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Vegetative growth and nutritional status as influenced by auxins and gibberellic acid, and their effect on fruit yield in lemon
Scientia Horticulturae, 26 (1985) 25--33
25
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
VEGETATIVE GROWTH AND NUTRITIONAL STATUS AS I N F L U E N C E D BY A U X I N S A N D G I B B E R E L L I C A C I D , A N D T H E I R EFFECT ON FRUIT YIELD IN LEMON
G.H.V. RATNA BABU and M.L. LAVANIA
Department of Horticulture, G.B. Pant University of Agriculture and Technology, Pantnagar, U.P. (India) Research Paper No. 3260 (Accepted for publication 27 November 1984)
ABSTRACT Ratna Babu, G.H.V. and Lavania, M.L., 1985. Vegetative growth and nutritional status as influenced by auxins and gibberellic acid, and their effect on fruit yield in lemon. Scientia Hortic., 26: 25--33. Application of 2,4-D and 2,4,5-T at concentrations ranging from 5 to 20 mg 1-1 to 5-year-old 'Pant L e m o n - l ' (Citrus limon Burro) trees reduced the vegetative growth in terms of height, spread, shoot length, number and size of the leaves in the autumn flush. Various NAA treatments (5--20 mg 1-1), however, enhanced growth, but not to the extent that was observed after GA 3 treatments. Application of GA~ at 10--40 mg 1-1 significantly enhanced all aspects of growth, and the effects were most pronounced at 20 and 40 mg 1-1. Nutritional status of the leaves showed a slight variation in relation to vegetative growth under various treatments. Some 2,4-D- and 2,4,5-T-treatments increased the fruit yield over the control, which could suggest mobilization of foods even at the expense of reduced vegetative growth. On the other hand, NAA, particularly at 10 mg 1-l, increased both vegetative growth and yield, suggesting that the transport of the photosynthates from the leaves to the fruits was not at the expense of new growth extension. Due to excessive growth enhancement under higher concentrations of GA s (20 and 40 mg 1-1), comparatively fewer nutrients were translocated to the fruit "sinks", thereby resulting in a non-significant decrease in yield. Keywords: auxins; flush; gibberellic acid ; mobilization; nutritional status; sink; vegetative growth; yield. ABBREVIATIONS CD = confidence difference; 2 , 4 - D = 2,4-dichlorophenoxyacetic acid; GA 3 = gibberellic acid; IAA = indole-3-acetic acid; NAA = Naphthaleneacetic acid; NS ffi non-significant; 2,4,5-T ffi 2,4,5-trichlorophenoxyacetic acid.
0304-4238/85/$03.30
© 1985 Elsevier Science Publishers B.V.
26 INTRODUCTION In citrus, vegetative growth occurs in 2--4 successive periods per year, known as flushes (Coit, 1919), and alternating with rest periods. Although the surface layers of the immature fruits contain chlorophyll and photosynthesis takes place in them, the quantity of photosynthates produced has been shown to be less than that required to replace the energy used in respiration (Bean and Todd, 1960; Todd et al., 1961) and thus fruit growth depends on the leaves (Kidd and West, 1947). As a result, a growing fruit becomes an active metabolic "sink" into which nutrients flow, even at the expense o f greatly curtailed vegetative growth (Loomis, 1953; Leonard, 1962). Thus a tree should put forth an o p t i m u m vegetative growth (leaf-to-fruit ratio) such that a major part of the assimilates is transported to the fruits and the remaining part is utilized for the production of new growth and other metabolic functions of the tree. Since the nutritional status within the tree also varies rapidly and significantly in view of the growth and fruiting, studies on the leaf nutrient status also provide a clear picture of the growth and fruiting behaviour of that particular tree. Various plant growth regulators were reported to act as "mobilizers" of nutrients and other substances from source to sink (Rood and Hamner, 1954; Moore, 1957; Crane, 1964) and also to affect mineral uptake and distribution within the plant (Brunstetter et al., 1948; Linck and Swanson, 1960; Bose and Hamner, 1961; Garcia and Guardiola, 1981), thus having a profound influence on the vegetative growth and fruiting of the tree. The fruit yield obtained in the a u t u m n flush of 'Pant L e m o n - l ' (flowering in O c t o b e r - N o v e m b e r ) under Tarai conditions of U.P. is low, but the fruits are harvested during the hotter m o n t h s (February--March) when there is high market demand for pickling and beverage production. Keeping the economic importance of this flush in view, attempts were made in the present investigation to increase the fruit yield with the aid of various plant growth regulator sprays. Whether an increase in yield by these growth regulators is at the expense of new vegetative growth at the end of the season, such that the succeeding crop may suffer to some extent, was also investigated. MATERIALS AND METHODS A field study was conducted on 5-year-old 'Pant L e m o n - l ' trees of uniform size and vigour in a randomized block design. There were in total 13 treatments each replicated 4 times. A single tree was taken as an experimental unit. The treatments comprised sprays with 2,4-D, 2,4,5-T and NAA each at 5, 10 and 20 mg 1-1 and GA3 at 10, 20 and 40 mg 1-1. Water spray was given as the control treatment. The sprays were given three times at fortnightly intervals starting from the third week of October, and their effects on autumn flush growth and fruiting were studied. Height and spread of the trees were measured at monthly intervals. T w e n t y shoots emerging within a
27 period of 7 days were tagged for observations on shoot length and leaf number. The areas of 25 leaves selected from the tagged shoots were measured with a planimeter. The total chlorophyll content of the leaves from the tagged shoots was measured by the method of Arnon (1948) with a SPECKOL spectrophotometer and expressed as mg g-1 of leaf on fresh mass basis. The total fruit yield from each tree was recorded at the end of the season. For the study of nutritional status, samples of 50 leaves, 5 months old, from the tagged shoots were collected from each tree. The nitrogen content was estimated by the Micro-Kjeldahl distillation method of Jackson (1960). For the determination of various nutrients, the dried leaf samples were digested in a ternary acid mixture (nitric acid:sulphuric acid:perchloric acid, 10:1:3 v/v/v) and a diacid mixture (nitric acid:perchloric acid, 10:3 v/v) separately. Phosphorus was estimated by the "Vanadomolybdophosphoric Yellow" colour method (Jackson, 1960), and potassium with TYPE 121 flame photometer from the ternary acid digest extracts. Calcium and magnesium contents of the leaves were determined from the diacid digests to avoid precipitation of calcium as calcium sulphate, using the method suggested by Jackson (1960). The micronutrients zinc, manganese and copper contents were determined by using a Higler and Watts Atomspek at the wavelengths of 213.9, 279.8 and 324.7 nm, respectively, as suggested by Issac and Kerber (1971). RESULTS Vegetative g r o w t h (Table I). -- 2,4-D and 2,4,5-T at most concentrations decreased shoot length. The minimum shoot length was observed after treatment with 20 mg 1-1 followed by 10 mg 1-1 of 2,4-D. However, NAA at 10 and 20 mg 1-1 and GA3 at all concentrations increased the shoot length significantly over control. Due to a decrease in the number of newly emerging vegetative shoots and also to their growth in terms of both length and diameter after 2,4-D and 2,4,5-T treatments, the overall tree height and spread at the end of the season tended to be reduced. NAA (10 and 20 mg 1-1) and GA3 (10--40 mg 1-1), on the other hand, enhanced tree growth both in terms of height and spread. The numbers of leaves per shoot in 2,4-D- and 2,4,5-T treated trees were reduced at 10 and 20 mg 1-1. Some of these leaves dropped prematurely, so that the differences in the number of leaves/shoot were more pronounced at the end of the season. The individual leaf areas also decreased, hence the total photosynthetic area/shoot was reduced. Mature leaves showed no apparent response, but young expanding leaves showed downward and inward rolling of margins. NAA (5 and 10 mg 11), on the other hand, increased the number of leaves/shoot, but not to the extent that was observed under higher concentrations of GA3. The individual leaf area was significantly
28
reduced at 20 mg-INAA, but not to the extent that was recorded with 20 mg 1-1 of the other t w o auxins. GA3 treatments resulted in the highest number of leaves/shoot and with maximum individual leaf area, so that the total photosynthetic area/shoot was maximum at 40 mg 1-1 concentration. Total chlorophyll c o n t e n t per gram fresh leaf increased with an increase in the concentration of the three auxins, but this increase was significant only at 20 mg 1-1 concentration. GA3 at all concentrations showed a significant decrease in chlorophyll content. Fruit yield (Table I). -- The total fruit yield per tree was comparatively low under all the treatments (Table I) because the autumn flush yields a lean crop. This is due to high utilization of the reserve food o f the tree for the spring and rainy-season crops, and comparatively little f o o d is diverted for the production o f the autumn flush crop. Further, the trees suffered a heavy fruit drop during the period of fruit growth, and the total fruit drop under the various treatments ranged from 61 to 84% (data not presented). In spite of all this, the growth-regulator-treated trees in several cases produced significantly higher fruit yield than the control trees. The lower concentrations were particularly effective. The highest fruit yield was recorded with 10 mg 1-1 NAA, followed b y 10 mg 1-1 2,4-D. It was observed that the increase in fruit yield was due to increases in both the number of marketable fruits harvested and the individual fruit weight and size. Nutritional status o f the leaves (Table II). -- Significant increases in leaf nitrogen were observed after treatments with 5 and 20 mg 1-1 each o f 2,4-D and 2,4,5-T, 5 and 10 mg 1-1 NAA, and 20 and 40 mg 1-1 GA3. No consistent effects on leaf phosphorus and potassium contents were observed. 2,4-D, 2,4,5-T at 5 mg 1-1 and GA3 at all concentrations decreased the leaf calcium significantly over control. With the increase in the concentration of GA3, there was a gradual decrease in the leaf calcium content. Treatments of 5 mg 1-1 each of 2,4,5-T, NAA and 20 and 40 mg 1-1 GA3 increased the leaf magnesium content. Leaf zinc content was often found to be comparatively higher in the growth-regulator-treated trees, while leaf manganese and copper contents were n o t appreciably influenced by various treatments.
DISCUSSION Shoot tips, including the young and enlarging leaves, are the centres o f most abundant auxin synthesis in w o o d y plants (Moore, 1979), and as the elongation o f internodes begins, auxin production b y the tip reaches a peak (Leopold, 1960). Thus, it m a y be assumed that the stem tip normally supplies sufficient auxin to the elongating internodes to maintain an optimal auxin concentration in those tissues for their growth (Wareing and Phillips, 1970). Thimann (1937) observed that stem segments showed increased growth with increasing concentrations of IAA up to 10 mg 1-I, and that
29 higher concentrations inhibited rather than promoted the growth. Since in this study these auxins were applied to the trees when the shoots had just started to emerge, the treatments might have augmented the already available optimal concentrations of endogenous auxin (IAA) making it supraoptimal, thus inhibiting growth. Further, auxin as a hormone is somewhat autocatalytic (Addicott, 1970). Thus it appears that too high an auxin concentration disorganizes the delicate machinery of growth. However, the promotion of growth by NAA, which is also an auxin, suggests that not all auxins are equally toxic, and a plant species may also show varying sensitivities to different auxins. Further, it is known that to produce a response of auxin, higher concentrations of NAA are necessary than of 2,4-D. Van Overbeek et al. (1951) proposed that 2,4-D toxicity may be a result of alteration of metabolism such that unsaturated toxic lactones like coumarin, scopoletin, etc., are accumulated in the tissues. Increased shoot length after GA3 treatment may be due to the increased length of certain internodes which were either in the process of elongation at the time of treatment or were differentiated soon thereafter. If the effect of GAs is auxin-mediated through synergism (Brian, 1958; Galston and Warburg, 1959), then the increased shoot growth by GAs -- even when optimal concentrations of auxin (IAA) are synthesized in the shoots -- supports the fact that gibberellins can still promote the growth when optimal concentrations of auxin are present (Hillman and Purves, 1961). On the other hand, Nanda and Purohit (1965) explained the enhancement of growth by GAs in relation to the mobilization of reserve starch. Due to enhanced mobilization by GAs, large amounts of food material are available over a shorter period, causing a spurt in the growth processes. The lower chlorophyll content of GAs-treated leaves has been explained as a "dilution" of the same amount of chlorophyll over the area of a larger leaf as the chlorophyll synthesis is not able to keep pace with excessive laminal growth (Wolf and Haber, 1960). The same explanation holds true for the higher chlorophyll content in the auxin-treated leaves, as their leaf area was decreased by these treatments, particularly at the highest concentration. The increased nitrogen uptake of GAs-treated leaves is the natural consequence of increased number of leaves during the early stages of shoot growth. The decreased calcium levels in the GA3-treated leaves may be caused by rapid utilization for the extensive growth of shoot and leaf, since calcium pectate is the most important constituent of the middle lamella of the cell wall. The increase in fruit yield by some of the auxin .treatments and by the lowest concentration of GA3 suggests that 2,4-D and 2,4,5-T acted as efficient mobilizers of nutrients to the fruit sinks, but that this mobilization was at the expense of greatly curtailed vegetative growth. However, if the tree is too exhausted for the fruiting in a flush, as was particularly observed under the higher concentrations of 2,4-D and 2,4,5-T, the production of further new growth and fruiting for the succeeding flush may suffer. NAA (5 and
30
J~
•~ ~
C4=4
oO C~
OO
CL,~
o~ ~
0
0
31
0 ~q
~
Z
~
0
0
32
10 mg 1-1) and GA3 (10 mg 1-1), conversely, increased both vegetative growth and fruit yield, suggesting that the mobilization was not at the expense of vegetative growth. Similarly, the slight non-significant decrease in yield observed under higher concentrations of GA3 can be explained on the basis of initial rapid mobilization of food reserves from the tree during the rapid extension of vegetative growth. ACKNOWLEDGEMENTS
The authors are grateful to Dr. S.C. Modgal, Director Research, and Dr. R.P. Singh, Professor and Head, Department of Horticulture, for providing necessary facilities and provocation during the period of this investigation.
REFERENCES
Addicott, F.T., 1970. Plant hormones in the control o f abscission. Biol. Rev., 45: 485-524. Arnon, D.I., 1948. Copper enzymes in isolated chloroplast. Polyphenoloxidase in Beta vulgaris. Plant Physiol., 24: 1--15. Bean, R.C. and Todd, G.W., 1960. Photosynthesis and respiration in developing fruits. I. 14CO2 uptake by young oranges in light and dark. Plant Physiol., 35: 425--429. Bose, P.C. and Hamner, C.L., 1961. Effect o f various concentrations of gibberellin on growth, flowers, fruits, mineral composition and mineral uptake of t o m a t o plants. Indian J. Agric. Sci., 31: 216--225. Brian, P.W., 1958. Role of gibberellin-like hormones in regulation o f plant growth and flowering. Nature (London), 181: 2122--2123. Brunstetter, B.C., Myers, A.T., Mitchell, J.W., Stewart, W.S. and Kaufman, M.W., 1948. Mineral composition o f bean stems treated with 3-indoleacetic acid. Bot. Gaz., 109: 268--276. Coit, J.E., 1919. Automatic disbudding in citrus. Calif. Citrogr., 5: 37. Crane, J.C., 1964. Growth substances in fruit setting and development. Annu. Rev. Plant Physiol., 16: 303--326. Galston, A.W. and Warburg, H., 1959. An analysis of auxin--gibberellin interaction in pea stem tissue. Plant Physiol., 34: 16--22. Garcia, L.A. and Guardiola, J.L., 1981. Effect o f gibberellic acid on ion uptake selectivity in pea seedlings. Planta, 153: 494--496. Hillman, W.S. and Purves, W.K., 1961. Does gibberellin act through an auxin-mediated mechanism. In: R.M. Klein (Editor), Plant Growth Regulation. Iowa State University Press, Ames, IA, pp. 589--600. Issac, R.A. and Kerber, J.O., 1971. Atomic absorption and flame p h o t o m e t r y - techniques and uses in soil, plant and water analyses. In: L.M. Walsh (Editor), Instrumental Methods for Analyses of Soil and Plant Tissue. American Soil Science Society, Madison, WI, pp. 18--37. Jackson, M.L., 1960. Soil Chemical Analysis. Constable, London, 498 pp. Kidd, F. and West, C., 1947. A note on the assimilation o f carbon dioxide by apple fruits during gathering. New Phytol., 46: 274--275. Leonard, E.R., 1962. Inter-relations o f vegetative and reproductive growth, with special reference to indeterminate plants. Bot. Rev., 28: 353--410. Leopold, A.C., 1960. Auxins and Plant Growth. University o f California Press, Berkeley and Los Angeles, 354 pp.
33 Linck, A.J. and Swanson, C.A., 1960. A study of several factors affecting the distribution of phosphorus-32 from the leaves o f P i s u m sativum. Plant Soil, 12: 57---68. Loomis, W.E., 1953. Growth and Differentiation in Plants. Iowa State College Press, Ames, IA, 458 pp. Moore, J.F., 1957. N-l-naphthylphthalamic acid, N-m-tolylphthalamic acid and other growth regulators applied as whole plant sprays to field-grown tomatoes. Proc. Am. Soc. Hortic. Sci., 70: 350--356. Moore, T.C., 1979. Biochemistry and Physiology of Plant Hormones. Narosa Publishing, New Delhi, 274 pp. Nanda, K.K. and Purohit, A.N., 1965. Effect of gibberellin on mobilization of reserve food and its correlation with extension growth. Planta, 66: 121--125. Rood, P. and Hamner, C.L., 1954. The use of 4-phthalimido-2,6-dimethylpyrimidine as a plant growth regulator. Proc. Am. Soc. Hortic. Sci., 63: 495--500. Thimann, K.V., 1937. On the nature of inhibitions caused by auxin. Am. J. Bot., 24: 407--412. Todd, G.W., Bean, R.C. and Propst, B., 1961. Photosynthesis and respiration in developing fruits. II. Comparative rates at various stages of development. Plant Physiol., 36: 69--73. Van Overbeek, J., Blondeau, R. and Home, V., 1951. Difference in activity between 2,4-D and other auxins. Plant Physiol., 26: 687--696. Wareing, P.F. and Phillips, I.D.J., 1970. The Control of Growth and Differentiation in Plants. Pergamon, Oxford, 303 pp. Wolf, F.T. and Haber, A.H., 1960. Chlorophyll content of gibberellin-treated wheat seedlings. Nature (London), 186: 217--218.
25
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
VEGETATIVE GROWTH AND NUTRITIONAL STATUS AS I N F L U E N C E D BY A U X I N S A N D G I B B E R E L L I C A C I D , A N D T H E I R EFFECT ON FRUIT YIELD IN LEMON
G.H.V. RATNA BABU and M.L. LAVANIA
Department of Horticulture, G.B. Pant University of Agriculture and Technology, Pantnagar, U.P. (India) Research Paper No. 3260 (Accepted for publication 27 November 1984)
ABSTRACT Ratna Babu, G.H.V. and Lavania, M.L., 1985. Vegetative growth and nutritional status as influenced by auxins and gibberellic acid, and their effect on fruit yield in lemon. Scientia Hortic., 26: 25--33. Application of 2,4-D and 2,4,5-T at concentrations ranging from 5 to 20 mg 1-1 to 5-year-old 'Pant L e m o n - l ' (Citrus limon Burro) trees reduced the vegetative growth in terms of height, spread, shoot length, number and size of the leaves in the autumn flush. Various NAA treatments (5--20 mg 1-1), however, enhanced growth, but not to the extent that was observed after GA 3 treatments. Application of GA~ at 10--40 mg 1-1 significantly enhanced all aspects of growth, and the effects were most pronounced at 20 and 40 mg 1-1. Nutritional status of the leaves showed a slight variation in relation to vegetative growth under various treatments. Some 2,4-D- and 2,4,5-T-treatments increased the fruit yield over the control, which could suggest mobilization of foods even at the expense of reduced vegetative growth. On the other hand, NAA, particularly at 10 mg 1-l, increased both vegetative growth and yield, suggesting that the transport of the photosynthates from the leaves to the fruits was not at the expense of new growth extension. Due to excessive growth enhancement under higher concentrations of GA s (20 and 40 mg 1-1), comparatively fewer nutrients were translocated to the fruit "sinks", thereby resulting in a non-significant decrease in yield. Keywords: auxins; flush; gibberellic acid ; mobilization; nutritional status; sink; vegetative growth; yield. ABBREVIATIONS CD = confidence difference; 2 , 4 - D = 2,4-dichlorophenoxyacetic acid; GA 3 = gibberellic acid; IAA = indole-3-acetic acid; NAA = Naphthaleneacetic acid; NS ffi non-significant; 2,4,5-T ffi 2,4,5-trichlorophenoxyacetic acid.
0304-4238/85/$03.30
© 1985 Elsevier Science Publishers B.V.
26 INTRODUCTION In citrus, vegetative growth occurs in 2--4 successive periods per year, known as flushes (Coit, 1919), and alternating with rest periods. Although the surface layers of the immature fruits contain chlorophyll and photosynthesis takes place in them, the quantity of photosynthates produced has been shown to be less than that required to replace the energy used in respiration (Bean and Todd, 1960; Todd et al., 1961) and thus fruit growth depends on the leaves (Kidd and West, 1947). As a result, a growing fruit becomes an active metabolic "sink" into which nutrients flow, even at the expense o f greatly curtailed vegetative growth (Loomis, 1953; Leonard, 1962). Thus a tree should put forth an o p t i m u m vegetative growth (leaf-to-fruit ratio) such that a major part of the assimilates is transported to the fruits and the remaining part is utilized for the production of new growth and other metabolic functions of the tree. Since the nutritional status within the tree also varies rapidly and significantly in view of the growth and fruiting, studies on the leaf nutrient status also provide a clear picture of the growth and fruiting behaviour of that particular tree. Various plant growth regulators were reported to act as "mobilizers" of nutrients and other substances from source to sink (Rood and Hamner, 1954; Moore, 1957; Crane, 1964) and also to affect mineral uptake and distribution within the plant (Brunstetter et al., 1948; Linck and Swanson, 1960; Bose and Hamner, 1961; Garcia and Guardiola, 1981), thus having a profound influence on the vegetative growth and fruiting of the tree. The fruit yield obtained in the a u t u m n flush of 'Pant L e m o n - l ' (flowering in O c t o b e r - N o v e m b e r ) under Tarai conditions of U.P. is low, but the fruits are harvested during the hotter m o n t h s (February--March) when there is high market demand for pickling and beverage production. Keeping the economic importance of this flush in view, attempts were made in the present investigation to increase the fruit yield with the aid of various plant growth regulator sprays. Whether an increase in yield by these growth regulators is at the expense of new vegetative growth at the end of the season, such that the succeeding crop may suffer to some extent, was also investigated. MATERIALS AND METHODS A field study was conducted on 5-year-old 'Pant L e m o n - l ' trees of uniform size and vigour in a randomized block design. There were in total 13 treatments each replicated 4 times. A single tree was taken as an experimental unit. The treatments comprised sprays with 2,4-D, 2,4,5-T and NAA each at 5, 10 and 20 mg 1-1 and GA3 at 10, 20 and 40 mg 1-1. Water spray was given as the control treatment. The sprays were given three times at fortnightly intervals starting from the third week of October, and their effects on autumn flush growth and fruiting were studied. Height and spread of the trees were measured at monthly intervals. T w e n t y shoots emerging within a
27 period of 7 days were tagged for observations on shoot length and leaf number. The areas of 25 leaves selected from the tagged shoots were measured with a planimeter. The total chlorophyll content of the leaves from the tagged shoots was measured by the method of Arnon (1948) with a SPECKOL spectrophotometer and expressed as mg g-1 of leaf on fresh mass basis. The total fruit yield from each tree was recorded at the end of the season. For the study of nutritional status, samples of 50 leaves, 5 months old, from the tagged shoots were collected from each tree. The nitrogen content was estimated by the Micro-Kjeldahl distillation method of Jackson (1960). For the determination of various nutrients, the dried leaf samples were digested in a ternary acid mixture (nitric acid:sulphuric acid:perchloric acid, 10:1:3 v/v/v) and a diacid mixture (nitric acid:perchloric acid, 10:3 v/v) separately. Phosphorus was estimated by the "Vanadomolybdophosphoric Yellow" colour method (Jackson, 1960), and potassium with TYPE 121 flame photometer from the ternary acid digest extracts. Calcium and magnesium contents of the leaves were determined from the diacid digests to avoid precipitation of calcium as calcium sulphate, using the method suggested by Jackson (1960). The micronutrients zinc, manganese and copper contents were determined by using a Higler and Watts Atomspek at the wavelengths of 213.9, 279.8 and 324.7 nm, respectively, as suggested by Issac and Kerber (1971). RESULTS Vegetative g r o w t h (Table I). -- 2,4-D and 2,4,5-T at most concentrations decreased shoot length. The minimum shoot length was observed after treatment with 20 mg 1-1 followed by 10 mg 1-1 of 2,4-D. However, NAA at 10 and 20 mg 1-1 and GA3 at all concentrations increased the shoot length significantly over control. Due to a decrease in the number of newly emerging vegetative shoots and also to their growth in terms of both length and diameter after 2,4-D and 2,4,5-T treatments, the overall tree height and spread at the end of the season tended to be reduced. NAA (10 and 20 mg 1-1) and GA3 (10--40 mg 1-1), on the other hand, enhanced tree growth both in terms of height and spread. The numbers of leaves per shoot in 2,4-D- and 2,4,5-T treated trees were reduced at 10 and 20 mg 1-1. Some of these leaves dropped prematurely, so that the differences in the number of leaves/shoot were more pronounced at the end of the season. The individual leaf areas also decreased, hence the total photosynthetic area/shoot was reduced. Mature leaves showed no apparent response, but young expanding leaves showed downward and inward rolling of margins. NAA (5 and 10 mg 11), on the other hand, increased the number of leaves/shoot, but not to the extent that was observed under higher concentrations of GA3. The individual leaf area was significantly
28
reduced at 20 mg-INAA, but not to the extent that was recorded with 20 mg 1-1 of the other t w o auxins. GA3 treatments resulted in the highest number of leaves/shoot and with maximum individual leaf area, so that the total photosynthetic area/shoot was maximum at 40 mg 1-1 concentration. Total chlorophyll c o n t e n t per gram fresh leaf increased with an increase in the concentration of the three auxins, but this increase was significant only at 20 mg 1-1 concentration. GA3 at all concentrations showed a significant decrease in chlorophyll content. Fruit yield (Table I). -- The total fruit yield per tree was comparatively low under all the treatments (Table I) because the autumn flush yields a lean crop. This is due to high utilization of the reserve food o f the tree for the spring and rainy-season crops, and comparatively little f o o d is diverted for the production o f the autumn flush crop. Further, the trees suffered a heavy fruit drop during the period of fruit growth, and the total fruit drop under the various treatments ranged from 61 to 84% (data not presented). In spite of all this, the growth-regulator-treated trees in several cases produced significantly higher fruit yield than the control trees. The lower concentrations were particularly effective. The highest fruit yield was recorded with 10 mg 1-1 NAA, followed b y 10 mg 1-1 2,4-D. It was observed that the increase in fruit yield was due to increases in both the number of marketable fruits harvested and the individual fruit weight and size. Nutritional status o f the leaves (Table II). -- Significant increases in leaf nitrogen were observed after treatments with 5 and 20 mg 1-1 each o f 2,4-D and 2,4,5-T, 5 and 10 mg 1-1 NAA, and 20 and 40 mg 1-1 GA3. No consistent effects on leaf phosphorus and potassium contents were observed. 2,4-D, 2,4,5-T at 5 mg 1-1 and GA3 at all concentrations decreased the leaf calcium significantly over control. With the increase in the concentration of GA3, there was a gradual decrease in the leaf calcium content. Treatments of 5 mg 1-1 each of 2,4,5-T, NAA and 20 and 40 mg 1-1 GA3 increased the leaf magnesium content. Leaf zinc content was often found to be comparatively higher in the growth-regulator-treated trees, while leaf manganese and copper contents were n o t appreciably influenced by various treatments.
DISCUSSION Shoot tips, including the young and enlarging leaves, are the centres o f most abundant auxin synthesis in w o o d y plants (Moore, 1979), and as the elongation o f internodes begins, auxin production b y the tip reaches a peak (Leopold, 1960). Thus, it m a y be assumed that the stem tip normally supplies sufficient auxin to the elongating internodes to maintain an optimal auxin concentration in those tissues for their growth (Wareing and Phillips, 1970). Thimann (1937) observed that stem segments showed increased growth with increasing concentrations of IAA up to 10 mg 1-I, and that
29 higher concentrations inhibited rather than promoted the growth. Since in this study these auxins were applied to the trees when the shoots had just started to emerge, the treatments might have augmented the already available optimal concentrations of endogenous auxin (IAA) making it supraoptimal, thus inhibiting growth. Further, auxin as a hormone is somewhat autocatalytic (Addicott, 1970). Thus it appears that too high an auxin concentration disorganizes the delicate machinery of growth. However, the promotion of growth by NAA, which is also an auxin, suggests that not all auxins are equally toxic, and a plant species may also show varying sensitivities to different auxins. Further, it is known that to produce a response of auxin, higher concentrations of NAA are necessary than of 2,4-D. Van Overbeek et al. (1951) proposed that 2,4-D toxicity may be a result of alteration of metabolism such that unsaturated toxic lactones like coumarin, scopoletin, etc., are accumulated in the tissues. Increased shoot length after GA3 treatment may be due to the increased length of certain internodes which were either in the process of elongation at the time of treatment or were differentiated soon thereafter. If the effect of GAs is auxin-mediated through synergism (Brian, 1958; Galston and Warburg, 1959), then the increased shoot growth by GAs -- even when optimal concentrations of auxin (IAA) are synthesized in the shoots -- supports the fact that gibberellins can still promote the growth when optimal concentrations of auxin are present (Hillman and Purves, 1961). On the other hand, Nanda and Purohit (1965) explained the enhancement of growth by GAs in relation to the mobilization of reserve starch. Due to enhanced mobilization by GAs, large amounts of food material are available over a shorter period, causing a spurt in the growth processes. The lower chlorophyll content of GAs-treated leaves has been explained as a "dilution" of the same amount of chlorophyll over the area of a larger leaf as the chlorophyll synthesis is not able to keep pace with excessive laminal growth (Wolf and Haber, 1960). The same explanation holds true for the higher chlorophyll content in the auxin-treated leaves, as their leaf area was decreased by these treatments, particularly at the highest concentration. The increased nitrogen uptake of GAs-treated leaves is the natural consequence of increased number of leaves during the early stages of shoot growth. The decreased calcium levels in the GA3-treated leaves may be caused by rapid utilization for the extensive growth of shoot and leaf, since calcium pectate is the most important constituent of the middle lamella of the cell wall. The increase in fruit yield by some of the auxin .treatments and by the lowest concentration of GA3 suggests that 2,4-D and 2,4,5-T acted as efficient mobilizers of nutrients to the fruit sinks, but that this mobilization was at the expense of greatly curtailed vegetative growth. However, if the tree is too exhausted for the fruiting in a flush, as was particularly observed under the higher concentrations of 2,4-D and 2,4,5-T, the production of further new growth and fruiting for the succeeding flush may suffer. NAA (5 and
30
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OO
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31
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32
10 mg 1-1) and GA3 (10 mg 1-1), conversely, increased both vegetative growth and fruit yield, suggesting that the mobilization was not at the expense of vegetative growth. Similarly, the slight non-significant decrease in yield observed under higher concentrations of GA3 can be explained on the basis of initial rapid mobilization of food reserves from the tree during the rapid extension of vegetative growth. ACKNOWLEDGEMENTS
The authors are grateful to Dr. S.C. Modgal, Director Research, and Dr. R.P. Singh, Professor and Head, Department of Horticulture, for providing necessary facilities and provocation during the period of this investigation.
REFERENCES
Addicott, F.T., 1970. Plant hormones in the control o f abscission. Biol. Rev., 45: 485-524. Arnon, D.I., 1948. Copper enzymes in isolated chloroplast. Polyphenoloxidase in Beta vulgaris. Plant Physiol., 24: 1--15. Bean, R.C. and Todd, G.W., 1960. Photosynthesis and respiration in developing fruits. I. 14CO2 uptake by young oranges in light and dark. Plant Physiol., 35: 425--429. Bose, P.C. and Hamner, C.L., 1961. Effect o f various concentrations of gibberellin on growth, flowers, fruits, mineral composition and mineral uptake of t o m a t o plants. Indian J. Agric. Sci., 31: 216--225. Brian, P.W., 1958. Role of gibberellin-like hormones in regulation o f plant growth and flowering. Nature (London), 181: 2122--2123. Brunstetter, B.C., Myers, A.T., Mitchell, J.W., Stewart, W.S. and Kaufman, M.W., 1948. Mineral composition o f bean stems treated with 3-indoleacetic acid. Bot. Gaz., 109: 268--276. Coit, J.E., 1919. Automatic disbudding in citrus. Calif. Citrogr., 5: 37. Crane, J.C., 1964. Growth substances in fruit setting and development. Annu. Rev. Plant Physiol., 16: 303--326. Galston, A.W. and Warburg, H., 1959. An analysis of auxin--gibberellin interaction in pea stem tissue. Plant Physiol., 34: 16--22. Garcia, L.A. and Guardiola, J.L., 1981. Effect o f gibberellic acid on ion uptake selectivity in pea seedlings. Planta, 153: 494--496. Hillman, W.S. and Purves, W.K., 1961. Does gibberellin act through an auxin-mediated mechanism. In: R.M. Klein (Editor), Plant Growth Regulation. Iowa State University Press, Ames, IA, pp. 589--600. Issac, R.A. and Kerber, J.O., 1971. Atomic absorption and flame p h o t o m e t r y - techniques and uses in soil, plant and water analyses. In: L.M. Walsh (Editor), Instrumental Methods for Analyses of Soil and Plant Tissue. American Soil Science Society, Madison, WI, pp. 18--37. Jackson, M.L., 1960. Soil Chemical Analysis. Constable, London, 498 pp. Kidd, F. and West, C., 1947. A note on the assimilation o f carbon dioxide by apple fruits during gathering. New Phytol., 46: 274--275. Leonard, E.R., 1962. Inter-relations o f vegetative and reproductive growth, with special reference to indeterminate plants. Bot. Rev., 28: 353--410. Leopold, A.C., 1960. Auxins and Plant Growth. University o f California Press, Berkeley and Los Angeles, 354 pp.
33 Linck, A.J. and Swanson, C.A., 1960. A study of several factors affecting the distribution of phosphorus-32 from the leaves o f P i s u m sativum. Plant Soil, 12: 57---68. Loomis, W.E., 1953. Growth and Differentiation in Plants. Iowa State College Press, Ames, IA, 458 pp. Moore, J.F., 1957. N-l-naphthylphthalamic acid, N-m-tolylphthalamic acid and other growth regulators applied as whole plant sprays to field-grown tomatoes. Proc. Am. Soc. Hortic. Sci., 70: 350--356. Moore, T.C., 1979. Biochemistry and Physiology of Plant Hormones. Narosa Publishing, New Delhi, 274 pp. Nanda, K.K. and Purohit, A.N., 1965. Effect of gibberellin on mobilization of reserve food and its correlation with extension growth. Planta, 66: 121--125. Rood, P. and Hamner, C.L., 1954. The use of 4-phthalimido-2,6-dimethylpyrimidine as a plant growth regulator. Proc. Am. Soc. Hortic. Sci., 63: 495--500. Thimann, K.V., 1937. On the nature of inhibitions caused by auxin. Am. J. Bot., 24: 407--412. Todd, G.W., Bean, R.C. and Propst, B., 1961. Photosynthesis and respiration in developing fruits. II. Comparative rates at various stages of development. Plant Physiol., 36: 69--73. Van Overbeek, J., Blondeau, R. and Home, V., 1951. Difference in activity between 2,4-D and other auxins. Plant Physiol., 26: 687--696. Wareing, P.F. and Phillips, I.D.J., 1970. The Control of Growth and Differentiation in Plants. Pergamon, Oxford, 303 pp. Wolf, F.T. and Haber, A.H., 1960. Chlorophyll content of gibberellin-treated wheat seedlings. Nature (London), 186: 217--218.