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Effects of non-destructive mechanical measurements on plant growth: a study with sweet pepper (Capsicum annuum L.)
Scientia Horticulturae 81 (1999) 369±375
Short communication
Effects of non-destructive mechanical measurements on plant growth: a study with sweet pepper (Capsicum annuum L.) H.-P. KlaÈring* Institute of Vegetable and Ornamental Crops Groûbeeren/Erfurt e.V., Theodor-Echtermeyer-Weg 1, D-14979 Groûbeeren, Germany Accepted 15 January 1999
Abstract The handling of plants in the measurement of plant growth and condition may disturb the alignment of stems, leaves, flowers or fruits and, thereby may affect plant growth. In this study of sweet pepper, the effects of repeated, non-destructive mechanical measurements on leaves and fruit were investigated, in three greenhouse experiments. In two experiments, plant growth was not affected by mechanical measurements. In a third experiment, the use of mechanical measurement reduced stem elongation, leaf area and yield. Retardation of stem growth was accompanied by increased dry matter content and changed nutrient content of stems. Obviously, thigmomorphogenic effects of mechanical measurements are difficult to predict. Retardation of stem elongation is suggested as a reliable and easy-to-measure indicator of mechanical perturbation of sweet pepper plants. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Dry matter; Fresh matter; Growth retardation; Mechanical perturbation; Leaf area; Stem elongation; Thigmomorphogenesis; Yield
1. Introduction In many experiments, plants are handled repeatedly in non-destructive measurement of plant properties. As a result, plants are disturbed and the * Corresponding author. fax: +49-33701-55391; e-mail: [email protected] 0304-4238/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 0 2 2 - 9
370
H.-P. KlaÈring / Scientia Horticulturae 81 (1999) 369±375
alignment of stems, leaves, flowers and fruits is altered. The most common measurements are of the length and width of leaves to estimate leaf area, the length or diameter of fruit to estimate fruit growth, and the use of chambers to measure CO2 and water vapour gas exchange of leaves or fruits. In general, it is assumed that such measurements do not perturb the plants. On the other hand, from Darwin (1881) to the present there have been many reports that plant growth is retarded by permanent or frequent mechanical effects (Hunt and Jaffe, 1980; Jaffe, 1985). Most commonly reduction of stem elongation was observed, often accompanied by thicker nodes (BuÈnning et al., 1948; Frizzell et al., 1960; Jaffe, 1976). Such thigmo- and seismomorphogenic effects have even been tested for the control of stem elongation in ornamental plants (Hammer et al., 1974; Jerzy and Piszczek, 1979; Jatzkowski and LuÈhmann, 1997). Retardation of stem elongation has been observed with greenhouse vegetables (Klapwijk and Wubben, 1975; Jerzy and Nowaczyk, 1977). Factors and substances controlling thigmomorphogenesis have been central to fundamental research (Goeschl et al., 1966; Pickard, 1971; Hiraki and Ota, 1975; Erner and Jaffe, 1982; Biro and Jaffe, 1984). However, investigations of the effects of mechanical perturbations on characteristics like biomass production or yield are rare (Frizzell et al., 1960; Beyl and Mitchell, 1977; Kraus et al., 1994). Therefore, the effects of mechanical perturbations caused by repeated measurements on leaves and fruits were investigated.
2. Materials and methods Three experiments were conducted with sweet pepper (Capsicum annuum L. cv. Mazurka), with a density of 3.3 plants mÿ2 on rockwool slabs. Normal horticultural practices were used in the control of greenhouse climate, water and nutrient supply and in the training of the plants (2 main stems per plant). As data were taken from experiments with distinct goals, different characteristics of plant growth were recorded from experiment to experiment. 2.1. Experiment 1 In 1994 plants were grown from 8 April to 20 September in a glasshouse (width 10 m, length 48 m, east±west orientation). Plant heights and the lengths of all the leaves were measured on eight plants every fortnight to calculate leaf area development (treatment). An additional four plants were harvested completely every fortnight to measure growth and dry matter distribution (KlaÈring et al., 1996). These plants were not perturbed by any measurement until the day of harvest (control).
H.-P. KlaÈring / Scientia Horticulturae 81 (1999) 369±375
371
2.2. Experiment 2 This experiment (1995) was similar to Experiment 1. The greenhouse was of the same design as in Experiment 1, but covered with double layer polyethylene sheets. Plant heights, lengths of the leaves, and lengths and diameters of the fruit were measured on eight plants every 4 weeks from 4 April to 21 August. Moreover, CO2 gas exchange was measured weekly on three leaves of each plant using a leaf chamber gas analyser LCA-4 (Analytical Development Company, UK) (treatment). Untreated plants (control) were harvested at 4-weekly intervals. 2.3. Experiment 3 In 1997, in both the greenhouses described above, strategies to control the nutrient solution supply were compared. No differences between greenhouses and nutrition treatments were found for most plant characteristics. Therefore, data were taken from all plots (16 plots with 96 plants each). At the start, two plants in the middle of each plot were marked. With these plants (treatment), lengths and diameters of all the fruit were measured weekly, and the lengths of leaves were measured 10 times during the growing season from 25 March to 29 October. Ripe fruit from these plants were harvested separately. At the end of the experiment, the plants were harvested completely. Fresh and dry weights of stems, leaves and fruit were recorded. Dry weights were taken after 4 days drying in a ventilated oven at 1058C. In addition, two other plants from each plot were harvested completely (control). These plants were located 1 m from the treated plants, and had not been disturbed during the season by any measurement. With these control plants the same measurements including those of plant height and leaf length were carried out. Leaf length was measured in all experiments by ruler, including all leaves down to a length of 0.06 m. Leaf area was calculated from leaf length (KlaÈring et al., 1996). Length and diameter of fruit were measured by a sliding ruler. 3. Results In Experiments 1 and 2 no significant effects of mechanical measurements on stem elongation and leaf area growth were observed (two way ANOVA, Factor 1 ± date, Factor 2 ± treatment or control, Fisher's F-test at significance level 0.05). Fig. 1 depicts the data of Experiment 1. In contrast, in Experiment 3, the treated plants reacted to mechanical perturbations with a changed morphology. Plant height, leaf area, yield, fresh weight of stems, leaves and (green) fruit were reduced significantly (Table 1). Reduced stem elongation was accompanied by a higher dry matter content of
372
H.-P. KlaÈring / Scientia Horticulturae 81 (1999) 369±375
Fig. 1. Plant height and leaf area of control plants and plants treated by repeated measurements of leaf area. Experiment 1.
stems while the dry matter content of leaves and (green) fruits seemed to be unaffected. Not only the dry matter content but also the nutrient content of stems was changed. In dry matter, concentrations of calcium (20.5%), magnesium (13.6%), nitrogen (9.6%) and phosphorus (8.2%) of the treated plants increased significantly while the concentration of potassium (ÿ5.4%) decreased significantly. The concentration of sulphur (3.7%) was less affected. To estimate the effect of changed leaf area on dry matter production of treated plants a photosynthesis-based model was used (KlaÈring et al., 1996; Heiûner, 1997). Dry matter production of treated plants was calculated from greenhouse Table 1 Selected characteristics of control plants and plants treated with repeated measurements Characteristic, measuring unit
Control
Treatment
Difference (per cent)
Yield, kg plantÿ2 Height, m Leaf area, m2 plantÿ1 Fresh matter of stems, g plantÿ1 Dry matter content of stems, % Fresh matter of leaves, g plantÿ1 Dry matter content of leaves, % Fresh matter green of fruits, g plantÿ1 Dry matter content of green fruits, %
4.47 1.92 1.18 468 21.54 620 14.98 390 6.67
3.95 1.81 1.08 427 23.57 556 15.24 313 6.51
ÿ11.6* ÿ5.7* ÿ8.5* ÿ8.8* 9.4* ÿ10.3* 1.7 ÿ19.7* ÿ2.4
Each figure is the mean of 32 plants. Differences marked by asterisks are significant (Student's t-test, significance level a 0.05), Experiment 3.
H.-P. KlaÈring / Scientia Horticulturae 81 (1999) 369±375
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climate and leaf area measured during the growing time. Dry matter production of control plants was estimated using the same calculation but with a 9.2% higher leaf area (Table 1). It resulted in a 2.5% higher value compared to the dry matter of treated plants. On the assumptions that at the start of the mechanical measurements difference in leaf area between control and treatment was 0 and that it increased linearly to the final value (9.2%) the calculated difference of dry matter production amounted only to 0.3%. In contradiction, measured difference in dry matter production was 9.7% (summarised from Table 1). 4. Discussion Mechanical measurements may affect the morphology and physiology of plants by thigmomorphogenic processes. As with many other crops, the degree of the reaction of sweet pepper plants (no measurable effect in Experiments 1 and 2, significant effects in Experiment 3) depended on the frequency and the nature of the perturbations (Hunt and Jaffe, 1980; Mitchell et al., 1975). However, it seems impossible to make exact quantitative predictions as different mechanical perturbations are not comparable and interactions with environmental factors must be expected (PoÈntinen and Voipio, 1992). The typical effect of mechanical perturbations observed in this study, as in most of the literature, was the reduction of stem elongation (Table 1). In Experiment 3, reduction of stem elongation was accompanied by an increase in dry matter content of stems, a reduction in leaf area and a decrease in fruit production (Table 1). Increasing dry matter content of stems was found in cauliflower (PoÈntinen and Voipio, 1992) and chrysanthemum (Beyl and Mitchell, 1977). The reduction of yield and total dry matter by 11.6% and 9.7%, respectively, could not be explained sufficiently by decreased leaf area. A possible overestimation of the reduction in total dry matter could be caused by higher dry matter of the roots of treated plants (Goodman and Ennos, 1997). However, as root dry matter of sweet pepper is less than 10% of dry matter of the total plant, this phenomenon by itself could not explain the remaining difference. Therefore, it was concluded that not only was morphology affected but also physiology. There are no clear results as to whether photosynthesis and respiration are involved in thigmomorphogenesis. In corresponding experiments with greenhouse vegetables, effects of mechanical perturbations on transpiration were found but there was no influence on photosynthesis (Latimer and Beverly, 1994; Van Iersel, 1997). However, the synthesis of several substances in the plant may increase significantly. In particular, an enhanced ethylene production was observed (Jaffe and Forbes, 1993; Smalle and Van Der Straeten, 1997). It is possible that considerable amounts of chemical energy are needed in these processes or the conversion factor of assimilates to structural dry matter might
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H.-P. KlaÈring / Scientia Horticulturae 81 (1999) 369±375
have changed. Different plant nutrient contents may be indicators of such changes. In the present study, a significant change of calcium concentration was found, suggesting the possible involvement of this ion in thigmomorphogenesis (Mitchell, 1996; Depege et al., 1997). In general, the frequency and intensity of perturbations of plants by mechanical measurements are insignificant compared to the treatments in experiments on thigmomorphogenesis. Therefore, the effects on morphology and physiology have often been neglected. However, as the effects of mechanical measurements on plant growth are not predictable, measurements on control plants should be carried out. In the literature and in our experiments, stem elongation was found to be involved whenever thigmomorphogenesis occurred. Therefore, this easy-tomeasure characteristic could be used to check a possible effect of mechanical measurements. Acknowledgements This study was supported by the Ministries of Agriculture of the Federal Republic of Germany, of the Land Brandenburg and of the Land ThuÈringen. I. Zwicker and B. LoÈffelbein helped in conducting the experiments, their assistance is gratefully acknowledged. References Beyl, C.A., Mitchell, C.A., 1977. Characterization of mechanical stress dwarfing in Chrysanthemum. J. Am. Soc. Hort. Sci. 102, 591±592. Biro, R.L., Jaffe, M.J., 1984. Thigmomorphogenesis: Ethylene evolution and its role in changes observed in mechanically perturbed bean plants. Physiol. Plantarum 62, 289±296. BuÈnning, E., Haag, L., Timmermann, G., 1948. Weitere Untersuchungen uÈber die formative Wirkung des Lichtes und mechanischer Reize auf Pflanzen. Planta 36, 178±187. Darwin, C., 1881. The power of movement in plants. D. Appleton, New York. Depege, N., Thonat, C., Coutand, C., Julien, J.-C., Boyer, N., 1997. Morphological responses and molecular modifications in tomato plants after mechanical stimulation. Plant Cell Physiol. 38, 1127±1134. Erner, Y., Jaffe, M.J., 1982. Thigmomorphogenesis: The involvement of auxin and abscisic acid in growth retardation due to mechanical perturbation. Plant Cell Physiol. 23, 935±941. Frizzell, J.L., Brown, L.C., Waddle, B.A., 1960. Some effects of handling on the growth and development of cotton. Agron. J. 52, 69±70. Goeschl, J.D., Rappaport, L., Pratt, H.K., 1966. Ethylene as a factor regulating the growth of pea epicotyls subjected to physical stress. Plant Physiol. 41, 877±884. Goodman, A.M., Ennos, A.R., 1997. The response of field grown sunflower and maize to mechanical support. Ann. Bot. 79, 703±711. Hammer, P.A., Mitchell, C.A., Weiler, T.C., 1974. Height control in greenhouse Chrysanthemum by mechanical stress. Hort. Science 9, 474±475.
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Heibner, A., 1997. Der CO2-Gaswechsel von Paprikapflanzen in AbhaÈngigkeit von der BestrahlungsstaÈrke, der CO2-Konzentration, der Lufttemperatur und dem DampfdrucksaÈttigungsdefizit der Luft: Messungen und Modell. Gartenbauwissenschaft 62, 78±90. Hiraki, Y., Ota, Y., 1975. The relationship between growth inhibition and ethylene production by mechanical stimulation in Lilium longiflorum. Plant Cell Physiol. 16, 665±672. Hunt, E.R., Jaffe, M.J., 1980. Thigmomorphogenesis: The interaction of wind and temperature in the field on the growth of Phaseolus vulgaris L.. Ann. Bot. 45, 665±672. Jaffe, M.J., 1976. Thigmomorphogenesis: A detailed characterization of the response of beans (Phaseolus vulgaris L.) to mechanical stimulation. Z. Pflanzenphysiol. 77, 422±436. Jaffe, M.J., 1985. Wind and other mechanical effects in the development and behavior of plants, with special emphasis on the role of hormones. In: Pharis and Reid (Eds.): Encyclopedia of plant Physiology: New Series, vol. 11. Springer, Berlin, pp. 444±484. Jaffe, M.J., Forbes, S., 1993. Thigmomorphogenesis ± the effect of mechanical perturbation on plants. Plant Growth Regulation 12, 313±324. Jatzkowski, M., LuÈhmann, B., 1997. Aufhebung der streckungsfoÈrdernden Wirkung einer dunkelrotbetonten Belichtung durch mechanische Reizung bei Argyranthemum frutescens `Silver Leaf'. Gartenbauwissenschaft 62, 218±224. Jerzy, M., Nowaczyk, P., 1977. Retardacja mechaniczna pomidoroÂw uprawianych w szklarni. Ogrodnictwo 11, 292±294. Jerzy, M., Piszczek, P., 1979. The retardation of Chrysanthemum by mechanical stress applied on different stages of growth and development. Acta Horticulturae 91, 377±381. Klapwijk, D., Wubben, C.F.M., 1975. The effects of shaking the shoot tips on growth of tomato plants. Glasshouse Crops Research and Experiment Station Naaldwijk, The Netherlands. Annu. Report 1975, 36±37. KlaÈring, H.-P., Heiûner, A., Fink, M., 1996. Growth of a sweet pepper crop ± measurement for modelling. Acta Horticulturae 417, 107±112. Kraus, E., KolloÈffel, C., Lambers, H., 1994. The effect of handling on photosynthesis, transpiration, respiration, and nitrogen and carbohydrate content of populations of Lolium perenne. Physiologia Plantarum 91, 631±638. Latimer, J.G., Beverly, R.B., 1994. Conditioning affects growth and drought tolerance of curcubit transplants. J. Am. Soc. Hort. Sci. 119, 943±948. Mitchell, C.A., Severson, C.J., Watt, J.A., Hammer, P.A., 1975. Seismomorphogenetic regulation of plant growth. J. Am. Soc. Hort. Sci. 100, 161±165. Mitchell, C.A., 1996. Recent advances in plant response to mechanical stress: theory and application. Hort. Science 31, 31±35. Pickard, B.G., 1971. Action potentials resulting from mechanical stimulation of pea epicotyls. Planta 97, 106±115. PoÈntinen, V., Voipio, I., 1992. Different methods of mechanical stress controlling the growth of lettuce and cauliflower seedlings. Acta Agricultura Scandinavivica Section B-Soil and Plant Sci. 42, 246±250. Smalle, J., Van Der straeten, D., 1997. Ethylene and vegetative development. Physiol. Plantarium 100, 593±605. Van Iersel, M., 1997. Tactile conditioning increases water use by tomato. J. Am. Soc. Hort. Sci. 122, 285±289.
Short communication
Effects of non-destructive mechanical measurements on plant growth: a study with sweet pepper (Capsicum annuum L.) H.-P. KlaÈring* Institute of Vegetable and Ornamental Crops Groûbeeren/Erfurt e.V., Theodor-Echtermeyer-Weg 1, D-14979 Groûbeeren, Germany Accepted 15 January 1999
Abstract The handling of plants in the measurement of plant growth and condition may disturb the alignment of stems, leaves, flowers or fruits and, thereby may affect plant growth. In this study of sweet pepper, the effects of repeated, non-destructive mechanical measurements on leaves and fruit were investigated, in three greenhouse experiments. In two experiments, plant growth was not affected by mechanical measurements. In a third experiment, the use of mechanical measurement reduced stem elongation, leaf area and yield. Retardation of stem growth was accompanied by increased dry matter content and changed nutrient content of stems. Obviously, thigmomorphogenic effects of mechanical measurements are difficult to predict. Retardation of stem elongation is suggested as a reliable and easy-to-measure indicator of mechanical perturbation of sweet pepper plants. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Dry matter; Fresh matter; Growth retardation; Mechanical perturbation; Leaf area; Stem elongation; Thigmomorphogenesis; Yield
1. Introduction In many experiments, plants are handled repeatedly in non-destructive measurement of plant properties. As a result, plants are disturbed and the * Corresponding author. fax: +49-33701-55391; e-mail: [email protected] 0304-4238/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 0 2 2 - 9
370
H.-P. KlaÈring / Scientia Horticulturae 81 (1999) 369±375
alignment of stems, leaves, flowers and fruits is altered. The most common measurements are of the length and width of leaves to estimate leaf area, the length or diameter of fruit to estimate fruit growth, and the use of chambers to measure CO2 and water vapour gas exchange of leaves or fruits. In general, it is assumed that such measurements do not perturb the plants. On the other hand, from Darwin (1881) to the present there have been many reports that plant growth is retarded by permanent or frequent mechanical effects (Hunt and Jaffe, 1980; Jaffe, 1985). Most commonly reduction of stem elongation was observed, often accompanied by thicker nodes (BuÈnning et al., 1948; Frizzell et al., 1960; Jaffe, 1976). Such thigmo- and seismomorphogenic effects have even been tested for the control of stem elongation in ornamental plants (Hammer et al., 1974; Jerzy and Piszczek, 1979; Jatzkowski and LuÈhmann, 1997). Retardation of stem elongation has been observed with greenhouse vegetables (Klapwijk and Wubben, 1975; Jerzy and Nowaczyk, 1977). Factors and substances controlling thigmomorphogenesis have been central to fundamental research (Goeschl et al., 1966; Pickard, 1971; Hiraki and Ota, 1975; Erner and Jaffe, 1982; Biro and Jaffe, 1984). However, investigations of the effects of mechanical perturbations on characteristics like biomass production or yield are rare (Frizzell et al., 1960; Beyl and Mitchell, 1977; Kraus et al., 1994). Therefore, the effects of mechanical perturbations caused by repeated measurements on leaves and fruits were investigated.
2. Materials and methods Three experiments were conducted with sweet pepper (Capsicum annuum L. cv. Mazurka), with a density of 3.3 plants mÿ2 on rockwool slabs. Normal horticultural practices were used in the control of greenhouse climate, water and nutrient supply and in the training of the plants (2 main stems per plant). As data were taken from experiments with distinct goals, different characteristics of plant growth were recorded from experiment to experiment. 2.1. Experiment 1 In 1994 plants were grown from 8 April to 20 September in a glasshouse (width 10 m, length 48 m, east±west orientation). Plant heights and the lengths of all the leaves were measured on eight plants every fortnight to calculate leaf area development (treatment). An additional four plants were harvested completely every fortnight to measure growth and dry matter distribution (KlaÈring et al., 1996). These plants were not perturbed by any measurement until the day of harvest (control).
H.-P. KlaÈring / Scientia Horticulturae 81 (1999) 369±375
371
2.2. Experiment 2 This experiment (1995) was similar to Experiment 1. The greenhouse was of the same design as in Experiment 1, but covered with double layer polyethylene sheets. Plant heights, lengths of the leaves, and lengths and diameters of the fruit were measured on eight plants every 4 weeks from 4 April to 21 August. Moreover, CO2 gas exchange was measured weekly on three leaves of each plant using a leaf chamber gas analyser LCA-4 (Analytical Development Company, UK) (treatment). Untreated plants (control) were harvested at 4-weekly intervals. 2.3. Experiment 3 In 1997, in both the greenhouses described above, strategies to control the nutrient solution supply were compared. No differences between greenhouses and nutrition treatments were found for most plant characteristics. Therefore, data were taken from all plots (16 plots with 96 plants each). At the start, two plants in the middle of each plot were marked. With these plants (treatment), lengths and diameters of all the fruit were measured weekly, and the lengths of leaves were measured 10 times during the growing season from 25 March to 29 October. Ripe fruit from these plants were harvested separately. At the end of the experiment, the plants were harvested completely. Fresh and dry weights of stems, leaves and fruit were recorded. Dry weights were taken after 4 days drying in a ventilated oven at 1058C. In addition, two other plants from each plot were harvested completely (control). These plants were located 1 m from the treated plants, and had not been disturbed during the season by any measurement. With these control plants the same measurements including those of plant height and leaf length were carried out. Leaf length was measured in all experiments by ruler, including all leaves down to a length of 0.06 m. Leaf area was calculated from leaf length (KlaÈring et al., 1996). Length and diameter of fruit were measured by a sliding ruler. 3. Results In Experiments 1 and 2 no significant effects of mechanical measurements on stem elongation and leaf area growth were observed (two way ANOVA, Factor 1 ± date, Factor 2 ± treatment or control, Fisher's F-test at significance level 0.05). Fig. 1 depicts the data of Experiment 1. In contrast, in Experiment 3, the treated plants reacted to mechanical perturbations with a changed morphology. Plant height, leaf area, yield, fresh weight of stems, leaves and (green) fruit were reduced significantly (Table 1). Reduced stem elongation was accompanied by a higher dry matter content of
372
H.-P. KlaÈring / Scientia Horticulturae 81 (1999) 369±375
Fig. 1. Plant height and leaf area of control plants and plants treated by repeated measurements of leaf area. Experiment 1.
stems while the dry matter content of leaves and (green) fruits seemed to be unaffected. Not only the dry matter content but also the nutrient content of stems was changed. In dry matter, concentrations of calcium (20.5%), magnesium (13.6%), nitrogen (9.6%) and phosphorus (8.2%) of the treated plants increased significantly while the concentration of potassium (ÿ5.4%) decreased significantly. The concentration of sulphur (3.7%) was less affected. To estimate the effect of changed leaf area on dry matter production of treated plants a photosynthesis-based model was used (KlaÈring et al., 1996; Heiûner, 1997). Dry matter production of treated plants was calculated from greenhouse Table 1 Selected characteristics of control plants and plants treated with repeated measurements Characteristic, measuring unit
Control
Treatment
Difference (per cent)
Yield, kg plantÿ2 Height, m Leaf area, m2 plantÿ1 Fresh matter of stems, g plantÿ1 Dry matter content of stems, % Fresh matter of leaves, g plantÿ1 Dry matter content of leaves, % Fresh matter green of fruits, g plantÿ1 Dry matter content of green fruits, %
4.47 1.92 1.18 468 21.54 620 14.98 390 6.67
3.95 1.81 1.08 427 23.57 556 15.24 313 6.51
ÿ11.6* ÿ5.7* ÿ8.5* ÿ8.8* 9.4* ÿ10.3* 1.7 ÿ19.7* ÿ2.4
Each figure is the mean of 32 plants. Differences marked by asterisks are significant (Student's t-test, significance level a 0.05), Experiment 3.
H.-P. KlaÈring / Scientia Horticulturae 81 (1999) 369±375
373
climate and leaf area measured during the growing time. Dry matter production of control plants was estimated using the same calculation but with a 9.2% higher leaf area (Table 1). It resulted in a 2.5% higher value compared to the dry matter of treated plants. On the assumptions that at the start of the mechanical measurements difference in leaf area between control and treatment was 0 and that it increased linearly to the final value (9.2%) the calculated difference of dry matter production amounted only to 0.3%. In contradiction, measured difference in dry matter production was 9.7% (summarised from Table 1). 4. Discussion Mechanical measurements may affect the morphology and physiology of plants by thigmomorphogenic processes. As with many other crops, the degree of the reaction of sweet pepper plants (no measurable effect in Experiments 1 and 2, significant effects in Experiment 3) depended on the frequency and the nature of the perturbations (Hunt and Jaffe, 1980; Mitchell et al., 1975). However, it seems impossible to make exact quantitative predictions as different mechanical perturbations are not comparable and interactions with environmental factors must be expected (PoÈntinen and Voipio, 1992). The typical effect of mechanical perturbations observed in this study, as in most of the literature, was the reduction of stem elongation (Table 1). In Experiment 3, reduction of stem elongation was accompanied by an increase in dry matter content of stems, a reduction in leaf area and a decrease in fruit production (Table 1). Increasing dry matter content of stems was found in cauliflower (PoÈntinen and Voipio, 1992) and chrysanthemum (Beyl and Mitchell, 1977). The reduction of yield and total dry matter by 11.6% and 9.7%, respectively, could not be explained sufficiently by decreased leaf area. A possible overestimation of the reduction in total dry matter could be caused by higher dry matter of the roots of treated plants (Goodman and Ennos, 1997). However, as root dry matter of sweet pepper is less than 10% of dry matter of the total plant, this phenomenon by itself could not explain the remaining difference. Therefore, it was concluded that not only was morphology affected but also physiology. There are no clear results as to whether photosynthesis and respiration are involved in thigmomorphogenesis. In corresponding experiments with greenhouse vegetables, effects of mechanical perturbations on transpiration were found but there was no influence on photosynthesis (Latimer and Beverly, 1994; Van Iersel, 1997). However, the synthesis of several substances in the plant may increase significantly. In particular, an enhanced ethylene production was observed (Jaffe and Forbes, 1993; Smalle and Van Der Straeten, 1997). It is possible that considerable amounts of chemical energy are needed in these processes or the conversion factor of assimilates to structural dry matter might
374
H.-P. KlaÈring / Scientia Horticulturae 81 (1999) 369±375
have changed. Different plant nutrient contents may be indicators of such changes. In the present study, a significant change of calcium concentration was found, suggesting the possible involvement of this ion in thigmomorphogenesis (Mitchell, 1996; Depege et al., 1997). In general, the frequency and intensity of perturbations of plants by mechanical measurements are insignificant compared to the treatments in experiments on thigmomorphogenesis. Therefore, the effects on morphology and physiology have often been neglected. However, as the effects of mechanical measurements on plant growth are not predictable, measurements on control plants should be carried out. In the literature and in our experiments, stem elongation was found to be involved whenever thigmomorphogenesis occurred. Therefore, this easy-tomeasure characteristic could be used to check a possible effect of mechanical measurements. Acknowledgements This study was supported by the Ministries of Agriculture of the Federal Republic of Germany, of the Land Brandenburg and of the Land ThuÈringen. I. Zwicker and B. LoÈffelbein helped in conducting the experiments, their assistance is gratefully acknowledged. References Beyl, C.A., Mitchell, C.A., 1977. Characterization of mechanical stress dwarfing in Chrysanthemum. J. Am. Soc. Hort. Sci. 102, 591±592. Biro, R.L., Jaffe, M.J., 1984. Thigmomorphogenesis: Ethylene evolution and its role in changes observed in mechanically perturbed bean plants. Physiol. Plantarum 62, 289±296. BuÈnning, E., Haag, L., Timmermann, G., 1948. Weitere Untersuchungen uÈber die formative Wirkung des Lichtes und mechanischer Reize auf Pflanzen. Planta 36, 178±187. Darwin, C., 1881. The power of movement in plants. D. Appleton, New York. Depege, N., Thonat, C., Coutand, C., Julien, J.-C., Boyer, N., 1997. Morphological responses and molecular modifications in tomato plants after mechanical stimulation. Plant Cell Physiol. 38, 1127±1134. Erner, Y., Jaffe, M.J., 1982. Thigmomorphogenesis: The involvement of auxin and abscisic acid in growth retardation due to mechanical perturbation. Plant Cell Physiol. 23, 935±941. Frizzell, J.L., Brown, L.C., Waddle, B.A., 1960. Some effects of handling on the growth and development of cotton. Agron. J. 52, 69±70. Goeschl, J.D., Rappaport, L., Pratt, H.K., 1966. Ethylene as a factor regulating the growth of pea epicotyls subjected to physical stress. Plant Physiol. 41, 877±884. Goodman, A.M., Ennos, A.R., 1997. The response of field grown sunflower and maize to mechanical support. Ann. Bot. 79, 703±711. Hammer, P.A., Mitchell, C.A., Weiler, T.C., 1974. Height control in greenhouse Chrysanthemum by mechanical stress. Hort. Science 9, 474±475.
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