Alternate bearing is a natural trait exhibited by the reproductive modules (i.e., shoot, branch, limb, and tree) of many tree species. This trait likely improves the probability of species survival in their native habitats but poses a major biological problem in horticultural enterprises as a result of excessive interannual variability in yield and quality of nutmeats. Alternate bearing in pecan is influenced by the tree’s processing of environmental and/or endogenous cues consistent with an autonomous flowering pathway involving phytohormones (Amasino, 2010; Rohla et al., 2007a, 2007b; Smith et al., 2007; Wilkie et al., 2008; Wood, 2011) and carbohydrate reserves (Wetzstein and Sparks, 1983).
Three distinct sequential phases of pistillate flower development appear involved in regulation of AB in pecan (Wood, 2011). These are: 1) foliar production of a phloem translocated florigen that initiates chromatin-modifying inductive processes in young bud primordia; 2) an interplay of foliar and fruit produced phytohormones acting on the primordia to regulate a second phase of chromatin modification; and 3) regulation of the final phase of chromatin modification by concentration of one or more non-structural carbohydrates (e.g., sucrose) acting in the environment of the axillary meristems during, or just after, vernalization preceding anthesis (Wood, 2011). 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 is also likely true for sugar concentration within the environment of the floral bud meristem in that sugar signaling is thought to play a key role in flowering (Gibson, 2003, 2005) through a complex interplay with phytohormones regarding their effects on gene expression (Leon and Sheen, 2003).
Non-structural simple carbohydrates can affect gene expression (Koch, 1996; Leon and Sheen, 2003; Li et al., 2003a, 2003b) and appear involved in one or more processes controlling pecan floral initiation (Smith and Waugh, 1938; Sparks, 1975; Wood, 1989; Wood et al., 2004; Worley 1979a, 1979b). For example, sugars appear to be major regulators of floral genes involved in certain vernalization and subsequent floral initiation and evocation processes (Wetzstein and Sparks, 1983; Wood, 1989, 1995). This raises the possibility that simple sugar components within the primordial environment exert a major regulatory role sufficient to influence chromatin-modifying events controlling floral initiation and evocation processes. If so, sugars supplied by xylem sap during the late winter to early spring transition period before and during budbreak likely influence floral initiation and evocation.
Positive pressure within a tree’s above-ground stem structure leads to late winter xylem sap flow. This pressure and flow is near maximum when there is substantial diurnal fluctuation of stem temperature above and below freezing (Marvin and Ericson, 1956; Marvin and Greene, 1951; Wiegand, 1906). Johnson et al. (1987) found xylem sap pressure, and subsequent sap flow, in relatively dormant late winter trees to depend on the xylem sap’s sucrose component. If sucrose affects dormant season sap pressure and flow in pecan, then the subsequent diurnal rhythmic-like pumping of sucrose-enriched sap into the apoplastic space of swelling buds likely influences gene regulation within floral meristems and therefore the final stage of floral initiation—i.e., Phase III (Wood, 2011). This might partially explain how carbohydrate reserves influence AB and raises the possibility that sufficient exposure to one or more simple carbohydrates made available just before budbreak, through early season sap flow, substantially regulates the “on” vs. “off” bearing state of pecan trees. The nature of this relationship is presently unknown for pecan and other angiosperms. This study assesses the relationship between xylem sap flow and its constituent simple carbohydrate composition at the time of spring budbreak to the subsequent cropload AB phase of pecan trees.
Finn, G.A., Straszewski, A.E. & Peterson, V. 2007 A general stage key for describing trees and woody plants Ann. Appl. Biol. 151 127 131
Hudson, W., Brock, J., Culpepper, S., Mitchem, W. & Wells, L. 2007 Georgia pecan pest management guide. Georgia Coop. Ext. Serv. Bul. No. 841
Johnson, R.W. & Tyree, M.T. 1992 Effect of stem water content on sap flow from dormant maple and butternut stems Plant Physiol. 100 853 858
Johnson, R.W., Tyree, M.T. & Dixon, M.A. 1987 A requirement for sucrose in xylem sap flow from dormant maple trees Plant Physiol. 84 495 500
Li, C.Y., Weiss, D. & Goldschmidt, E.E. 2003b Girdling affects carbohydrate-related gene expression in leaves, bark and roots of alternate-bearing citrus trees Ann. Bot. (Lond.) 92 137 143
Marvin, J.W. & Ericson, R.O. 1956 A statistical evaluation of some of the factors responsible for the flow of sap from the sugar maple Plant Physiol. 31 57 61
Rohla, C.T., Smith, M.W. & Maness, N.O. 2007a Influence of cluster thinning on return bloom, nut quality, and concentrations of potassium, nitrogen, and non-structural carbohydrates in pecan J. Amer. Soc. Hort. Sci. 132 158 165
Rohla, C.T., Smith, M.W., Maness, N.O. & Reid, W. 2007b A comparison of return bloom and nonstructural carbohydrates, nitrogen, and potassium concentrations in moderate and severe alternate-bearing pecan cultivars J. Amer. Soc. Hort. Sci. 132 172 177
Schmidt, T., Elfving, D.C., McFerson, J.R. & Whiting, M.D. 2009 Crop load overwhelms effects of gibberellic acid and ethephon on floral initiation in apple HortScience 44 1900 1906
Smith, C.L. & Waugh, J.G. 1938 Seasonal variations in the carbohydrate and nitrogen content of roots of bearing pecan trees J. Agr. Res. 57 449 460
Smith, M.W., Rohla, C.T. & Maness, N.O. 2007 Correlations of crop load and return bloom with root and shoot concentrations of potassium, nitrogen, and nonstructural carbohydrates in pecan J. Amer. Soc. Hort. Sci. 132 158 165
Tyree, M.T. 1983 Maple sap uptake, exudation, and pressure changes correlated with freezing exotherms and thawing endotherms Plant Physiol. 73 277 285
Wood, B.W. 1995 Relationship of reproductive and vegetative characteristics of pecan to previous-season fruit development and post-ripening foliation period J. Amer. Soc. Hort. Sci. 120 635 642
Wood, B.W. 2011 Influence of plant bioregulators on pecan flowering and implications for regulation of pistillate flower initiation HortScience 46 870 877
Worley, R.E. 1979a Pecan yield, quality, nutlet set, and spring growth as a response to time of fall defoliation J. Amer. Soc. Hort. Sci. 104 192 194
Worley, R.E. 1979b Fall defoliation date and seasonal carbohydrate concentration of pecan wood tissue J. Amer. Soc. Hort. Sci. 104 195 199