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Annals of Botany 78 : 163–168, 1996 Changes of Carbohydrates in Pepper (Capsicum annuum L.) Flowers in Relation to Their Abscission Under Different Shading Regimes B. ALONI*, L. K A R N I, Z. Z A I D M AN and A. A. S C H A F F E R Department of Vegetable Crops, Institute of Field and Garden Crops, Agricultural Research Organization, The Volcani Center, P.O.B. 6, Bet Dagan 50250, Israel Received : 24 July 1995 Accepted : 25 January 1996 Abscission of pepper flowers is enhanced under conditions of low light and high temperature. Our study shows that pepper flowers accumulate assimilates, particularly in the ovary, during the day time, and accumulate starch, which is then metabolized in the subsequent dark period. With the exception of the petals, the ovary contains the highest total amounts of sugars and starch, compared with other flower parts and contains the highest total activity, as well as activity calculated on fresh mass basis, of sucrose synthase, in accordance with the role of this enzyme in starch biosynthesis. Low light intensity or leaf removal decreased sugar accumulation in the flower and subsequently caused flower abscission. The threshold of light intensity for daily sugar accumulation in the sink leaves was much lower than in flowers, resulting in higher daytime accumulation of sugars in the sink leaves than in the adjacent flower buds under any light intensity, suggesting a competition for assimilates between these organs. Flowers of bell pepper cv. ‘ Maor ’ and ‘ 899 ’ (sensitive to abscission) accumulated less soluble sugars and starch under shade than the flowers of bell pepper cv. ‘ Mazurka ’ and of paprika cv. ‘ Lehava ’ (less sensitive). The results suggest that the flower capacity to accumulate sugars and starch during the day is an important factor in determining flower retention and fruit set. # 1996 Annals of Botany Company Key words : Pepper, Capsicum annuum L., abscission, shading, pepper flowers, ovary, leaves, sugars, starch, acid invertase, sucrose synthase. INTRODUCTION In most agricultural crops, flower retention and fruit set are highly sensitive to environmental stresses. High temperature and low light conditions are known to enhance flower abscission and to affect photosynthetic rates (Gent, 1986 ; Shishido, Chala and Krupa, 1987 ; Kitroongruang et al., 1992 ; Havaux, 1993), assimilate partitioning (Dinar and Rudich, 1985 b ; Aloni, Pashkar and Karni, 1991 a) and sugar metabolism in source and sink tissues (Hawker, 1982 ; Dinar and Rudich, 1985 a; Bhuller and Jenner, 1986 ; Macleod and Duffus, 1988). In addition, hormonal balance, particularly of ethylene and auxin, is important in controlling abscission (Beyer and Morgan, 1971 ; Wien, Turner and Yang, 1989 ; Ofir et al., 1993). Low light intensity enhances pepper flower abortion and thus reduces fruit yield. While fertilization is sensitive to high temperature (ElAhmadi and Stevens, 1979 ; Kao, Chen and Ma, 1986 ; Mutters and Hall, 1992), this process may not be important in the abortion of flower buds (pre-anthesis) and fruitlets (post-fertilization). Both flower retention and fruit set depend on assimilate supply to the developing reproductive organ, as shown in many studies in which leaf area, CO enrichment, leaf # removal and alterations of competing sinks were found to * For correspondence. Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 1655–E, 1995 series. 0305-7364}96}08016306 $18.00}0 affect flower development, flower abortion and fruit set (see review by Kinet, Sachs and Bernier, 1985 and references therein). More recently it was shown that flower retention in pepper decreases under shading stress (Wien, Turner and Yang, 1989) and flower abscission was higher if shading was applied in combination with high temperature. It has been suggested (Aloni, Pashkar and Karni, 1991 b) that competition for assimilates between the flowers and the adjacent young leaves may be important in determining flower retention. Only recently Turner and Wien (1994) have shown that susceptibility of pepper cultivars may be related to assimilate partitioning to the flower buds. In order to investigate the relationships between assimilate concentration and metabolism in peppers further we investigated the daily changes in carbohydrate concentration of the flowers in relation to their abscission when assimilate supply was limited by removal of source leaves or by low light intensity in pepper cultivars with different sensitivity to flower abortion. MATERIALS AND METHODS Plant material, growth conditions and shading treatments Seedlings of pepper plants of several different cultivars (Bell pepper cvs. ‘ Maor ’, ‘ 899 ’, from Hazera, Israel, ‘ 11480’ from Zeraim Gedera, Israel, Mazurka’ from Rijk Zvaan, Holland, Paprika ‘ Lehava ’, from The Volcani Center, Israel) were transplanted on 1 Jul. 1993 into 10 l pots filled # 1996 Annals of Botany Company 164 Aloni et al.—Carbohydrate Content of Pepper Flowers 1 2 3 4 Each treatment was applied to 24 plants, divided among three replicates. After the trimming treatments the rate of abscission of the primary and secondary flower buds of internode 8 was recorded. Abscission of reproductive organs was determined by counting vacant internodes on the whole plant and calculating the percentage of aborted organs. Photosynthesis measurements F. 1. Schematic drawing of leaf pruning. The arrow points to internode 8 at which the main stem flower bud (E) was maintained. The (*) symbolize the attached leaves. The broken line symbolizes the stem continuation. Nos 1–4 are the treatments as explained in the Materials and Methods. with a peat : perlite mixture (60 : 40, v}v) and irrigated with commercial nutrient solution (‘ Shefer ’, Deshanim Co. Israel, N : P : K, 7 : 3 : 7 and microelements). The plants were grown in a greenhouse with minimum and maximum temperatures of 22 and 28 °C, respectively. After 30 d each plant was allowed to develop two main stems, which were supported by training threads. The flowers of each internode, on both stems, were maintained with two adjacent source leaves. All the side shoots were removed. Sixty days after transplantation, pepper plants of different cultivars developed flowers, fruitlets and fruits, at different developmental stages. The mature fruits were removed in order to avoid any effects of fruit load. At that stage the plants were placed under nets of several different meshes in order to reduce the maximum photosynthetically active radiation (PAR) intensity (range 200–920 µmol m−# s−") and different daily integrated total radiation, 2–8 MJ d−". PAR was measured continuously by LI-COR (Lincoln, Nebraska, USA), LI-185B, radiometer. Each shading treatment was applied to 20 plants (5 plants¬4 replicates) of each cultivar for a period of 15 d. Control plants were unshaded and received an average maximum PAR of 920 µmol m−# s−". Photosynthesis, as net carbon exchange rate (CER), was determined by infra red gas analysis (IRGA) measurements (ADC—The Analytical Development Co, Hoddesdon, Herts, UK). The measurements were made between 1100 and 1200 h on fully expanded leaves, 24 h from the onset of the shading treatments. Samples of sink leaves (approximately 5 cm long) were taken for sugar and starch determinations at dawn (0600 h) and dusk (1800 h) of that day. Determinations of soluble sugars and starch Five samples of leaf discs (each of 1 cm in diameter) from sink leaves 5 cm long, or of flowers or flower parts of approximately 200 mg fresh mass each, were extracted three times with 80 % ethanol at 80 °C. The combined extracts were evaporated to dryness and redissolved in 2 ml distilled water, from which aliquots of 50–200 µl were taken for sugar determination. Reducing sugars were determined colorimetrically using dinitrosalicylic acid (Miller, 1959) ; sucrose was determined by using the anthrone reagent method as modified for determination of non-reducing sugars (Van Handel, 1968). Starch was determined by measuring glucose following digestion of the ethanolinsoluble residue with amyloglucosidase (Dinar, Rudich and Zamski, 1983). Total non-structural carbohydates (NSC) is the sum of soluble sugars and starch and daily accumulation of them was calculated as the difference between sugar concentrations at dawn and dusk. Calculations of significant differences at the 5 % level were carried out by analysis of variance and least significant differences. Manipulations of source}sink ratio Plants of cv. ‘ Mazurka ’ were grown in an unshaded greenhouse and trimmed as described above. Leaves and flowers were trimmed at the eighth internode of each stem, as follows (see Fig. 1) : (1) The primary (main stem) flower bud (3 d before anthesis) was retained but the source leaf and the side shoots at this internode were cut off. This treatment was designated as : Flower (®). (2) The primary flower bud retained with one adjacent source leaf attached ; Flower (1). (3) The primary flower bud retained with two adjacent source leaves attached ; Flower (2). (4) The primary flower bud retained with two source leaves and secondary flower bud (present on the side shoot of the same internode) with its adjacent leaf attached ; Flower (2SF). Enzyme assays Determination of soluble acid invertase activity in pepper flowers was as described by Aloni et al. (1991 a, b). In short, tissue samples of approximately 300 mg were ground with a Polytron homogenizer in 5 ml ice-cold grinding medium containing : 25 m HEPES buffer (N-2-hydroxyethylpiperazine-N2-2-ethanesulphonic acid) pH 7±2, 5 m MgCl , 2 m DDT (- Dithiothreitol) and 3 m DIDCA # (diethyldithiocarbamic acid) as antioxidant. The mixture was centrifuged at 20 000 g for 20 min at 4 °C. Aliquots of 100 µl of the supernatant were incubated in 1±0 ml 0±1 N phosphate citrate buffer, pH 5±0 and 20 m sucrose (Km of the enzyme found to be 5 m sucrose). The incubation was Aloni et al.—Carbohydrate Content of Pepper Flowers carried out for 30 min at 37 °C and was terminated by addition of 1 ml dinitrosalicylic acid reagent. After boiling for 5 min, the resulting sugars were determined colorimetrically as described above. Sucrose synthase activity was determined according to Schaffer, Aloni and Fogelman (1987). Following extraction as described for acid invertase the mixture was dialysed overnight in order to remove the internal sugars. The enzymatic activity was determined as sucrose breakdown on aliquots of 200 µl incubated in incubation medium containing 0±1 phosphate-citrate buffer pH 7±0, 200 m sucrose (with Km being 125 m) and 5 m UDP. After incubation at 37 °C for 30 min the resulting fructose was determined by the dinitrosalicylic acid reaction. The data were expressed on fresh mass basis. RESULTS Effect of light intensity on flower abscission Table 1 shows that reduction of maximum light intensity from 920 to 200 µmol m−# s−" by shading, enhanced pepper flower abscission in several cultivars, but there were differences in their susceptibility. Paprika, cv. ‘ Lehava ’ and bell pepper ‘ Mazurka ’ appeared to be the least sensitive while bell pepper cvs. ‘ 899 ’, ‘ Maor ’ and ‘ 11480 ’ were highly susceptible. Shading reduced CER in all the tested cultivars but it was not related to the flower abscission in these cultivars under the shading treatments. Effect of source size The effect of source reduction was studied by removing different numbers of source leaves from the internode of given primary flower buds and by monitoring daily sugar accumulation in these flowers on the day which followed the treatment. Subsequently, the abscission rate was monitored. Table 2 shows that when a single source leaf was present, the flower at the same internode [Flower (1)] accumulated T 1. The effect of maximum daily light intensity on photosynthesis, measured 24 h after the onset of the shading treatments, and on flower abscission (in the whole plant), determined 15 d later Light ( µmol m−# s−") 200 Cultivar 500 920 Flower abscission (%) ‘ Maor ’ 86 ‘ 899 ’ 100 ‘ 11480 ’ 93 ‘ Mazurka ’ 26 ‘ Lehava ’ 21 l.s.d. (P ¯ 0±05) 40 71 32 10 12 8 12 15 8 0 0 200 500 920 Photosynthesis ( µmol CO m−# s−") # 2±5 2±3 2±8 3±0 1±7 4±2 4±0 4±5 5±7 4±2 2±2 12±0 11±8 8±2 9±2 7±0 165 T 2. The effect of pruning treatments on flower abscission, 21 d from leaf remoal and on daytime gain or loss of soluble sugars, starch and non-structural sugars (NSC, soluble sugarsstarch) in flowers of pepper plants c. ‘ Mazurka ’ Change in carbohydrates during the light period (mg per flower) Pruning Treatment Flower (®) Flower (1) Flower (2) Flower (2SF) SF l.s.d. (P ¯ 0±05) Flower abscission reducing (%) sucrose sugars 65 10 10 40 100 35 ®0±1 0±2 0±4 — ®0±1 0±05 ®0±3 0±0 1±3 — ®0±2 0±08 starch total NSC ®0±8 1±1 1±2 — ®0±2 0±09 ®1±2 1±3 2±9 — ®0±5 0±55 Sugar was determined 24 h after the pruning treatments. and ® signs indicate gain and loss of sugars, respectively. Explanation for the lettering of the pruning treatments is given in the Materials and Methods section. T 3. The distribution of carbohydrates (NSC is non reduced carbohydratesucrosestarch) between arious pepper flower parts (c. ‘ Mazurka ’). Data are means (n ¯ 5) Flower part Petals Ovary Sepals Stylestamen l.s.d. (P ¯ 0±05) Reducing sugars Sucrose (mg per organ) Starch Total NSC 2±4 0±8 0±3 0±5 0±4 0±3 0±6 0±1 0±2 0±3 0±4 1±7 0±2 0±5 0±4 3±1 3±1 0±6 1±2 0±6 1±3 mg sugars d−". Detaching this leaf [Flower (®)] caused a loss of 1±2 mg d−" of sugars from this flower. When two leaves were attached [Flower (2)], the adjacent primary flower gained 2±9 mg sugars d−" on the following day. The secondary flower (SF) lost 0±5 mg in the same period. Table 2 shows that 21 d after the onset of the pruning, abscission of flowers without adjacent source leaf [Flower (®)] was 65 %. In Flower (1) and (2), abscission had diminished to only 10 % during that time. When a flower bud was present on the side shoot [Flower (2SF)], abscission of the primary flower was increased to 40 %. All the secondary flower buds were aborted 21 d after leaf pruning. Distribution of sugars in pepper flowers Flower parts differ in the amount of carbohydrates (Table 3). The petals and the ovary had the largest amounts of reducing sugars and starch. Sucrose appeared only in minute amounts in all flower parts, indicating that if translocated it was rapidly metabolized. There were differences in sugar accumulation between sink leaves and 166 Aloni et al.—Carbohydrate Content of Pepper Flowers T 4. The effect of accumulatie daily PAR, measured inside the greenhouse on the concentration and increase in concentration between dusk and dawn of total NSC (solublestarch) in sink leaes (5 cm long) and flower buds of pepper c. ‘ Mazurka ’ T 6. The actiities of acid inertase and sucrose synthase per gram fresh mass or per organ in arious parts of pepper, c. ‘ Mazurka ’, flowers at anthesis. Data are means (n ¯ 5) Enzymatic activity, sucrose hydrolysed Total NSC (mg g f. wt−") Light (MJ d−") Dawn Dusk 7±2 7±1 12±3 15±0 9±6 18±2 23±8 40±4 2±4 11±1 11±5 25±4 3±5 16±0 17±8 19±7 25±1 27±3 ®2±3 ®2±4 ®1±5 0±7 6±1 1±4 Leaves 1±6 2±66 5±15 8±31 l.s.d. (P ¯ 0±05) Flower buds 1±6 2±66 3±11 5±15 8±31 l.s.d. (P ¯ 0±05) 3±9 18±3 20±2 21±2 24±4 21±2 0±8 Acid invertase Flower part Dusk®Dawn (difference) ( µmol g f. wt min−") Activity per unit fresh mass Petals Ovary Sepals Stamenstyle l.s.d. (P ¯ 0±05) Total activity per gram Petals Ovary Sepals Stamen l.s.d. (P ¯ 0±05) flowers under different daily integrated PAR (Table 4). Sugar concentration at the end of the day was increased in 5 cm long sink leaves as light in the greenhouse increased from 1±6 to 8±31 MJ d−". These values were typical of cloudy and sunny days, respectively, during the experiment. At a total PAR of 1±6 MJ d−", sugar accumulated in sink leaves during the day. On the other hand, in flowers, positive sugar balance was observed only when the daily integrated PAR was approximately 5±0 MJ d−". Cultiar differences The ‘ Lehava ’ and ‘ Mazurka ’ cultivars, which were the Sucrose synthase 0±46 0±09 0±47 0±87 0±24 0±14 0±58 0±72 0±12 0±24 ( µmol per organ min−") 52 32 7 15 15 10 60 4 18 21 least sensitive to abscission (as shown in Table 1) gained sugars and starch in their flowers during the day even under shade, while the more susceptible cultivars (‘ Maor ’ and ‘ 899 ’) had negative daily sugar balance in their flowers under shade (Table 5). Enzymatic actiity Acid invertase and sucrose synthase (the sucrose cleaving enzymes) are both present in the pepper flower bud. There were no significant differences in acid invertase activity between different flower parts except for the low enzymatic activity in the sepals (Table 6). On the other hand, the T 5. The concentration and daily accumulation of starch and total NSC in flower buds of arious pepper cultiars in control (maximum irradiation of 920 µmol m−# s−") and shading (500 µmol m−# s−") treatments. Diff. indicates the difference between dusk and dawn alues Carbohydrate concentration (mg g f. wt−") Starch Cultivar Total NSC dawn dusk Diff. dawn dusk Diff. control shade l.s.d. (P ¯ 0±05) 4±2 3±2 7±8 2±8 3±6 ®0±4 18±6 18±3 27±5 13±4 8±9 ®4±9 control shade l.s.d. (P ¯ 0±05) 4±1 2±2 14±6 5±1 6±7 ®1±1 control shade l.s.d. (P ¯ 0±05) 3±5 2±9 33±6 30±4 12±8 8±0 control shade l.s.d. (P ¯ 0±05) 5±1 3±3 17±0 11±5 8±9 4±5 ‘ Maor ’ 1±2 3±8 ‘ 899 ’ 6±1 1±6 2±0 ®0±6 7±9 6±2 1±1 3±8 ‘ Lehava ’ 12±4 8±5 8±9 5±6 20±8 22±4 2±8 5±2 ‘ Mazurka ’ 10±3 7±2 3±1 5±2 2±9 8±1 7±0 2±4 Aloni et al.—Carbohydrate Content of Pepper Flowers sucrose synthase concentration was greatest in the ovary, followed by stamenstyle and sepals with the least being in the petals. DISCUSSION Photosynthesis is an important factor in determining pepper flower abortion, since its reduction by shading enhanced flower abortion in several pepper cultivars (Table 1). However, differences in photosynthetic rates do not explain the differences in cultivar susceptibility. For example, the photosynthesis measured in cvs. ‘ Maor ’ and ‘ 899 ’, which are highly susceptible to shading-induced flower abscission, was greater than that measured in the less sensitive cultivars ‘ Mazurka ’ and ‘ Lehava ’. This agrees with the results of Turner and Wien (1994) who found no differences in photosynthetic rates between pepper cultivars with different sensitivities to shading, and with a report by Aloni et al. (1991 b) that flower abortion in pepper, caused by high temperatures, was not accompanied by changes in photosynthesis. It is suggested that assimilate allocation to the flower and its metabolism within the flower is a more important factor for its retention than photosynthesis. Nevertheless, when the availability of assimilates to the developing flower is reduced (e.g. by leaf pruning or shading) abscission is greatly enhanced. Based on leaf pruning and shading experiments, it is calculated that to set properly, a ‘ Mazurka ’ flower has to accumulate about 1±2 mg sugars d−" until pollination takes place. This amount can be provided if a given flower has at least one source leaf attached at its internode and the plant is exposed to light of approximately 5 MJ d−" (Table 4). If irradiation is less, the daily balance of sugar accumulation in the flower becomes negative, resulting in abscission. The sink leaves, on the other hand, start to accumulate sugars at a daily light imput of only 1±6 MJ d−", indicating a stronger sink capacity than that of the flower bud. These results reinforce our previous suggestion that the sink leaves and the adjacent flower buds may compete for translocated assimilates (Aloni et al., 1991 b) and that the sink leaves are stronger sinks than the adjacent flowers. In contradiction, Turner and Wien (1994) showed that removal of sink leaves did not affect the abscission of pepper flower buds under low light conditions. They also showed that pepper cultivars of differing susceptibility of flower abscission to low light did not differ in dry-matter partitioning between flower buds and sink leaves. However, since these studies were carried out on small numbers of cultivars and in plants subjected to a single low light treatment (30–35 µmol m−# s−") this hypothesis requires more tests. Within the flower, the ovary and the petals accumulate most of the incoming sugars. However, while petals accumulate reducing sugars, the ovary metabolizes a large proportion of the sugars into starch (Table 3). Walker and Hawker (1976) suggested that the sink capacity of pepper fruit is initiated only after pollination takes place and that acid invertase and sucrose synthase activities are associated with sink activity. Sun et al. (1992) suggested that sucrose synthase may serve as an indicator for sink strength in growing tomato fruits. Robinson, Hewitt and Bennett (1988) have shown that both sucrose synthase and ADP- 167 glucose pyrophosphorylase activities correlate with a transient starch accumulation found in developing tomato fruits. Our work shows that sucrose synthase activity is present predominantly in the flower ovary while acid invertase activity is similar (with the exception of the sepals) in all flower parts (Table 6), suggesting that the ovary serves as the main active sink for assimilates, possibly due to its high capacity for starch biosynthesis. The differences in cultivar susceptibility to flower abscission caused by shade (Table 1), and the differences between cultivars in daily capacity to accumulate sugarstarch under shade (Table 5) indicate that pepper cultivars with differing susceptibilities to flower abscission may also differ in their capacity to metabolize sucrose and accumulate starch during the light periods until pollination takes place. The daily accumulation of starch may increase the sucrose gradient between the source leaf and the developing flower, therefore driving sucrose transport toward the flower. Sucrose synthase, located in the ovary, is a sucrose cleaving enzyme producing ADP glucose which is subsequently metabolised to starch. This enzyme may be regulatory in the pathway to starch biosynthesis, as has been previously suggested (Claussen, 1983 ; Claussen, Hawker and Loveys, 1985 ; Sung, Xu and Black, 1988). Possibly, pepper cultivars of different susceptibility to flower abscission may differ in the concentration and activity of this enzyme in the developing flower. This possibility is presently being investigated. A C K N O W L E D G E M E N TS This research was supported by Grant No. US-1706-89 from BARD, the United States-Israel Binational Agricultural Research and Development Fund and by The Cooperative Arid Land Agricultural Research Program (CALAR II), an AID Fund, contract no : GNE-0158-G0017-00. LITERATURE CITED Aloni B, Pashkar T, Karni L. 1991 a. Nitrogen supply influences carbohydrate partitioning in pepper seedlings and transplant development. Journal of the American Society of Horticultural Science 116 : 995–999. Aloni B, Pashkar T, Karni L. 1991 b. Partitioning of [14]-C sucrose and acid invertase activity in reproductive organs of pepper plants in relation to their abscission under heat stress. Annals of Botany 67 : 371–377. Beyer EM, Morgan PW. 1971. 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