Black spruce [Picea mariana (Mill.) B.S.P.] and tamarack [Larix laricina (Du Roi) K. Koch] are the predominant tree species in boreal peatlands. The effects of 34 days of flooding on morphological and physiological responses were investigated in the greenhouse for black spruce and tamarack seedlings in their second growing season (18 months old). Flooding resulted in reduced root hydraulic conductance, net assimilation rate and stomatal conductance and increased needle electrolyte leakage in both species. Flooded tamarack seedlings maintained a higher net assimilation rate and stomatal conductance compared to flooded black spruce. Flooded tamarack seedlings were also able to maintain higher root hydraulic conductance compared to flooded black spruce seedlings at a comparable time period of flooding. Root respiration declined in both species under flooding. Sugar concentration increased in shoots while decreasing in roots in both species under flooding. Needles of flooded black spruce appeared necrotic and electrolyte leakage increased over time with flooding and remained significantly higher than in flooded tamarack seedlings. No visible damage symptoms were observed in flooded tamarack seedlings. Flooded tamarack seedlings developed adventitious roots beginning 16 days after the start of flooding treatment. Adventitious roots exhibited significantly higher root hydraulic conductivity than similarly sized flooded tamarack roots. Flooded black spruce lacked any such morphological adaptation. These results suggest that tamarack is better able to adjust both morphologically and physiologically to prolonged soil flooding than black spruce seedlings.

Black spruce (Picea mariana (Mill.) BSP) and tamarack (Larix laricina (Du Roi) K. Koch) are the predominant tree species in the boreal peatlands of Alberta, Canada, where low nutrient availability, low soil temperature and a high water table limit their growth. Effects of flooding for 28 days on morphological and physiological responses were investigated in greenhouse-grown black spruce and tamarack seedlings in a growth chamber. Flooding reduced root hydraulic conductance, net assimilation rate and stomatal conductance, and increased water-use efficiency (WUE) and needle electrolyte leakage in both species. Although flooded black spruce seedlings maintained higher net assimilation rates and stomatal conductance than flooded tamarack seedlings, flooded tamarack seedlings were able to maintain higher root hydraulic conductance than flooded black spruce seedlings. Needles of flooded black spruce developed tip necrosis and electrolyte leakage after 14 days of flooding, and these symptoms were subsequently more prominent than in needles of flooded tamarack seedlings. Flooded tamarack seedlings exhibited no visible injury symptoms and developed hypertrophied lenticels at their stem base. Application of exogenous ethylene resulted in a significant reduction in net assimilation, stomatal conductance and root respiration, whereas root hydraulic conductivity increased in both species. Thus, although flooded black spruce seedlings maintained a higher stomatal conductance and net assimilation rate than tamarack seedlings, black spruce did not cope with the deleterious effects of prolonged soil flooding and exogenous ethylene as well as tamarack.

Abstract• Fall fertilization may increase plant nutrient reserves, yet associated impacts on seedling cold hardiness are relatively unexplored.• Bareroot red pine (Pinus resinosa Ait.) seedlings in north-central Minnesota, USA were fall fertilized at the end of the first growing season with ammonium nitrate (NH 4 NO 3 ) at 0, 11, 22, 44, or 89 kg N ha −1 . Seedling morphology and cold hardiness [assessed by freeze induced electrolyte leakage (FIEL)] were evaluated six weeks after fertilization and following the second growing season.• Seedling height and number of needle primordia increased with fertilizer rate for both sampling years. Seedlings fertilized with 44 and 89 kg N ha −1 attained target height (15 cm) after the second growing season. Shoot and root N concentration increased after the first growing season in fall fertilized seedlings compared to controls. Fall fertilized seedlings had lower FIEL (i.e., increased cold hardiness) compared to controls when tested at -40 • C after the first growing season, but no significant differences in FIEL of control and fertilized seedlings were observed after the second growing season.• Results suggest that fall fertilization of red pine seedlings can help render desired target height in the nursery, while maintaining or increasing cold hardiness levels. Mots-clés :rusticité au froid / croissance / fertilisation azotée / primordiums d'aiguilles Résumé -Fertilisation automnale des plants de Pinus resinosa : absorption des éléments nutritifs, rusticité au froid, et développement morphologique.• La fertilisation automnale peut augmenter les réserves en éléments nutritifs des plants, mais les répercussions sur la rusticité au froid des semis sont encore relativement inexplorées.• Des plants à racines nues de Pinus resinosa Ait.dans le centre-nord du Minnesota (USA) ont été fertilisés à l'automne à la fin de la première saison de croissance avec du nitrate d'ammonium (NH 4 NO 3 ) à 0, 11, 22, 44, ou 89 kg N ha −1 . La morphologie des plants et la rusticité au froid [estimée par la fuite d'électrolyte (FIEL) induite par le gel] ont été évaluées six semaines après la fertilisation et à la suite dans la deuxième saison de croissance.• La hauteur des plants et le nombre de primordiums d'aiguilles ont augmenté avec le taux de fertilisation pour les deux années d'échantillonnage. Les plants fertilisés avec 44 et 89 kg N ha −1 ont atteint l'objectif de hauteur (15 cm) après la deuxième saison de croissance. La concentration en N des tiges et des racines a augmenté après la première saison de croissance chez les plants fertilisés à l'automne par rapport aux témoins. Les plants fertilisés à l'automne ont eu un plus faible FIEL (c'est-à-dire, une augmentation de rusticité), comparativement aux témoins lors du test à -40 • C après la première saison de croissance, mais aucune différence significative de FIEL entre plants fertilisés et témoins n'a été observée après la deuxième saison de croissance.• Les résultats suggèrent que la fertilisation d'automne des plants de Pinus resinosa...

-We evaluated gas exchange, chlorophyll index (CI) using SPAD-502 chlorophyll meter, and leaf nutritional responses in one-year-old northern red oak (Quercus rubra L.) container seedlings transplanted into control (unfertilized) or fertilized (0.86 g N plant −1 ) sand culture and grown in a greenhouse for 90 days. We labeled current nitrogen (N) uptake with ( 15 NH 4 ) 2 SO 4 and directly quantified proportional contributions of N derived from fertilizer (NF) compared with retranslocation or N derived from plant (NP) in leaf growth of red oak seedlings. NF met a greater N demand in leaf growth of fertilized plants while unfertilized seedlings relied entirely on NP for their leaf growth. Fertilization increased leaf dry mass by 67% and new stem dry mass by 253% 90 days after transplanting compared to control seedlings. Specific leaf area (SLA) was not significantly altered but CI increased 90 days after transplanting. Higher leaf N concentration and content in fertilized compared with control seedlings was linked to greater chlorophyll concentrations in the former plants. The higher coefficient of determination (r 2 = 0.72) noted between leaf N concentrations and CI suggests that the SPAD meter could be a useful tool for assessing leaf N status in northern red oak seedlings. Fertilized seedlings exhibited higher net assimilation, stomatal conductance, and transpiration rates compared with controls. Increased seedling growth in response to fertilization was related to maintenance of higher gas exchange and greater nutrient uptake, which could improve outplanting success. fertilization / gas exchange / northern red oak / nitrogen / photosynthesis / retranslocation / SPAD meter Résumé -Croissance, échanges gazeux et réponses nutritionnelles de jeunes semis de Quercus rubra soumis à une fertilisation par ( 15 NH 4 ) 2 SO 4 . Nous avons estimé les échanges gazeux foliaires, un index de teneurs en chlorophylles (IC) avec un chlorophylle-mètre SPAD-502 et les teneurs en nutriments dans les feuilles de jeunes plants de chêne rouge d'Amérique (Quercus rubra L.) âgés d'un an. Les plants ont été transplantés dans du substrat sableux non fertilisé (témoins) ou fertilisé avec 0.86 g N par plante, et cultivés pendant 90 jours sous serre. L'azote apporté par la fertilisation était marqué avec ( 15 NH 4 ) 2 SO 4 et nous avons directement quantifié les contributions à la croissance foliaire de N apporté par la fertilisation (NF) par rapport à celle de N remobilisé depuis les pools de réserve de la plante (NP). NF constituait la fraction la plus importante d'azote foliaire de plants fertilisés, alors que l'azote foliaire des plants non fertilisés était exclusivement constitué de NP. La fertilisation s'est traduite par une augmentation, par rapport aux plantes témoins, de 67 % de la biomasse foliaire et de 253 % de la biomasse de tiges nouvellement formées 90 jours après la transplantation. La surface spécifique des feuilles n'était pas affectée par la fertilisation alors que CI avait significativement augmenté. Des teneur...

The ability of antistress compounds to enhance the drought tolerance of conifer seedlings was tested by feeding plant growth regulators (PGRs) to 1-year-old white pine (Pinus strobus L.), which were then subjected to either a moderate (11 day) or a more severe (16 day) drought. The following PGRs were either fed directly into the xylem or applied as a root drench: the antioxidant Ambiol (2-methyl-4-[dimethylaminomethyl]-5-hydroxybenzimidazole dihydrochloride), the polyamine, spermine, an anti-ethylene agent, aminoethoxyvinylglycine (AVG), and the inhibitor, abscisic acid (ABA). Leaf water potentials (Y l ) declined in untreated seedlings when they were exposed to drought. Preconditioning with PGRs postponed water deficits and prevented membrane leakage under drought. The specific physiological adjustments observed were found to vary, depending on the type of compound. Ambiol, AVG and spermine caused transpirant rates to decline under drought. Although the antitranspirant effects of Ambiol and spermine would explain the increase in water use efficiency under drought, spermine also enhanced photosynthesis. The same compounds promoted osmotic adjustment, which would help to maintain turgor under drought. This was shown by the decline in osmotic potential at full turgor, and at zero turgor, in Ambiol and spermine-treated seedlings. Seedlings treated with Ambiol and ABA could sustain a greater water loss before turgor declined to zero. The possibility that preconditioning may help to maintain leaf physiological functioning under drought by reducing water stress and stress-ethylene production is discussed.