Growth responses of chrysanthemum and bell pepper
transplants to photoselective plastic ®lms
$Shumin Li
a, Nihal C. Rajapakse
a,*, Roy E. Young
b,1, Ryu Oi
ca
Department of Horticulture, Clemson University, Clemson, SC 29634, USA
b
Department of Agricultural and Biological Engineering, Clemson University, Clemson, SC 29634, USA
c
Organic Performance Materials Laboratory, Mitsui Chemicals, 1190 Kasama-cho, Sakae-ku, Yokohama 247, Japan
Accepted 8 November 1999
Abstract
Plant response to photoselective plastic ®lms with three concentrations of a far-red (FR) light absorbing dye (named as YCE-1 #80, YCE-1 #75 and YCE-1 #65) was tested using chrysanthemum (Dendranthemagrandi¯orum (Ramat.) Kitamura) and bell pepper (Capsicum annuum L.) as model plants. The dye in ®lms intercepted FR wavelengths of sunlight with maximum interception at 760 nm. FR light interception increased and transmission of photosynthetic photon ¯ux (PPF) decreased as the dye concentration increased. The R:FR ratio and estimated phytochrome photoequilibrium (fc) of transmitted light increased from 1.1 to 3.7 and from 0.72 to 0.81, respectively, with increase in dye concentration. Light transmitted through photoselective ®lms reduced plant height and internode length by 10±35% depending on the crop and dye concentration in the ®lm. Photoselective ®lms reduced the leaf area and shoot dry weight of plants. Speci®c leaf dry weight (dry weight per unit leaf area) and speci®c stem dry weight (dry weight per unit length of stem) were also slightly reduced in plants grown inside photoselective ®lm chambers suggesting that both small plants and reduced dry matter assimilation may have contributed to the reduction in shoot dry weight. Reduction in plant height was apparent within 2 weeks after initiation of the treatment. Plant height progressively decreased as the dye concentration increased. Although ®lms with higher dye concentrations are more effective in height reduction, the reduction in PPF with
$Technical contribution No. 4559 of the South Carolina Agricultural Experiment Station,
Clemson University. *
Corresponding author. Tel.:1-864-656-4970; fax:1-864-656-4960.
E-mail address: nrjpks@clemson.edu (N.C. Rajapakse). 1
Present address: Department of Agricultural and Biological Engineering, Pennsylvania State University, University Park, PA 16802-1909, USA.
increasing dye concentration can adversely affect plant growth and development, and this fact should be considered in commercial production of photoselective ®lms. Our results indicate that a photoselective ®lm with a R:FR ratio of 2.2 (or fc of 0.78, which corresponds to 75% light transmission) caused about 20% height reduction in chrysanthemum and 30% height reduction in bell pepper after 4 weeks of treatment. This initial work demonstrates that the use of greenhouse ®lms with FR light absorbing dyes to control plant height is as effective as chemical growth regulators or CuSO4®lters. With the commercial development of photoselective greenhouse covers or shade material, nursery and greenhouse industry could reduce costs for growth regulating chemicals, reduce health risks to their workers and consumers, and reduce potential environmental pollution.#2000 Elsevier Science B.V. All rights reserved.
Keywords: Spectral ®lters; Greenhouse covers; Photomorphogenesis; Height control
1. Introduction
Plants can perceive subtle changes in red (R) and far-red (FR) light composition in their environment and make physiological and morphological adjustment through phytochrome. Upon prolonged exposure to a given light environment, a photoequilibrium (f) develops between activePfrlevel relative to total phytochrome. In general, an environment with high R light relative to FR light results in establishing a high f. Morgan and Smith (1976, 1979) reported that stem elongation rate of herbaceous plants were inversely proportioned to the
f. Therefore, plants grown in an environment with high R light can be shorter than those produced in high FR light.
In greenhouse industry, growers often place plants close together and hang baskets over the benches to increase production capacity. This results in a relative increase in FR light in lower canopy due to the absorption of R light by the upper canopy. Therefore, a low f can be established in the plant under overcrowded conditions, resulting in spindly and tall plants. Growers often control this undesirable growth by chemical growth retardants. Because of the human safety issues, the use of growth regulating chemicals has been subjected to strict regulations on ornamental crops and banned on food crops. Currently, there are no chemicals available for height control of vegetable transplants in the USA. Growers in other countries are facing similar restrictions on using chemical growth regulators on food crops. This has led to increased interest in non-chemical alternatives. Manipulation of greenhouse light quality to establish a highfoffers a non-chemical alternative for height control of greenhouse crops. Relative amount of R light in a greenhouse can be increased to establish a high
fby using electric light sources that are high in R wavelengths and low in FR wavelengths. Although there is a growing demand for arti®cial lighting for plant growth, the initial cost of establishing such a system could be high and some arti®cial lighting sources may lead to irregular plant growth due to uneven
spectral distribution (spectral gaps) of the lighting source (Protasova et al., 1990). Spectral ®lters that can ®lter out FR light can be used to alter greenhouse light quality relatively inexpensively. In earlier work, it was shown that liquid copper sulfate (4% CuSO45H2O) ®lters were effective in removing FR light and establishing a high phytochrome photoequilibrium (fc) within plants (Mortensen and Strùmme, 1987). FR light ®ltering by CuSO4 ®lters was effective in controlling height of a wide range of greenhouse crops (Mortensen and Strùmme, 1987; McMahon et al., 1991; Rajapakse and Kelly, 1992). Although effective, liquid ®lters have limited value to commercial growers because of its high initial cost, dif®culty in liquid handling and phytotoxicity in the event of spill.
The development of plastic photoselective greenhouse covering with FR light absorbing dyes could facilitate the commercialization of spectral ®lters as a non-chemical alternative for greenhouse crop height control. To the knowledge of the authors, such greenhouse ®lms are not commercially available. In this paper, we report the effectiveness of plastic greenhouse covers with varying concentrations of a FR light absorbing dye in controlling height of chrysanthemum and bell pepper plants. The objectives of this work were to test the effectiveness of photoselective ®lms and select a dye concentration that gives an optimum height control while minimizing the reduction in radiation entering the greenhouse.
2. Materials and methods
2.1. Photoselective ®lms
Polyethylene (PE) ®lms with varying concentrations of a FR light absorbing dye were produced by Mitsui Chemicals, Tokyo, Japan. These ®lms are identi®ed by the following code names: BCE-1 (control, dye at 0 g mÿ2), YCE-1 #80 (dye at 0.08 g mÿ2), YCE-1 #75 (dye at 0.13 g mÿ2) and YCE-1 #65 (dye at 0.22 g mÿ2). The photosynthetic photon ¯ux (PPF) transmission of BCE-1 (control), YCE-1 #80, #75, and #65 ®lms were about 90, 80, 75 and 65%, respectively. Four PVC framed growth chambers (1.21.21.3 m3; one for
each ®lm) were covered with the above experimental ®lms. Another chamber with the same dimension as above was covered with sealed double-layered polycarbonate panels ®lled with liquid CuSO4(4%) to compare the effectiveness of the ®lms. The PPF transmission of CuSO4 ®lter was about 75%. All growth chambers were placed inside a glasshouse.
instantaneous PPF inside chambers was 620120mmol mÿ2sÿ1. Daily PPF integral was recorded with LI-1000 data logger ®tted with an LI-190SB quantum sensor. Plants in the chambers received an average daily PPF integral of 162 mol mÿ2 on a clear day.
Spectral distribution in 10 nm increments from 330 to 1100 nm was measured at the middle of each chamber with a 1800 spectroradiometer ®tted with a LI-1800-10 remote cosine sensor at the beginning and end of the experiments. The cheesecloth did not alter the quality of light transmitted. Multiple light scans within a chamber indicated that the spectral distribution was uniform inside the chamber. The R:FR ratio was determined as the ratio of photon ¯ux density between 600 and 700 nm (R) and 700 and 800 nm (FR). fc was estimated as described by Sager et al. (1988).
2.2. Plant material and culture
Fifty uniformly rooted `Bright Golden Anne' chrysanthemum shoot cuttings with six to seven leaves were planted individually in 0.6 l square pots containing a commercial potting mix (Metro Mix-360, Scotts-Sierra Horticultural Products, Marysville, OH). Plants were allowed to establish as single stem plants in the greenhouse for 1 week before transferring to the experimental chambers. Bell pepper `Capistrano' seeds were sown in 98-cell plug trays containing the same potting mix and germinated under an intermittent mist. When cotyledons were fully expanded, 50 seedlings were transplanted individually in 0.6 l square pots and allowed to establish for 1 week. After 1 week establishment, chrysanthemum and bell pepper plants (10 plants of each experiment per treatment) were transferred to experimental chambers and were grown for 4 weeks. When treatment was initiated, chrysanthemum and bell pepper plants were 7.0 and 1.5 cm tall, respectively. All plants were irrigated with 200 mg lÿ1N from 20 N± 4.4 P±16.7 K fertilizer (Peter's 20-10-20 Peat-lite special, Scotts-Sierra Horti-cultural Products) as needed. Daily maximum and minimum air temperatures inside chambers were recorded during the experiment. Average daily maximum or minimum temperatures during experimental period was not different among chambers and were 282 and 2228C, respectively.
2.3. Experimental design, data collection and analyses
Experimental chambers were randomly placed inside the glasshouse. Because of the limited number of chambers, experiments were repeated to replicate (September±November 1997). In each replicate, 10 plants were grown for each crop. Plants were randomly placed with approximate spacing of 1818 cm.
Plant height (height from soil level to apex) and the number of fully expanded leaves were recorded weekly. Average internode length was calculated as plant
height divided by number of leaves. Total leaf area (LI-3100 Area Meter, Lincoln, NE) and dry weights of stems and leaves were measured at the end of the 4-week-treatment. For dry weight measurements, tissue was oven dried at 858C for 48± 72 h. Data were analyzed using analysis of variance procedure (SAS institute, Cary, NC) and differences among treatment means were tested by Duncan's multiple range test atP0.05.
3. Results and discussion
3.1. Light quality
Spectral distribution curves of light transmitted through unshaded photo-selective ®lms are shown in Fig. 1. The dye in YCE-1 photophoto-selective ®lms intercepted FR wavelengths of sunlight with maximum interception at 760 nm. The interception of FR wavelengths increased as the dye concentration increased. In contrast, CuSO4 ®lter intercepted almost all wavelengths beyond 700 nm
Fig. 1. Photon distribution of light transmitted through photoselective ®lms. BCE-1 is the control ®lm. YCE-1 #80, #75, and #65 are photoselective ®lms with the FR light absorbing dye at 0.08, 0.13 and 0.22 g mÿ2
(capacity of the spectroradiometer was 330±1100 nm). The CuSO4chamber had more blue and less R light than photoselective ®lm chambers. The R:FR ratio increased from 1.1 to 3.7 and estimatedfcincreased from 0.72 to 0.81 as the dye concentration in ®lms increased (Table 1). The R:FR ratio andfcof transmitted light did not change during the experiment (data not shown).
3.2. Plant growth under photoselective ®lms
Photoselective ®lms reduced height of chrysanthemum plants and bell pepper seedlings (Fig. 2). The height reduction by ®lms increased as the dye concentration increased. For example, the ®nal height of chrysanthemum plants was reduced to 11, 19 and 22% inside YCE-1 #80, #75, and #65 chambers (lowest to highest R:FR ratio orfc), respectively, compared to control plants. A similar trend was observed in bell pepper height reduction. Both chrysanthemum and bell pepper plants grown in CuSO4 chamber were the shortest, but they were not signi®cantly different from plants grown in YCE-1 #65 chamber, indicating that ®lms with highest dye concentration (tested here) were as effective as CuSO4 ®lter. Our results are in agreement with Murakami et al. (1995, 1996a,b) who reported a reduction of plant height of cucumber (Cucumis sativus L.), tomato (Lycopersicon esculentumMill.), and sun¯ower (Helianthus annuusL.) under FR light intercepting ®lters. Similar to ®ndings of Morgan and Smith (1976), height of chrysanthemum and pepper plants were inversely proportional to the fc. Although the R:FR ratio andfcwere similar in YCE-1 #65 and CuSO4chambers, plants grown in CuSO4chamber were slightly shorter than those grown in YCE-1 #65 chamber (Fig. 2). This may be explained by the fact that CuSO4chamber had more blue wavelengths compared to photoselective ®lms. Blue light has been shown to reduce plant height (Adamse et al., 1988; Warpeha and Kaufman, 1989). In addition to the blue light, CuSO4®lters also removed almost all of FR
Table 1
R:FR ratios and estimatedfcof light transmitted through photoselective ®lms and liquid CuSO4 ®ltera
Treatmentb R:FR ratio fc
BCE-1 1.1 0.72
BCE-1 is the control ®lm. YCE-1 #80, #75, and #65 are photoselective ®lms with the FR light absorbing dye at 0.08, 0.13 and 0.22 g mÿ2, respectively. CuSO4is the chamber covered with panels ®lled with 4% CuSO45H2O liquid.
wavelengths whereas photoselective ®lm let some FR wavelengths into the chamber. Therefore, it is dif®cult to compare the role of each wavelength band on height control by ®lms and CuSO4 ®lters.
The effectiveness of ®lms also varied with species. Final height of chrysanthemum was reduced 22% by YCE-1 #65 ®lm whereas that of bell peppers was reduced 35% by the YCE-1 #65 ®lm, suggesting that bell pepper seedlings were more responsive to ®ltered light than the chrysanthemum plants.
Fig. 2. Weekly height increase of chrysanthemum and bell pepper seedlings grown in different photoselective ®lm chambers. Vertical bars indicate standard error. Each point is the mean of 20 plants. BCE-1 is the control ®lm. YCE-1 #80, #75, and #65 are photoselective ®lms with the FR absorbing dye at 0.08, 0.13 and 0.22 g mÿ2
, respectively. CuSO4is the chamber covered with panels ®lled with 4% CuSO45H2O liquid. Plants in all treatment chambers received the same amount of
The difference in responsiveness could be due to the stage of development of plants when they were exposed to photoselective ®lms. Bell pepper seedlings only had one or two true leaves when they were placed in photoselective ®lm chambers whereas chrysanthemum cuttings had six to seven leaves when they were placed in the chambers.
The dye concentration in ®lms in¯uenced the time it takes for plants to respond to altered light environment (Fig. 2). Height reduction in chrysanthemum was observed after 1 week exposure to YCE-1 #65 compared to 2 weeks in YCE-1 #75 and 3 weeks in YCE-1 #80 chambers. A similar pattern was observed for bell pepper seedlings, but they responded sooner; height reduction was achieved under YCE-1 #80 ®lm after 2 weeks of exposure. Average internode length followed a pattern similar to height reduction (data not shown). Number of leaves was not signi®cantly affected by the photoselective ®lms (data not shown), indicating that height reduction was a result of internode length reduction, but not due to the delay in stage of development.
Photoselective ®lms and CuSO4 ®lter reduced total leaf area and leaf size in both chrysanthemums and bell peppers (Table 2). Chrysanthemums grown in YCE-1 #65 and CuSO4 had smallest individual leaves and total leaf area. In chrysanthemum, total leaf area or leaf size was not different between YCE-1 #80 or #75 chambers and the control chamber. However, in bell peppers total leaf area of control plants was greater than plants grown in YCE-1 chambers. Reduction in leaf size gives a compact appearance to the plant but can result in reduction of photosynthetic area which further results in a reduction in dry matter accumulation.
Total shoot dry weight of chrysanthemums and bell pepper seedlings decreased progressively as the dye concentration in ®lms (R:FR ratio orfc) increased. Total shoot dry weight of chrysanthemums and peppers grown in the CuSO4and YCE-1 #65 chamber was reduced over 40% (Table 2). Photoselective ®lms reduced leaf and stem dry weights of chrysanthemum and bell pepper, but the stem dry weight reduction was greater than the leaf dry weight reduction. Speci®c leaf dry weight (SLDW, dry weight per unit leaf area) and speci®c stem dry weight (SSDW, dry weight per unit length of stem) were reduced in plants grown inside photoselective ®lms. Speci®c dry weight reduction increased as the dye concentration increased. Reduction in speci®c dry weights indicates that dry matter assimilation was affected by photoselective ®lms and that both small plants and the reduction in dry matter assimilation may have contributed to the reduction in total shoot dry weight.
The photoselective ®lms affected dry matter partitioning into leaves and stems (Table 2). In chrysanthemums, plants grown in the control chamber had 66% of total shoot dry matter in leaves and 34% in stems. Photoselective ®lms reduced percentage dry matter accumulation in stems from 34 to 24% and increased dry matter accumulation in leaves from 66 to 76%. Percentage dry matter
In¯uence of photoselective ®lms on total leaf area (LA), average leaf size (LS), leaf dry weight (LDW), speci®c leaf dry weight (SLDW), stem dry weight (SDW), speci®c stem dry weight (SSDW), and total shoot dry weight (TDW) of chrysanthemum and bell pepper seedlings
Treatmenta LA (cm2) LS (cm2) LDW (g) SLDW
BCE-1 681ab 28a 2.96a (66)c 0.0044a 1.55a (34) 0.0431a 4.51a
YCE-1 #80 649a 28a 2.53b (71) 0.0039b 1.06b (29) 0.0332a 3.59b
YCE-1 #75 630a 27a 2.16c (73) 0.0034c 0.79c (27) 0.0271b 2.95c
YCE-1 #65 561b 24b 1.78d (76) 0.0032c 0.56d (24) 0.0213bc 2.34cd
CuSO4 522b 23b 1.63d (79) 0.0031c 0.44e (21) 0.0178c 2.07d
Bell pepper
BCE-1 555a 50a 1.96a (70) 0.0034a 0.82a (30) 0.0703a 2.78a
YCE-1 #80 485b 49a 1.76ab (72) 0.0036a 0.69b (28) 0.0707a 2.45b
YCE-1 #75 430bc 43b 1.34bc (72) 0.0031ab 0.51c (28) 0.0617b 1.85c
YCE-1 #65 417cd 42b 1.20bc (73) 0.0029b 0.44cd (27) 0.0584bc 1.64cd
CuSO4 378d 38c 1.18c (77) 0.0031ab 0.35d (23) 0.0497c 1.53d
a
BCE-1 is the control ®lm. YCE-1 #80, #75, and #65 are photoselective ®lms with the FR light absorbing dye at 0.08, 0.13 and 0.22 g mÿ2 , respectively. CuSO4is the chamber covered with panels ®lled with 4% CuSO45H2O liquid.
b
Each number is the mean of 20 plants. Mean comparison within a column by Duncan's multiple range test atP0.05. Means with the same letter are not signi®cantly different.
c
Numbers in parentheses are % dry weight.
accumulation in leaves increased as the R:FR ratio increased. In peppers, plants grown in the control chamber had 70% of total shoot dry matter in leaves and 30% in stems. Photoselective ®lms reduced percentage dry matter accumulation in stems from 30 to 27% and increased dry matter accumulation in leaves from 70 to 73%. In both chrysanthemum and peppers, the greatest change in dry matter partitioning was found in CuSO4 ®lter-grown-plants. Quality of light can in¯uence the translocation of photosynthates. Hurd (1974) reported that light low in R:FR ratio increased stem dry weight of tomato plants. Kasperbauer (1987) reported that lowering R:FR ratio increased photosynthate partitioning into shoots and developing seeds. The greater dry matter accumulation into leaves under photoselective ®lms may be because of the relatively high R and low FR light in these treatments. Britz and Sager (1990) reported that plants grown under blue light de®cient sources had less translocation of photosynthate out of leaves, thus increasing leaf dry matter content.
4. Conclusions
In summary, photoselective plastic ®lms with FR light intercepting dyes were effective in regulating plant height of chrysanthemum and bell pepper plants without the use of chemical growth regulators. The ®lm with the highest dye concentration (used in this study) was as effective as 4% CuSO4 ®lters in controlling plant height. Although ®lms with higher dye concentration are more effective in height reduction, the reduction in PPF transmission with increased dye concentration may adversely affect plant growth and development, and this fact should be considered in commercial development of photoselective ®lms. Our results indicate that a photoselective ®lm with a R:FR ratio of 2.2 (which corresponds to 75% light transmission) caused about 20% height reduction in chrysanthemum and 30% height reduction in bell pepper after 4 weeks of treatment. This initial work demonstrates that the use of greenhouse ®lms with FR light absorbing dyes is as effective as chemical growth regulators or CuSO4 ®lters in controlling plant height of chrysanthemums and bell pepper seedlings. With the commercial development of photoselective greenhouse covers or shade material, nursery and greenhouse industry could reduce costs for growth regulating chemicals, reduce health risks to their workers and consumers, and reduce potential environmental pollution.
References
Adamse, P., Jaspers, P.A.P.M., Bakker, J.A., Wesselius, J.C., Heeringa, G.H., Kendrick, R.E., Koornneef, M., 1988. Photophysiology of tomato mutant de®cient in labile phytochrome. J. Plant Physiol. 133, 436±440.
Britz, S.J., Sager, J.C., 1990. Photomorphogenesis and photoassimilation in soybean and sorghum grown under broad spectrum or blue-de®cient light sources. Plant Physiol. 94, 448±454. Hurd, R.G., 1974. The effect of an incandescent supplement on the growth of tomato plants in low
light. Ann. Bot. 38, 613±623.
Kasperbauer, M.J., 1987. Far-red light re¯ection from green leaves and effects on phytochrome-mediated assimilate partitioning under ®eld conditions. Plant Physiol. 85, 350±354.
McMahon, M.J., Kelly, J.W., Decoteau, D.R., Young, R.E., Pollock, R.K., 1991. Growth of
Dendranthemagrandi¯orum (Ramat.) Kitamura under various spectral ®lters. J. Am. Soc. Hort. Sci. 116, 950±954.
Morgan, D.C., Smith, H., 1976. Linear relationship between phytochrome photoequilibrium and growth in plants under simulated natural radiation. Nature 262, 210±212.
Morgan, D.C., Smith, H., 1979. A systematic relationship between phytochrome-controlled development and species habit, for plants grown in simulated natural radiation. Planta 145, 253± 258.
Mortensen, L.M., Strùmme, E., 1987. Effect of light quality on some greenhouse crops. Sci. Hort. 33, 27±36.
Murakami, K., Cui, H., Kiyota, M., Aiga, I., 1995. The design of special covering materials for greenhouses to control plant elongation by changing spectral distribution of daylight. Acta Hort. 399, 135±142.
Murakami, K., Cui, H., Kiyota, M., Yamane, T., Aiga, I., 1996a. The effects of covering materials for greenhouses to control plant growth by changing spectral distribution of daylight. Acta Hort. 435, 123±130.
Murakami, K., Cui, H., Kiyota, M., Takemura, Y., Oi, R., Aiga, I., 1996b. Covering materials to control plant growth by modifying the spectral balance of daylight. Plasticulture 110, 2±14. Protasova, N.N., Welles, J.M., Dobrovolskii, M.W., Tsoglin, L.N., 1990. Spectral characteristics of
light sources and plant growth peculiarities under arti®cial illumination. Soviet Plant Physiol. 37, 293±303.
Rajapakse, N.C., Kelly, J.W., 1992. Regulation of chrysanthemum growth by spectral ®lters. J. Am. Soc. Hort. Sci. 117, 481±485.
Sager, J.C., Smith, W.O., Edwards, J.C., Cyr, K.L., 1988. Photosynthetic ef®ciency and phytochrome photoequilibria determination using spectral data. Trans. ASAE 31, 1882±1887. Warpeha, K.M., Kaufman, L.S., 1989. Blue-light regulation of epicotyl elongation inPisum sativus.
