agronomy Article From Emergence to Flowering: Four Beet (Beta vulgaris ssp.) Cultivars’ Phenological Response to Seed Priming Zahra Salimi and Birte Boelt * Department of Agroecology, Aarhus University, 4200 Slagelse, Denmark; [email protected] * Correspondence: [email protected]; Tel.: +45-8715-8276 Received: 18 November 2019; Accepted: 6 December 2019; Published: 10 December 2019 Abstract: Beta vulgaris comprise various variety groups that are cultivated for vegetative or reproductive production. As a biennial crop, beets have a photothermal induction requirement for the transition from the vegetative to the reproductive stage. Bolting and flowering are not desirable when beets are cultivated for vegetative production and are suppressed during breeding programs, though these structures are important in seed production. Therefore, the potential of seed hydro priming as an enhancement technique to partially induce vernalization in seeds was evaluated. Following hydro priming, seeds were sown in October 2018 and evaluated during three selected phenological stages. Treating seeds with hydro priming significantly improved the emergence rate in all four tested cultivars. Moreover, treatments significantly lowered the required growing degree days (GDD) for transition to the reproductive stage in all four tested cultivars. Regardless of the treatment effect on an individual developmental stage, the treatment efficiency should be evaluated on the whole production process, depending on the purpose of production. The focus of this study was mainly on improvement of seed production performance. The outcome showed the potential of priming to influence the reproductive stages of the plant life cycle rather than just the germination. Keywords: vernalization; bolting; seed enhancement; seed production; photothermal induction; GDD; topping 1. Introduction In the earliest records, beets were described as leafy plants (chards). Later, swollen root cultivars were developed. Cultivation of red fodder beet dates back to the 15th century, while the sugar beet has been cultivated since the end of the 18th century [1]. Beta vulgaris is a member of the Amaranthaceae family [2], and comprises a variety of groups of Swiss chard, garden beet (beetroot and red beet), leaf beet, fodder beet, and sugar beet [3]. Each of these groups has a different usage in industry and consumption. In Europe, sugar beet is the main crop for sugar production [3,4]. Moreover, it provides clean energy through bioethanol and the production of hydrogen gas. The high concentrations of betaine and betalain in beetroot has made it a valuable resource for pharmaceutical and food companies [5]. As a biennial crop, beets require a photothermal induction of the transition from the vegetative to the reproductive stage. The first element in this induction is vernalization. Vernalization refers to the plant’s requirement of low temperatures during the rosette stage for the induction of flowering. The range (0–15 ◦C) and duration (5–20 weeks) of low temperatures play an important role in the extent of vernalization [6–10]. If the extent of vernalization is insufficient, the plant can be devernalized by temperatures above 15 ◦C immediately following vernalization [11,12]. Agronomy 2019, 9, 863; doi:10.3390/agronomy9120863 www.mdpi.com/journal/agronomy Agronomy 2019, 9, 863 2 of 14 Agronomy 2019, 9, x FOR PEER REVIEW 2 of 14 The completion of vernalization initiates stem el elongation.ongation. The The main main stem stem in in beets is called the bolter and the process of stem elongation is termedtermed bolting.bolting. The tendency of beets toward bolting varies in different different groups of cultivars and even be betweentween cultivars cultivars of of the the same same variety variety group group [13–15]. [13–15]. Rather than vernalization initiation in the plant du duringring the rosette stage, seeds can be be affected affected by the environment, while they are still on the mother plant.plant. If the temperatures drop considerably during floweringflowering and seed ripening, seeds can be partially vernalized [[16].16]. Following bolting, the plants require long days to to induce induce flowering flowering [10,15], [10,15], and and interior interior (genetic) (genetic) and exterior (environment and agricultural management) elements can affect affect this developmental stage [17]. [17]. The The acquisition acquisition of of flowering flowering ability ability can can be be established established in incell cell memory memory by bydegradation degradation of macromoleculesof macromolecules and and thus thus maintained maintained until until the the optimal optimal conditions conditions are are available available [18]. [18]. Generally, Generally, floweringflowering of beets occurs over 3–4 weeks. The The durati durationon of of this this stage stage can can affect affect the range of maturity levels among the harvested seeds [[3,8].3,8]. Production of beet seeds has various challenges. First, First, plants plants need need to fulfil fulfil the photothermal induction requirementrequirement prior prior to to the the reproduction reproduction stage, stage, which which leads leads to ato long a long seed seed production production process process and anda delayed a delayed breeding breeding cycle. Furthermore,cycle. Furthermore, photothermal photothermal induction requirementsinduction requirements limit the geographical limit the geographicaldistribution ofdistribution seed production of seed [ 19production]. Since breeding [19]. Since eff breedingorts have efforts focused have mostly focused on themostly vegetative on the vegetativeproduction production of beets, they of beets, have they centered have oncentered the suppression on the suppression of bolting, of due bolting, to the due negative to the negative effect of effectbolters of on bolters yield on [3, 13yield–15 ,[3,13–15,18].18]. To increase the number of seed bearers, clipping or topping has been applied on the primary bolters in the seed production process. The The purpos purposee of of this this common technique technique is is to improve the development ofof side side branches, branches, more more homogeneous homogene flowering,ous flowering, enhanced enhanced synchronization synchronization of flowering, of flowering,and reduced and plant reduced height. plant This height. technique This istechniqu usuallye carriedis usually out carried during out the boltingduring the stage bolting by hand stage or bymechanically hand or mechanically to improve to seed improve production seed production [3,8]. [3,8]. In Beta vulgaris,, seedsseeds developdevelop inside inside the the fruit. fruit. This This irregular irregular dry dry body body consists consists of of a pericarp a pericarp and and an anoperculum operculum forming forming the ovarythe ovary cap, whichcap, which covers covers the true the seed. true The seed. true The seed true can seed consist can of consist 2–5 embryos, of 2–5 embryos,which are which supported are supported nutritionally nutritionally by a diploid by perisperm,a diploid perisperm, instead of instead a triploid of endosperma triploid endosperm [1,3,20,21]. [1,3,20,21].They contain They more contain than more one embryo than one and embryo therefore and the therefore seedsare theconsidered seeds are considered multigerm multigerm (Figure1). (FigureHowever, 1). seedsHowever, with seeds one embryo with one (monogerm embryo (monogerm seeds) have seeds) been have produced been produced mechanically mechanically from the frommultigerm the multigerm ones [1]. ones Eventually, [1]. Eventually, genetically genetically monogerm monogerm cultivars cultivars were released were released commercially commercially in the 1960sin the [1960s22]. [22]. Figure 1. MonogermMonogerm ( A)) and and multigerm multigerm ( (B)) beet beet seed. Using seeds of a good good quality quality is is a a crucial crucial factor factor in in attaining attaining fast, fast, uniform, uniform, and and steady steady emergence, emergence, especially with monogerm seeds [3,23]. [3,23]. Seed Seed enhanc enhancementement techniques techniques are are beneficial beneficial in improving improving the seed performance. Techniques Techniques that that can can be be applied to initiate the preliminary process of metabolite activation in the reversible part ofof germinationgermination cyclecycle cancan alsoalso bebe beneficial.beneficial. Priming Priming is is a seed enhancement techniquetechnique which which has has proved proved to to have have reliable reliable eff ectseffects on seedon seed performance performance [21,23 [21,23–29].–29]. In fact, In fact,priming priming regulates regulates hydration hydration of the seeds,of the but seeds, not tobu thet not point to the of radicle point protrusionof radicle [protrusion21,23]. During [21,23]. this Duringprocedure, this metabolic procedure, processes metabolic begin, processes but do begi not proceedn, but do far not enough proceed for actualfar enough germination for actual [30]. germination [30]. Timing is important in this process to avoid “overpriming”. The probable reason of the occurrence of this phenomena is degradation of Late embryogenesis abundant (LEA) proteins [31]. Agronomy 2019, 9, 863 3 of 14 Timing is important in this process to avoid “overpriming”. The probable reason of the occurrence of this phenomena is degradation of Late embryogenesis abundant (LEA) proteins [31]. Different priming techniques can be applied to seeds. The most important factor in these is the priming agent, which can be either water or chemical.