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IntroductionFlowers are a critical determinate of even-tual yield of perennial fruit trees. Therefore, an understanding of the underlying mecha-nisms and influencing factors, as well as the general phenology of flower development in citrus, is critically important to the sustaina-bility of successful commercial citrus produc-tion. This series of articles will aim to provide South African citriculturists with a concise, up-to-date and comprehensive overview of the various stages of reproductive phenology of Citrus. This current article will be a general introduction to the physiology of citrus flow-ering, with the following two articles dealing with the phenology of flower development and morphogenesis from a flower to a fruit. InleidingDie aantal en kwaliteit van blomme in meer-jarige vrugte bome is een van die belangrikste faktore wat oeslading en vrugkwaliteit bepaal. Uiteraard is dit uiters belangrik om die onder-liggende meganismes en faktore wat blom-ontwikkeling beinvloed, asook die algemene fenologie van blom-ontwikkeling in Citrus te verstaan. Die doel van die reeks artikels is om die wetenskaplie literatuur oor reproduktiewe fenologie van sitrus in ‘n opgedateerde, same-vattende asook oorsigtelikewyse aan al die rol-spelers in die Suid-Afrikaanse sitrus-industrie aan te bied. Hierdie artikel is die eerste in ‘n reeks van drie bydraes. Why do plants flower?Reproductive development is a response in flowering plants that is initiated by the trans-port of an endogenous signal from the area of perception (leaves or roots) to plant mer-istematic tissues which are capable of under-

going morphogenesis; the net result being a change from a vegetative to a reproductive state (Bangerth, 2009). Under conditions of sufficient plant metabolic energy, water and nutrient availability, continuous perception of an environmental stimulus and uninter-rupted endogenous signaling to a suscepti-ble meristem will eventually, under growth-promoting environmental conditions, lead to the meristem’s morphological transition from a vegetative to a reproductive state (Thomas et al., 2000).

Due to the differences in genetic composi-tion resulting from millions of years of vary-ing evolutionary adaptations to the prevailing environment, plant species differ in their sen-sitivity to the same environmental conditions, and as a result exhibit different physiological

responses (Battey, 2000). Annual or biennial plants such as pumpkin, cabbage and squash (Fig. 1), convert all of their meristems to form flowers at the expense of new leaf and shoot production (Bangerth, 2009). To optimize the potential for reproduction and thus produc-ing the next generation of offspring, the ma-jority of these plants’ meristems experience a total identity transition from a vegetative to a reproductive state, and produce flowers with viable seeds, with plant mortality following 1-2 seasons, as an imminent result (Battey, 2000). However, in perennial plants such as fruit trees of peach, pear and citrus (Fig. 1), buds are pre-destined to produce both new shoots and flow-ers, and more importantly, the apical meristem of at least one of its shoots remain indetermi-nate beyond one growing season, thereby in-

THE REPRODUCTIVE PHENOLOGY OF CITRUS. I:

Introduction to the physiology of citrus flowering Jakkie (OPJ) StanderCitrus Research International, Department of Horticultural Science, University of Stellenbosch | E-mail: jakkie@sun.ac.za

Figure 1: Annual or biennial plants such as pumpkin, cabbage and squash, convert all of their meristems to a reproductive phase at the expense of new leaf and shoot production (note the Brussel sprout). Perennial plants such as peach, pear and citrus, partition buds to produce both new shoots and flowers (note the orange tree). (Adapted from Smith Brother, 1898)

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creasing potential for plant longevity (beyond two growing seasons) (Thomas et al., 2000).

In most angiosperms (flowering plant species bearing seeds inside a fruit), after pol-lination and fertilization of the ovule, a flow-er eventually develops into a fruit and carries part of the genetic information of the mother plant, preserved in a seed. Flowering plants play an important role in the natural ecosys-tem, as source of nectar for bees, and more importantly, fruit developing from flowers are a major source of important nutritional elements to herbivores (Janick, 2005). Do-mestication of certain perennial fruit trees such as apple, peach and citrus have led to their commercial production across con-tinents, and today these plants provide the major source of vitamins to an ever-growing human population. The citrus flowerCitrus originated in South East Asia and be-longs to Aurantiodeae, the ‘Orange’ subfami-ly of Rutaceae (Spiegel-Roy and Goldschmidt, 1996). This subfamily is characterized by producing fruit in the form of a hesperidium – a modified berry characterized by a leather and oily rind which encloses internal swollen juice sacs. The trees vary from small to large (tree height of 1-4 meters) and the leaves and bark have oil glands enclosed in the epider-mal cell layer (rind). Citrus trees are peren-nial evergreens, which sustain a complex tree structure with one to three distinctive annual vegetative growth flushes (Monselise, 1985). In most citrus cultivars, with the ex-ception of lemons and limes, flowering and fruit development is an annual event under subtropical conditions with flowering in the spring, whereas in tropical areas, flowering is a continuous event, mostly determined by the availability of sufficient rainfall or water supply (Schneider, 1968).

In a citrus tree, after a sufficient flower in-ductive period (triggered by low tempera-tures and/or water stress) (Albrigo and Sauco, 2004), a flower (Fig. 2) develops from a receptive terminal (tip of a shoot) and/or axillary bud (a bud located between the leaf and the shoot) on a vegetative shoot, when environmental conditions favour initiation of bud-break (such as with the onset of in-

creased temperatures) (Davenport, 1990; Krajewski and Rabe, 1995a).

The fruit stem (pedicel) (Fig. 2A) origi-nates from the terminal or axillary bud (Fig. 2B), situated between the shoot (Fig. 2C) and leaf (Fig. 2D). It is about 10 mm long, and provides the vascular connectivity between the shoot and flowers, via an attachment to the receptacle and calyx (Fig. 2E). The pedicel

OPSOMMinG:

• Blom-ontwikkeling is dié belangrikste bepalende faktor van oes-lading en –kwaliteit.• Eenjarige gewasse transformeer al sy knoppe na blomme; meerjarige gewasse allokeer knoppe om beide blomme en nuwe lote te vorm.• Sitrus ondergaan drie sterk onderskeibare periodes van vegetatiewe groei – in die lente, somer en herfs (September, Desember en Maart).• Vrugte inhibeer knopbreek en die vorming van nuwe vegetatiewe lote (nuwe dra-posi-sies). Groot oesladings kan nadelig wees vir die vorming van nuwe dra-posisies en kan lei tot alternerende drag.• ‘n Loot moet ten minste ses weke oud wees om blomme te kan inisieer en ontwikkel.• In meeste kultivars ontwikkel blomme op lote van die somer-groei (Desember-Januarie).• ‘n Blom-loot het gewoonlik agt nodes (dus ook agt knoppe), ‘n driehoekige vorm en ‘n sterk regop groeiwyse.• Terminale blom-knoppe ontwikkel eerste en blomme is meer gekonsentreer op die deel van die loot wat die verste van die stam (apikale posisies) is.• ‘n Gemengde bloeiwyse [“groenblom” (een of meer blaar en blom uit een knop)] is meer algemeen op die apikale posisies (punte) van blom-lote, terwyl enkel-blomme [wit-blom (een- of meer blomme)] meer geneig is om op die onderste helfte van ‘n blom-loot te vorm.• ‘n “Groenblom” lei tot beter vrugset en –grootte, as “witblom”. ‘n Tipiese loot van ‘n “groenblom” is dikker, het ‘n beter ontwikkelde vervoerstelsel, dus meer vaskulêre bundels en ‘n beter ontwikkelde xileem silinder as dié van “witblom”.• 0“Groenblom” is ‘n gevolg van hoё temperature tydens blom-differensiasie en knopbreuk (Julie-Augustus) terwyl “witblom” dikwels volg na blootstelling aan lae temperature.

Figure 2: A mature citrus flower and its different organs as connected to a shoot via the pedicel.

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of a flower that sprouts from axillary buds has two abscission zones – one at the base of the pedicel and another located more distally to-wards the receptacle, whereas a terminal flow-er only has one abscission zone. The receptacle is a cup-like structure to which the different flowering parts are attached and forms the base of the calyx. From the calyx, vascular bundles connect the five sepals (Fig. 2F) that enclose the anther-carrying stamens (Fig. 2G) and pistil [stigma (Fig. 2H), style (Fig. 2I) and ovary (Fig. 2J)]. Like the citrus leaf, the sepals have stomata on the abaxial surface (the sur-face facing the shoot) (Schneider, 1968).

The different sections of the fruit pulp in-itiate during flowering as small ovule-carry-ing locules within the ovary, which are each connected to a stylar canal that extends to-wards, and terminates, just below the surface of the stigma (Schneider, 1968). Anthers bear pollen, and with successful transfer of pol-len granules to the stigma by bees (pollina-tion), pollen tube growth (germination) via the stylar canal towards the ovules, results in seed development (fertilization). On modern commercial citrus farms, citrus producers sometimes avoid seed development by using physical barriers during the bloom period, such as netting-structures (Fig. 3), to avoid the transfer of viable pollen by bees from an-thers of one cultivar to the stigma of another. After pollination, morphogenesis from flow-er to fruit initiates with the abscission of the sepals, stamens and style, and by a process of cell division and enlargement, the ovary de-velop into a mature fruit (Schneider, 1968). Bearing unit: age, position and general characteristicsUnder subtropical conditions citrus exhibits three distinct periods of vegetative growth, that alternate with root growth, during which new leafy shoots originate from terminal (Fig.

4A) and axillary buds (Fig. 4B) on older veg-etative stems (Monselise, 1985). Vegetative growth initiates once the developing shoot pushes through the bud scales and elongates via lengthening of the shoot area between the apical meristem and the bud (Schneider, 1968). New, immature leaves are a strong sink for assimilates, nutrients and water and develop from primordia that were present in the buds prior to growth initiation. After an active growth period of 3-4 weeks, the leaves start to harden off and only then become a source of photosynthetic assimilates to newly developing sink organs, such as fruit and new shoots (Ruan, 1993).

In contrast to annual or biennial flower-ing plants, perennial fruit trees possess the ability to allow buds to produce flowers and/or new vegetative shoots (Bangerth, 2009), thereby enabling citrus to coincide its first vegetative flush with the time of flowering in

the spring, as ambient temperatures starts to increase in July-September. Thereafter, a sec-ond and third vegetative growth flush in the summer and autumn consists of newly devel-oping vegetative shoots sprouting from buds on new and old shoots, as well as internode elongation and hardening off of previous growth flushes from the current season. Dur-ing the spring (with the exception of lemon and lime), after a sufficient inductive period, flowers develop in varying intensities on 6 weeks or older vegetative shoots (Albrigo and Chica, 2011) that originated from vegetative stems in the three different growth flushes of the previous 12-month season. Fruit load in-hibits bud sprouting and when a heavy crop load is present during time of bud sprouting, the formation of new vegetative shoots from axillary buds is inhibited and restricts the formation of new flower-bearing units (Ver-reynne and Lovatt, 2009).

Figure 3: Covering of trees with temporary nets during flowering provides a barrier that restricts bees from carrying viable pollen to the stigma of flowers (pollination), which could lead to unwanted seed development.

Figure 4: New vegetative shoots originate from terminal (A) and axillary buds (B) by pushing through the bud scales and elongating via intercalary growth.

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The characteristics of a typical flower-bear-ing shoot differ among various cultivars (Table 1), as well as in response to factors such as crop load (Verreynne and Lovatt, 2009) and tree canopy density, as influ-enced by cultural practices such as prun-ing (Krajewski and Pittaway, 2000). How-ever, a flowering shoot generally tends to have a length of approximately eight nodes (Schneider, 1968) (Fig. 5A) and an age of 5-12 months (Guardiola, 1981). The typical flowering shoot has triangular internodes (Fig. 5B); compared to older non-flowering shoots that are round, thicker and shorter than flowering shoots (Schneider, 1968). Flowering shoots towards the inner canopy are more shaded and mostly hanging down, producing small fruit which are prone to various quality disorders (Cronje et al., 2011), whereas strong flowering shoots which exhibit an upright growth habit arise from strong limbs and are situated towards the outer parts of the tree canopy (Krajewski and Pittaway, 2000). Terminal flower buds sprout first (Guardiola et al., 1982). Flowers are more abundant towards the apical region of a shoot (Valiente and Albrigo, 2004), with a predominantly leafy (mixed) inflorescence sprouting from the apical buds and generally single reproduc-tive (leafless) inflorescence sprouting from the buds towards the base (Abbott, 1935).

Inflorescence typesDuring bud-break in the spring, developing flowers arise from buds either as leafy (Fig. 6A) or leafless (Fig. 6B) inflorescences. A leafy inflorescence produces a combination of one or more flowers and leaves, whereas a leafless inflorescence is characterized by a bud sprouting one or more flowers with no

leaves (Krajewski and Rabe, 1995a). Leafy in-florescences predominantly sprout from the apical buds of a shoot, whereas leafless in-florescences are more abundant towards the base (Abbott, 1935).

Delayed bud-break and sprouting usually produces leafy inflorescence (Lenz, 1966), which could be explained by increased tem-peratures at the time of bud-break, as warm temperatures prior to and during flower dif-ferentiation are associated with the sprouting of predominantly leafy inflorescence, and low temperatures with the development of leafless inflorescence (Moss, 1969). In most citrus cultivars, with the exception of lemon, lime, grapefruit and satsuma mandarin, fruit set is higher and eventual fruit size larger for fruit arising from leafy inflorescences (Kra-jewski and Rabe, 1995a).

Figure 5: In citrus a typical flower-bearing shoot generally tends to have approximately eight nodes (A), is triangular in shape (B: cross-sectional cut) and has an age of 6-12 months.

Cultivar time of the vegetative flush that region becomes a typical flower-bearing shoot

‘Clementine’ mandarin Summer flush (8-9 months) (KrajewsKi and rabe, 1995b) Stellenbosch, SA

‘Washington Navel’ Spring flush (12 months) (Lovatt et aL., 1984) California, USAsweet orange Summer flush (8-9 months) (GuardioLa et aL., 1982) Valencia, Spain

‘Shamouti’ sweet orange Spring flush (12 months) (personaL correspondence) Jaffa, Israel

‘Hamlin’ sweet orange Summer flush (8-9 months) (vaLiente and aLbriGo, 2004) Florida, USA

‘Valencia’ sweet orange Summer flush (8-9 months) (vaLiente and aLbriGo, 2004) Florida, USA

Table 1: The origin of flowering shoots of various citrus cultivars.

Figure 6: During bud break in the spring, developing flowers arise from buds either as leafy (A) or leafless (B) inflorescences. A leafy inflorescence produces a combination of one or more flowers and leaves, whereas a leafless inflorescence is characterized by a bud sprouting one or more flow-ers with no leaves.

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86TEGNOLOGIE | CRI JUNIE | JULIE 2015

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87TECHNOLOGY | CRI JUNE | JULY 2015

It’s our mission to make you a preferred supplier of perishable products world–wide through innovation and quality control.

Making you the preferred choice of international perishable produce

Making you the preferred choice of international perishable produce

www.ppecb.com

Head OfficeT +27 21 930 1134F +27 21 939 6868E ho@ppecb.com

45 Silwerboom AvePlattekloof, Cape Town

7560

Not only do leafy inflorescences increase photo-assimilate supply to developing ova-ries, but flowers on leafy inflorescences are also a stronger sink than those of leafless in-florescences, due to a better developed xylem cylinder and a higher number of vascular bundles (Erner and Shomer, 1996). Concluding commentsFlowering is the most important determinate of yield and quality of citrus fruit production. The understanding of the biology of flowering, or reproductive biology, is of critical impor-tance to sustain and increase the productivity of the South African citrus industry. The pur-pose of this article is to introduce the reader to the basic biology of citrus flowering as an evergreen perennial in relation to annuals and biennials. The next articles will focus on the phenology of citrus flower development, as well as the events and factors leading to the morphogenesis of a flower to a fruit. References citedAbbott, C. E. 1935. Blossom-bud differentiation in citrus trees. American Journal of Botany 22(4):476–485.

Albrigo, L. G. and V. G. Sauco. 2004. Flower bud induction, flowering and fruit-set of some tropical and subtropical fruit trees with special reference to Citrus. Acta Horticulturae. 632:81–90.

Albrigo, L. G. and E. J. Chica. 2011. Citrus shoot age requirements to fulfill flowering potential. Proc. Fla. State Hort. Soc. 124:56–59.

Bangerth, K.F. 2009. Floral induction in mature, perennial angiosperm fruit trees: Similarities and discrepancies with annual/biennial plants and the involvement of plant hormones. Scientia Horticul-turae. 122:153–163.

Battey, N. H. 2000. Aspects of seasonality. Journal of experimental Botany. (51) 352:1769–1780.

Cronje, P.J.R., G.H. Barry, and M. Huysamer. 2011. Fruit position during development of ‘Nules

Clementine’ mandarin affects the concentration of K, Mg and Ca in the flavedo. Scientia Horticultu-rae.130:829–837.

Davenport T.L. 1990. Citrus flowering. In: Horti-cultural Reviews. Vol. 12 (ed. J. Janick), pp. 349–408. Timber Press, Portland.

Erner, Y. and L. Shomer. 1996. Morphology and anatomy of stems and pedicels of spring flush shoots associated with citrus fruit set. Annals of Botany (London). 77:537–545.

Guardiola, J.L. 1981. Flower initiation and devel-opment in Citrus. Proc. Intl. Soc. Citricult. 2:242–246.

Guardiola, J.L., C. Moneri, and M. Agusti. 1982. The inhibitory effect of gibberellic acid on flowering in Citrus. Physiologia Plantarum. 55:136–142.

Janick. J. 2005. The origins of fruit, fruit growing, and fruit breeding. Plant breeding reviews. 25:255–320.

Krajewski, A. and T. Pittaway. 2000. Manipu-lation of citrus flowering and fruiting by pruning. Proc. Int. Soc. Citric. 1:357–360.

Krajewski, A. and E. Rabe. 1995a. Citrus flower-ing: a critical review. Journal of Horticultural Sci-ence. 70:357–374.

Krajewski, A. and E. Rabe. 1995b. Effect of head-ing and its timing on flowering and vegetative shoot development in Clementine mandarin (Citrus re-ticulata Blanco). Journal of Horticultural Science. 70:445–451.

Lovatt, CJ., S.M. Streeter, T.C. Minter, N.V. O'Connel, D.L. Flaherty, M.W. Freeman and P.B. Goodell. 1984. Phenology of flowering in Citrus sinensis L. Osbeck, cv. 'Washington' navel or-

ange. Proc. Int. Soc. Citriculture: 1: 186–190.

Monselise, S.P. 1985. Citrus and related genera, p.275–294. In: A.H. Halevy (ed.). CRC handbook of flowering, vol. II. CRC Press, Boca Raton, Fla.

Moss, G.I. 1969. Influence of temperature and pho-toperiod on flower induction and in florescence development in sweet orange (Citrus sinensis L. Os-beck). Journal of Horticultural Science. 44:311–320.

Ruan, Y. 1993. Fruit set, young fruit and leaf growth of Citrus unshiu in relation to assimilate supply. Sci-entia Horticulturae. 53:99–107.

Schneider, H. 1968. Anatomy of Citrus. In: Re-uther, W. (Ed.), The citrus industry. Univ. Calif Div Agr Sci, Berkeley, Vol. 2, p. 1–85.

Smith Bros. 1898. Horticulture: A guide to ama-teurs in the fruit, vegetable and flower garden, greenhouse, conservatory and stoep. Pollet & Co. Johannesburg, South Africa.

Spiegel-Roy, P. and E.E. Goldshmidt. 1996. Biol-ogy of Citrus. Cambridge Univ. Press, Cambridge, U.K.

Thomas, H.M. and H. Ougham. 2000. Annuality, perenniality and cell death. Journal of experimental Botany. 51(352):1781–1788.

Valiente, J.I. and L.G. Albrigo. 2004. Flower bud induction of sweet orange trees [Citrus sinensis (L.) Osbeck]: Effect of low temperatures, crop load, and bud age. Journal of the American Society of Horti-cultural Science. 129:158–164.

Verreynne, J.S., Lovatt, C.J. 2009. The effect of crop load on budbreak influences return bloom in alternate bearing 'Pixie' mandarin. Journal of the American Society of Horticultural Science. 134 (3):299–307.

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