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B. Dansereau, Y. Zhang, S. Gagnon, and H.L. Xu

We examined effects of single-layer glass and double-layer antifog polyethylene films on growth and flowering of stock (Matthiola incana L.) and snapdragon (Antirrhinum majalis L.) in a 3-year period. Stock produced more buds/spike with shorter but thicker stems under single-layer glass and under antifog 3-year polyethylene, and showed higher photosynthetic capacity (P c) under single-layer glass than under other covers regardless of light regimes. Similarly, growth and flowering of snapdragon were significantly better under single-layer glass than in polyethylene houses. A supplemental light of 60 μmol·m-2·s-1 accelerated flowering by 20 to 25 days, improved flower quality, and eliminated differences in plant growth and quality of snapdragon between covering treatments. The P c of stock was lower under all polyethylene covers than under single-layer glass. Among the three antifog polyethylene films, a slightly higher P c was measured for plants under antifog 3-year polyethylene. However, there was no difference among covering treatments in the net photosynthetic rate (P N) at low light level (canopy level). Supplemental lighting reduced P c of stock leaves, especially under single-layer glass, and diminished differences in P c among covering treatments. Dry mass was more influenced by larger leaf area caused by higher leaf temperature than by P N. Overall, antifog 3-year polyethylene was a good covering material when both plant quality and energy saving were considered.

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J. Caron, H.L. Xu, P.Y. Bernier, I. Duchesne, and P. Tardif

In nursery plant production, optimum water use is important to maintain productivity and make this production environmentally sound. Water should be supplied when it becomes difficult to extract for the plant, at a bulk soil water potential threshold value that may vary with environmental conditions, species and substrate properties. The objective of this study was to determine the threshold value at which availability of water rapidly drops for three newly developed substrates to be used in the production of Prunus ×cistena. Xylem water potential and potential at the soil-root interface were used as indices of water availability and were compared with bulk soil water potential. Water was easily available (no drop in xylem or soil-root interface water potential) from container capacity down to a bulk soil water potential of about-10 kPa when xylem water potential was used as an indicator and -8 kPa when the soil-root interface water potential was chosen as the indicator. No significant differences in the threshold values were found between substrates, consistent with the absence of differences in the substrate physical properties. The differences in water availability among substrates were consistent with an observed difference in salt content. The important variability observed in the threshold suggests that plant based measures may be preferred to soil based measures in assessing water availability in artificial mixes.