Coriander (Coriandrum sativum L.) or cilantro, as it is commonly called in the United States, is an herbaceous, annual plant with a basal rosette growth habit that belongs to the family Apiaceae and is widely cultivated throughout the world (Morales-Payan, 2011). Both the seeds and vegetative parts of the plant contain high levels of essential oils and organic compounds with broad medicinal properties (Laribi et al., 2015). Cilantro is grown mostly for its dried fruits, although its highly aromatic leaves, which are used to flavor a variety of dishes from salsas to curries, are the main part of the plant consumed in the United States. Its origin has not been conclusively determined, but most accessions can be placed into one of two broad categories: 1) those that originated in the Caucasus and Central Asia and are characterized by a prolonged vegetative growth state with many basal rosette leaves and 2) those that originated in the Near East and the Indian subcontinent where cultural selection favored quick flowering and large fruits over leaf production (Diederichsen, 1996).
Commercially grown cilantro is commonly cultivated in open field systems and suffers from relatively few production problems, although bacterial leaf spot, Phytophthora root rot, and Fusarium wilt can sometimes cause economic damage (Dugan et al., 2017; Pernezny et al., 1997). Several studies have explored various treatments and storage methods to increase its postharvest shelf life (Hassan and Mahfouz, 2012; Luo et al., 2004), and the effects of fertilization and water use have also been investigated extensively (Angeli et al., 2016). However, one persistent physiological trait of cilantro that has received proportionately little attention is its propensity to “bolt” or initiate rapid stem elongation from the rosette and flower under certain environmental conditions, causing the leaves to assume an unpleasant flavor (Bashtanova and Flowers, 2011). This averse flavor, which is most likely due to changes in the essential oil composition in the leaves (Potter, 1996), renders the crop unmarketable. During the winter in south Florida, it is possible to harvest a crop twice before bolting commences, but this becomes increasingly difficult as temperatures start to rise (Chuck Obern, personal communication, 2016). The relationship of cilantro bolting to increases in temperature and LD environmental conditions has been suggested in several extension publications; however, these provide little more than general observations on yield and early flowering (Blade et al., 2016; Smith et al., 2011).
Putievsky (1983) showed that LD conditions accelerate cilantro flowering but reduce overall yield, and Nawata et al. (1995), in their survey of cilantro landraces in southeast Asia, gathered evidence suggesting that cilantro is a facultative LD plant. Tomitaka et al. (2001) later provided support for these results by examining the stages of floral development in relation to changes in daylength through a series of controlled greenhouse experiments. Their results showed a strong, negative linear relationship between photoperiod and the elapsed time until flowering, even though flowering eventually occurred in all photoperiods tested. In their study, flower bud pre-differentiation occurred at an average of 49 d after seeding under daylength neutral conditions. This is in agreement with values published elsewhere (Diederichsen and Hammer, 2003).
Plant growth, development, and organ ontogenesis are complex processes, which are highly regulated by endogenous plant hormones and genetic and epigenetic control of regulatory pathways (Bishopp et al., 2006; Johnson and Lenhard, 2011; Pikaard and Scheid, 2014). Floral development and regulation in several other Apiaceous species are well understood and were shown to be regulated mainly through photoperiod and vernalization. Dill (Anethum graveolens), an LD plant, can be induced to flower once exposed to a single 11-hour daylength cycle, and early flowering is accompanied by lower vegetative yields (Hälvä et al., 1993; Hamner and Naylor, 1939). Similarly, fennel (Foeniculum vulgare) will bolt after being exposed to several photoperiod cycles for a minimum of 13.5 h (Peterson et al., 1993). Several other studies reported that celery (Apium graveolens) is a day-neutral species that is induced to flower by vernalization (Jenni et al., 2005), although LD conditions after vernalization treatments greatly reduced the time to bolting (Ramin and Atherton, 1994).
Floral induction in Arabidopsis thaliana, also a facultative LD plant with basal rosette growth, has been studied extensively and is highly regulated by several integrated pathways that monitor both environmental and plant internal conditions (Simpson and Dean, 2002). Bioactive gibberellins (GA) are a class of endogenous phytohormones that positively direct seed germination, stem elongation, and floral initiation and significantly interact with the auxin–cytokinin pathways for root and shoot meristem development (Vanstraelen and Benková, 2012). Arabidopsis mutants deficient in GA synthesis genes show extreme dwarfing phenotypes with tight rosette patterns and are nonresponsive to LD inductive conditions, supporting the role that GA plays in integrating these two pathways (Reeves and Coupland, 2001).
Several studies have investigated the effects of exogenously applied GA in cilantro to increase yield. Das et al. (2018) found that a 20 ppm GA application at 25 and 30 d after seeding increased leaf yield by 47%, but they did not comment on the incidence of preharvest flowering. Morales-Payan and Stall (2004) also observed a significant yield increase with GA applications. GA applied at rates of 10 and 20 g·ha−1 to plots of cilantro at 7 WAS had no effect on overall yield but significantly increased preharvest bolting and plant height (Kahn and Maness, 2010). When applied to plots at rates of 50 to 75 ppm, GA increased the number of flowering umbels, plant height, seed yield, and size and decreased the days to flowering and seed maturity (Yugandhar et al., 2016). Although early flowering and increased seed yield are beneficial traits for cultivation of coriander fruit, this is not the case when leaf production is the objective. Applying auxin or cytokinin to the foliage to prevent early bolting has been proposed, but no evidence was provided for its efficacy (Morales-Payan, 2011), and to date, no studies have specifically examined any methods to reduce the incidence of preharvest bolting in the field.
PGRs are synthetically produced compounds which alter growth and development and can attenuate the effects of environmental conditions and of endogenous phytohormones such as GA (Rademacher, 2015). Some compounds such as the diethanolamine salt of mefluidide (MFL) are cell division inhibitors and are used strictly to inhibit seedhead formation in turfgrass (Haguewood et al., 2013). Other PGRs, such as chlormequat chloride (CQC), paclobutrazol (PBZ), flurprimidol, and prohexadione calcium (PCa), directly inhibit GA biosynthesis (Rademacher, 2016). Currently, PCa has widespread use in controlling fire blight in apples (McGrath et al., 2009), reducing vegetative growth in peanuts (Beam et al., 2002), and decreasing shoot growth in apples and cherries (Cline, 2017). Although PBZ is mainly used in the ornamental plant industry, it has some efficacy in reducing shoot growth in tropical and subtropical tree crops (Rademacher, 2016).
It is possible to control certain environmental conditions within a greenhouse setting; however, it is currently not practical to do so in open production systems. Given our assumptions that cilantro is induced under LD conditions and that GA at least partially controls stem elongation and floral initiation in cilantro, we hypothesized that PGRs that inhibit GA biosynthesis would decrease the incidence of early bolting in the field. The objective of our study was therefore to determine the efficacy of several commercially available GA inhibitors to prolong the vegetative stages of cilantro in the field and to identify the appropriate time at which to apply these products.
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