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The effects of a decrease in red to far-red (R/FR) ratio on shoot growth of two-week-old Arabidopsis thaliana (L.) Heynh. seedlings were examined in the context of possible causal involvement of key plant growth hormones. Decreasing the R/FR ratio significantly increased petiole elongation and leaf area expansion of the Columbia (Col) line seedlings. In contrast, seedlings of the Landsberg erecta (Ler) line showed no significant change in leaf area and only a marginal increase in petiole growth. This low R/FR ratio-induced growth was accompanied by significant increases in concentrations of the growth 'effector' gibberellin ([GA.sub.4]) and an auxin (indole-3-acetic acid (IAA)) in shoot tissues of Col. However, cytokinins (CKs) in Col shoot tissues were decreased and ethylene evolution was reduced when the R/FR ratio was decreased from that of normal sunlight to a low R/FR ratio. Several A. thaliana genotypes with plant hormone-related mutations were also assessed, including auxin resistant, axr2-1; GA insensitive, gai-1; and ethylene over-producing, eto2. None of these increased their petiole length or leaf area growth in response to lowering the R/FR ratio. We thus conclude that both G[A.sub.4] and IAA are causally involved in the increased shoot growth of A. thaliana Col seedlings that occurs in response to a lower than normal R/FR ratio. Key words: red to far-red ratio, Arabidopsis thaliana, gibberellins, cytokinins, auxin, ethylene. Dans le cadre d'une possible implication causale des hormones vegetales cles, les auteurs ont examine les effets d'une diminution du rapport rouge a rouge sombre (R/RS) sur la croissance des tiges de plantules de l'Arabidopsis thaliana agees de deux semaines. Une diminution du rapport R/RS augmente significativement l'elongation du petiole et l'expansion du limbe foliaire des plantules de la lignee Columbia (Col). Au contraire, les plantules de la lignee Landsberg erecta (Ler) ne montrent aucun changement significatif de l'expansion foliaire et une augmentation marginale de la croissance du petiole. Cette croissance reduite sous l'effet d'un faible rapport R/RS s accompagne d'augmentations significatives dans les teneurs en gibberelline ([GA.sub.4]) << enclencheur >> de croissance et d'une auxine (l'acide indole-3-acetique (AIA)) dans les tissus caulinaire du Col. Cependant, les cytokinines (CKs) des tissus caulinaire du Col diminuent lorsque le rapport R/RS diminue, a partir de celui de la lumiere solaire vers un faible rapport R/RS. Les auteurs ont egalement evalue plusieurs genotypes de l'A. thaliana comportant des mutations reliees aux hormones, incluant la resistance aux auxines, axr2-1; l'insensibilite a la GA, gai-1; et la surproduction d'ethylene, eto2. Aucun de ceux ci n'augmente la longueur de leur petiole ou leur croissance du limbe foliaire en reaction a une reduction du R/RS. Les auteurs concluent qu'il y a une implication causale a la fois de la GA et de l'AIA dans l'augmentation de croissance de la tige chez les plantules Col survenant suite a une diminution du R/RS par rapport a la normale. Mots-cles : rapport rouge a rouge sombre, Arabidopsis thaliana, gibberellines, cytokinines, auxine, ethylene. [Traduit par la Redaction], Introduction Plants growing under canopy shade or in the shade of neighbouring, proximate vegetation are subjected to shade light, i.e., light with lower red to far-red (R/FR) ratios (Ballare et [...]

Plants growing in canopy shade typically exhibit increased stem elongation and reduced leaf growth. This is as a result of direct interactions between plant photoreceptors sensing the change (reduction) in the ratio of red to far-red (R/ FR) light and photosynthetically active radiation (PAR) and plant hormones, and regulating these morphological traits. The effect of the varying light conditions found in shade on endogenous salicylic acid (SA) content was tested, and the possible role of SA in shade avoidance by sunflower (Helianthus annuus L.) hypocotyls was examined. A logarithmic increase in PAR irradiance levels increased endogenous SA levels roughly 10-fold. Separation of individual light wavelengths (R, FR, and blue) constituting the PAR irradiance of sunlight, established that only FR light had significant and positive effects on endogenous SA levels. Further, a low R/FR ratio significantly increased the endogenous SA content in hypocotyls compared with normal and high R/FR ratios. Uncoupling the effect of R/FR ratio and PAR irradiance on endogenous SA content demonstrated that PAR irradiance is a much stronger signal than FR light-enrichment. Thus, while a low R/FR ratio increases the SA content in sunflower hypocotyls, low PAR, the other component of canopy shade, decreases the SA content much more effectively than low R/FR ratio increases it. Therefore, it appears that SA probably has no direct role in shade avoidance effects. Key words: light quality, light irradiance, narrow-band light, salicylic acid, sunflower. Les plantes qui poussent a l'ombre de la canopeee montrent de facon typique une elongation de la tige et une reduction de la croissance foliaire. Ceci resulte d'interactions directes entre les photorecepteurs de la plante percevant le changement (reduction) du rapport rouge/rouge sombre (R/FR) ainsi que les radiations photosynthetiquement actives (PAR) et les hormones regularisant ces caracteres morphologiques. Les auteurs ont experimente l'effet de diverses conditions ombragees sur la teneur en acide salicylique (SA) et ils ont examine; le riale possible du SA dans la reaction a l'ombre, dans des hypocotyles de tournesol (Helianthus annuus L.). Une augmentation logarithmique des degrees d'irradiance augmente les teneurs en SA endogene d'environ dix fois. Une separation des longueurs d'ondes individuelles (R, FR et bleue) constituant l'irradiance PAR de la lumiere solaire montre que seule la lumiere FR exerce un effet significatif et positif sur les teneurs internes en SA. De plus un faible rapport R/FR augmente significativement la teneur endogene en SA dans les hypocotyles, comparativement aux rapports normaux de R/FR. Un decouplage de l'effet du rapport R/FR et de l'irradiance PAR sur la teneur endogene en SA demontre que l'irradiance PAR constitue un signal beaucoup plus puissant que l'enrichissement en FR. Ainsi, un faible rapport R/FR augmente la teneur en SA chez les hypocotyles du tournesol, un faible PAR, l'autre constituant de l'ombre sous la canopeie, diminue la teneur en SA beaucoup plus efficacement que le faible rapport R/FR n'arrive a l'augmenter. Consequemment, il apparait que la SA ne joue aucun role direct dans les effets d'eevitement de l'ombrage. Mots-cles: qualite de la lumiere, irradiance lumineuse, lumiere a bandes eetroites, acide salicylique, tournesol., Introduction Plants grown under canopy shade or in the shade of neighbouring taller vegetation are subjected to a lowered ratio of red to far-red (R/FR) light and lower levels of [...]

Abstract: Plants growing in vegetative shade are subjected to reductions in both red to far-red (R/FR) light ratio and in irradiance levels of photosynthetically active radiation (PAR). In this report [...]

Abstract: The objective of this study was to evaluate the effects of elevated carbon dioxide and sucrose concentrations on Arabidopsis thaliana L. Heynh root growth, morphology, and architecture. Two levels [...]

Abstract: Plant ecotypes of Stellaria longipes Goldie from competitive, shade-adapted prairie habitat and less competitive, nonshaded alpine habitat were subjected to shortened or extended photoperiod conditions. Increasing daylength was positively [...]

Abstract: Four phenotypically different genotypes from an alpine population of Stellaria longipes Goldie s.l. (Caryophyllaceae) were collected from neighbouring sites at the top of the Plateau Mountain in southeastern Alberta, [...]

High latitude forests will experience large changes in temperature and CO2 concentrations this century. We evaluated the effects of future climate conditions on 2 dominant boreal tree species, Pinus sylvestris L. and Picea abies (L.) H. Karst, exposing seedlings to 3 seasons of ambient (430 ppm) or elevated CO2 (750 ppm) and ambient temperatures, a + 4 °C warming or a + 8 °C warming. Pinus sylvestris responded positively to warming: seedlings developed a larger canopy, maintained high net CO2 assimilation rates (Anet), and acclimated dark respiration (Rdark). In contrast, carbon fluxes in Picea abies were negatively impacted by warming: maximum rates of Anet decreased, electron transport was redirected to alternative electron acceptors, and thermal acclimation of Rdark was weak. Elevated CO2 tended to exacerbate these effects in warm-grown Picea abies, and by the end of the experiment Picea abies from the +8 °C, high CO2 treatment produced fewer buds than they had 3 years earlier. Treatments had little effect on leaf and wood anatomy. Our results highlight that species within the same plant functional type may show opposite responses to warming and imply that Picea abies may be particularly vulnerable to warming due to low plasticity in photosynthetic and respiratory metabolism. [ABSTRACT FROM AUTHOR]