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The influence of light period on carbon partitioning, translocation and growth in tomato
Scientia Horticulturae, 42 (1990) 75-83 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
75
T h e I n f l u e n c e o f L i g h t P e r i o d on C a r b o n P a r t i t i o n i n g , T r a n s l o c a t i o n a n d G r o w t h in Tomato*
S I T H E S W A R Y LOGENDRA, J A M E S D. P U T M A N and H A R R Y W. J A N E S
Department o/Horticulture and Forestry, Rutgers, The State University o/New Jersey, New Brunswick, N J 08903 (U.S.A.) (Accepted for publication 19 July 1989 )
ABSTRACT Logendra, S., Putman, J.D. and Janes, H.W., 1990. The influence of light period on carbon partitioning, translocation and growth in tomato. Scientia Hortic., 42: 75-83. Carbon partitioning, translocation and growth were examined in young tomato (Lycopersicon esculentum Mill. ) plants grown for 38 days under a 12-h light period and a photosynthetic photon flux density ( P P F D ) of 150 #moles m -2 s -1 and then transferred and grown under 8-, 16- or 20h light periods for an additional 7 days under similar PPFD. The third leaf from the base of the plant was used for carbon partitioning and translocation studies. The longer the length of the light period, the greater were the net assimilation rate (NAR), relative growth rate (RGR) and amount of carbon fixed and translocated. The shoot-to-root ratio and leaf area ratio (LAR) decreased with an increase in the light period. No significant differences were observed in photosynthetic rate among the three light treatments. From the total amount of carbon fixed, a certain amount stayed in the leaf independent of light period and the surplus was translocated. The 8-h light treatment had the highest accumulation rates of hexose, and starch. The starch accumulation rate in the 8- and 16-h light treatments were twice as high as that in the 20-h light treatment. The amount of hexose, sucrose and starch at any one time increased with an increase in the length of the light period. The carbon fixed in excess of demand was stored in the leaf as starch, and this accumulated over time in the 20-h light treatment. The leaves appeared to have a maximum capacity to store starch. Keywords: carbon partitioning; leaf area ratio; light period; Lycopersicon esculentum; net assimilation rate; relative growth rate; tomato; translocation. Abbreviations: L A R - - l e a f area ratio; N A R - - n e t assimilation rate; P P F D - - p h o t o s y n t h e t i c photon flux density; R G R - - relative growth rate. *New Jersey Agricultural Experiment Station Publication No. D - 12150-1-89.
0304-4238/90/$03.50
© 1990 Elsevier Science Publishers B.V.
76
S. LOGENDRAET AL.
INTRODUCTION
The effect of light duration on photosynthate partitioning in leaves is of considerable interest as it affects the amount of photosynthate that is available for translocation to the sink tissues. In many crop species the plants grown under short light periods have greater starch accumulation rates (Chatterton and Silvius, 1979, 1980b; Sicher et al., 1982; Robinson, 1984; Huber et al., 1984), starch content (Chatterton and Silvius, 1980a; Sicher et al., 1982; Robinson, 1984 ) and low sucrose content (Grange, 1985 ) when compared with those grown under long light periods. The high starch accumulation rates observed under short light periods are associated with low translocation rates (Chatterton and Silvius, 1980b; Huber et al., 1984) and decreased amounts of carbon translocated (Grange, 1985). It was suggested that, under short light periods plants accumulate higher amounts of starch to satisfy the carbohydrate requirement of the subsequent longer nights (Chatterton and Silvius, 1979). The relative growth of plant organs is also affected by the length of the light period as observed in soybeans (Chatterton and Silvius, 1979). Here the shoot growth was favored over the root growth under short light periods, suggesting that the photosynthates were preferentially used for the growth of the shoot under those conditions. The present study was initiated to examine the effect of duration of light on photosynthate partitioning and translocation in source leaves of young tomato plants and its overall influence on growth. MATERIALS AND METHODS
T r e a t m e n t s . - Seeds of tomato ( L y c o p e r s i c o n e s c u l e n t u m cultivar 'Dombito' ) were sown in vermiculite and germinated under mist in the greenhouse. When the first true leaf was about 0.5 cm long (approximately 2 weeks after sowing), uniform seedlings were transplanted into 10-cm pots containing a peat: vermiculite: perlite mix, 40: 40: 20 by volume with a pH of 6.5. To measure the growth parameters, seedlings were also grown in a hydroponic growing system containing a half-strength Hoagland solution as described by Hurewitz and Janes (1983). This system was employed to facilitate root weight determinations. All seedlings were grown in an environmentally controlled chamber with a 12 h day/12 h night cycle, a photosynthetic photon flux density (PPFD) of 150 #moles m - 2 s- 1, and a temperature of 26 ° C/23 ° C. The seedlings in the mix were irrigated twice weekly with half-strength Hoagland solution. Approximately 38 days after seeding, i.e. when the third leaf (from the base of plant) was about 70-80% expanded and was a net source (Ho and Shaw, 1976 ), the plants were moved into different growth chambers and were grown under 8-, 16- and 20-h light periods with a P P F D of 150 #moles m -2 s -1 for 7 days.
INFLUENCE OF LIGHT ON TOMATO
77
Observations were made on the third leaf, over a 24-h period starting with the onset of light on the eighth day after transfer. For each observation 4-6 plants were sampled and the data are the averages from two separate experiments. M e a s u r e m e n t o f n e t p h o t o s y n t h e s i s a n d translocation. - Net photosynthesis
was measured using a Beckman 215A IR gas analyzer (Hurewitz and Janes, 1983). The intact third leaf was placed in a clear plexiglass leaf assimilation chamber and the area around the petiole was sealed with clay. Temperature and light intensity were kept similar in the sample and growth chambers. There was a parallel air flow through both the sample and reference chambers and the difference in COz concentration between the two chambers was determined. Translocation was measured as per Terry and Mortimer (1972). Leaf discs (0.67 cm 2) for dry weight determinations were removed at the beginning of the light period, at the beginning of the dark period, and at the end of the dark period. Discs from the same leaf were used for determining dry weight changes between the different sampling times. These changes were used in determining translocation. C a r b o h y d r a t e assay. - Leaf samples collected for carbohydrate assay were freeze dried and ground to a powder. The extraction of sugars was done as per Jones et al. (1977). Freeze-dried leaf tissue (20 mg) was extracted in 0.3 M HC104 for 5 min and the mixture was centrifuged. The supernatent was used for the sugar assay, while the residue was saved for starch determinations. An aliquot (0.25 ml) of the supernatant was neutralized with 0.14 N NaOH and the glucose present in a portion of this aliquot was measured enzymatically (Bergmeyer, 1963; Jones et al., 1977; Schmitt et al., 1985). Another portion of the same aliquot was used for determining fructose. Fructose was converted to glucose enzymatically and the glucose was measured (Jones et al., 1977). The sucrose was assayed by heating another aliquot (0.25 ml) of the extraction solution in a water bath at 95 °C for 30 min to hydrolyze the sucrose to glucose. It was then neutralized with 0.14 N NaOH and the sucrose-derived glucose was measured. The starch was assayed following the method of MacRae (1971). The residue containing the starch was washed in distilled water and incubated with 0.2 N sodium acetate buffer and amyloglucosidase at 55 ° C for 48 h to break down the starch to glucose completely. This glucose was measured as before. G r o w t h analysis. - The shoots and roots of plants grown in a hydroponic growing system were dried separately at 60 ° C for 7 days, and the dry weights were measured on the day of transfer and on the eighth day of transfer for all the treatments.
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S. LOGENDRA ET AL.
RESULTS AND DISCUSSION
The total amount of carbohydrate fixed during the light period increased with an increase in the length of the light period (Table 1). No significant differences were observed among the three treatments in photosynthetic rate. This is in agreement with the findings of Chatterton and Silvius (1980b) on pangola and Sicher et al. (1982) on maize and wheat. The amount of carbohydrate translocated during the light period and over 24 h in the 8-h light treatment was significantly lower than that of the 16- and 20-h light treatments (Table 1). Even though significant differences were not observed in the amount of carbohydrate translocated during the light period and over 24 h between the 16- and 20-h light treatments, they had an increasing trend with an increase in the light period. A similar observation was made by Grange (1985) in mature pepper leaves. No significant differences were observed among the treatments in the amount of carbohydrate not translocated during the light period and over 24 h (Table 1). This suggests that from the amount of carbon fixed a certain amount stays in the leaf, independent of light period and all the surplus is translocated. It also indicates that a higher proportion of the fixed carbon is translocated under longer light periods. The lower light period translocation rate observed in the 8-h light treatment was associated with higher accumulation rates of hexose and starch (Table 2 ). The starch accumulation rates of 8- and 16-h light treatments were similar. However, the starch accumulation rates of these two light treatments were much greater than that of the 20-h light treatment (Table 2). This is in agreem e n t with the findings of Huber et al. (1984) on soybean, Robinson (1984) on spinach, Chatterton and Silvius (1980a) on soybean, spinach, sugarbeet, corn and pangola grass. These researchers all observed higher starch accumulation rates in plants grown under short light periods than those grown under long light periods. The amount of hexose and sucrose accumulated during the light period inTABLE1 T h e effect of length of light period on C H 2 0 fixation a n d translocation Length of light period (h)
Total C H 2 0 fixed during light period (g m - 2 )
Photosynthetic rate ( g m -2 h - 1)
Total C H 2 0 translocated during light period ( g m -2)
Total C H 2 0 translocat e d over 24 h (g m -2)
Total C H 2 0 n o t translocated during light period ( g m -2)
Total C H 2 0 n o t translocated over 24 h ( g m -~)
8 16 20
3.35 a 5.665 7.01 c
0.42 a 0.35 a 0.35 a
0.43 a 2.90 b 3.805
1.73 a 3.915 4.88 b
2.92 a 2.76 a 3.21 a
1.62 a 1.75 a 2.13 ~
Values in c o l u m n s followed by t h e s a m e superscript are n o t significantly different at P - 0.05.
INFLUENCE OF LIGHT ON TOMATO
79
TABLE 2 T h e effect o f l e n g t h of l i g h t p e r i o d o n c a r b o h y d r a t e a c c u m u l a t i o n a n d p a r t i t i o n i n g Length of light period
Amount accumulated during light period (gm -2)
A c c u m u l a t i o n r a t e d u r i n g light p e r i o d ( g m - 2 h -1 )
(h)
Hexose
Sucrose
Starch
Hexose
Sucrose
Starch
8 16 20
0.495 a 0.743 b 0.992 ¢
0.106 a 0.160 b 0.212 °
0.984 a 1.843 b 1.070 a
0.062 b 0.046 a 0.050 a
0.013 a 0.010 ~ 0.011 a
0.123 b 0.115 b 0.054 ~
Values in c o l u m n s followed b y t h e s a m e s u p e r s c r i p t are n o t s i g n i f i c a n t l y d i f f e r e n t a t P-< 0.05.
creased with an increase in the light periods (Table 2), but the amount of starch accumulated was highest in the 16-h light treatment, while the 8- and 20-h light treatments accumulated similar amounts of starch. Thus, even though the 20-h light treatment had the highest amount of carbohydrate fixed (see Table 1 ), the amount of starch accumulated on the eighth day of transfer was low. However, the amount of starch stored at the beginning of the light period on the eighth day of transfer was highest in this treatment (Fig. 1 ). This may be due to the fact that in the leaves of plants grown under the 20-h light treatment, the carbon fixed in excess of sink demand was stored as starch each day and this accumulated over the 7-day experimental period. Thus, the leaves accumulated less starch on the eighth day as they had almost reached their maximum capacity to accumulate starch, and the excess carbon was made available for translocation and for the formation of hexose and storage of sucrose. Work done by Bradley and Janes (1985) revealed that tomato plants grown under 24 h of light exhibited leaf chlorosis and reduced leaf growth after 5-7 days. In that same study, the starch levels in the third leaf increased over time in continuous light as compared with that of the 12-h control. The high starch content of the 20-h light period observed in our experiment support their data and leads us to speculate further that plants under the 20-h light period may exhibit chlorotic symptoms if left for more days, as the excess starch accumulated in the leaves could disrupt the chloroplast, as observed by Kalucheva and Vinarova (1969). The amounts of either hexose, sucrose or starch at any one time were directly proportional to the length of the light period (Fig. 1). The lower levels of carbohydrates observed at the beginning of the light period and end of the dark period compared with that at the beginning of the dark period in the 8- and 16-h light treatments (Fig. 1) indicated that the carbohydrate that was accumulated during the day was mostly consumed (translocation or respiration) in the night. However, in the 20-h light treatment (Fig. 1 ), little or no starch was lost during the night, suggesting that the
S. LOGENDRA ET AL.
80
I
8 H LIGHT
SUCROSE
EZ:2] HEXOSE STARCH
_BL
Z LLJ
0
Z O (D
I 4
LJ cw k-- Z <( L~ >ZE O G3 E~ <( (D
C9
8
24
16 H LIGHT
SUCROSE
IZ:3 HEXOSE k~q STARCH
3 2
0
II
I6
SUCROSE
24
2 0 H LIGHT
S
HEXOSE
T
STARCH
0
q
20
24
TIME (H) Fig. 1. Carbohydrate content at different times in the 8-, 16- and 20-h light treatments at the beginning of light period (0 h), beginning of dark period (8, 16 or 20 h), and end of dark period (24 h). Vertical lines represent the standard errors.
INFLUENCE OF LIGHT ON TOMATO
81
translocation and respiration requirement during the night was satisfied by the hexose and sucrose pool in the leaves. The net assimilation rate (NAR) increased with an increase in length of light period (Table 3). Even though there was no significant difference in relative growth rate (RGR) and leaf area ratio (LAR) between the 16- and 20-h light periods, these two parameters had an increasing and a decreasing trend, respectively, with an increase in the length of the light period. This is in agreement with the findings of Hurd and Thornley (1974) in tomato plants. However, the shoot-to-root ratio decreased with an increase in light duration (Table 3). The favored growth of shoot over the root in the 8-h light treatment revealed that more photosynthates were translocated to the shoot than to the root. This may be an adjustment by the plant to fix and translocate more carbon under short light periods by increasing the photosynthetic area. A similar observation had been made in soybean (Chatterton and Silvius, 1979). In experiments conducted by Bruggink and Heuvelink (1987) to study the influence of the mean daily light integral on the growth of young tomato plants, it was observed that within a plant dry weight interval of 740-2460 mg, the NAR and RGR increased and LAR decreased with an increasing rate up to an approximate mean daily light integral of 50 J c m - 2 d a y - 1 and with a decreasing rate from 50 to 300 J cm -2 day -1. Our data agree with these, as the plants grown under 8-, 16- and 20-h light periods in our experiment were exposed to a daily total radiation integral of 94, 190 and 235 J cm -2 day -1, respectively. Our results suggest that light period influences photosynthate partitioning, translocation and growth in young tomato. However, whether the differences noticed among the treatments are owing to the length of the light period or the amount of carbohydrate fixed still needs to be addressed. The amount of carbohydrate fixed increased with an increase in the length of the light period. This was due to the differences in the total amount of light energy given to each treatment. Thus, when more carbohydrate was fixed under longer light duration, more of it would have translocated out of the source leaves and the whole plant growth would have been affected. Moreover, the differences in the TABLE 3 NAR, RGR, L A R a n d shoot: root ratio as affected by the length of light period L e n g t h of light period
NAR (g m - 2 day-1 )
RGR (g g-1 day-1 )
LAR (m 2 g-1 )
Shoot: root ratio
3.36 a 6.945 8.74 c
0.111 a 0.2015 0.2285
0.033 b 0.028 a 0.026 a
9.45 4.71 4.28
(h) 8 16 20
Values in columns followed by t h e same superscript are n o t significantly different a t P-< 0.05.
82
S. LOGENDRAET AL.
total amount of carbohydrate fixed among the different treatments may also have affected the carbohydrate partitioning pattern in the source leaves. Our data provide basic insight into environmental influences on carbon metabolism in tomato and have applied implication, particularly where greenhouse production of tomatoes or seedling transplants may be influenced with the use of supplemental light. It also indicates that during the low-light times of the year, the extension of the day with even moderate levels of supplemental light may hold promise as a tool for influencing carbon partitioning and whole plant architecture. However, further investigations are being done to separate the light period effects from those caused by the amount of carbohydrate fixed and to elucidate the short- and long-term effects of altering light period on tomato growth and carbon partitioning. ACKNOWLEDGEMENTS
This work was performed as a part of NJAES Project No. 12150, supported by the New Jersey Agricultural Experiment Station, Hatch Act Funds, and Public Service Electric and Gas.
REFERENCES Bergmeyer, H.U., 1963. Methods of Enzymatic Analysis. Academic Press, New York, NY, p. 117. Bradley, F.M. and Janes, W., 1985. Carbon partitioning in tomato leaves exposed to continuous light. Acta Hortic., 174: 293-302. Bruggink, G.T. and Heuvelink, E., 1987. Influence of light on the growth of young tomato, cucumber and sweet pepper plants in the greenhouse: effects on relative growth rate, net assimilation rate and leaf area ratio. Scientia Hortic., 31: 161-174. Chatterton, N.J. and Silvius, J.E., 1979. Photosynthate partitioning into starch in soybean leaves. 1. Effects of photoperiod versus photosynthetic period duration. Plant. Physiol., 64: 749-753. Chatterton, N.J. and Silvius, J.E., 1980a. Photosynthate partitioning into leaf starch as affected by daily photosynthetic period duration in six species. Physiol. Plant., 49: 141-144. Chatterton, N.J. and Silvius, J.E., 1980b. Acclimation of photosynthate partitioning and photosynthetic rates to changes in length of the daily photosynthetic period. Ann. Bot., 46: 739-745. Grange, R.I., 1985. Carbon partitioning in mature leaves of pepper: effects of daylength. J. Exp. Bot., 36: 1749-1759. Ho, L.C. and Shaw, A.F., 1976. Carbon economy and translocation of 14C in leaflets of the seventh leaf of tomato during leaf expansion. Ann. Bot., 41: 833-848. Huber, S.C., Rufty, T.W. and Kerr, P.S., 1984. Effect of photoperiod on photosynthate partitioning and diurnal rhythms in sucrose phosphate synthase activity in leaves of soybean (Glycine max L. [Merr] ) and tobacco (Nicotiana tobacum L.). Plant Physiol., 75: 1080-1084. Hurd, R.G. and Thornley, J.H.M., 1974. An analysis of the growth of young tomato plants in water culture at different light integrals and CO2 concentrations. 1. Physiological aspects. Ann. Bot., 38: 375-378. Hurewitz, J. and Janes, H.W., 1983. Effect of altering the root zone temperature on growth, translocation, carbon exchange rate, and leaf starch accumulation in the tomato. Plant Physiol., 73: 46-50.
INFLUENCE OF L I G H T ON TOMATO
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Jones, M.G.K., Outlaw, W.H., Jr. and Lowry, O.H., 1977. Enzymic assay of 10 -7 to 10 -14 moles of sucrose in plant tissues. Plant Physiol., 60: 379-383. Kalucheva, I. and Vinarova, K., 1969. Deformation of chloroplasts upon illumination and darkening of tomato leaves. C. R. Acad. Bulgarae Sci., 22: 93-96. MacRae, J.C., 1971. Quantitative measurement of starch in very small amounts of leaf tissue. Planta, 96: 101-108. Robinson, J.M., 1984. Photosynthetic carbon metabolism in leaves and isolated chloroplasts from spinach plants grown under short and intermediate photosynthetic periods. Plant Physiol., 75: 397-409. Schmitt, M.R., McKelvey, S.A., Fellows, R.J. and Giaquinta, R.T., 1985. Invertase and sucrose metabolism in source and sink leaves. In: J. Cronshaw, W.J. Lucas, R.T. Giaquinta (Editors), Plant Biology. Vol. I. Phloem Transport. Alan R. Liss, New York, NY, pp. 411-418. Sicher, R.C., Harris, W.G., Kremer, D.F. and Chatterton, N.J., 1982. Effects of shortened day length upon translocation and starch accumulation by maize, wheat, and pangola grass leaves. Can. J. Bot., 60: 1304-1309. Terry, N. and Mortimer, D.C., 1972. Estimation of the rates of mass carbon transfer by leaves of sugar beet. Can. J. Bot., 50: 1049-1054.
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T h e I n f l u e n c e o f L i g h t P e r i o d on C a r b o n P a r t i t i o n i n g , T r a n s l o c a t i o n a n d G r o w t h in Tomato*
S I T H E S W A R Y LOGENDRA, J A M E S D. P U T M A N and H A R R Y W. J A N E S
Department o/Horticulture and Forestry, Rutgers, The State University o/New Jersey, New Brunswick, N J 08903 (U.S.A.) (Accepted for publication 19 July 1989 )
ABSTRACT Logendra, S., Putman, J.D. and Janes, H.W., 1990. The influence of light period on carbon partitioning, translocation and growth in tomato. Scientia Hortic., 42: 75-83. Carbon partitioning, translocation and growth were examined in young tomato (Lycopersicon esculentum Mill. ) plants grown for 38 days under a 12-h light period and a photosynthetic photon flux density ( P P F D ) of 150 #moles m -2 s -1 and then transferred and grown under 8-, 16- or 20h light periods for an additional 7 days under similar PPFD. The third leaf from the base of the plant was used for carbon partitioning and translocation studies. The longer the length of the light period, the greater were the net assimilation rate (NAR), relative growth rate (RGR) and amount of carbon fixed and translocated. The shoot-to-root ratio and leaf area ratio (LAR) decreased with an increase in the light period. No significant differences were observed in photosynthetic rate among the three light treatments. From the total amount of carbon fixed, a certain amount stayed in the leaf independent of light period and the surplus was translocated. The 8-h light treatment had the highest accumulation rates of hexose, and starch. The starch accumulation rate in the 8- and 16-h light treatments were twice as high as that in the 20-h light treatment. The amount of hexose, sucrose and starch at any one time increased with an increase in the length of the light period. The carbon fixed in excess of demand was stored in the leaf as starch, and this accumulated over time in the 20-h light treatment. The leaves appeared to have a maximum capacity to store starch. Keywords: carbon partitioning; leaf area ratio; light period; Lycopersicon esculentum; net assimilation rate; relative growth rate; tomato; translocation. Abbreviations: L A R - - l e a f area ratio; N A R - - n e t assimilation rate; P P F D - - p h o t o s y n t h e t i c photon flux density; R G R - - relative growth rate. *New Jersey Agricultural Experiment Station Publication No. D - 12150-1-89.
0304-4238/90/$03.50
© 1990 Elsevier Science Publishers B.V.
76
S. LOGENDRAET AL.
INTRODUCTION
The effect of light duration on photosynthate partitioning in leaves is of considerable interest as it affects the amount of photosynthate that is available for translocation to the sink tissues. In many crop species the plants grown under short light periods have greater starch accumulation rates (Chatterton and Silvius, 1979, 1980b; Sicher et al., 1982; Robinson, 1984; Huber et al., 1984), starch content (Chatterton and Silvius, 1980a; Sicher et al., 1982; Robinson, 1984 ) and low sucrose content (Grange, 1985 ) when compared with those grown under long light periods. The high starch accumulation rates observed under short light periods are associated with low translocation rates (Chatterton and Silvius, 1980b; Huber et al., 1984) and decreased amounts of carbon translocated (Grange, 1985). It was suggested that, under short light periods plants accumulate higher amounts of starch to satisfy the carbohydrate requirement of the subsequent longer nights (Chatterton and Silvius, 1979). The relative growth of plant organs is also affected by the length of the light period as observed in soybeans (Chatterton and Silvius, 1979). Here the shoot growth was favored over the root growth under short light periods, suggesting that the photosynthates were preferentially used for the growth of the shoot under those conditions. The present study was initiated to examine the effect of duration of light on photosynthate partitioning and translocation in source leaves of young tomato plants and its overall influence on growth. MATERIALS AND METHODS
T r e a t m e n t s . - Seeds of tomato ( L y c o p e r s i c o n e s c u l e n t u m cultivar 'Dombito' ) were sown in vermiculite and germinated under mist in the greenhouse. When the first true leaf was about 0.5 cm long (approximately 2 weeks after sowing), uniform seedlings were transplanted into 10-cm pots containing a peat: vermiculite: perlite mix, 40: 40: 20 by volume with a pH of 6.5. To measure the growth parameters, seedlings were also grown in a hydroponic growing system containing a half-strength Hoagland solution as described by Hurewitz and Janes (1983). This system was employed to facilitate root weight determinations. All seedlings were grown in an environmentally controlled chamber with a 12 h day/12 h night cycle, a photosynthetic photon flux density (PPFD) of 150 #moles m - 2 s- 1, and a temperature of 26 ° C/23 ° C. The seedlings in the mix were irrigated twice weekly with half-strength Hoagland solution. Approximately 38 days after seeding, i.e. when the third leaf (from the base of plant) was about 70-80% expanded and was a net source (Ho and Shaw, 1976 ), the plants were moved into different growth chambers and were grown under 8-, 16- and 20-h light periods with a P P F D of 150 #moles m -2 s -1 for 7 days.
INFLUENCE OF LIGHT ON TOMATO
77
Observations were made on the third leaf, over a 24-h period starting with the onset of light on the eighth day after transfer. For each observation 4-6 plants were sampled and the data are the averages from two separate experiments. M e a s u r e m e n t o f n e t p h o t o s y n t h e s i s a n d translocation. - Net photosynthesis
was measured using a Beckman 215A IR gas analyzer (Hurewitz and Janes, 1983). The intact third leaf was placed in a clear plexiglass leaf assimilation chamber and the area around the petiole was sealed with clay. Temperature and light intensity were kept similar in the sample and growth chambers. There was a parallel air flow through both the sample and reference chambers and the difference in COz concentration between the two chambers was determined. Translocation was measured as per Terry and Mortimer (1972). Leaf discs (0.67 cm 2) for dry weight determinations were removed at the beginning of the light period, at the beginning of the dark period, and at the end of the dark period. Discs from the same leaf were used for determining dry weight changes between the different sampling times. These changes were used in determining translocation. C a r b o h y d r a t e assay. - Leaf samples collected for carbohydrate assay were freeze dried and ground to a powder. The extraction of sugars was done as per Jones et al. (1977). Freeze-dried leaf tissue (20 mg) was extracted in 0.3 M HC104 for 5 min and the mixture was centrifuged. The supernatent was used for the sugar assay, while the residue was saved for starch determinations. An aliquot (0.25 ml) of the supernatant was neutralized with 0.14 N NaOH and the glucose present in a portion of this aliquot was measured enzymatically (Bergmeyer, 1963; Jones et al., 1977; Schmitt et al., 1985). Another portion of the same aliquot was used for determining fructose. Fructose was converted to glucose enzymatically and the glucose was measured (Jones et al., 1977). The sucrose was assayed by heating another aliquot (0.25 ml) of the extraction solution in a water bath at 95 °C for 30 min to hydrolyze the sucrose to glucose. It was then neutralized with 0.14 N NaOH and the sucrose-derived glucose was measured. The starch was assayed following the method of MacRae (1971). The residue containing the starch was washed in distilled water and incubated with 0.2 N sodium acetate buffer and amyloglucosidase at 55 ° C for 48 h to break down the starch to glucose completely. This glucose was measured as before. G r o w t h analysis. - The shoots and roots of plants grown in a hydroponic growing system were dried separately at 60 ° C for 7 days, and the dry weights were measured on the day of transfer and on the eighth day of transfer for all the treatments.
78
S. LOGENDRA ET AL.
RESULTS AND DISCUSSION
The total amount of carbohydrate fixed during the light period increased with an increase in the length of the light period (Table 1). No significant differences were observed among the three treatments in photosynthetic rate. This is in agreement with the findings of Chatterton and Silvius (1980b) on pangola and Sicher et al. (1982) on maize and wheat. The amount of carbohydrate translocated during the light period and over 24 h in the 8-h light treatment was significantly lower than that of the 16- and 20-h light treatments (Table 1). Even though significant differences were not observed in the amount of carbohydrate translocated during the light period and over 24 h between the 16- and 20-h light treatments, they had an increasing trend with an increase in the light period. A similar observation was made by Grange (1985) in mature pepper leaves. No significant differences were observed among the treatments in the amount of carbohydrate not translocated during the light period and over 24 h (Table 1). This suggests that from the amount of carbon fixed a certain amount stays in the leaf, independent of light period and all the surplus is translocated. It also indicates that a higher proportion of the fixed carbon is translocated under longer light periods. The lower light period translocation rate observed in the 8-h light treatment was associated with higher accumulation rates of hexose and starch (Table 2 ). The starch accumulation rates of 8- and 16-h light treatments were similar. However, the starch accumulation rates of these two light treatments were much greater than that of the 20-h light treatment (Table 2). This is in agreem e n t with the findings of Huber et al. (1984) on soybean, Robinson (1984) on spinach, Chatterton and Silvius (1980a) on soybean, spinach, sugarbeet, corn and pangola grass. These researchers all observed higher starch accumulation rates in plants grown under short light periods than those grown under long light periods. The amount of hexose and sucrose accumulated during the light period inTABLE1 T h e effect of length of light period on C H 2 0 fixation a n d translocation Length of light period (h)
Total C H 2 0 fixed during light period (g m - 2 )
Photosynthetic rate ( g m -2 h - 1)
Total C H 2 0 translocated during light period ( g m -2)
Total C H 2 0 translocat e d over 24 h (g m -2)
Total C H 2 0 n o t translocated during light period ( g m -2)
Total C H 2 0 n o t translocated over 24 h ( g m -~)
8 16 20
3.35 a 5.665 7.01 c
0.42 a 0.35 a 0.35 a
0.43 a 2.90 b 3.805
1.73 a 3.915 4.88 b
2.92 a 2.76 a 3.21 a
1.62 a 1.75 a 2.13 ~
Values in c o l u m n s followed by t h e s a m e superscript are n o t significantly different at P - 0.05.
INFLUENCE OF LIGHT ON TOMATO
79
TABLE 2 T h e effect o f l e n g t h of l i g h t p e r i o d o n c a r b o h y d r a t e a c c u m u l a t i o n a n d p a r t i t i o n i n g Length of light period
Amount accumulated during light period (gm -2)
A c c u m u l a t i o n r a t e d u r i n g light p e r i o d ( g m - 2 h -1 )
(h)
Hexose
Sucrose
Starch
Hexose
Sucrose
Starch
8 16 20
0.495 a 0.743 b 0.992 ¢
0.106 a 0.160 b 0.212 °
0.984 a 1.843 b 1.070 a
0.062 b 0.046 a 0.050 a
0.013 a 0.010 ~ 0.011 a
0.123 b 0.115 b 0.054 ~
Values in c o l u m n s followed b y t h e s a m e s u p e r s c r i p t are n o t s i g n i f i c a n t l y d i f f e r e n t a t P-< 0.05.
creased with an increase in the light periods (Table 2), but the amount of starch accumulated was highest in the 16-h light treatment, while the 8- and 20-h light treatments accumulated similar amounts of starch. Thus, even though the 20-h light treatment had the highest amount of carbohydrate fixed (see Table 1 ), the amount of starch accumulated on the eighth day of transfer was low. However, the amount of starch stored at the beginning of the light period on the eighth day of transfer was highest in this treatment (Fig. 1 ). This may be due to the fact that in the leaves of plants grown under the 20-h light treatment, the carbon fixed in excess of sink demand was stored as starch each day and this accumulated over the 7-day experimental period. Thus, the leaves accumulated less starch on the eighth day as they had almost reached their maximum capacity to accumulate starch, and the excess carbon was made available for translocation and for the formation of hexose and storage of sucrose. Work done by Bradley and Janes (1985) revealed that tomato plants grown under 24 h of light exhibited leaf chlorosis and reduced leaf growth after 5-7 days. In that same study, the starch levels in the third leaf increased over time in continuous light as compared with that of the 12-h control. The high starch content of the 20-h light period observed in our experiment support their data and leads us to speculate further that plants under the 20-h light period may exhibit chlorotic symptoms if left for more days, as the excess starch accumulated in the leaves could disrupt the chloroplast, as observed by Kalucheva and Vinarova (1969). The amounts of either hexose, sucrose or starch at any one time were directly proportional to the length of the light period (Fig. 1). The lower levels of carbohydrates observed at the beginning of the light period and end of the dark period compared with that at the beginning of the dark period in the 8- and 16-h light treatments (Fig. 1) indicated that the carbohydrate that was accumulated during the day was mostly consumed (translocation or respiration) in the night. However, in the 20-h light treatment (Fig. 1 ), little or no starch was lost during the night, suggesting that the
S. LOGENDRA ET AL.
80
I
8 H LIGHT
SUCROSE
EZ:2] HEXOSE STARCH
_BL
Z LLJ
0
Z O (D
I 4
LJ cw k-- Z <( L~ >ZE O G3 E~ <( (D
C9
8
24
16 H LIGHT
SUCROSE
IZ:3 HEXOSE k~q STARCH
3 2
0
II
I6
SUCROSE
24
2 0 H LIGHT
S
HEXOSE
T
STARCH
0
q
20
24
TIME (H) Fig. 1. Carbohydrate content at different times in the 8-, 16- and 20-h light treatments at the beginning of light period (0 h), beginning of dark period (8, 16 or 20 h), and end of dark period (24 h). Vertical lines represent the standard errors.
INFLUENCE OF LIGHT ON TOMATO
81
translocation and respiration requirement during the night was satisfied by the hexose and sucrose pool in the leaves. The net assimilation rate (NAR) increased with an increase in length of light period (Table 3). Even though there was no significant difference in relative growth rate (RGR) and leaf area ratio (LAR) between the 16- and 20-h light periods, these two parameters had an increasing and a decreasing trend, respectively, with an increase in the length of the light period. This is in agreement with the findings of Hurd and Thornley (1974) in tomato plants. However, the shoot-to-root ratio decreased with an increase in light duration (Table 3). The favored growth of shoot over the root in the 8-h light treatment revealed that more photosynthates were translocated to the shoot than to the root. This may be an adjustment by the plant to fix and translocate more carbon under short light periods by increasing the photosynthetic area. A similar observation had been made in soybean (Chatterton and Silvius, 1979). In experiments conducted by Bruggink and Heuvelink (1987) to study the influence of the mean daily light integral on the growth of young tomato plants, it was observed that within a plant dry weight interval of 740-2460 mg, the NAR and RGR increased and LAR decreased with an increasing rate up to an approximate mean daily light integral of 50 J c m - 2 d a y - 1 and with a decreasing rate from 50 to 300 J cm -2 day -1. Our data agree with these, as the plants grown under 8-, 16- and 20-h light periods in our experiment were exposed to a daily total radiation integral of 94, 190 and 235 J cm -2 day -1, respectively. Our results suggest that light period influences photosynthate partitioning, translocation and growth in young tomato. However, whether the differences noticed among the treatments are owing to the length of the light period or the amount of carbohydrate fixed still needs to be addressed. The amount of carbohydrate fixed increased with an increase in the length of the light period. This was due to the differences in the total amount of light energy given to each treatment. Thus, when more carbohydrate was fixed under longer light duration, more of it would have translocated out of the source leaves and the whole plant growth would have been affected. Moreover, the differences in the TABLE 3 NAR, RGR, L A R a n d shoot: root ratio as affected by the length of light period L e n g t h of light period
NAR (g m - 2 day-1 )
RGR (g g-1 day-1 )
LAR (m 2 g-1 )
Shoot: root ratio
3.36 a 6.945 8.74 c
0.111 a 0.2015 0.2285
0.033 b 0.028 a 0.026 a
9.45 4.71 4.28
(h) 8 16 20
Values in columns followed by t h e same superscript are n o t significantly different a t P-< 0.05.
82
S. LOGENDRAET AL.
total amount of carbohydrate fixed among the different treatments may also have affected the carbohydrate partitioning pattern in the source leaves. Our data provide basic insight into environmental influences on carbon metabolism in tomato and have applied implication, particularly where greenhouse production of tomatoes or seedling transplants may be influenced with the use of supplemental light. It also indicates that during the low-light times of the year, the extension of the day with even moderate levels of supplemental light may hold promise as a tool for influencing carbon partitioning and whole plant architecture. However, further investigations are being done to separate the light period effects from those caused by the amount of carbohydrate fixed and to elucidate the short- and long-term effects of altering light period on tomato growth and carbon partitioning. ACKNOWLEDGEMENTS
This work was performed as a part of NJAES Project No. 12150, supported by the New Jersey Agricultural Experiment Station, Hatch Act Funds, and Public Service Electric and Gas.
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Jones, M.G.K., Outlaw, W.H., Jr. and Lowry, O.H., 1977. Enzymic assay of 10 -7 to 10 -14 moles of sucrose in plant tissues. Plant Physiol., 60: 379-383. Kalucheva, I. and Vinarova, K., 1969. Deformation of chloroplasts upon illumination and darkening of tomato leaves. C. R. Acad. Bulgarae Sci., 22: 93-96. MacRae, J.C., 1971. Quantitative measurement of starch in very small amounts of leaf tissue. Planta, 96: 101-108. Robinson, J.M., 1984. Photosynthetic carbon metabolism in leaves and isolated chloroplasts from spinach plants grown under short and intermediate photosynthetic periods. Plant Physiol., 75: 397-409. Schmitt, M.R., McKelvey, S.A., Fellows, R.J. and Giaquinta, R.T., 1985. Invertase and sucrose metabolism in source and sink leaves. In: J. Cronshaw, W.J. Lucas, R.T. Giaquinta (Editors), Plant Biology. Vol. I. Phloem Transport. Alan R. Liss, New York, NY, pp. 411-418. Sicher, R.C., Harris, W.G., Kremer, D.F. and Chatterton, N.J., 1982. Effects of shortened day length upon translocation and starch accumulation by maize, wheat, and pangola grass leaves. Can. J. Bot., 60: 1304-1309. Terry, N. and Mortimer, D.C., 1972. Estimation of the rates of mass carbon transfer by leaves of sugar beet. Can. J. Bot., 50: 1049-1054.