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Yoshiaki Kitaya, Tsutomu Moriya, and Makoto Kiyota

Supplemental lighting and CO2 enrichment have been employed to promote plant growth in commercial plant production in greenhouses. In a semi-closed plant production system with a large number of plants at a high density, the relative humidity in the air around growing plants could be in excess of 80%. This research was initiated to determine the effects of CO2 concentration and photoperiod on the growth of plants under relatively high humidity conditions. In the experiment, lettuce plants were grown for 13 days under eight combinations of two CO2 levels (CO2, 0.38 and 0.76 mmol·mol-1), two photoperiods (PP, 16 and 24 h/day), and two relative humidity levels (RH, 80% and 90%) in growth chambers. The air temperature was 25 °C. Plants were illuminated with fluorescent lamps at a photosynthetic photon flux of 0.23 mmol·m-2·s-1. The dry mass of lettuce shoots (leaves and stems) grown in 0.76 mmol·mol-1 CO2, 24 h/day PP, and 80% to 90% RH was greatest in all treatments and was five times the least value obtained in 0.38 mmol·mol-1 CO2, 16 h/day PP and 90% RH. The dry mass of lettuce shoots decreased to 40% as RH increased from 80% to 90 % under 0.38-0.76 mmol·mol-1 CO2 and 16 h/day PP. Growth suppression by excess humidity was less significant in longer PP and higher CO2. Supplemental lighting and CO2 enrichment would be more effective for promoting growth of plants grown under higher humidity conditions.

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Toshio Shibuya, Ryoko Terakura, Yoshiaki Kitaya, and Makoto Kiyota

Application of a low-relative-humidity treatment (LHT) to seedlings can reduce water stress on cuttings harvested from the seedlings, after the cuttings are planted. Effects of illumination during LHT and LHT duration on leaf water potential and leaf conductance in cucumber (Cucumis sativus L.) seedlings used as the model plant material and on growth of harvested cuttings were investigated to determine optimal LHT conditions. The seedlings received LHT for 12 or 24 h in a lighted or dark growth chamber at air temperatures of 28 to 31 °C and relative humidity of 12% to 25%. Cuttings including a foliage leaf and two cotyledons were harvested by cutting the hypocotyl of the seedlings immediately after the treatment, and then the cuttings were planted in vermiculite medium. Four days after planting, the total fresh weight of the cuttings from seedlings that had received LHT in the lighted chamber was 2.2 times that of cuttings from seedlings that had not received LHT, whereas the total fresh weight of those that had received LHT in the dark increased by 1.3 to 1.8 times. Significant effects of illumination during LHT were also observed in the transpiration rate and growth of the cuttings, harvested following the treatment, after they were planted. By varying LHT duration, it was also found that leaf water potential and leaf conductance of the seedlings decreased as LHT duration increased up to 18 h. Thus, illumination during LHT increased the growth of cuttings taken following the treatment, and optimal treatment duration of around 18 h was estimated from the seedlings' leaf conductance and leaf water potential.

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Toshio Shibuya, Akihito Sugimoto, Yoshiaki Kitaya, and Makoto Kiyota

To evaluate the effects of plant density on gas exchanges under water stress resulting from high vapor-pressure deficit (VPD), we measured net photosynthetic rate (P n), transpiration rate, and leaf conductance (g l) of cucumber (Cucumis sativus L.) seedlings before and after raising the VPD at different plant densities. Measurements were conducted continuously using a chamber and weighing method. Five, nine, or 12 seedlings with leaf area index (LAI) of 0.39, 0.73, and 1.10, respectively, were placed in the assimilation chamber. The average VPD in the chamber was raised from 1.1 to 3.7 kPa 30 min after the starting measurement. The P n and g l decreased after raising the VPD above the plant community from 1.1 to 3.7 kPa. The VPD near the leaf surface (measured with 3-mm diameter humidity sensors) decreased with increasing LAI of the plant community, whereas average VPD in the whole chamber did not change with LAI. We noted significant negative correlations between the VPD near the leaf surface and P n and g l. These results indicate that higher plant density mitigates the inhibition of photosynthesis resulting from high VPD by maintaining a lower VPD near the leaf surface with the development of a thicker boundary layer above the canopy.

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Toshio Shibuya, Akihito Sugimoto, Yoshiaki Kitaya, Makoto Kiyota, Yuichiro Nagasaka, and Shinya Kawaguchi

We estimated leaf vapor conductance (g l) of cucumber grafted transplants under greenhouse growing conditions. Fifty-six transplants were placed on a bench in the greenhouse. The transpiration rate (Tr) of the canopy was estimated by weighing the 16 transplants in the center using an electronic balance. The total vapor diffusion resistance (R l+b) from inside the leaf to the atmosphere was estimated based on the vapor diffusion model, which incorporates the absolute humidity near the leaf surface and that inside the leaf as well as Tr. Next, g l was estimated from R l+b and the resistance of leaf boundary layer evaluated with a model leaf. The Tr in the afternoon tended to be larger than that in the morning at the same photosynthetic photon flux (PPF) level. By contrast, the g l in the afternoon tended to be smaller than that in the morning at the same PPF level. The decrease of g l in the afternoon seems to be induced by the excessive transpiration resulting from an increase of vapor pressure deficit at the leaf surface.