Michèle Bernier-Cardou

Influence of gelling agents on culture medium gel strength, water availability, tissue water potential, and maturation response in embryogenic cultures of Pinus strobus L.

SummaryMaturation of somatic embryos of Pinus strobus L. was evaluated on media containing various types (agars and gellan gum), brands and concentrations of gelling agents in the presence of 80 μM ABA and 0.09 M sucrose. The media were characterized with respect to gel strength, water potential and water availability. Embryogenic tissue and somatic embryos cultured on medium with various concentrations of gellan gum were used to determine their water potential (Ψ). Regardless of the type of gelling agent used, gel strength increased with gelling agent concentration and was critical to the maturation response. High gel strength was associated with reduced water availability from the medium to the cultures. The water potential of gelled maturation medium remained constant between 0.4 and 1.0% gellan gum. It is concluded that the embryogenic tissue was exposed to varying amounts of water at the onset of and during the culture period, and that the amount of water in the culture environment in turn influenced the maturation response. Cotyledonary somatic embryos derived from gellan gum medium of high gel strength had a lower Ψ than somatic embryos matured on medium of lower gel strength. Once somatic embryos developed to the cotyledonary stage on the maturation medium, they were transferred to the germination medium. The germination frequency and the number of morphologically normal germinants were higher for somatic embryos matured on medium of high gel strength. Raising the concentration of the gelling agent in the maturation medium may be an alternative to the use of solutes to restrict water available to the embryogenic cultures.

Forest succession over a 20-year period following clearcutting in balsam fir-yellow birch ecosystems of eastern Québec, Canada

Abstract Vegetation development over a 20-year period following clearcutting in balsam fir (Abies balsamea (L.) Mill.)-yellow birch (Betula alleghaniensis Britt.) ecosystems was examined in a study area located in eastern Quebec, Canada. Vegetation, physiographic and soil data were collected in 10 mature ecosystems and in 30 ecosystems harvested 5 years ago (10), 10 years ago (10), or 20 years ago (10). The 40 ecosystems had similar physiographic and soil characteristics. They were typically located on mesic sites situated on ground moraines thicker than 50 cm. Following harvesting, sites were invaded by competing species. Mountain maple (Acer spicatum Lamb.) was the most important competing species. Twenty years after logging, it fully occupied the sites with 7040 stems ha−1 (diameter at breast height ≥ 1 cm). Its regeneration stocking reached 88% with a density of 22775 stems ha−1. Wild red raspberry (Rubus idaeus L. var. strigosus (Michx.)) and fireweed (Epilobium angustifolium L.) were abundant during a 10-year period after logging, but disappeared almost completely afterwards. The abundance of competing species has considerably reduced site production for a period of 20 years and will probably continue to do so for 20 to 30 more years. The proportion of commercial deciduous species increased from 36% of the total number of stems (diameter at breast height ≥ 1 cm) in mature stands to 89% in stands harvested 20 years ago. Balsam fir and white spruce (Picea glauca (Moench) Voss) advanced regeneration was considerably reduced. Stocking of these species went down from 76% in mature stands to only 27% in 20-year-old stands. As a result, it is unlikely that the harvested areas will naturally evolve toward the original climax balsam fir-yellow birch forest type in the foreseeable future.

Effect of sugars, amino acids, and culture technique on maturation of somatic embryos of Pinus strobus on medium with two gellan gum concentrations

Maturation of five embryogenic lines of Pinus strobus L. was tested on media with various sugars and sources of organic nitrogen, and solidified with two gellan gum concentrations (0.6 and 1.0%). Mature somatic embryo production was more abundant at 1.0% gellan gum than at 0.6%. Complex combinations of amino acids had little effect on mature embryo production of most tested embryogenic lines. Increasing glutamine concentration of the maturation medium from 1.7 to 7.3 g l−1 was beneficial to one embryogenic line. Increasing sucrose concentration or substituting part of the sucrose with mannitol or sorbitol had variable effects on somatic embryo maturation depending on the embryogenic line. A medium with 88 mM sucrose plus 175 mM sorbitol solidified with 1.0% gellan gum produced high numbers of somatic embryos in four out of five embryogenic lines tested.

The combined effect of photoperiod and temperature on egg dormancy in an island and a mainland population of the hemlock looper, Lambdina fiscellaria.

Embryonic dormancy characteristics of the hemlock looper (HL), Lambdina fiscellaria (Guenée) (Lepidoptera: Geometridae), an important coniferous defoliator, were investigated using eggs from an island (Newfoundland) and a mainland (Quebec) population in eastern Canada. We determined (1) the pre‐diapause duration or time required for eggs to change color at 15 °C, under L16:D8 and L12:D12, (2) the percent hatch and time to hatch of early‐diapausing eggs that were, as of 1 September, either (a) incubated under each combination of three photoperiods (L16:D8, L12:D12, or L8:D16) and three temperatures (15, 20, or 25 °C) or (b) stored outdoors prior to their monthly incubation (October–May) under the same treatments, and (3) the diapause duration at 15 °C. The two HL ecotypes completed their pre‐diapause phase in ca. 15 days under long‐day photoperiod. In the mainland ecotype, photoperiod did not influence the pre‐diapause duration. Regardless of photoperiod and ecotype, only eggs incubated continuously at 15 °C hatched successfully after ca. 120 days. Temperature was the most important factor modulating the dormancy of eggs acclimated outdoors, in both ecotypes. From October to December (diapause phase), percent egg hatch at 20 and 25 °C increased from low (20%) to levels similar to those obtained at 15 °C (70%). These percentages remained stable throughout the post‐diapause phase (January–May). Time to hatch, which was shorter at warmer temperatures, decreased from October to December at all temperatures. It remained stable from January to March (quiescence), however, and declined thereafter. Eggs from the island were heavier than those from the mainland and their odds of hatching were 2.3 times higher. At 15 °C, diapause lasted ca. 90 days. Our findings indicate that eggs from the two HL ecotypes (1) undergo an obligatory diapause, (2) complete diapause without pre‐exposure to cold, (3) respond similarly to photoperiod and temperature during diapause and post‐diapause, and (4) do not hatch successfully after prolonged exposure to high temperatures.

Impact of short-term exposure to low subzero temperatures on egg hatch in the hemlock looper, Lambdina fiscellaria

The frequency of extreme events, such as cold spells, is expected to increase under global warming. Therefore, the ability of insects to survive rapid changes in temperature is an important aspect to investigate in current population ecology. The hemlock looper (HL), Lambdina fiscellaria (Guenée) (Lepidoptera: Geometridae), a defoliator of boreal balsam fir forests in eastern Canada, overwinters at the egg stage on tree trunks and branches where eggs can be exposed to very low subzero air temperatures. Using eggs from the island of Newfoundland (NL) and Quebec mainland (QC), we undertook field and laboratory experiments to determine: (1) their supercooling point (SCP) in mid‐January and mid‐February; (2) overwintering mortality; (3) cold tolerance to various combinations of subzero temperatures (−25, −30, −33, −35, or −37 °C) and exposure durations (2, 4, 8, 12, or 16 h); and (4) potential causes of death at subzero temperatures above the SCP. Regardless of population or sampling date, eggs supercooled on average at −40.1 °C. In the field, 59% of eggs from either population that overwintered in Sainte‐Foy (QC) and Corner Brook (NL) hatched successfully, whereas none did in Armagh (QC) or Epaule (QC). In the laboratory, 50% of eggs survived after 4 h at −34.4 °C or after 14 h at −32.9 °C. In contrast, regardless of exposure duration, >50% of eggs hatched at temperatures ≥−33 °C, but <50% did so at ≤−35 °C, suggesting high pre‐freeze mortality. However, when eggs were attached to thermocouples and exposed to temperatures ranging from −25 to −37 °C for 16 h, 69% froze at temperatures of −35 to −37 °C, but only 2% did at −25 or −30 °C. Time to freeze decreased as subzero temperatures declined, and this was more evident in island eggs than in mainland eggs. Overall, eggs can freeze after a brief exposure to subzero temperatures higher than the standard SCP, and are thus highly vulnerable to cold spells.

Reproductive performance of the hemlock looper, Lambdina fiscellaria, as a function of temperature and population origin

In a recent study of the hemlock looper (HL), Lambdina fiscellaria (Guenée) (Lepidoptera: Geometridae), long exposure of early‐diapausing eggs to high temperatures considerably reduced their ability to hatch. This finding raised the possibility that adults could also be negatively affected by increasing temperatures if they reproduced too early in the season in response to global warming. To investigate this hypothesis, newly formed HL pupae from three populations of eastern Canada ‐ Quebec (QC), Newfoundland (NL), and Labrador (LB) ‐ were submitted to four constant temperatures (10, 15, 20, or 25 °C) during pupal and adult development. The effect of population origin on HL reproduction was generally negligible. Mating probability was high at 15 and 20 °C (0.86 and 0.83, respectively), quite low at 10 °C (0.53), and even lower at 25 °C (0.38). Mating started earlier in the night and lasted longer as temperature decreased. Both productivity and absolute fecundity increased when temperature increased from 10 to 15 °C and then decreased slowly as temperature increased further. Over populations and temperatures, relative fecundity averaged 0.95, indicating that females had enough time to lay most of their eggs before they died. High temperatures had a deleterious effect on egg fertility: between 10 and 20 °C, relative fertility was about 0.90, but it dropped to 0.51 at 25 °C. The average proportion of fertile eggs declined from 0.88 in the first quarter of the egg‐laying period to 0.57 in the last quarter, suggesting lower sperm count or viability, or deterioration of the oocytes as the egg‐laying period progresses. Based on these findings, we argue that the production of an additional fifth instar among HL populations of southern origin can be viewed as an adaptive mechanism allowing adults to postpone reproduction or the egg‐laying period in order to mitigate the detrimental effect of high temperatures on their probability of mating successfully or that of laying fertile eggs.