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Stomatal Responses of the Cowpea (Vigna unguiculata L.) to Light Intensity
Bioehem. Physiol. Pflanzen 172, 541-545 (1978)
Short Communication
Stomatal Responses of the Cowpea (Vigna unguiculata L.) to Light Intensity A. G. I. TARILA, D. P. 0RMROD, and N. 0. AnEDIPE Department of Horticultural Science, University of Guelph, Guelph, Ontario, Canada and Department of Agricultural Biology, University of Ibadan, Ibadan, Nigeria Key Term Index: stomata response, light intensity; Vigna unguiculata.
Summary The influence of light intensity (7 and 27 klux) on the stomatal responses of two cowpea cultivars (Adzuki an<)_ Early Ramshorn) was studied in sand and solution culture experiments in controlled environment chambers. Stomata were far more numerous on the abaxial than on the adaxial surface of the leaf in both cultivars. High light intensity increased stomatal length, width and density in both cultivars with different magnitudes of increases. At both light intensities there were no significant differences between the stomatal length and width of the two cultivars. There was a significant interaction between light intensity and cultivar on stomatal density. While Early Ramshorn had more stomata per unit area than Adzuki at low light intensity, Adzuki had greater stomatal density than Early Ramshorn at the high light intensity.
A major physiological problem of the cowpea plant is the premature abscission of floral buds, flowers, and young fruits. The magnitude of the problem is much greater than in the morphologically similar temperate species like field beans (OJEHOMON 1968). The abscission of young fruits has been found to be related, in part, to the mobilization of assimilates by the older fruits to the "starvation detriment" of the young fruits (ADEDIPE and 0RMROD 1975a, ADEDIPE et al. 1976). It is, however, known that certain cultivars exhibit this trait more than others (OJEHOMON 1968, 1970, ADEDIPE and 0RMROD 1975a, ADEDIPE et al. 1976). In tropical areas, environmental factors, particularly light intensity patterns during the year, play a major role in determining crop productivity differences in rainforest and savanna zones (KASSAM and KowAL 1973). In an earlier study, ADEDIPE and 0RMROD (1975 b) found that cultivars representative of the two magnitudes of fruit abscission responded differently to light intensity, in terms of growth characteristics, chlorophyll concentration and carbohydrate content. These responses were later substantially confirmed with mature plants (TARILA et al. 1977). Such studies, which indicated marked differences in responses to light intensity, suggested that cultivars of cowpea might differ in their stomatal characteristics, since it is known that light influences stomatal behaviour in many species (CuTTER 1971). The objectives of the present study were, therefore, to (a) provide hitherto lacking data on stomatal characteristics of the cowpea plant, and (b) relate the stomatal charac-
542
A. G. J. TARILA, D.P. 0RMROD, and N. 0. ADEDIPE
teristic responses to light intensity in two cowpea cultivars representative of two premature abscission magnitudes. This study, therefore, forms part of continuing physiological studies of the premature fruit abscission problem. Plant cultivation Seeds of cowpea (Vigna unguiculata L.) cultivars "Adzuki" and "Early Ramshorn" were sown in flats containing sand and germinated at 30/25 ± 1 oc day/night temperatures in "Controlled Environment" Model EY 15 growth chambers. Ten days after planting, when the first tri-foliate leaves were about to emerge, the seedlings were selected for uniformity and transplanted one plant to a 23 em dia. plastic pot containing 7.7 kg of sand. The plants were grown at two light intensities (7 and 27 klux), measured with a "Weston" Model 756 light meter. Light was provided from an adjustable height panel. Wattage inputs were 79% fluorescent (cool-white) and 21% incandescent. Plants were re-randomized in the cabinet, once a week, to minimize differences in intra-chamber micro-environments. Light duration was 12 h and relative humidity 85 ± 5 %· The plants were watered thrice daily with 40 ml deionized water, and fed once daily with 500 ml Hoagland's solution containing 62 ppm P. Stomatal impressions and measurements At 6 weeks, cellulose acetate film impressions of both surfaces of the terminal leaflets were taken immediately after the first watering, according to the description by RASCHKE (1970) and modifications by KHATAMIAN et al. (1973). Impressions were taken in situ from 6 terminal leaflets on 6 consecutive nodes of each plant, starting from the 4th node basipetally. Stomatal distribution, expressed as the number of stomata per square centimeter, was computed, and mean stomatal apertures and stomatal lengths calculated from observations of twenty stomata on each leaflet, using an eyepiece micrometer. The data were subjected to analysis of variance (2 x 7 factorial), and differences between treatment means detected by the Duncan's multiple range test.
Stomata were far more numerous on the abaxial surface (Fig. 1 A, C) than on the adaxial surface (Fig. 1 B, D). At the high light intensity of 27 klux, stomatal density was greater, particularly on the abaxial surface, in Adzuki (Fig. 1 C) than in Early Ramshorn (Fig.1A). Based on these observations, detailed studies of stomatal responses to light intensities were restricted to the abaxial surface. High light intensity increased stomatal length (Fig. 2A), stomatal width (Fig. 2B) and stomatal density (Fig. 2C), in both cultivars. However, the magnitudes of such increases depended on the cultivar. There was a significant interaction between light intensity and cultivar, in regard to stomatal density. While Early Ramshorn had more stomata per unit than Adzuki under low light intensity, Adzuki had a greater density at the high light intensity (Fig. 2 C). The magnitudes of the increases in stomatal density due to the high light intensity were 119% in Adzuki, and 28% in Early Ramshorn. The values for stomatal length were 22 and 29% (Fig. 2 A), and for the width, 27 and 50% (Fig. 2 B), in Adzuki and Early Ramshorn, respectively. The ratio of stomatal length to width was lower in Adzuki (59) than in Early Ramshorn (65) at low light intensity, but the cultivars were similar (57 and 56) at high light intensity. The observation that there were more stomata per unit area on the abaxial surface of cowpea leaves is in agreement with reports for a wide variety of species (CuTTER 1971). Increase in stomatal density at the high light intensity also supports the results obtained with other species (ADEDIPE et al. 1973, PENFOUND 1931). The present study shows the
Stomutal Response to Light Intensity
543
cowpea plant to resp:md to light intensity, in terms of stomatal pore size, in the same direction as, but differing in magnitude from other species (AnEDIPE et al. 1973, DuGGER et al. 1961, HEATH 1959, WENT 1944). Of the 3 stomatal characteristics (length, width, density), the two cultivars differed significantly only in stomatal density (Fig. 2). It is noteworthy that these stomatal density responses are similar to the relative dry matter distribution (particularly in terms of seed weight as per cent of total dry matter) observed in an earlier study (TARILA et al. 1977). In the present study, at the low light intensity, Adzuki had a lower stomatal density than did Early Ramshorn, just as the seed weight as a per cent of t3tal plant weight was lower in Adzuki than in Early Ramshorn (TARILA et al. 1977). At the high light intensity, Adzuki had greater stomatal density than did Early Ramshorn; which, complements the dry matter distribution patterns. The study indicates that, while Early Ramshorn is more adapted to low light intensity in terms of the dry matter accumulated by the seed relative to other parts of the plant, it is limited by lack of stomatal responsiveness to high light intensity. Adzuki would,
Fig. 1. Distribution of stomata on the leaves of two cowpea cultivars at 27 klux light intensity. A: abaxial surface of Early Ramshorn; B: adaxial surface of Early Ramshorn; C: abaxial surface of Adzuki; D: adaxial surface of Adzuki (Magnification, X 300).
544
A. G. I. T \HIL,\, D.P. 0RMROD, and N. 0. ADEDIPE
120
A. Stoma al length
B. Stomatal width
"
"
07klux El27 klux
100
BO
Fig. 2. Stomatal characteristics of leaves of tu;o cowpea cultivars at two light intensities. Each bar is the mean of 6 plant replicates. Bars with the same letter within a stomatal characteristic and cultivar, are not significantly different at the 5% level.
therefore, be more adaptable to varied intensities of light. Consequently, it would be more useful for breeding purposes aimed at developing new cultivars suited to a range of ecological zones with different light intensities, for example, rainforest as well as savanna zones in which light intensity patterns vary considerably during the year (KASSAM and KowAL 1973). This stomatal responsiveness of Adzuki to increasing light intensity is an additional attribute in its physiological efficiency in P utilization (ADEDIPE and Ommon 1975 b, TARILA et al. 1977), as well as on a more even distribution of assimilates in fruits of different ontogeny, which factor bas been found to be associated with a lower degree of premature abscission of young fruits (ADEDIPE and 0RMROD 1975a, ADEDIPE et al. 1976). The seeds used in this study were kindly supplied by the National Cereals Research Institute, Ibadan, Nigeria.
References ADEDIPE, N. 0., FLETCHER, R. A., and 0RMROD, D. P.: Distribution of 14 C-assimilates in the cowpea (Vigna unguiculata L.) in relation to fruit abscission and treatment with benzyladenine. Ann. Bot. 40, 731-737 (1976). - KHATAMIAN, H., and 0RMROD, D.P.: Stomatal regulation of ozone phytotoxicity in tomato. Z. Pflanzenphysiol. 68, 323-328 (1973). - and 0RMROD, D.P.: Absorption of foliar-applied 32 P bY successive leaves, and distribution patterns in relation to early fruiting and abscission in the cowpea (Vigna unguiculata L.). Ann. Bot. 39, 639-646 (1975a). - - Effects of light intensity on growth and on chlorophyll, carbohydrate and phosphorus contents of the cowpea (Vigna unguiculata L.). Biochem. Physiol. Pflanzen 167, 301-309 (1975 b). Cu'I'TER, E. G.: Plant Anatomy. Part II. Edward Arnold (Pub.) Ltd., London, 343 pp. (1971).
Stomatal Response to Light Intensity
545
DuGGER, W. M., TAYLOR, 0. C., CARDIFF, E., and THOMPSO='I, C. R.: Stomatal action in plants as related to damage from photochemical oxidants. Plant Physiol. 37, 487-491 (19Gl). HEATH, 0. V. S.: Light and carbon dioxide in stomatal movements. In: Encycl. Plant Physiol. Vol. 17, 415-465 (1959). KAssAM, A. H., and KowAL, J. M.: Productivity of crops in the savanna and rainforest zones in Nigeria. Savanna 2, 39-49 (1973). KHATAMIAN, H., ADEDIPE, N. 0., and OR~mon, D.P.: Soil-plant-water aspects of ozone phytotoxicity in tomato plants. Plant Soil 38, 531-541 (1973). 0JEHOMON, 0. 0.: Flowering, fruiting production and abscission in cowpea, Vigna unguiculata (L.) Walp. J. West Afr. Sci. Ass. 13, 227-234 (1968). - Effect of continuous removal of open flowers on the seed yield of two varieties of cowpea, Vigna unguiculata (L.) Walp. J. Agr. Sci., Camb. 74, 375-381 (1970). PENFOUND, W. T.: Plant anatomy as conditioned by light intensity and soil moisture. Amer. J. Bot. 18, 558-572 (1931). RASCHKE, K.: Stomatal responses to pressure changes and interruptions in the water supply of detached leaves of Zea mays L. Plant Physiol. 41), 415-423 (1970). TARILA, A. G. 1., 0RMROD, D.P., and ADEDIPE, N. 0.: Effects of phosphorus nutrition and light intensity on growth and development of the cowpea (Vigna unguiculata L.). Ann. Bot. 41, 75-83 (1977). WENT, F. W.: Plant growth under controlled conditions. III. Correlation between various physiological processes and growth in the tomato plant. Amer. J. Bot. 31, 537-596 (1944). Received November 2, 1977. Authors' address: Mr. A. G. I. TARILA and Dr. D.P. 0RMROD, Department of Horticultural Science, University of Guelph, Guelph, Ontario (Canada) and Dr. N. 0. AnEDIPE, Department of Agricultural Biology, University of Ibadan, lbadan (Nigeria).
Verantwortlich fiir die Redaktion: Prof. Dr. B. Parthier, 401 Halle (Saale). Ver·ag: VEB Gustav Fischer Verlag, 69 Jena, Villengang 2, Telefon 2 7332. Satz und Druck: Druckerei ,Magnus Poser 69 Jena.- Veroffentlicht unter der Lizenznummer 1067 des Presseamtes beim Vorsitzenden des Ministerrates der Deutschen Demokratischen Republik. Aile Rechte beim Verlag. Naohdruck (auch auszugsweise) nur mit Genehmigung des Verlages und des Verfassers sowie mit Angabe der Quelle gestattet. Printed in the German Democratic Republic. Artikel·Nr. (EDV) 5~916
Short Communication
Stomatal Responses of the Cowpea (Vigna unguiculata L.) to Light Intensity A. G. I. TARILA, D. P. 0RMROD, and N. 0. AnEDIPE Department of Horticultural Science, University of Guelph, Guelph, Ontario, Canada and Department of Agricultural Biology, University of Ibadan, Ibadan, Nigeria Key Term Index: stomata response, light intensity; Vigna unguiculata.
Summary The influence of light intensity (7 and 27 klux) on the stomatal responses of two cowpea cultivars (Adzuki an<)_ Early Ramshorn) was studied in sand and solution culture experiments in controlled environment chambers. Stomata were far more numerous on the abaxial than on the adaxial surface of the leaf in both cultivars. High light intensity increased stomatal length, width and density in both cultivars with different magnitudes of increases. At both light intensities there were no significant differences between the stomatal length and width of the two cultivars. There was a significant interaction between light intensity and cultivar on stomatal density. While Early Ramshorn had more stomata per unit area than Adzuki at low light intensity, Adzuki had greater stomatal density than Early Ramshorn at the high light intensity.
A major physiological problem of the cowpea plant is the premature abscission of floral buds, flowers, and young fruits. The magnitude of the problem is much greater than in the morphologically similar temperate species like field beans (OJEHOMON 1968). The abscission of young fruits has been found to be related, in part, to the mobilization of assimilates by the older fruits to the "starvation detriment" of the young fruits (ADEDIPE and 0RMROD 1975a, ADEDIPE et al. 1976). It is, however, known that certain cultivars exhibit this trait more than others (OJEHOMON 1968, 1970, ADEDIPE and 0RMROD 1975a, ADEDIPE et al. 1976). In tropical areas, environmental factors, particularly light intensity patterns during the year, play a major role in determining crop productivity differences in rainforest and savanna zones (KASSAM and KowAL 1973). In an earlier study, ADEDIPE and 0RMROD (1975 b) found that cultivars representative of the two magnitudes of fruit abscission responded differently to light intensity, in terms of growth characteristics, chlorophyll concentration and carbohydrate content. These responses were later substantially confirmed with mature plants (TARILA et al. 1977). Such studies, which indicated marked differences in responses to light intensity, suggested that cultivars of cowpea might differ in their stomatal characteristics, since it is known that light influences stomatal behaviour in many species (CuTTER 1971). The objectives of the present study were, therefore, to (a) provide hitherto lacking data on stomatal characteristics of the cowpea plant, and (b) relate the stomatal charac-
542
A. G. J. TARILA, D.P. 0RMROD, and N. 0. ADEDIPE
teristic responses to light intensity in two cowpea cultivars representative of two premature abscission magnitudes. This study, therefore, forms part of continuing physiological studies of the premature fruit abscission problem. Plant cultivation Seeds of cowpea (Vigna unguiculata L.) cultivars "Adzuki" and "Early Ramshorn" were sown in flats containing sand and germinated at 30/25 ± 1 oc day/night temperatures in "Controlled Environment" Model EY 15 growth chambers. Ten days after planting, when the first tri-foliate leaves were about to emerge, the seedlings were selected for uniformity and transplanted one plant to a 23 em dia. plastic pot containing 7.7 kg of sand. The plants were grown at two light intensities (7 and 27 klux), measured with a "Weston" Model 756 light meter. Light was provided from an adjustable height panel. Wattage inputs were 79% fluorescent (cool-white) and 21% incandescent. Plants were re-randomized in the cabinet, once a week, to minimize differences in intra-chamber micro-environments. Light duration was 12 h and relative humidity 85 ± 5 %· The plants were watered thrice daily with 40 ml deionized water, and fed once daily with 500 ml Hoagland's solution containing 62 ppm P. Stomatal impressions and measurements At 6 weeks, cellulose acetate film impressions of both surfaces of the terminal leaflets were taken immediately after the first watering, according to the description by RASCHKE (1970) and modifications by KHATAMIAN et al. (1973). Impressions were taken in situ from 6 terminal leaflets on 6 consecutive nodes of each plant, starting from the 4th node basipetally. Stomatal distribution, expressed as the number of stomata per square centimeter, was computed, and mean stomatal apertures and stomatal lengths calculated from observations of twenty stomata on each leaflet, using an eyepiece micrometer. The data were subjected to analysis of variance (2 x 7 factorial), and differences between treatment means detected by the Duncan's multiple range test.
Stomata were far more numerous on the abaxial surface (Fig. 1 A, C) than on the adaxial surface (Fig. 1 B, D). At the high light intensity of 27 klux, stomatal density was greater, particularly on the abaxial surface, in Adzuki (Fig. 1 C) than in Early Ramshorn (Fig.1A). Based on these observations, detailed studies of stomatal responses to light intensities were restricted to the abaxial surface. High light intensity increased stomatal length (Fig. 2A), stomatal width (Fig. 2B) and stomatal density (Fig. 2C), in both cultivars. However, the magnitudes of such increases depended on the cultivar. There was a significant interaction between light intensity and cultivar, in regard to stomatal density. While Early Ramshorn had more stomata per unit than Adzuki under low light intensity, Adzuki had a greater density at the high light intensity (Fig. 2 C). The magnitudes of the increases in stomatal density due to the high light intensity were 119% in Adzuki, and 28% in Early Ramshorn. The values for stomatal length were 22 and 29% (Fig. 2 A), and for the width, 27 and 50% (Fig. 2 B), in Adzuki and Early Ramshorn, respectively. The ratio of stomatal length to width was lower in Adzuki (59) than in Early Ramshorn (65) at low light intensity, but the cultivars were similar (57 and 56) at high light intensity. The observation that there were more stomata per unit area on the abaxial surface of cowpea leaves is in agreement with reports for a wide variety of species (CuTTER 1971). Increase in stomatal density at the high light intensity also supports the results obtained with other species (ADEDIPE et al. 1973, PENFOUND 1931). The present study shows the
Stomutal Response to Light Intensity
543
cowpea plant to resp:md to light intensity, in terms of stomatal pore size, in the same direction as, but differing in magnitude from other species (AnEDIPE et al. 1973, DuGGER et al. 1961, HEATH 1959, WENT 1944). Of the 3 stomatal characteristics (length, width, density), the two cultivars differed significantly only in stomatal density (Fig. 2). It is noteworthy that these stomatal density responses are similar to the relative dry matter distribution (particularly in terms of seed weight as per cent of total dry matter) observed in an earlier study (TARILA et al. 1977). In the present study, at the low light intensity, Adzuki had a lower stomatal density than did Early Ramshorn, just as the seed weight as a per cent of t3tal plant weight was lower in Adzuki than in Early Ramshorn (TARILA et al. 1977). At the high light intensity, Adzuki had greater stomatal density than did Early Ramshorn; which, complements the dry matter distribution patterns. The study indicates that, while Early Ramshorn is more adapted to low light intensity in terms of the dry matter accumulated by the seed relative to other parts of the plant, it is limited by lack of stomatal responsiveness to high light intensity. Adzuki would,
Fig. 1. Distribution of stomata on the leaves of two cowpea cultivars at 27 klux light intensity. A: abaxial surface of Early Ramshorn; B: adaxial surface of Early Ramshorn; C: abaxial surface of Adzuki; D: adaxial surface of Adzuki (Magnification, X 300).
544
A. G. I. T \HIL,\, D.P. 0RMROD, and N. 0. ADEDIPE
120
A. Stoma al length
B. Stomatal width
"
"
07klux El27 klux
100
BO
Fig. 2. Stomatal characteristics of leaves of tu;o cowpea cultivars at two light intensities. Each bar is the mean of 6 plant replicates. Bars with the same letter within a stomatal characteristic and cultivar, are not significantly different at the 5% level.
therefore, be more adaptable to varied intensities of light. Consequently, it would be more useful for breeding purposes aimed at developing new cultivars suited to a range of ecological zones with different light intensities, for example, rainforest as well as savanna zones in which light intensity patterns vary considerably during the year (KASSAM and KowAL 1973). This stomatal responsiveness of Adzuki to increasing light intensity is an additional attribute in its physiological efficiency in P utilization (ADEDIPE and Ommon 1975 b, TARILA et al. 1977), as well as on a more even distribution of assimilates in fruits of different ontogeny, which factor bas been found to be associated with a lower degree of premature abscission of young fruits (ADEDIPE and 0RMROD 1975a, ADEDIPE et al. 1976). The seeds used in this study were kindly supplied by the National Cereals Research Institute, Ibadan, Nigeria.
References ADEDIPE, N. 0., FLETCHER, R. A., and 0RMROD, D. P.: Distribution of 14 C-assimilates in the cowpea (Vigna unguiculata L.) in relation to fruit abscission and treatment with benzyladenine. Ann. Bot. 40, 731-737 (1976). - KHATAMIAN, H., and 0RMROD, D.P.: Stomatal regulation of ozone phytotoxicity in tomato. Z. Pflanzenphysiol. 68, 323-328 (1973). - and 0RMROD, D.P.: Absorption of foliar-applied 32 P bY successive leaves, and distribution patterns in relation to early fruiting and abscission in the cowpea (Vigna unguiculata L.). Ann. Bot. 39, 639-646 (1975a). - - Effects of light intensity on growth and on chlorophyll, carbohydrate and phosphorus contents of the cowpea (Vigna unguiculata L.). Biochem. Physiol. Pflanzen 167, 301-309 (1975 b). Cu'I'TER, E. G.: Plant Anatomy. Part II. Edward Arnold (Pub.) Ltd., London, 343 pp. (1971).
Stomatal Response to Light Intensity
545
DuGGER, W. M., TAYLOR, 0. C., CARDIFF, E., and THOMPSO='I, C. R.: Stomatal action in plants as related to damage from photochemical oxidants. Plant Physiol. 37, 487-491 (19Gl). HEATH, 0. V. S.: Light and carbon dioxide in stomatal movements. In: Encycl. Plant Physiol. Vol. 17, 415-465 (1959). KAssAM, A. H., and KowAL, J. M.: Productivity of crops in the savanna and rainforest zones in Nigeria. Savanna 2, 39-49 (1973). KHATAMIAN, H., ADEDIPE, N. 0., and OR~mon, D.P.: Soil-plant-water aspects of ozone phytotoxicity in tomato plants. Plant Soil 38, 531-541 (1973). 0JEHOMON, 0. 0.: Flowering, fruiting production and abscission in cowpea, Vigna unguiculata (L.) Walp. J. West Afr. Sci. Ass. 13, 227-234 (1968). - Effect of continuous removal of open flowers on the seed yield of two varieties of cowpea, Vigna unguiculata (L.) Walp. J. Agr. Sci., Camb. 74, 375-381 (1970). PENFOUND, W. T.: Plant anatomy as conditioned by light intensity and soil moisture. Amer. J. Bot. 18, 558-572 (1931). RASCHKE, K.: Stomatal responses to pressure changes and interruptions in the water supply of detached leaves of Zea mays L. Plant Physiol. 41), 415-423 (1970). TARILA, A. G. 1., 0RMROD, D.P., and ADEDIPE, N. 0.: Effects of phosphorus nutrition and light intensity on growth and development of the cowpea (Vigna unguiculata L.). Ann. Bot. 41, 75-83 (1977). WENT, F. W.: Plant growth under controlled conditions. III. Correlation between various physiological processes and growth in the tomato plant. Amer. J. Bot. 31, 537-596 (1944). Received November 2, 1977. Authors' address: Mr. A. G. I. TARILA and Dr. D.P. 0RMROD, Department of Horticultural Science, University of Guelph, Guelph, Ontario (Canada) and Dr. N. 0. AnEDIPE, Department of Agricultural Biology, University of Ibadan, lbadan (Nigeria).
Verantwortlich fiir die Redaktion: Prof. Dr. B. Parthier, 401 Halle (Saale). Ver·ag: VEB Gustav Fischer Verlag, 69 Jena, Villengang 2, Telefon 2 7332. Satz und Druck: Druckerei ,Magnus Poser 69 Jena.- Veroffentlicht unter der Lizenznummer 1067 des Presseamtes beim Vorsitzenden des Ministerrates der Deutschen Demokratischen Republik. Aile Rechte beim Verlag. Naohdruck (auch auszugsweise) nur mit Genehmigung des Verlages und des Verfassers sowie mit Angabe der Quelle gestattet. Printed in the German Democratic Republic. Artikel·Nr. (EDV) 5~916