependent substrate binding ability of NAD malate dehydrogenase in four populations of Lathyrus japonicus willd. (Leguminosae)

The substrate-binding ability of NAD malate dehydrogenase for oxaloacetic acid was analysed in four clonal populations of Lathyrus japonicus collected from sites of contrasting climates and acclimated to three experimentally-induced thermoperiods. The Km values and Q10 estimates at different substrate concentrations indicate that enzyme-substrate affinity is temperature dependent and is modified by acclimation in a pattern which suggests efficient compensatory regulation for this enzyme property. Positive thermal modulation was observed for plants of all clones acclimated at 7°/15°C and 15°/25°C and for plants of the two cold-adapted clones grown at a 22°/30°C thermo-period. At the high temperature regime, plants from the two warm summer sites showed negative thermal modulation, suggesting that adaptational modifications of Km-temperature relationships are not compensated completely by acclimation.

Effects of temperature and CO2 enrichment on kinetic properties of phospho-enol-pyruvate carboxylase in two ecotypes of Echinochloa crus-galli (L.) Beauv., a C4 weed grass species

SummaryTwo populations of Echinochloa crus-galli (Québec, Mississippi) were grown at the Duke University Phytotron under 2 thermoperiods (28°/22°C, 21°/15°C day/night) and 2 CO2 regimes (350 and 675 μl l-1). Thermostability, energy of activation (Ea),Km (PEP), Km (Mg++), and specific activity of phospho-enol-pyruvate carboxylase (PEPc) were analyzed in partially purified enzyme preparations of plants grown for 5 weeks. Thermostability of PEPc from extracts (in vitro) and leaves (in situ) was significantly higher in Mississippi plants. In vitro denaturation was not appreciably modified by thermal acclimation but CO2 enrichment elicited higher thermostability of PEPc. In situ thermostability was significantly higher than that of in vitro assays and was higher in Mississippi plants acclimated at 28°/22°C and in plants of the two ecotypes grown at 675 μl l-1 CO2. Ea (Q10 30°/20°C) for PEPc was significantly lower in Québec plants as compared to Mississippi and no acclimatory shifts were observed. Significantly higher Kms (PEP) in 20°C assays were obtained for Mississippi as compared to Québec plants but values were similar at 30°C and 40°C assays. Km (Mg++) decreased at higher assay temperatures and were significantly lower for PEPc of the Québec ecotype. No significant changes in Km (Mg++) values were associated with modifications in temperature on CO2 regimes. PEPc activity measured at 30°C was significantly higher for Québec plants when measured on a leaf fresh weight, leaf area or protein basis but not on a chlorophyll basis. Significantly higher PEPc activity for both genotypes was observed for plants acclimated at 21°/15°C or grown at 675 μl l-1 CO2. Net photosynthesis (Ps) and net assimilation rates (NAR) were higher in Québec plants and were enhanced by CO2 enrichment. NAR was higher in plants acclimated at low temperature, while an opposite trend was observed for Ps. PEPc activities were always in excess of the amounts required to support observed rates of CO2 assimilation.

Isoenzymatic variability among populations and varieties of wild leek (Allium tricoccum)

Abstract Six populations of wild leek (Allium tricoccum) containing two sympatric varieties (var. tricoccum and var. burdickii) were collected in the range of habitats found at the northern limit of the distribution of the species in Quebec. Seven enzyme systems comprising 14 loci were successfully resolved by polyacrylamide gel electrophoresis. Variation among individuals and populations was detected for malate dehydrogenase and esterases, but the electrophoretic analysis was not able to discriminate between individuals of the two varieties. The mean Hedricks genetic identity for pairwise comparisons among populations was /H=0.982. The reduced genetic variation of this forest perennial herb at the northern limit of its distribution may be explained by the autogamous sexual reproduction and the predominant contribution of vegetative propagation to population maintenance.

ADAPTATION AND ACCLIMATION OF HIGHER PLANTS AT THE ENZYME LEVEL: KINETIC PROPERTIES OF NAD MALATE DEHYDROGENASE IN THREE SPECIES OF VIOLA

SUMMARY (1) Kinetic properties of NAD malate dehydrogenase (MDH) were investigated in three species of Viola: V. blanda, V.fimbriatula and V. sororia. Plants from three clones of each species, collected over the latitudinal range of each species in eastern North America, were acclimated under controlled conditions at three different thermoperiods: 7/15 IC, 15/22 IC and 22/28 OC. (2) The apparent Michaelis constant (Km.) for oxalacetic acid increased with rising assay temperatures, and cold acclimation reduced Km values at low assay temperatures in plants of the cold-adapted Quebec clones of V. blanda and V. sororia acclimated at 7/15 OC and 15/22 OC. (3) The Vm.ax/Km ratios for MDH were higher at assay temperatures closely related to growth conditions although this trend was expressed differentially depending on the species or the clone analysed. (4) Differential trends were observed in between-species comparisons and can be correlated with the microclimate in the habitat of the species.

Protein and enzyme uniformity in a new, isolated population of the sharp-tailed sparrow

Abstract Tissue samples of three breeding male Sharp-tailed sparrows, taken from a small, recently established population, were analysed by starch gel and polyacrylamide gel electrophoresis. We found no variation at 46 isozyme loci and two globin activity sites. The probability that our sample has been taken from a population having an average heterozygosity level of 0.058, the mean level for an emberizine sparrow population, is shown to be 0.00019. Banding patterns for general proteins, resolved on isoelectric focusing gels, were identical for all individuals. The apparent lack of protein variability in the population is most probably the result of a bottleneck that occurred at the time of founding. Sharp-tailed sparrows are promiscuous, and their populations frequently show unbalanced sex ratios. The potential infiuence of these life history traits on the rate of deciine in genetic variability is discussed.