Through its impact on photosynthesis and morphogenesis, light is the environmental factor that most affects plant architecture. Using light rather than chemicals to manage plant architecture could reduce the impact on the environment. However, the understanding of how light modulates plant architecture is still poor and further research is needed. To address this question, we examined the development of two rose cultivars, Rosa hybrida'Radrazz' and Rosa chinensis'Old Blush', cultivated under two light qualities. Plants were grown from one-node cuttings for 6 weeks under white or blue light at equal photosynthetic efficiencies. While plant development was totally inhibited in darkness, blue light could sustain full development from bud burst until flowering. Blue light reduced the net CO(2) assimilation rate of fully expanded leaves in both cultivars, despite increasing stomatal conductance and intercellular CO(2) concentrations. In 'Radrazz', the reduction in CO(2) assimilation under blue light was related to a decrease in photosynthetic pigment content, while in both cultivars, the chl a/b ratio increased. Surprisingly, blue light could induce the same organogenetic activity of the shoot apical meristem, growth of the metamers and flower development as white light. The normal development of rose plants under blue light reveals the strong adaptive properties of rose plants to their light environment. It also indicates that photomorphogenetic processes can all be triggered by blue wavelengths and that despite a lower assimilation rate, blue light can provide sufficient energy via photosynthesis to sustain normal growth and development in roses.

Caron, J., Rivière, L.-M. and Guillemain, G. 2005. Gas diffusion and air-filled porosity: Effect of some oversize fragments in growing media. Can. J. Soil Sci. 85: 57-65. Large particles are often added to growing media to improve air-filled porosity, an often-reported growth-limiting factor. Previous studies have shown that large fragments can leave air-filled porosity unchanged and actually decrease the air exchange characteristics in peat:bark and pure peat media, with an adverse effect on plant growth. Thirteen different growing mixes composed of perlite, bark, peat, sand, rockwool, wood and coco fibre, in which the size of the larger fractions was varied, were tested. Air-filled porosity, easily available water, pore effectiveness coefficient, and diffusivity were examined in these mixes. Air-filled porosity was found to be the highest in mixes where the largest fraction was composed of coarser particles. Pore effectiveness coefficient was found to decrease with the increasing coarseness of the largest fractions. Gas diffusivity was found to be highest when intermediate (2-4 mm) fragments were used as the largest fraction of the mixes. These findings applied to impermeable, disk-or spherically shaped, coarse fragments (perlite, coco hulls, bark), but did not apply to threadlike coarse material (wood fibres). Gas diffusivity values (at a water potential of -0.75 kPa) obtained for rockwool, the peat:bark mix containing 2-to 4-mm bark particles, the peat:medium wood fibre mix, the peat:fine perlite (0.8-2 mm) mix and the pure fine coconut medium were not statistically different one from another. Air-filled porosity was negatively correlated to the fractions less than 1 mm, between 0 and 0.2 mm and between 0.2 and 0.8 mm. Easily available water, pore effectiveness coefficient and gas diffusivity were not correlated to particle size distribution, indicating that these properties cannot be estimated based on particle size and must instead be measured directly.Key words: Aetion, peat substrates, perlite, bark, available water Caron, J., Rivière, L.-M. et Guillemain, G. 2005. La diffusion des gaz et la porosité en air : incidence des particules surdimensionnées dans le milieu de culture. Can. J. Soil Sci. 85: 57-65. On ajoute souvent de grosses particules au milieu de culture afin d'en accroître la porosité en air, facteur souvent blâmé pour une croissance plus lente. Or, des études antérieures révèlent que la présence de grosses particules pourrait ne pas modifier la porosité en air voire altérer les propriétés des échanges gazeux dans les milieux composés de mousse de sphaigne et d'écorce ou de mousse de sphaigne pure, donc affecter la croissance des plantes. Les auteurs ont testé treize milieux de culture composés de perlite, d'écorce, de mousse de sphaigne, de sable, de laine minérale, de bois et de fibres de noix de coco en faisant varier le calibre des plus gros fragments. Ensuite, ils ont établi la porosité en air, l'eau facilement disponible, le coefficient d'efficacité des pores et la diffusivité des gaz d...

Caron, J., Morel, P., Rivie`re, L.-M. and Guillemain, G. 2010. Identifying appropriate methodology to diagnose aeration limitations with large peat and bark particles in growing media. Can. J. Soil Sci. 90: 481Á494. Large-sized particles (coarse peat, bark or sawdust) are often added to growing media to improve substrate aeration properties (gas storage and exchange). Recent studies have shown that large fragments mixed with fines may create barriers that restrict gas diffusion or create competition for oxygen even if they improve air storage. An experiment was carried out to compare the growth performances of growing media containing large fragments and to assess their aeration status using different methods. Different mixes were made of a fine sphagnum peat (average size 2.4 mm) and a coarse (1Á2, 2Á4, 4Á6, 6Á10, and 10Á20 mm particles) sphagnum peat or bark (2Á4 and 10Á20 mm). These substrates had different aeration properties and were used to grow Poinsettia and Impatiens 'New Guinea' in a greenhouse, resulting in differences in plant growth. The results show that air-filled porosity remained relatively unaffected by fragment size. Gas relative diffusivity differed significantly between treatments and was highest in the mix with the 2Á4 mm particles and diminished rapidly as fragment size increased from 4 to 20 mm or decreased to 1Á2 mm. Diffusivity was clearly lower in the bark/peat mixes but showed the same trend with coarse fragments. Root and shoot growth parameters were significantly and positively correlated to gas relative diffusivity. Moreover, the growth reduction observed in the bark/peat mixes relative to pure peat was most likely linked to limited gas exchange. Air-filled porosity assessments performed in situ (in the pot itself) or prior to potting, in cylinders, gave inconsistent results or were not significantly correlated to plant growth, indicating that aeration limitations are better diagnosed with gas diffusivity in growing media.Key words: Air-filled porosity, gas relative diffusivity, gas diffusion, peat substrates, bark, Euphorbia pulcherima, Impatiens ()novae-guinea)Caron, J., Morel, P., Rivie`re, L.-M. et Guillemain, G. 2010. Identifying appropriate methodology to diagnose aeration limitations with large peat and bark particles in growing media. Can. J. Soil Sci. 90: 481Á494. Des fragments de fortes tailles (tourbe, e´corce ou sciure brute) sont souvent ajoute´s aux substrats de culture de serres ou de pe´pinie`res pour ame´liorer leurs proprie´te´s d'ae´ration (stockage et e´change de gaz). Les e´tudes re´centes ont prouve´que les larges fragments me´lange´s aux fins peuvent cre´er des barrie`res qui limitent la diffusion de gaz ou cre´ent une compe´tition pour l'oxyge`ne meˆme s'ils ame´liorent le stockage d'air. Deux expe´riences ont e´te´effectue´es pour comparer les performances en serre de diffe´rents milieux de culture contenant des fragments larges et pour e´valuer leur statut d'ae´ration selon diffe´rentes me´thodes. Ces substrats ont e´te´faits d'un me´lange d'un volume de t...

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