Odontonema Strictum): Effects of Photoperiod and Duration of Short Day Exposure
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Int.J.Curr.Microbiol.App.Sci (2019) 8(5): 2384-2390 International Journal of Current Microbiology and Applied Sciences ISSN: 2319-7706 Volume 8 Number 05 (2019) Journal homepage: http://www.ijcmas.com Original Research Article https://doi.org/10.20546/ijcmas.2019.805.282 Floral Initiation in Red Firespike (Odontonema strictum): Effects of Photoperiod and Duration of Short Day Exposure Amir Rezazadeh1* and Richard L. Harkess2 1Mississippi State University, Coastal Research and Extension Center, Poplarville, MS 39470, 2Mississippi State University, Department of Plant and Soil Sciences, Starkville, MS 39759 *Corresponding author ABSTRACT Red firespike (Odontonema strictum) is a tropical flowering plant that has a potential as a K e yw or ds new flowering potted plants for its attractive red inflorescence. Flowering and morphology of red firespike were evaluated in response to photoperiod. In Expt. 1, morphology and Floral initiation, growth characteristics of firespike were evaluated in response to photoperiod. Plants were Floral bud propagated from semi-softwood cuttings and grown under long days (LD). Plants were differentiation, exposed to short days (SD) of 9 h for 0, 4, 8, 12, 16, 20, 24, 28 or 32 days. Plant height and Reproductive phase, growth index increased as the number of SD increased. Plants that received 16 or more SD Short -day reached anthesis earlier than plants grown under fewer SD. The number of plants that Article Info flowered was higher in 32 SD than in lower SD. In Expt. 2, plants were placed in SD for 0, 8, 16, 24 or 32 days and then apical meristems were examined histologically to determine Accepted: the pattern of flowering in firespike during the time prior to anthesis. Plants that received 18 April 2019 only 8 SD showed early floral initiation and after 16 SD, meristems continued Available Online: reproductive differentiation even when returned to LD. Under the experimental conditions 10 May 2019 tested, the most rapid, uniformed, and complete flowering of firespike occurred when plants received 24-32 days of inductive signal (SD). Introduction valuable in production and in planning the appropriate time for marketing (Challa, 1985). Initiation of flowering is a highly regulated The current market often requires flowering process. Plants switch from vegetative to plants produced for a specific date or holiday reproductive development by floral induction, such as Christmas. For commercially which depends on endogenous and important plants, it is inevitable to construct environmental signals such as day length prediction models for the flowering of these (Erwin, 2006; Noy-Porat, et al., 2009). Thus, plants (Erwin and Heins, 1990). time of floral initiation and anthesis can be predictable that is important for commercial One of the most important events in the life growers. The ability to predict when a certain cycle of flowering plants is the transition from plant developmental stage will occur is vegetative to reproductive growth. Floral 2384 Int.J.Curr.Microbiol.App.Sci (2019) 8(5): 2384-2390 induction and initiation are the first stages factors controlling floral induction and preceding floral bud differentiation. Induction initiation (Colasanti and Sundaresan, 1996). A is a key developmental stage controlled by small change in daylength leads to change external and internal factors acting to direct from vegetative growth to reproductive flowering (Wang et al., 2014). Many plant development (Runkle and Heins, 2006). In species have the ability to initiate flowering in some tropical plants, floral induction starts in response to environmental factors such as response to decline in photoperiod of 30 min change in daylength (photoperiod). Daylength or less. is a controlling factor for flower induction and initiation for many plants. Plants flowering in Red firespike is an attractive plant commonly response to photoperiod have been classified cultivated as a background plant in gardens as long-day (LD), day neutral (DN) and short- and sometimes is used to make hedges in day (SD) plants (Thomas and Vince-Prue, tropical and subtropical climates (Watkins, 1997). Long day plants flower when the 1975; Whistler, 2000). It is native to the daylength exceeds their critical photoperiod. tropical and sub-tropical regions of Central Day neutral plants are insensitive to daylength America (Daniel, 1995). The plant has and SD plants flower when the daylengths are potential as a flowering potted plant due to its less than a critical photoperiod. Short day and attractive red inflorescence, dark green LD plants may also be categorized as foliage, and natural bloom time (Rezazadeh et facultative or obligate for photoperiod al.,. 2015). Wang et al., (2014) showed that requirement where flowering is hastened by floral transition is roughly predictable photoperiod in facultative plants or a critical according to vegetative growth characteristics. photoperiod is required in obligate plants The correlation between vegetative growth (Thomas and Vince-Prue, 1997). Critical morphology (i.e. plant height, growth index, photoperiod is the factor that if met, induce number of branches, etc.) and meristematic flowering in both long day and short day changes in the apical meristem during floral plants. transition is important to understand production scheduling. However, if plants receive insufficient inductive signal, floral meristem revers to Previous studies on environmental factors producing leaves instead of developing affecting flowering of red firespike only reproductive organs, a phenomenon called discussed time to visible flower (Garofalo, flower reversion (Battey and Lydon, 1990). If 2002). Red firespike is reported to be a SD short-day plants are transferred to the long-day plant (Garofalo, 2002). However, there is condition after floral induction or initiation, currently no information available on floral development can be inhibited, photoperiod requirements of firespike for particularly if the transfer occurs early in flower induction. Meristematic development floral development (Adams et al., 1998). One during transition to flowering in red firespike way to determine the minimum number of has not been characterized nor has the timing inductive photoperiodic cycles necessary for of the events during transition from vegetative floral initiation is to limit the inductive to reproductive stage been reported. The aims photoperiod or transferring deferent numbers of the present investigation were to determine of plants back to the non-inductive condition the minimum number of short-days needed to (Damann and Lyons, 1993). Daylength induce full flowering and to evaluate the effect response varies widely between species and is of photoperiod on growth and flowering on one of the most important environmental red firespike. 2385 Int.J.Curr.Microbiol.App.Sci (2019) 8(5): 2384-2390 Materials and Methods cuttings), and number of plants reaching first floret opening (anthesis) were counted. Data Vegetative stock plants of red firespike were were analyzed using the GLM procedure of maintained under LD using 4-hour night SAS (version 9.3; SAS Institute, Cary, NC). interruption lighting with 60-W (3 to 4 Means were separated using the least µmol.m-2.s-1) incandescent lights from 22:00 significant difference (LSD) test at P ≤ 0.05. to 02:00 HR for 2 weeks to prevent floral initiation occurring before start of In experiment 2, red firespike cuttings were experimental treatments. On January 2014, rooted and grown under LD as previously medial, single-stemmed plants were described. Plants then received different propagated from semi-softwood cuttings of photoperiod treatments of 0, 4, 8, 12, 16, 20, red firespike. Four weeks later, the rooted 24, 28 or 32 SD before transfer back to LD. cuttings were transplanted to 15 cm diameter One group of plants as control was exposed to pots containing a commercial potting substrate continuous LD. (Sunshine Mix 1; SunGro Horticulture, Bellevue, WA) and grown under natural After each SD treatments, plants were exposed daylength (12 day-light hours) plus a 4-hour to 0 or 8 LD treatments to see if floral night interruption as described before. Plants reversion occurs. Ten plants were used in a were grown at 23±0.5 °C in greenhouse and completely randomized design for each liquid fertilizer at 200 ppm nitrogen was photoperiod treatment. Three plants from each provided with at each irrigations (Peters Peat- treatment were sampled for histological study Lite 20–10–20; Scotts, Marysville, OH). after a subsequent exposure to 0 or 8 LD. The apical meristem of the plants was excised and Two experiments were performed. In immediately fixed in FAA (5 parts formalin, 5 experiment 1, plants were exposed to parts glacial acetic acid, 90 parts 50% ethanol) photoperiod of 0, 8, 16, 24 or 32 SD before for a minimum of 14 days at ~5 °C before being returned to LD. Long day photoperiod dehydrating in an ethanol series and was provided using 100 W incandescent light embedding in paraffin for sectioning. The mounted above each bench in greenhouse. In embedded meristems were sectioned at 10 µm, addition, control plants were exposed to stained with hemalum/erythrosine, and continuous LD until termination of examined under light microscopy for experiment. Short-day treatments were anatomical differences (Harkess and Lyons, provided in different growth chambers using a 1994). The morphological characteristics of combination of cool-white fluorescent and each sample were used to describe bud incandescent lamps from 08:00 to 17:00 HR differentiation during transition to flowering and irradiance was 200 µmol.m-2.s-1. The and each differentiation phase classified temperature and relative humidity (RH) in numerically into different stages. Apical growth chambers were fixed at 23˚C and 70%, meristematic development was evaluated and respectively. Ten plants were used in a classified into one of five stages: Stage 1) completely randomized design. Plant height vegetative (Fig. 1A); Stage 2) floral initiation (from the soil level to the top point of the with upward rise of the tunica and corpus (Fig. plant) was measured at termination of the 1B); Stage 3) periclinal division of cells in the study.