Natural selection operating over evolutionary time has produced pecan as an economically important species that exhibits pronounced biennial-like alternations in seed production as a strategy for ensuring long-term reproductive success. This year-to-year variation in pistillate flowering, and subsequent cropload, is termed AB. Although AB-linked variation in pistillate flowering likely increases individual fitness in natural habitats, it is also a major impediment to greater horticultural domestication and is the primary biology-based impediment to horticultural enterprises (Wood, 1991). Excessive year-to-year variability in pistillate flowering limits tree and orchard profitability, thus adversely affecting producers, processors, and consumers through instabilities in nutmeat supply, quality, and price (Sparks, 1974, 1975; Wood, 1991). Although the specific processes regulating AB in pecan remain ambiguous, the trait tightly links to floral initiation processes occurring within bud meristems within the tree's canopy.
Horticultural manipulation of pistillate flowering and mitigation of AB in commercial pecan orchards currently targets minimization of tree stress with orchard management strategies directly or indirectly targeting key exogenous biotic and abiotic stressors (Wood et al., 2003). These include sunlight, nutrient elements, and water as essential resources and pathogens, arthropods, and weeds as potentially harmful pests. Cropload thinning before, or at the time of, inception of kernel (i.e., primarily cotyledon) filling of developing seeds also acts to moderate AB by increasing subsequent year pistillate flowering (Smith and Gallott, 1990; Smith et al., 1993; Wood, 1995). This fruit/seed association implicates one or more seed-associated phloem mobile phytohormones in regulation of floral initiation (i.e., the production of meristems of clearly recognizable flower primordia and includes all preceding reactions that are required if flowers are to be initiated).
Regulation of floral initiation in trees depends on processing of environmental and/or endogenous cues (Amasino, 2010) with initiation in most large-seeded temperate woody perennial angiosperms being primarily controlled by endogenous cues consistent with processing through an autonomous flowering pathway involving phytohormones (Wilkie et al., 2008). Floral initiation in pecan is therefore likely to involve an autonomous flowering pathway as a key step in its floral initiation process (Wood et al., 2003). Like with many other tree-fruit species (Schmidt et al., 2009), florally induced bud primordia on heavy cropload trees (i.e., “on” year of AB cycle) are likely exposed to different phytohormonal environments than are primordia of induced buds on light cropload trees (i.e., “off” year of AB cycle). This raises the possibility that timely application of phytohormones or bioregulators to tree canopies might alter the phytohormonal environment of primordia in such a way as to enable control of pistillate flowering by pecan farmers.
The efficacy and horticultural potential of bioregulators to control the “on” and “off” flowering phases of pecan trees has not been reported despite considerable circumstantial evidence that endogenous phytohormones are involved in floral initiation processes (Barnett and Mielke, 1981; Rohla et al., 2007a, 2007b; Wood, 1982, 1984a, 1984b, 1991, 2003; Wood and McMeans, 1981; Wood et al., 2003). A variety of natural and synthetic bioregulators are efficacious for control of floral initiation processes in several polycarpic perennial crops and involve timely use of floral promoters [generally ethephon and P-Ca and NAA or GA4 in certain situations (Looney et al., 1985)] in “on” years to promote return flowering the following “off” year and use of floral inhibitors [gibberellic acids (GA3,4,7) (Greene, 2000) and auxin analogs (e.g., NAA) in certain cases] in “off” years to decrease subsequent year flowering. It is unknown whether these promoters and inhibitors similarly affect pecan flowering in “on” and “off” years.
Commercial pecan production enterprises need better horticultural tools for managing flowering and AB. Successful development and exploitation of such tools depend on acquiring better understanding of floral initiation processes operating in pecan. This study assesses certain promising bioregulators for activity and/or influence on pecan flowering and examines how their interactions influence pistillate flower initiation. It reports that several synthetic bioregulators possess potential as horticultural tools for controlling pistillate flowering and AB in pecan and, based on observed influence of bioregulators on flowering, presents a “three-level signaling” model explaining regulation of pistillate flower initiation in pecan trees.
AmlingH.J.AmlingK.A.1983Physiological differentiation of pistillate flowers of pecan and cold requirements for their initiationJ. Amer. Soc. Hort. Sci.108195198
BangerthF.LiC.-J.GruberJ.2000Mutual interaction of auxin and cytokinins in regulating correlative dominancePlant Growth Regulation32205217
BangerthK.F.2009Floral induction in mature, perennial angiosperm fruit trees: Similarities and discrepancies with annual/biennial plants and the involvement of plant hormonesSci. Hort.122153163
BertlingI.BangerthF.1995Changes in hormonal pattern of the new growth of Sclerocarya birrea after rejuvenation treatment with GA3 and heading backGartenbauwissenschaf60119124
BukovacM.J.SabbatiniP.SchwallierP.G.2006Modifying alternate bearing of spur-type ‘Delicious’ apple with ethephonHortScience4116061611
DennisF.G.Jr.NeilsenJ.C.1999Physiological factors affecting biennial bearing in tree fruit: The role of seeds in appleHortTechnology9317322
EvansJ.R.EvansR.R.RegusciC.L.RademacherW.1999Mode of action, metabolism, and uptake of BAS-125W, prohexadione–calciumHortScience3412001201
ItoA.HayamaH.KashimuraY.YoshiokaH.2001Effect of maleic hydrazide on endogenous cytokinin contents in lateral buds, and its possible role in flower bud formation on the Japanese pear shootSci. Hort.87199205
LiC.BangerthF.2003Stimulatory effect of cytokinins and interaction with IAA on the release of lateral buds of pea plants from apical dominanceJ. Plant Physiol.16010591063
LooneyN.E.PharisR.P.NomaM.1985Promotion of flowering in apple trees with gibberellins A4 and C-3 epi-gibberellin A4Planta165292294
NordstromA.TarkowskiP.TarkowskaD.NorbaekR.AstotC.DolzalK.SandbergG.2004Auxin regulation of cytokinin biosynthesis in Arabidopsis thaliana: A factor of potential importance for auxin–cytokinin-regulated developmentProc. Natl. Acad. Sci. USA10180398044
RohlaC.T.SmithM.W.ManessN.O.2007aInfluence of cluster thinning on return bloom, nut quality, and concentrations of potassium, nitrogen, and non-structural carbohydrates in pecanJ. Amer. Soc. Hort. Sci.132158165
RohlaC.T.SmithM.W.ManessN.O.ReidW.2007bA comparison of return bloom and nonstructural carbohydrates, nitrogen, and potassium concentrations in moderate and severe alternate-bearing pecan cultivarsJ. Amer. Soc. Hort. Sci.132172177
SchmidtT.ElfvingD.C.McFersonJ.R.WhitingM.D.2009Crop load overwhelms effects of gibberellic acid and ethephon on floral initiation in appleHortScience4419001906
ShalitA.RozmanA.GoldschmidtA.AlvarrezJ.P.BowmanJ.L.EshedY.LifschitzE.2009The flowering hormone florigen functions as a general systemic regulator of growth and terminationProc. Natl. Acad. Sci. USA10683928397
SmithC.L.WaughJ.G.1938Seasonal variations in the carbohydrate and nitrogen content of roots of bearing pecan treesJ. Agr. Res.57449460
SmithM.W.ReidW.CarrollB.ChearyB.1993Mechanical fruit thinning influences fruit quality, yield, return fruit-set, and cold injury of pecanHortScience2810811084
SmithM.W.RohlaC.T.ManessN.O.2007Correlations of crop load and return bloom with root and shoot concentrations of potassium, nitrogen, and nonstructural carbohydrates in pecanJ. Amer. Soc. Hort. Sci.132158165
VasconcelosC.M.GrevenM.WinefieldC.C.TroughtM.C.T.RawV.2009The flowering process of Vitis vinifera: A reviewAmer. J. Enol. Viticult.60411434
WoodB.W.1984aFree and bound abscisic acid and free gibberellin-like substances in pecan kernel tissue during seed developmentJ. Amer. Soc. Hort. Sci.109626629
WoodB.W.1991Alternate bearing in pecan180190WoodB.W.PayneJ.A.Pecan husbandry: Challenges and opportunities. First National Pecan Workshop Proc., U.S. Department of Agriculture, Agricultural Research Service, ARS-96
WoodB.W.1995Relationship of reproductive and vegetative characteristics of pecan to previous-season fruit development and post-ripening foliation periodJ. Amer. Soc. Hort. Sci.120635642
WoodB.W.ConnerP.J.WorleyR.E.2003Relationship of alternate bearing intensity in pecan to fruit and canopy characteristicsHortScience38361366
WorleyR.E.1979aPecan yield, quality, nutlet set, and spring growth as a response to time of fall defoliationJ. Amer. Soc. Hort. Sci.104192194