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Nsalambi Nkongolo, Jean Caron and Fabienne Gauthier

Increasing rates (5%, 10%, 25%, and 40%, v/v) of six sources of organic wastes were substituted for peat to assess changes on the physical properties of peat–perlite media and the subsequent plant response. Wastes were both fresh and composted bio-filter, sewage sludge, and de-inked paper sludge. Geranium plants (Pelagornium ×hortum `Orbit Hot Pink') were grown in the media. Saturated hydraulic conductivity (Ksat) and air-filled porosity (AFP) were successively measured with a Cote infiltrometer and by time-domain reflectometry. Pore space tortuosity (PST) and gas relative diffusivity (Dp/Do) were calculated. Both physical and plant growth parameters were significantly affected by the source and rate of application of waste. Ksat (P = 0.0001, r = 0.937), AFP (P = 0.001, r = 0.984), PST (P = 0.0001, r = 0.935), Dp/Do (P = 0.0001, r - 0.872) linearly increased as the rate of waste increased in the media. However, plant height (P = 0.0001, r = 0.856), root dry weight (P = 0.0001, r = 0.994), and shoot dry weight (P = 0.0001, r = 0.963) either linearly or quadratically decreased as the rate of waste increased. Decreases in plant growth parameters were most likely due to high salinity of organic wastes.

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Hui-lian Xu, Jean Caron and André Gosselin

Water potential at soil–root interface (ψ s-r) indicates soil water availability to the plants. It is related to plant water potential and transpiration. To know the characteristics of ψ s-r, in the plants under a subirrigation system, hysteresis of ψ s-r, as well as xylem water potential (ψ x) and transpiration were examined in response to soil dehydration for Prunus × cistena grown in three soil mixes: mix 1-composted bark, peat, and sand; mix 2—peat, bark, sand, and compost; and mix 3—peat, sawdust, and sand. When the soil mixes were dried from high to low water potential (ψ s), plants grown in mix 2 maintained higher ψ s-r, as well as higher ψ x and higher transpiration. However, when the soil mixes were dehydrated from the bottom, the relationships of ψ s-r, ψ x, and transpiration to ψ s showed strong hysteresis effect. ψ s-r was always lower at a given ψ s when soil was rewetted from dry to wet conditions than when soil was dried from wet conditions. ψ x and transpiration also showed hysteresis in response to soil dehydration. The extent of hysteresis was the largest in mix 2 and the smallest in mix 3. Hysteresis of ψ X or transpiration showed a similar trend to that of ψ s-r. This suggests that ψ s-r is a good indicator of soil water availability to the plants and more directly related to ψ X and transpiration than is ψ s. The difference in hysteresis of ψ s-r among soil mixes might be related to the properties of hydraulic conductance, which are determined by the soil texture. Hence, further study is needed to elucidate the mechanism of the hysteresis phenomenon.

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Isabelle Lemay, Jean Caron, Martine Dorais and Steeve Pepin

Ongoing research on organic growing media for greenhouse tomato production is driven by the constant changes in the quality, stability, and form of the organic byproducts used in the manufacturing of these media. This study was undertaken to determine appropriate irrigation set points for a sawdust–peat mix (SP) under development given that the performance of this substrate appeared to be strongly dependent on appropriate irrigation management. A greenhouse tomato experiment was conducted to compare different irrigation management approaches for a SP substrate in the spring and summer. Using preliminary measurements from an initial experiment (Expt. 1), different irrigation strategies for the SP substrate were compared in a second experiment (Expt. 2): 1) a variable irrigation regime using a timer control (with frequency adjusted as a function of irradiance); 2) tensiometer control at –1.5 kPa; and 3) two constant substrate water potential devices: –1.1 kPa and –0.9 kPa. An irrigation timer/controller using solar radiation input was used with a rockwool control (RC) substrate. Measurements of plant activity [photosynthesis rate and stomatal conductance (g S)], substrate physical and chemical properties, biomass, and yield were obtained. For all irrigation strategies, results indicated that 10% to 20% higher photosynthesis rates and g S values were obtained with the SP substrate compared with RC. Data indicated that moderate drying conditions (matric potential ranging from –2.2 kPa to –1.5 kPa in Expt. 1 and Expt. 2, respectively) relative to container capacity (–0.6 kPa) were beneficial for improving plant photosynthetic activity and allowed the highest yields for the SP substrate. Variable irrigation management showed higher levels of plant activity than constant watering and increased the oxygen concentration in the substrate by ≈2% in absolute value relative to the constant water potential device. Lower CO2 and N2O levels were also observed with the variable irrigation strategy. On the other hand, maximum nutrient solution savings were achieved with the constant matric potential devices (8% to 31% relative to the RC). This study showed high productivity potential for the SP substrate with suitable irrigation management. Replacing conventional growing media with organic waste-based products using an appropriate irrigation strategy may help to increase the sustainability of the greenhouse industry.

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Vincent Pelletier, Steeve Pepin, Thomas Laurent, Jacques Gallichand and Jean Caron

Cranberries were grown under controlled environmental conditions to determine the effects of soil waterlogging on cranberry gas exchange in three growth stages and to investigate the recovery time after waterlogging. Photosynthesis declined by 28% after the first day of waterlogging at the bud elongation stage and was 46% lower after the fifth day. At the flowering stage, the reduction in photosynthesis started to be significant only after the fifth day, whereas no reduction was observed at the fruit development stage. Stomatal limitations were responsible, in part, for the observed decrease in photosynthesis since stomatal conductance (g S) declined by 68% and 45% after the fifth day of waterlogging during bud elongation and flowering, respectively. After water drained away in the saturated treatments, leaf photosynthesis remained lower than in the unsaturated control treatment for 1 to more than 10 days at the bud elongation stage. Our results demonstrate that short-term hypoxic soil conditions can alter cranberry gas exchange depending on plant growth stage, and suggest that adequate drainage or control of the water table depth is required to avoid the negative effects of soil waterlogging on cranberry yield.

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Philippe Jobin, Jean Caron, Pierre-Yves Bernier and Blanche Dansereau

Hydrophilic polymers or hydrogels have shown potential to increase water retention of media and to reduce irrigation frequency. This property would be particularly useful in the production of fast growing species in which large amounts of water are needed. This study evaluated the effect of two acrylic-based hydrogels on water desorption curve and hydraulic conductivity of substrates and on plant growth. The duration of their effects was also investigated. Rooted cuttings of Surfinia (Petunia ×hybrida `Brilliant Pink') were transplanted into 30-cm pots containing one of three different substrates amended with one of two types of hydrogels, a commercial acrylic polymer, and a commercial acrylic-acrylamide copolymer, and grown for 9 weeks under well watered conditions and then imposed with a drought. Results indicated that both polymer types gave similar results. The substrates' physical properties (air-filled porosity, available water) at potting time were significantly affected by hydrogel addition, but differences vanished within 9 weeks of growth. Hydrogels had no significant effect on the point at which plant wilted and on the substrate's unsaturated hydraulic conductivity. Shoot dry weight was affected by substrate and hydrogel and was positively correlated to water content between container capacity and -10 kPa of water potential, or between container capacity and the soil water potential at plant turgor loss.

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Laurence Gendron, Guillaume Létourneau, Julien Cormier, Claire Depardieu, Carole Boily, Raymond Levallois and Jean Caron

Quebec, Canada, is the third largest strawberry (Fragaria ×ananassa) producer in North America, behind Florida and California. In view of increasing global water scarcity and the high water requirements of strawberry production, there is a critical need for growers to optimize irrigation practices to improve crop water productivity (CWP). In Quebec, pulsed irrigation has been shown to increase yields in strawberry crops while using the same volume of water as standard (nonpulsed) irrigation, thus improving CWP. However, more frequent and shorter-duration water applications (pulsed irrigation) might be more complex to manage manually; therefore, it could be of interest to automate the irrigation process at the farm scale. The first objective of our study was to assess the economic impact of pulsed irrigation compared with the standard irrigation procedure (nonpulsed irrigation) in a strawberry crop grown in a highly permeable clay loam soil in Quebec. The second aim was to determine whether pulsed irrigation would generate enough benefits to offset the cost of an automated irrigation system. We used data from three sites to determine the effect of pulsed irrigation on marketable yields and gross revenues compared with nonpulsed irrigation. We conducted a cost–benefit analysis to assess the cost-effectiveness of an automated irrigation system based on net gains associated with pulsed irrigation. Our results showed that pulsed irrigation was appropriate in strawberry crops grown in a highly permeable soil because it led to significant gross revenue increases relative to the standard irrigation procedure. Our results also revealed that pulsed irrigation generated enough additional benefits to cover the cost of an automated irrigation system, with a short payback period of about 1 year.

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Suzanne E. Allaire, Jean Caron, Isabelle Duchesne, Léon-Étienne Parent and Jacques-André Rioux

A 2-year experiment with Prunus ×cistena sp. was conducted in pots using seven substrates composed of various proportions of primarily peat, compost and bark. Peat substrates significantly affected root and shoot dry weight. Water desorption characteristics and saturated hydraulic conductivity were measured in situ to estimate the pore tortuosity factor and the relative gas diffusion coefficient. The pH, electrical conductivity, C/N ratio, total and hydrolyzable N, as well as NO3 --N and NH4 +-N in solution were also measured. Estimates of the physical properties suggest that a lack of aeration limited plant growth. Plant growth was significantly correlated with both the gas relative diffusivity and the pore tortuosity factor. Among the chemical factors, pH and soil nitrate level were also correlated with plant growth. No significant correlation was found between plant growth and air-filled porosity or any other measured chemical properties. This study indicates that an index of gas-exchange dynamics could be a useful complementary diagnostic tool to guide substrate manufacturing.