INFLUENCE OF PHOTOPERIOD ON IMPROVED 'WHITE SIM' CARNATION (DIANTHUS C A R Y O P H Y L L U S L.) BRANCHING AND FLOWERING

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1 INFLUENCE OF PHOTOPERIOD ON IMPROVED 'WHITE SIM' CARNATION (DIANTHUS C A R Y O P H Y L L U S L.) BRANCHING AND FLOWERING R. D. Heins and H. F. Wilkins Department of Horticultural Science University of Minnesota St. Paul, Minnesota U.S.A. Paper No. 9835, Scientific Journal Series, Minnesota Ag. Exp. Sta. Abstract Plants were rotated between long day (LD), normal days (ND), and short days (SD) 0, 21, 42, 63, or 84 days after removing the terminal shoot from cuttings. SD stimulated lateral branching which is required for rapid future flower production while LD inhibited it. The SD inhibited stem elongation and flowering while LD stimulated both. The SD given to shoots that were not sufficiently developed or that were reproductive had no affect on subsequent lateral branching. Introduction The time from planting to anthesis of a cutting is economically important to growers. The time required to produce a return crop and the number of flowers are equally important. Two factors affecting the above are the length and number of lateral shoots present when the first flower crop is harvested. Although a lateral bud differentiates at every node, active elongation does not necessarily occur. Ideally, growers would like the lateral buds on the lower stems to be actively growing when the first flowers are harvested, as a 5 cm shoot will flower 30 to 60 days earlier than a blind cut (Koon, 1958). Blake (1955) classified the carnation as a facultative LD plant. Many researchers (Blake, 1955; Pokorny and Kamp, 1960; Harris and Griffin, 1961; and Freeman and Langhans, 1965) have shown that flowering is faster under LD when compared to SD. However, LD treatments reduce lateral shoot development (Novovesky, 1967). Pokorny and Kamp (1960) found that plants under SD had many laterals and flowering was delayed. Thus, we asked, "Was it possible to give SD to Lhe plants at one time of development and LD at another to get both lateral shoot development and rapid flowering?" Materials and Methods 'Improved White Sim' cuttings were potted (7.5 cm) on 14 June 1976 and grown under normal day (ND) lengths at 45 N. Temperatures were 18.5 /12.5 C (day/night) or as low as could be controlled by fan and pad cooling. The terminal shoot was removed 14 days after planting. Twenty-one days later plants were selected with four primary (1 ) lateral shoots and repotted (12.7 cm). There were 5 plants in each of 2 blocks for the 15 treatments (figure 1) in a Randomized Complete Block design. SD were from 1600 to 0800 and LD were provided by a 5 hr night interruption (2200 to 0300) of 100 watt incandescent light bulbs spaced 90 cm apart and 90 cm above the plant tops (45 lux; 8.5 microeinsteins/ Acta Horticulturae 71, 1977 Carnations 69

2 m^). The data were taken with respect to I o lateral shoot origin on the plant. The top bud was designated as shoot number one and the bottom as number four. Results Development of the four primary (I o ) lateral shoots did not proceed at the same rate after apical shoot removal. A decreasing rate occurred from the acropetal to the basipetal I o lateral shoots. Basipetal shoots took longer to flower, had longer stems, more nodes, and fewer vegetative laterals (table 1) when compared to the acropetal. LD plants flowered earliest and had the lowest number of nodes and actively growing lateral shoots. SD plants flowered last and had the highest number of nodes and actively growing lateral shoots. ND plants were intermediate in these respects (table 2). Shoots were most sensitive to photoperiod from the 7th to the 9th week after the terminal shoot removal. Treatments Ic and lie are comparable except the SD and subsequent LD were given 21 days later in lie. This 21 day delay resulted in a 28 flowering delay and increased the number of actively growing vegetative 2 lateral shoots from 2.5 to 2.9 per stem (table 1). The contrast^ig treatments (LD first, then SD), IIIc and IVc, show the opposite respoiwe. The 21 day delay in LD treatment resulted in a 24 day flowering advance and decreased the number of actively growing vegetative laterals from 3.1 to 2.7. Both Ic and IVc (LD - 7th to 9th week) advanced anthesis dates and decreased the number of actively growing lateral shoots when compared to lie and IIIc which received SD during this period. An interaction between time - treatment showed that a shoot's response to any photoperiod is dependent on its stage of development. Treatments Illb and IVb are comparable except LD and subsequent SD were given 21 days later in IVb. The I o lateral shoots in IVb had developed more leaf pairs than Illb when given LD. Therefore, all the I o laterals responded to LD during the 7th to 9th week in IVb. In Illb only the top two I o lateral shoots were responsive to LD during the 4th to 6th week (table 3). The SD that followed in Illb stimulated 2 vegetative shoots on the basipetal I o shoots which had not initiated a flower bud but did not stimulate such a response in IVb as flower initiation had occurred. The results of IIIc and IVc were similar but not dramatic. As with LD, shoots must reach a certain developmental state to perceive SD. If SD are given prior to this stage and LD follow, the lateral shoots will not respond to the SD. Because of slower basipetal development, these shoots did not perceive the SD (4th to 6th week) as the acropetal shoots did in lb (table 4). Thus, when LD followed SD, rapid flower initiation occurred and 2 laterals did not develop in the basipetal shoots (3.1 vs 0.6 vegetative 2 ). Under continuous LD, shoots developed 0.3 to 1.5 vegetative 2 lateral shoots (total data not presented). When SD are delayed 21 days (lib), basipetal I o laterals developed sufficiently to perceive SD and more 2 shoots developed when compared to lb (2.1 in lib vs 0.6 in lb). 70

3 Discussion Many photoperiod studies on flowering have neglected its affect on lateral branching. Porkorny and Kamp (1960) reported SD plants had more lateral shoots than LD. Novovesky (1967) reported 4-hr night interruptions inhibited lateral shoots. Thus, LD have had their greatest commercial advantage with two year old plants soon to be discarded (Holley and Rudolph, 1969). However, judicious use of LD is feasible year-round (Koon, 1976). These data show that photoperiod duration - timing can control vegetative lateral shoot numbers. Appr >riate photoperiods must be given at critical shoot development stages which appears to be immediately prior to floral initiation. After initiation temperature and irradiance become the dominant factor in flower development, not photoperiod. We would like to present the following model on lateral shoot development. Under a given environmental condition (radiant energy, nutrition, and temperature) a shoot will become reproductive depending on photoperiod after the formation of a certain leaf number. SD will delay while LD will hasten initiation. By slowing this process, active lateral bud development occurs. Once the shoot is reproductive, the flower is the dominant sink and lajferal shoot activation is inhibited. An analogy is the lily meristem, where slowing its development rate by lower temperatures during the vegetative to reproductive period permits more floral buds to differentiate (Roh and Wilkins, 1977). If this hypothesis is correct, slowing development during the transition from the vegetative to the reproductive state in the carnation should also increase lateral shoot numbers. Once active growth is started, a rapid transition to the reproductive state using LD should be feasible. This should allow for rapid cropping without the loss of future lateral breaks. Slowing the development while the shoot is very small or after it is reproductive will have little effect on lateral shoot activation and subsequent cropping. We feel that SD given 5 to 7 weeks after pinching followed by 3 weeks of long days would give an optimal response of both lateral shoots and future flower initiation and production. Flower production over a two year period tends to follow total solar radiation curves, decreasing in winter, increasing in the spring. We ask when were these flowering shoots originally initiated? Since it takes 5 to 6.5 months for a lateral shoot to flower (Homan, 1974), we counted back and found that winter production was initiated during the LD of summer and the larger spring and summer production was initiated under the SD of winter. This supports the hypothesis that LD and rapid flower initiation inhibits lateral shoot activity while short days and slow flower initiation stimulate lateral shoot formation. The 1 basipetal shoots on a pinched stem by the very nature of their lower position on the plant, receive less solar radiation. Further, leaves are red filters allowing more far red light to pass (Federer and Tanner, 1966). Far-red light stimulates stem elongation and inhibit lateral shoots (Tucker, 1976). Result shown in table i be partly 71

4 explained by photoperiod and stage of shoot development. be explained by the leaf filtering and far-red light. Others could Lastly, the far-red light ratio increases at sunrise and sunset (Shorpshire, 1971). The atmosphere is a filter and the further north (or south) from the equator, the greater the filtering effect. In the summer, long twilight periods are high in far-red light (higher latitudes as in northern Europe). A dual effect of long photoperiods and high far red light at dusk and dawn could greatly reduce lateral shoots and flowering in the winter. We can only postulate the effect on summer cutting production if these data and theories are correct. References Blake, J., Photoperiodism in the Perpetual Flowering Carnation. 14th Intern. Hort. Cong. 33: Freeman, R., and Langhans, R.W., Photoperiod Affects Carnations. N.Y. State Fl. Grow. Bull. 231:1-3. Federer, C.A., and Tanner, C.B., Spectral Distribution of Light in the Forest. Ecology 47(4): Harris, G.P., and Griffin, J.E., Flower Initiation in the Carnation in Response to Photoperiod. Nature 191:614. Holley, W.D., and Rudolph, C., Lighting of Carnations. Col. Fl! Grow. Ass. Bull. 224:1-4. Homan, T.P., and Holley, W.D., The Flowering of Return Carnation Crops from Multiple Breaks. Col. Fl. Grow. Ass. Bull. 287:1-2. Koon, G.C., Continuous Culture of Carnations. Col. Fl. Grow. Ass. Bull. 108:1-3. Koon, G., Personal communication. Novovesky I, M.P., Effects of Photoperiod and CO2 Enrichment on Carnations. Col. Fl. Grow. Ass. Bull. 209:1-2. Pokorny, F.A., and Kamp, J.R., Photoperiodic Control of Growth and Flowering of Carnations State Flor. Ass. Bull. 202:6-8. Shropshire Jr., W., Photoinduced Parental Control of Seed Germination and the Spectral Quality of Solar Radiation. Solar Energy, 15: Tucker, D.J., Effects of Far-red Light on the Hormonal Control of Side Shoot Growth in the Tomato. Ann. Bot. 40: Roh, S.M., and Wilkins, H.F., Even Higher Flower Bud Numbers Are Now Possible in Easter Lilies by Dipping Your Greenhouse Temperatures. Florists' Review 159(4127):33, Table 1 - The effect of 1 lateral shoot position on further development. Data averaged over all treatments. 1 lateral position Days to anthesis Node numbers Stem (cm) Total 2 vegetative laterals Vegetative laterals below cut

5 Table 2 - Days to anthesis from date terminal bud was removed and the number of nodes and vegetative lateral shoots. Data is averaged over all four 1 shoot positions. Days to Vegetative 2 lateral Node Treatment anthesis shoot numbers numbers ND SD LD la b c Ha b c Ilia b c IVa b c Table 3 - Effect of reversing SD and LD photoperiods at different development stages on the number of days to anthesis, node number, and total 2 vegetative laterals. Total Total 1 Days Number 2 Days Number 2 lateral to of veg to of veg position anthesis nodes laterals anthesis nodes laterals 11 lb IVb IIIc IV c 73

6 Table 4 - Effect of reversing LD and SD photoperiods at different developmental stages on the number of days to anthesis, node number, and total 2 vegetative laterals. 0 Total Total 1 Days Number 2 Days Number 2 lateral to of veg to of veg position anthesis nodes laterals anthesis nodes laterals lb Ic ""Ti lib lie Week TREATMENT IV Date # ; ND SD LD A B C A B C A B C A B C 6/28-7/4 1 7/5-11 :2 7/12-18 :3 i 7/ /26-8/1 :5 8/2-8 :6 8/9-15 :7 8/16-22 :8 8/23-29 :9 8/30-9/5 :10 9/6-12 :11 9/13-19 : 12 9/20-26 :13 9/27-10/3:14 10/4-10 :15 1 Figure 1 - Time and date of photoperiod. 7k