Abstract
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.
High-performance production systems unconstrained by weather conditions have recently been developed to produce high-quality transplants under artificial light (Kozai, 2007; Kozai et al., 2006). Seedlings grown under light provided by the fluorescent lamps used in these systems have less shoot elongation than those grown under natural light (Ohyama et al., 2003). The reduced shoot elongation seems to be the result of the high red to far-red ratios (R:FR) of typical commercial fluorescent lamps, which emit little FR irradiance. Elongation of plant shoots could be improved by increasing the FR content of the light from these lamps (Murakami et al., 1991). In natural ecosystems, shoot elongation and leaf expansion are increased and leaf thickness and chlorophyll content reduced by a reduction in the R:FR resulting from absorption of red irradiation by neighboring vegetation (Smith and Whitelam, 1997). This morphological response is called shade avoidance and is used by plants to tolerate or avoid shading (Franklin, 2008). In contrast, the higher-than-natural R:FR of fluorescent lamps may improve adaptability to high irradiation levels because seedlings under such lamps show morphological responses that are inverse to shade avoidance. At high irradiation levels, leaves adapted to high irradiation (sun leaves) have greater photosynthetic light-use efficiency than shaded leaves (Lichtenthaler et al., 1981). If adaptation of seedlings to high irradiation could be improved by growth under fluorescent lamps, then the photosynthetic efficiency and consequent growth at high irradiation could be improved. However, photosynthetic acclimation to light at higher-than-natural R:FR has not been well investigated, although that at lower R:FR has been investigated in ecological science studies (Corré, 1983; Maliakal et al., 1999; Murchie and Horton, 1997; Sleeman and Dudley, 2001; Sleeman et al., 2002; Turnbull et al., 1993). To evaluate the effect of adaptation to fluorescent light with high R:FR light on the potential photosynthetic advantage of transplants, we investigated the photosynthetic light responses of cucumber (Cucumis sativus L.) seedlings grown under fluorescent lamps with high R:FR light and compared them with the responses of the seedlings grown under metal-halide lamps that provided a spectrum similar to that of natural light and under a fluorescent lamp with low R:FR light.
Materials and Methods
Expt. 1: Comparison of high red:far-red fluorescent lamps with metal-halide lamps.
Cucumis sativus L. ‘Hokushin’ was sown in plastic pots (60 mm diameter, 55 mm height) containing vermiculite medium and then germinated in a chamber maintained at an air temperature of 26 to 28 °C, a relative humidity of 60%, and a photosynthetic photon flux density (PPFD) of 250 μmol·m−2·s−1 under fluorescent lamps (FHF32EX-N-H; Panasonic Corp., Kadoma, Japan) with a light:dark photoperiod of 12:12 h. After the cotyledons had fully expanded, the seedlings were moved to growth chambers illuminated with fluorescent lamps with high R:FR light (FPL55EX-N; Panasonic Corp.) (FLH) or with metal-halide lamps (DR400/TL; Toshiba Lighting & Technology Corp., Yokosuka, Japan) (ML) providing a spectrum similar to that of natural light. The R (wavelength 600 to 700 nm):FR (700 to 800 nm) of FLH was 7.0 and that of ML was 1.2. The spectrum of ML was similar to that of natural light, except that ML had strong peaks at wavelengths of 540 to 570 nm (Fig. 1). The spectra were measured by a spectrometer (BLK-CXR-SR; StellarNet Inc., Tampa, FL).
Fluorescent lamp with high red:far-red (R:FR) light (FLH), fluorescent lamp with low R:FR light (FLL), and metal-halide lamp (ML) spectra used in the experiment. Photon fluxes per unit wavelength are expressed in values relative to a maximum. R:FR was estimated by dividing the accumulated photon flux from red (wavelength 600 to 700 nm) by that from far-red (700 to 800 nm) light. The spectrum of ML was measured below the water filter.
Citation: HortScience horts 45, 4; 10.21273/HORTSCI.45.4.553
Nutrient solution (A-type recipe of Otsuka House Solution; Otsuka Chemical Co. Ltd., Osaka, Japan) was supplied to the bottoms of the pots continuously by standing the pots in a solution 5 to 10 mm deep. The composition of the solution in grams per 1000 L of tap water was: total nitrogen, 260; P2O5, 120; K2O, 405; CaO, 230; MgO, 60; MnO, 1.5; B2O3, 1.5; iron, 2.7; copper, 0.03; zinc, 0.09; and molybdenum, 0.03. Electrical conductivity was ≈2.6 mS/cm, and the pH value was ≈6.5. The growth conditions were: air temperature 27 to 28 °C, relative humidity 50%, and PPFD 350 μmol·m−2·s−1 at the canopy surface with a light:dark photoperiod of 12:12 h. The PPFD at the canopy was maintained by adjusting the distance between the light sources and the plant canopy during the growing period. A water filter (20-mm depth) was placed under the ML to prevent increases in leaf temperature from the long-wave radiation from the ML.
After being grown for 8 d under FLH or 6 d under ML, sample seedlings from each treatment group were taken to measure the net photosynthetic rate (Pn) and quantum yield of photosystem II photochemistry (ФPSII; Maxwell and Johnson, 2000). Because of the faster development of leaves under ML than under FLH, FLH seedlings took 2 d longer to equal the growth stage of ML seedlings for measurement. The seedlings under each treatment had one full-expanded foliage leaf, one unexpanded foliage leaf, and two cotyledons. Pn and ФPSII of the first foliage leaves were measured with a photosynthesis and fluorescence measuring system (LI-6400-40; LI-COR Inc., Lincoln, NE) at PPFDs of 1000, 500, 250, 100, and 50 μmol·m−2·s−1; the PPFD was changed in the order of high to low. After the measurement of Pn and ФPSII, the dark respiration rate of the leaves was measured with the same system at a PPFD of 0 μmol·m−2·s−1. Gross photosynthetic rate (Pg) was estimated by summing the dark respiration rate and the Pn. The minimum waiting time for measurement at each PPFD was 3 min, and the maximum was 10 min. Illumination was supplied by red (peak = 635 nm) and blue (peak = 465 nm) light-emitting diodes in a combination ratio of 9:1. In the leaf chamber of the measuring system, the air temperature was maintained at 30 ± 3 °C, leaf temperature at 28 °C, relative humidity at 50% ± 10%, and CO2 concentration at 400 μmol·mol−1. The photosynthetic electron transport rate (ETR) was then estimated from ФPSII, PPFD, and leaf absorption in accordance with the method of Genty et al. (1989). The absorbance of leaves was estimated from a relative chlorophyll content determined with a chlorophyll meter (SPAD-502; Konica Minolta Sensing Inc., Sakai, Japan) as follows. The relative chlorophyll content of several seedlings’ leaves was evaluated with the chlorophyll meter in a preliminary experiment. Absorbance of the leaves was also evaluated from spectral reflectance and transmittance determined with the spectrometer. From these values, the linear relationship between relative chlorophyll content and leaf absorption was determined. Finally, we could estimate the absorbance of sampled leaves from the relative chlorophyll content with this relationship. The measurements of the photosynthetic parameters were conducted for five seedlings in each treatment without replication.
Samples of 10 seedlings in each treatment group were taken to measure the fresh weight, area, and chlorophyll content of the first foliage leaf and the shoot length. The relative chlorophyll content was evaluated with the chlorophyll meter. Fresh weight per leaf area was estimated as an index of leaf thickness. Significance between means of the growth parameters was determined by Student's t test at P = 0.05 and 0.01.
Expt. 2: Comparison of high red:far-red fluorescent lamps with low red:far-red fluorescent lamps.
Cucumber seedlings were grown in growth chambers illuminated with FLH or fluorescent lamps with low R:FR light (FL20S·FR·P; Panasonic Corp.) (FLL), and then the photosynthetic light-response curve and growth characteristics of the seedlings were evaluated in the same way as Expt. 1. The spectrum of FLH was similar to that of FLL, except that there was higher photon flux at FR (Fig. 1). The R:FR of FLL was 1.1. The growing conditions (with the exception of the light source) and measuring methods were the same as in Expt. 1. It took 6 d under FLL and 8 d under FLH for seedlings to reach an equal growth stage for measurement.
Expt. 3: Effects of seedlings’ age on net photosynthetic rate at high photosynthetic photon flux density.
We evaluated Pn of seedlings at varied age to prove that the difference in age of seedlings does not affect the photosynthetic light response. The seedlings was grown under FLH or FLL for 4, 6, and 8 d after the cotyledons had fully expanded, and then Pn of them was measured at a PPFD of 1000 μmol·m−2·s−1. Measuring methods and conditions (with the exception of PPFD level) were the same as in Expt. 1.
Results
Expt. 1.
The Pg of seedlings grown under either FLH or ML increased with a tendency of saturation as PPFD was raised from 0 to 1000 μmol·m−2·s−1 (Fig. 2A). The tendency of saturation was stronger in ML than FLH. The Pg of FLH seedlings was 1.18, 1.15, 1.20, 1.31, and 1.38 times that of ML seedlings at PPFDs of 50, 100, 250, 500, and 1000 μmol·m−2·s−1, respectively. The light compensation point, estimated from the light-response curve of Pn (data not shown), was 35 μmol·m−2·s−1 in FLH seedlings and 33 μmol·m−2·s−1 in ML seedlings. The ratio of Pg of the FLH seedlings to that of the ML seedlings increased with increasing PPFD. The ETR of seedlings grown under FLH or ML increased with increasing PPFD from 0 to 1000 μmol·m−2·s−1 in the same way as Pg (Fig. 3A). The ETR of FLH seedlings was 1.08, 1.10, 1.14, 1.22, and 1.32 times that of ML seedlings at PPFDs of 50, 100, 250, 500, and 1000 μmol·m−2·s−1, respectively. An almost linear relationship between ETR and Pg was observed in each treatment group (Fig. 4A). The Pg in ML seedlings tended to be lower than in FLH seedlings at the same ETR. The ФPSII of FLH seedlings was 1.05, 1.07, 1.11, 1.19, and 1.28 times that of ML seedlings at PPFDs of 50, 100, 250, 500, and 1000 μmol·m−2·s−1, respectively (Fig. 5A). The ratio of the ФPSII of the FLH seedlings to those of the ML seedlings increased with increasing PPFD. There was no significant difference in leaf fresh weight or leaf area between FLH and ML seedlings (P > 0.05; Table 1). The fresh weight per leaf area, relative chlorophyll content, and shoot length in FLH seedlings were 1.11, 1.25, and 0.47 times, respectively, those of ML seedlings (P < 0.01; Table 1). The absorbance of red and blue mixed light in FLH leaves was 89.4% and that in ML leaves was 86.9%.
Light-response curves of gross photosynthetic rate (Pg) of (A) cucumber seedlings grown under fluorescent lamps with high red:far-red (R:FR) light (FLH) or metal-halide lamps (ML) (Expt. 1); and (B) those grown under FLH or fluorescent lamps with low R:FR light (FLL) (Expt. 2). Values represent means ± se (n = 5). Measurements were conducted at a leaf temperature of 28 °C and a CO2 concentration of 400 μmol·mol−1. Illumination was supplied by red and blue light-emitting diodes in a combination ratio of 9:1 during the measurement.
Citation: HortScience horts 45, 4; 10.21273/HORTSCI.45.4.553
Light-response curves of photosynthetic electron transfer rate (ETR) of (A) cucumber seedlings grown under fluorescent lamps with high red:far-red (R:FR) light (FLH) or metal-halide lamps (ML) (Expt. 1); and (B) those grown under FLH or fluorescent lamps with low R:FR light (FLL) (Expt. 2). Values represent means ± se (n = 5). For measurement conditions, see Figure 2.
Citation: HortScience horts 45, 4; 10.21273/HORTSCI.45.4.553
Relationships between photosynthetic electron transfer rate (ETR) and gross photosynthetic rate (Pg) of (A) cucumber seedlings grown under fluorescent lamps with high red:far-red (R:FR) light (FLH) or metal-halide lamps (ML) (Expt. 1); and (B) those grown under FLH or fluorescent lamps with low R:FR light (FLL) (Expt. 2). Values represent means ± se (n = 5).
Citation: HortScience horts 45, 4; 10.21273/HORTSCI.45.4.553
Light-response curves of quantum yield of photosystem II (ΦPSII) of (A) cucumber seedlings grown under fluorescent lamps with high red:far-red (R:FR) light (FLH) or metal-halide lamps (ML) (Expt. 1); and (B) those grown under FLH or fluorescent lamps with low R:FR light (FLL) (Expt. 2). Values represent means ± se (n = 5). For measurement conditions, see Figure 2.
Citation: HortScience horts 45, 4; 10.21273/HORTSCI.45.4.553
Leaf fresh weight, leaf area, relative chlorophyll content of first leaf, and shoot length in cucumber seedlings grown under fluorescent lamps with high red:far-red light (FLH) or metal-halide lamps (ML) (Expt. 1).
Expt. 2.
The ETR and ФPSII of FLH seedlings in Expt. 2 were greater than those in Expt. 1 (Figs. 3 and 5), although the environmental conditions were almost the same for FLH seedlings in the two experiments. The growth parameters of FLH seedlings in each experiment also differed (Tables 1 and 2). These differences were probably the result of differences in the timing of the experiment, although the true reasons are not clear. The absolute values obtained in each experiment could not be compared directly.
The Pg of seedlings grown under either FLH or FLL increased with a tendency of saturation as PPFD was raised from 0 to 1000 μmol·m−2·s−1 (Fig. 2B). The tendency of saturation is stronger in FLL than FLH. The ratio of the Pg of the FLH seedlings to that of the FLL seedlings increased with increasing PPFD in the same way as the relationship between FLH and ML in Expt. 1. The Pg of FLH seedlings was 0.97, 1.04, 1.11, 1.28, and 1.40 times that of FLL seedlings at PPFDs of 50, 100, 250, 500, and 1000 μmol·m−2·s−1, respectively. The light compensation point, estimated from the light-response curve of Pn (data not shown), was 50 μmol·m−2·s−1 in FLH seedlings and 47 μmol·m−2·s−1 in FLL seedlings. The ETR of seedlings grown under FLH or FLL increased with increasing PPFD from 0 to 1000 μmol·m−2·s−1 in the same way as Pg (Fig. 3B). A strong correlation was observed between ETR and Pg in each treatment group (Fig. 4B). Pg increased linearly with ETR; however, the slope became gentler in FLL seedlings at higher ETR, whereas it was almost constant in FLH seedlings. The Pg in FLL seedlings was lower than in FLH seedlings at high ETRs (from ≈100 μmol·m−2·s−1). The ФPSII of FLH seedlings was greater than that of FLL seedlings at PPFDs of 500 and 1000 μmol·m−2·s−1 but smaller than that of FLL seedlings at PPFDs of 50 and 100 μmol·m−2·s−1 (Fig. 5B). The ratios of the ETR and ФPSII of FLH seedlings to those of FLL seedlings increased with increasing PPFD in the same way as these ratios for FLH to ML in Expt. 1. The relationships between the photosynthetic light response of FLH and FLL were therefore similar to those of FLH and ML in Expt. 1, but ETR and ФPSII of FLH were greater than in Expt. 1.
There were no significant differences in leaf fresh weight and leaf area between FLH and FLL seedlings (P > 0.05; Table 2). The fresh weight per leaf area in FLH seedlings was 1.09 times that in FLL seedlings (P < 0.05; Table 2). The relative chlorophyll content and shoot length in FLH seedlings were 1.28 and 0.56 times, respectively, those in FLL seedlings (P < 0.01; Table 2). The relationship between the growth characteristics of FLH and FLL were therefore similar to those of FLH and ML in Expt. 1, but the growth parameters of FLH seedlings were greater than in Expt. 1. The absorbance of red and blue mixed light in FLH leaves was 90.0% and that in FLL leaves was 89.6%.
Leaf fresh weight, leaf area, relative chlorophyll content of first leaf, and shoot length in cucumber seedlings grown under fluorescent lamps with high red:far-red (R:FR) light (FLH) or fluorescent lamps with low R:FR light (FLL) (Expt. 2).
Expt. 3.
The Pn of seedlings grown under either FLH or ML was increased from Day 4 to Day 6 and then was maintained almost constant from Day 6 to Day 8 (Fig. 6). The Pn of FLH seedlings was greater than those of FLL regardless of their age and was ≈1.2 times those of FLL at the same age. The Pn in this experiment was greater than those in Expt. 2 (data not shown).
Changes in net photosynthetic rate (Pn) of cucumber seedlings grown under fluorescent lamps with high red:far-red (R:FR) light (FLH) or fluorescent lamps with low R:FR light (FLL) after starting treatment (Expt. 3). Values represent means ± se (Day 4, n = 5; Day 6, n = 4; Day 8, FLH: n = 5, FLL: n = 4). Measurements were conducted at a photosynthetic photon flux density of 1000 μmol·m−2·s−1, a leaf temperature of 28 °C, and a CO2 concentration of 400 μmol·mol−1. Illumination was supplied by red and blue light-emitting diodes in a combination ratio of 9:1 during the measurement.
Citation: HortScience horts 45, 4; 10.21273/HORTSCI.45.4.553
Discussion
The differences in the photosynthetic light response and morphological characteristics between FLH and ML seedlings seemed to be the result of the difference in R:FR between the two lamps because similar relationships were observed between FLH and FLL in Expt. 2 in which virtually only the FR photon flux was modified.
The FLH seedlings took 2 d longer (8 d) to equal the growth stage of ML or FLL seedlings for measurement. This difference in age of seedlings probably did not affect their photosynthetic light response because the Pn of FLH seedlings at high PPFD on Day 6 and Day 8 was almost same and was greater than those of FLL seedlings at the same age (Fig. 6). However, why the Pn in Expt. 3 was greater than those in Expt. 2 is unknown.
Comparison with the results of another experiment revealed that the photosynthetic light response of FLH seedlings was similar to those of sun leaves, and the curves of ML and FLL seedlings were similar to those of shade leaves (Lichtenthaler et al., 1981); photosynthetic light-use efficiency at high PPFDs was higher in FLH seedlings than in ML and FLL seedlings grown at the same PPFDs (Fig. 2). This means that, at high PPFDs, seedlings grown in a closed system under FLH would fix more CO2 per leaf area than seedlings grown under natural light at the same PPFD. In contrast, a low PPFD would be unfavorable to FLH seedlings because, like in sun leaves versus shade leaves, the light compensation point of FLH seedlings was higher than those of ML and FLL seedlings. Illumination with low R:FR light is advantageous for improving the growth of seedlings because plants often grow faster at lower R:FR (Murakami et al., 1991; Pausch et al., 1991), as we also observed here. On the other hand, the photosynthetic efficiency in FLH seedlings was greater than that in FLL seedlings at high PPFDs, although the growth rate in the former was lower. Therefore, a high R:FR light is probably advantageous when seedlings are to be moved to high PPFD conditions, for example, as transplants. There is, therefore, a tradeoff between faster growth and improved photosynthetic efficiency at high PPFDs. In selecting artificial light for plant production, we need to consider which of the two advantages is more important.
The higher Pg of FLH seedlings at high PPFDs was probably the result of the higher ETR, because Pg was closely correlated with ETR (Fig. 4). One reason for the higher ETR of FLH seedlings than of ML and FLL seedlings was the higher relative chlorophyll content per leaf area. Another reason is probably the higher efficiency of PSII photochemistry in FLH seedlings than in ML seedlings and in FLL seedlings at high PPFDs (Fig. 5). The function of chlorophyll may differ with the R:FR in the same way as it differs in sun and shade leaves (Öquist et al., 1992). However, we do not know the reason for the improved ΦPSII in FLH seedlings compared with ML and FLL seedlings at high PPFDs (Fig. 5). A linear relationship between ETR and Pg was observed over a wide range of them in Expt. 1, meaning that CO2 fixation efficiency per electron transport remained constant. The efficiency of CO2 fixation in C3 plants is reduced because of photorespiration when CO2 fixation is saturated (Kato et al., 2003). In Expt. 1, strong nonphotochemical heat dissipation was probably induced under high PPFD; consequently, the linear relationship between ETR and Pg was maintained. This would be supported by the decrease in ФPSII under higher PPFD. On the other hand, the development of a nonlinear relationship in Expt. 2 was probably the result of the greater Pg and ETR than those in Expt. 1. The gentle slope in the relationship between ETR and Pg in FLL seedlings at high ETR (Fig. 4B) seemed to be the result of the reduction of the CO2 fixation efficiency at a high PPFD. The reduction of CO2 fixation efficiency could also be reduced by acclimating to high PPFD (Kato et al., 2003). Our results suggest that both a high R:FR light and high PPFD act by the same mechanism to reduce the reduction of CO2 fixation efficiency. To clarify these mechanisms, a detailed analysis of chlorophyll function will be necessary.
From the values of fresh weight per leaf area and relative chlorophyll content (Tables 1 and 2), we assumed that leaf thickness and chlorophyll content per leaf area were greater in growth under FLH than under ML or FLL. The R:FR under FLH (7.0) is much higher than that of solar radiation and does not exist in nature. With a lower R:FR than occurs with natural light, shoot elongation and leaf expansion rates increase so that the plant can avoid shade and chlorophyll content declines (Smith and Whitelam, 1997). In addition, plant leaves adapted to low R:FR light have low net photosynthetic rates (Maliakal et al., 1999; Sleeman and Dudley, 2001; Sleeman et al. 2002). In FLH seedlings, the inverse physiological and morphological responses (i.e., increased leaf thickness and chlorophyll content and reduced shoot elongation) to those typical of shade avoidance seemed to be the result of heightened adaptation to the higher PPFD induced by the high R:FR of FLH. The photosynthetic efficiency of FLH seedlings at high PPFDs was probably improved as a result of this heightened adaptation to sun.
From these data, we concluded that growth of cucumber seedlings under fluorescent lamps with high R:FR light can improve potential photosynthesis compared with that under low R:FR illumination. Photosynthetic responses under light of different qualities have been well investigated (Goins et al., 1997; Korbee et al., 2005; Pausch et al., 1991; Yorio et al., 2001). The fact that light quality affects photosynthetic light response should be considered in selecting a light source for plant production under artificial light when postproduction characteristics are vital such as in transplant production.
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