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.
Yoshiaki Kitaya, Tsutomu Moriya, and Makoto Kiyota
Toshio Shibuya, Ryosuke Endo, Yoshiaki Kitaya, and Mizuki Tsuchida
The light competition in dense plant stands may be disadvantageous in transplant production because competition stimulates stem elongation and can reduce photosynthate allocation to leaves; this, in turn, may reduce the early growth rate after transplanting. In this study, we focused on how the proportion of far-red (FR) light affected light competition among cucumber (Cucumis sativus L.) seedlings and investigated the effects of the plant density × FR interaction on photosynthate allocation and subsequent early growth after transplanting. Seedlings at the cotyledon stage were planted into plug trays at densities ranging from 109 to 1736 plants/m2; then they were grown for 4 days under light-emitting diode (LED) light containing FR light (FR+) at approximately the same red-to-FR ratio as in sunlight (1.2) or under light containing no FR (FR−). The higher density significantly stimulated stem elongation under both FR+ and FR−, but the effect was small under FR−; this indicates that light competition in the dense stands was inhibited by reducing FR light. The higher plant density significantly increased photosynthate allocation to the stem and decreased allocation to the leaves under both FR+ and FR−; however, again, the effect was smaller under FR−. After transplanting the seedlings to pots, early growth decreased in the seedlings that allocated less photosynthate to their leaves. Our results indicate that light with reduced FR can mitigate the disadvantageous photosynthate allocation of transplants and the reduction of early growth after transplanting that are likely to occur as a result of light competition at high plant density.
Kaori Itagaki, Toshio Shibuya, Motoaki Tojo, Ryosuke Endo, and Yoshiaki Kitaya
The present study evaluated the development of powdery mildew fungus (Podosphaera xanthii) on leaves of cucumber (Cucumis sativus L.) acclimatized to different CO2 concentrations ([CO2]) to examine plant–pathogen interactions under the wide range of [CO2] that can occur in greenhouse cultivation. Seedlings of resistant and nonresistant cultivars were acclimatized to reduced (200 µmol·mol−1), ambient (400 µmol·mol−1), or elevated (1000 µmol·mol−1) [CO2]. Powdery mildew spores were inoculated onto the adaxial surface of cotyledons or first true leaves, and colonization was measured after 7 days. Colony density decreased as acclimatization [CO2] increased at the cotyledon stage but increased at the first-true-leaf stage in both cultivars. This result implies that when the effects of [CO2] on plant–pathogen interactions are described, growing stage must be specified. The acclimatization [CO2] was correlated positively with leaf mass per area, dry matter content, and carbon (C) content and negatively with nitrogen (N) content at both stages. Therefore, these leaf properties could not explain the changes in host-plant susceptibility between stages. The effect of acclimatization [CO2] was greater on the resistant cultivar than on the nonresistant cultivar, indicating that the resistant cultivar was more responsive.
Kaori Itagaki, Toshio Shibuya, Motoaki Tojo, Ryosuke Endo, and Yoshiaki Kitaya
The development of powdery mildew fungus (Podosphaera xanthii) is suppressed on cucumber (Cucumis sativus L.) seedlings acclimatized to higher red-to-far-red ratio (R:FR) than natural R:FR (≈1.2), but its early development and any limiting factors are still unclear. The present study evaluated conidial germination, initial invasion, and subsequent development of P. xanthii on cucumber seedlings raised under light-emitting diode (LED) lights with R:FRs of 1.2, 5.0, or 10. There were no differences in conidial germination or initial invasion between the treatments, so there was no effect of acclimatization to R:FR on either. But, the development of hyphae, hyphal cells, and haustoria after inoculation were suppressed on seedlings acclimatized to higher R:FR. Because differences occurred only after the initial invasion, nonstructural properties of the host leaves may have affected conidial development. Higher R:FR also suppressed conidial development under natural light filtered through a photo-selective film, which absorbs near-infrared (NIR)-light. However, this effect was reduced when the plants were moved to natural R:FR after inoculation, possibly because of reacclimatization of the seedlings.
Yoshiaki Kitaya, Genhua Niu, Maki Ohashi, and Toyoki Kozai
Artificial lighting is widely used in controlled environment plant production to enhance plant growth and quality. However, high light intensity with artificial lighting is costly, and often causes increase of leaf temperature and, thus, leaf burn. We investigated the effects of photosynthetic photon flux (PPF) and photoperiod on the growth and morphogenesis of lettuce plug transplants under ambient and enriched CO2 levels. Three days after seeding, the plants were cultured under four PPF levels (100, 150, 200, and 300 μmol·m–2·s–1), two photoperiods (16 and 24 hr), and two CO2 levels (400 and 800 μmol·mol–1) for 18 days in growth chambers. Light source was fluorescent lamps. The air temperature around the plants was kept at 20°C. The results showed that dry weight of the plants increased linearly as PPF and daily integrated PPF (product of PPF and photoperiod) increased under both CO2 levels. At the same daily integrated PPF, higher CO2 level and longer photoperiod led to higher dry weight of the plants. CO2 enrichment increased significantly dry weight of the plants. The ratio of T/R and specific leaf area of the plants decreased quadratically as daily integrated PPF increased under both CO2 levels. The ratio of leaf length to leaf width of the plants decreased quadratically as PPF increased under the two photoperiods and CO2 levels.
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.
Toshio Shibuya, Ryosuke Endo, Yuki Kitamura, Yoshiaki Kitaya, and Nobuaki Hayashi
To evaluate the effect of fluorescent lamps with a high red:far-red (R:FR) light on the potential photosynthesis of transplants, we investigated the photosynthetic light responses of cucumber (Cucumis sativus L.) seedlings grown under fluorescent lamps with high R:FR light (FLH) and compared them with the responses of the seedlings grown under metal-halide lamps (ML) that provided a spectrum similar to that of natural light and under a fluorescent lamp with low R:FR light (FLL). The seedlings were grown under FLH (R:FR = 7.0), ML (R:FR = 1.2), or FLL (R:FR = 1.1) at a photosynthetic photon flux density (PPFD) of 350 μmol·m−2·s−1. The gross photosynthetic rate (Pg), quantum yield of photosystem II (ΦPSII), and photosynthetic electron transfer rate (ETR) of the foliage leaves were then evaluated at PPFDs ranging from 0 to 1000 μmol·m−2·s−1. The photosynthetic light response of FLH seedlings was similar to those of sun leaves, and the responses of ML and FLL seedlings were similar to those of shade leaves. The Pg, ETR, and ΦPSII of FLH seedlings at PPFD of 1000 μmol·m−2·s−1 was 1.38, 1.32, and 1.28 times, respectively, those of ML seedlings, and was 1.40, 1.23, and 1.22 times, respectively, those of FLL seedlings. The Pg was closely correlated with ETR in each treatment. FLH seedlings had thicker leaf and greater chlorophyll content per leaf area than ML and FLL seedlings. The greater Pg of FLH seedlings than in the other two groups of seedlings at high PPFD was probably the result of the improved ETR resulting from physiological and morphological changes in response to the high R:FR light.
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.
Toshio Shibuya, Yoshiaki Kitaya, Toyoki Kozai, and Masaichi Nakahara
Net photosynthetic and evapotranspiration rates of tomato (LAI = 2.3) and lettuce (LAI = 6.6) plug sheets were estimated based on measurements of the weight of plug sheets and vertical profiles of CO2 concentration above the plug sheets. The measurements were continued in situ for several days in a greenhouse when plugs were at transplant stage. The maximum net photosynthetic rates of tomato and lettuce plug sheets were 0.8 and 2.0 mg CO2/m2 per sec on a plug sheet area basis, respectively. The maximum evapotranspiration rates of those sheets were 100 mg·m–2·s–1. Net photosynthetic and evapotranspiration rates of tomato and lettuce plug sheets increased linearly with an increase in solar radiation flux, with a correlation coefficient of 0.9.
Yoshiaki Kitaya, Genhua Niu, Toyoki Kozai, and Maki Ohashi
Lettuce (Lactuca sativa L. cv. Summer-green) plug transplants were grown for 3 weeks under 16 combinations of four levels (100, 150, 200, and 300 μmol·m-2·s-1) of photosynthetic photon flux (PPF), two photoperiods (16 and 24 h), and two levels of CO2 (400 and 800 μmol·mol-1) in growth chambers maintained at an air temperature of 20 ±2 °C. As PPF increased, dry mass (DM), percent DM, and leaf number increased, while ratio of shoot to root dry mass (S/R), ratio of leaf length to leaf width (LL/LW), specific leaf area, and hypocotyl length decreased. At the same PPF, DM was increased by 25% to 100% and 10% to 100% with extended photoperiod and elevated CO2 concentration, respectively. Dry mass, percent DM, and leaf number increased linearly with daily light integral (DLI, the product of PPF and photoperiod), while S/R, specific leaf area, LL/LW and hypocotyl length decreased as DLI increased under each CO2 concentration. Hypocotyl length was influenced by PPF and photoperiod, but not by CO2 concentration. Leaf morphology, which can be reflected by LL/LW, was substantially influenced by PPF at 100 to 200 μmol·m-2·s-1, but not at 200 to 300 μmol·m-2·s-1. At the same DLI, the longer photoperiod promoted growth under the low CO2 concentration, but not under the high CO2 concentration. Longer photoperiod and/or higher CO2 concentration compensated for a low PPF.