Petal Abscission in Rose Flowers: Effects of Water Potential, Light Intensity and Light Quality

Annals of Botany 78: 619 623, 1996 Petal Abscission in Rose Flowers: Effects of Water Potential, Light Intensity and Light Quality W. G. VAN DOORN* and A. VOJINOVIC Agrotechnological Research Institute (ATO-DLO), P.O. Box 17, 67 AA Wageningen, The Netherlands Received: 27 February 1996 Accepted: 16 May 1996 Petal abscission was studied in roses (Rosa hybrida L.), cvs. Korflapei (trade name Frisco), Sweet Promise (Sonia) and Cara Mia (trade name as officially registered cultivar name). Unlike flowers on plants in greenhouses, cut flowers placed in water in the greenhouse produced visible symptoms of water stress, depending on the weather during the experiment and on the cultivar. Cut Frisco roses showed no visible signs of water stress and the time to petal abscission was as in uncut flowers. In Sonia roses the symptoms of water stress varied from mild to severe, and the number of flowers in which the petals abscised varied from 1% (mild stress) to % (severe stress). An antimicrobial compound in the vase water of Sonia roses, or removal of the leaves, alleviated the symptoms of water stress and increased the number of stems in which the petals abscised. Cut Cara Mia roses showed severe symptoms of water stress in all experiments and petal abscission was found in only a few flowers, even when the stems were placed at 2 C and low photon flux (15 µmol m s ). Abscission in Sonia and Cara Mia roses was low or absent when the water potential of the leaves reached values below 2 MPa within the first 5 d of the experiment; such low values were not reached in Frisco roses. Addition of sucrose to the vase solution, together with an effective antimicrobial compound, had no effect on the time to petal abscission, at any light intensity. Placing flowers in far-red light also had no effect on abscission, compared with flowers placed in red light or white light of the same photon fluence. It is concluded that petal abscission in the rose cultivars studied is not affected by their water status unless the plants reach a low water potential (about 2 MPa) early on during vase life. Petal abscission is not inhibited by low light intensity nor affected by the Pr Pfr ratio. 1996 Annals of Botany Company Key words: Abscission, light intensity, petals, phytochrome, Rosa hybrida L., rose, sugars, water potential. INTRODUCTION In previous research it was found that the time to petal shedding in cut roses placed in water, in a climate-controlled room or in a greenhouse, was the same as in intact plants, provided that the temperatures were similar. These experiments were in winter, when the incident radiation was low. Cutting of the flowers and placing them in water resulted in visible symptoms of water stress, depending on the cultivar. Endogenous carbohydrate levels were reduced in plants placed in the climate-controlled room, owing to the low light levels. These results suggested that petal abscission in roses was remarkably insensitive to a decrease in both water potential and carbohydrate level, at least when these two factors concurred (van Doorn and Schro der, 1995). Other (preliminary) experiments on cut rose flowers showed, however, that severe wilting may occur within a few days of vase life, and that the petals of such flowers do not abscise. Petal abscission in roses is hastened by exogenous ethylene (Woltering and van Doorn, 1988), and as water stress may increase the rate of ethylene production in various plant parts (Apelbaum and Yang, 1981), including flowers (van Doorn and Reid, 1991), it may stimulate abscission. A low water potential can also directly affect abscission, as low water availability may inhibit metabolic processes such as * For correspondence. 35-7364 96 11619 5 $. the production and action of enzymes involved in the cell separation process. Placing roses under low light intensity (15 µmol m s ) in a climate-controlled chamber reduced the endogenous content of sucrose, glucose and fructose in the petal cells (van Doorn et al., 1991). In other experimental systems, external application of sucrose reduced flower abscission in Lathyrus odoratus (Mor, Reid and Kofranek, 1984), delayed leaf abscission in Phaseolus ulgaris (Martin, 1954), and had no effect on leaf abscission in Vigna radiata (Mao and Craker, 199), hence the effect of carbohydrates on stem and leaf abscission is as yet unclear, and little is apparently known about its effect on petal abscission. Placing the flowers at low photon flux density can alter their Pr Pfr ratio. Leaf abscission in Vigna radiata was hastened by far-red light and delayed by red light (Decoteau and Craker, 1987; Mao and Craker, 199). Petal abscission may similarly be increased under low photon flux density, through phytochrome control. In the present paper an attempt is made to separate the effects of water stress, photon flux density and light quality on rose petal abscission. We hypothesized that abscission would be hastened when the water stress is moderate and delayed or prevented after severe water stress. A series of flowers was placed in darkness which prevented water stress but resulted in low endogenous carbohydrate levels and in a change in the Pr Pfr ratio. Placing flowers in red and far- 1996 Annals of Botany Company Downloaded from https://academic.oup.com/aob/article-abstract/78/5/619/833 on 9 December 217

62 an Doorn and Vojino ic En ironmental Effects on Rose Petal Abscission red light further separated these effects. Tests included Frisco, Sonia and Cara Mia roses, cultivars which show little, intermediate and severe symptoms of water stress when cut and placed in water, respectively. MATERIALS AND METHODS Plant material Rose flowers (Rosa hybrida L.) of the cultivars Korflapei (trade name Frisco), Sweet Promise (trade name Sonia), and Cara Mia (trade name as officially registered cultivar name) were investigated. The cultivars are indicated by their trade names in this paper. Flowers were left on the plant in the glasshouses of the Department of Horticulture, Agricultural University at Wageningen, The Netherlands. The experiments occurred between Apr. and Oct. 1993. Temperatures and relative humidity (RH) were recorded constantly, on an hourly basis, and averaged 2 24 C and 55 71% in the various experiments, respectively. Other flowers were cut at the commercial cutting stage, at a stem length of 4 cm. Stems were placed in water within minutes of cutting, after recutting the basal 5 cm in air. Leaves on the basal 1 cm segment of the stem, when present, were removed. Individual stems were then placed in about 3 ml of vase solutions made from deionized water, at 2 C, 6% RH and a photon flux of 15 µmol m s (cool white fluorescent light; Philips 36W 84 lamps) for 12 h each day (7 to 19 h). Effects of darkness and light quality In order to investigate effects of light and RH, some cut flowers were placed under a polyethylene bag and compared with flowers outside the bag. The RH inside the bag was more than 9%, but some ventilation was allowed in order to prevent condensation. Other series were placed at 2 C, 6% RH in darkness, red or far-red light, and in a regime where red and far-red light were alternated (3 s red and 45 s far-red) or red and darkness were alternated (3 s red and 45 s darkness), as previously described (van Doorn and van Lieburg, 1993). Photosynthetically active photon fluence (4 7 nm) at the level of the uppermost leaves was 12 µmol m s averaged over s, both in the cyclic red darkness and the red far-red treatments, determined with a Li-Cor 18 spectroradiometer (Li-cor, Lincoln, NE, USA). Water potential The water potential was determined in leaf 3 below the flower head, using a Scholander pressure chamber. Leaves were wrapped in plastic bags prior to determination of the potential, with only the petiole extruding, and the bag was left around the leaf during the measurement. Five replicate stems were used. Chemicals Aluminium sulphate (Merck, Darmstadt, Germany) was included in the vase water at 8 gl, and silver nitrate (Sigma, St. Louis, MO, USA) at 2 mg l. Sucrose (Merck) was added at 2 g l. Aluminium sulphate and silver nitrate, at the concentrations used, have previously been found to adequately control bacterial growth in the vase water of rose stems (van Doorn, de Witte and Perik, 199). Flower diameter The maximum diameter of the flowers was measured at intervals using a digital micrometer (type 5-31, Mitutoyo, Veenendaal, The Netherlands). Abscission, wilting The time between the commercial cutting stage and abscission (more than 5% of petals lost) was measured by daily observation of individual flowers, using ten replications. The percentage petal abscission refers to the number of flowers showing 5% abscission of their petals. Similarly, the time between cutting and visible symptoms of petal wilting was measured in ten replications, and recorded daily. Wilting was considered mild when the petals and the leaves were somewhat flaccid, and severe when the leaves and the petals had clearly lost turgor and the pedicel showed bending. Statistics Results were compared by analysis of variance and F test at P 5, using the GENSTAT V Statistical Package (Rothamsted, UK). All experiments were repeated at least once. RESULTS Effect of cutting and placement in water in the greenhouse Frisco roses placed in water in the greenhouse opened and grew as non-cut flowers, did not show visible signs of wilting, and the petals of all cut flowers abscised as in the non-cut ones (Fig. 1A). Abscission occurred even in experiments in which the temperature in the greenhouse and the incident radiation was high. In cut Sonia roses placed in water in the greenhouse, flower growth and symptoms of water stress were largely dependent on the climate. During a cool and overcast period, growth and opening was similar to non-cut flowers, the flowers showed slight wilting, and the petals of all flowers abscised, with a time to abscission similar to that of non-cut flowers. During warm and sunny weather, petal growth was inhibited with respect to the uncut controls, the leaves and petals showed severe wilting, and only approx. 2% of the flowers showed petal abscission (Fig. 1B). In cut Cara Mia roses placed in water in the greenhouse symptoms of severe water stress were even found during a period of cool weather, and abscission in this cultivar was always low (Fig. 1C). Even when placed in water in the climate-controlled room (in the light) the number of stems showing abscission was very low (Table 1). Removal of the leaves of Sonia roses, prior to placement in water, decreased the symptoms of water stress in the flower heads, and increased the number of flowers with Downloaded from https://academic.oup.com/aob/article-abstract/78/5/619/833 on 9 December 217

an Doorn and Vojino ic En ironmental Effects on Rose Petal Abscission 621 A 1 B 1 12 8 4 5 12 8 4 5 1 2 3 1 2 3 C 1 12 8 4 5 1 2 FIG. 1. Flower diameter (, ) and petal abscission (percentage of flowers showing 5% abscission;, ) in Frisco (A), Sonia (B) and Cara Mia (C) rose flowers, left attached to the plant (, ) or cut and placed in water in the greenhouse (, ). Experiments in summer, during sunny and warm weather. 3 TABLE 1. Effects of fluorescent light (15 µmol m s ), of darkness at arious relati e humidities, and of red and far-red light at low photon fluence flux, on petal abscission in cut rose flowers of three culti ars placed in water in a climate-controlled room at about 2 C. The number in parenthesis refers to the percentage of flowers showing petal abscission Time to petal abscission (d) Treatment Frisco Sonia Cara Mia Fluorescent light (6% RH) 26 8 1 9 (1%) 14 5 2 3 (1%) (2 3%) Darkness (high humidity 27 4 2 3 (1%) 13 6 2 1 (1%) 17 8 3 1 (1%) due to plastic bag) Darkness (6% RH) 27 8 2 9 (1%) 14 1 2 (1%) (3%) Red (6% RH) 26 9 2 2 (1%) 13 2 1 8 (1%) (2%) Far-red (6% RH) 27 1 2 7 (1%) 14 4 2 2 (1%) (3%) Red Far-red (6% RH) 26 5 2 3 (1%) 14 7 2 (1%) (2%) Red Darkness (6% RH) 27 2 6 (1%) 13 9 2 (1%) (2%) abscission (Fig. 2A). The inclusion of antibacterial compounds, such as aluminium sulphate (Fig. 2B) or silver nitrate (not shown), in the vase solution also alleviated the symptoms of water stress and increased the number of flowers of which the petals abscised. The inclusion of sucrose (together with aluminium sulphate at 8 gl ) in the vase solution had no effect on abscission (Fig. 2B). Effects of placement in darkness or in red and far-red light Placement of cut Frisco or Sonia roses in darkness at 6% RH or at high humidity (more than 9%, due to placement under a polyethylene bag) did not change the time to petal abscission nor the number of stems showing petal fall (Table 1). When cut Cara Mia flowers were placed Downloaded from https://academic.oup.com/aob/article-abstract/78/5/619/833 on 9 December 217

622 an Doorn and Vojino ic En ironmental Effects on Rose Petal Abscission 1 A 1 5 5 1 5 B 1 2 1 2 FIG. 2. Petal abscission in Sonia roses, cut and placed in water in the greenhouse, during warm weather. A, Effect of leaf removal (, leaves removed;, leaves attached); B, Effect of aluminium sulphate ( 8 gl, ) or sucrose (2 g l ) and aluminium sulphate ( 8 gl, ) in the vase solution., Leafy control flowers. in darkness at high RH no symptoms of water stress were observed and all flowers shed their petals, but in darkness at 6% RH petal abscission occurred in a few stems only (Table 1). Red and far-red light, at low photon fluence, given at 6% RH, had no effect on the time to abscission in Frisco and Sonia roses, nor did it change the number of flowers exhibiting petal abscission (Table 1). Such treatments also did not change the number of Cara Mia flowers showing abscission, but as the number of abscising flowers was low the time to abscission could not be adequately determined (Table 1). Water potential The water potential of the leaves of well watered plants in the greenhouse was about 6 MPa, in all cultivars investigated. In cut flowers placed in water in the greenhouse the water potential always dropped and the onset of this drop and its rate depended on the cultivar and the weather conditions. A low water potential reached during the last 5 d prior to petal fall showed no correlation with abscission. 3 3. 1. 2. Water potential (MPa) 3. FIG. 3. Relationship between the lowest leaf water potential during the first 5 d of vase life and petal abscission in cut rose flowers of cultivars Frisco (, ), Sonia (, ) and Cara Mia (,,, ). (,, ) Control flowers, left uncut. (,, ) Flowers cut and placed in water next to the intact plants, in a greenhouse. The tests represent a range of weather conditions, from overcast and cool to sunny and warm. Also shown are Cara Mia flowers placed in water in a climate-controlled room at 2 C, in the light ( ) or in darkness at high RH ( ). In contrast, when a low water potential occurred within the first 5 d of vase life it was correlated with a low number of stems showing abscission. During the first 5 d of the experiment, the water potential of the leaves of cut Frisco flowers placed in the greenhouse fell to 1 1 MPa in experiments in which the number of sunshine hours per day was high. Under such conditions the water potential of Sonia roses fell to 2 4 MPa. The water potential of the leaves of Cara Mia roses was as low as 1 9 MPa even in experiments during cool weather. The relationship between the lowest leaf water potential during the first 5 d of vase life and petal abscission is shown in Fig. 3. Cut Frisco and Sonia roses placed in water in the climatecontrolled room reached a water potential usually not lower than 1 5 MPa (results not shown). In cut Cara Mia roses placed in water in the climate-controlled room, the water potential fell to 2 2 5 MPa in flowers placed in the light, but remained below 2 MPa when placed in darkness at high RH (Fig. 3). DISCUSSION The present results indicate that petal senescence, although insensitive to moderate water stress, is inhibited by severe water stress. In previous experiments, carried out in Jan. and Feb., during a period with little sunshine, no inhibition of petal abscission was observed in cut Sonia roses placed in water in the greenhouse (van Doorn and Schro der, 1995). This is now corroborated, but when the weather during the experiment was sunny and warm, Sonia roses placed in water in the greenhouse, next to the uncut flowers, exhibited little or no petal abscission. Whenever the water potential of the leaves in these experiments decreased to approx. 2 MPa during the first 5 d of the experiments, petal Downloaded from https://academic.oup.com/aob/article-abstract/78/5/619/833 on 9 December 217

an Doorn and Vojino ic En ironmental Effects on Rose Petal Abscission 623 abscission was strongly inhibited or absent. The inhibition was alleviated by removal of the leaves or the inclusion of an antibacterial compound in the water, which prevented the water potential falling to below 2 MPa. Rose cultivars differ greatly in the degree of water stress during vase life. Frisco roses usually show no symptoms of water stress, Sonia intermediate symptoms, such as lack of opening and slight wilting, and Cara Mia roses strong symptoms, including wilting and desiccation. The water potential of Frisco roses placed in water in the climatecontrolled room did not reach 1 5 MPa, but in Cara Mia roses it dropped to below 2 MPa in most experiments. Only when cut Cara Mia roses were placed in darkness at a high RH, abscission occurred in all stems and the water potential of the leaves did not drop below 2 MPa. In all three cultivars studied, therefore, petal abscission of cut flowers was as in uncut ones, provided that the water potential did not reach values below about 2 MPa. In flowers placed in the climate-controlled room for a few days the levels of sucrose, glucose and fructose in the petals is much lower than in flowers left uncut (van Doorn et al., 1991). Excised flowers placed in a climate-controlled room showed the same time to petal abscission as uncut flowers, which could relate to a combined effect of slight water stress and carbohydrate stress. When water stress was excluded, as in the experiments where cut flowers were placed under a polyethylene bag in the climate-controlled room, abscission was still not affected. Similarly, abscission was not changed when sucrose was fed via the vase solution, in cut flowers placed in the climate-controlled room. (It should be noted that the data of Fig. 2 show an increase of abscission in sucrose-treated flowers over controls, an effect due to the aluminium sulphate given with the sucrose. A sucrose treatment by itself is not relevant as it stimulates bacterial growth, which results in early water stress.) These results, therefore, indicate that the carbohydrate status of the petals has little effect on the abscission process. Placing plants in darkness often changes their phytochrome status. Far-red light has been reported to increase the rate of flower abscission, for example in Hibiscus (Heindl and Brun, 1983; van Lieburg, van Doorn and van Gelder, 199). Experiments in which cut roses were placed in red light, far-red light, and in alternating red and far-red, however, showed no effect of the treatments on petal abscission. A change in the Pr Pfr ratio, therefore, apparently has little effect on the process of cell separation in petals of cut roses. Petal abscission of roses is sensitive to ethylene (Woltering and van Doorn, 1988) and ethylene production is often stimulated by water stress (Apelbaum and Yang, 1981), by sucrose feeding (Mayak and Dilley, 1976) and by far-red light (Goeschl, Pratt and Bonner, 1967; Samimy, 1968). In carnation flowers, a sucrose treatment increased the rate of ethylene synthesis, but simultaneously reduced the sensitivity to ethylene (Mayak and Dilley, 1976). We did not determine the effects of the present treatments on ethylene synthesis and ethylene sensitivity, but if increased ethylene production occurred, it did not stimulate abscission. It is concluded that severe water stress inhibits petal fall in roses. Petal abscission was not affected by a reduction in the level of metabolically active carbohydrates, nor by its reversal by sucrose feeding. A change in the phytochrome status of the plants had no effect either. In roses, cutting and placement in water often inhibits petal abscission, depending on the cultivar, an effect which is apparently mainly due to early water stress. ACKNOWLEDGEMENTS We thank Prof. Daniel Co me (Universite Pierre et Marie Curie, Paris, France), for facilitating the visit of A. V. to ATO-DLO, and Dr. Peter van de Pol (Department of Horticulture, Agricultural University, Wageningen, The Netherlands) for allowing us to work in the Departments greenhouses. LITERATURE CITED Apelbaum A, Yang SF. 1981. Biosynthesis of stress ethylene induced by water deficit. Plant Physiology 68: 594 596. Decoteau DR, Craker LE. 1987. Abscission: ethylene and light control. Plant Physiology 83: 97 972. Goeschl JD, Pratt HK, Bonner BA. 1967. An effect of light on the production of ethylene and the growth of the plumular portion of etiolated pea seedlings. Plant Physiology 42: 177 18. Heindl JC, Brun WA. 1983. 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