Flow Volume and Carbohydrate Composition of Late Winter Xylem Sap Influences Subsequent Crop Load in Pecan

Alternate bearing (AB) by individual trees is a major biological problem for producers of pecan [Carya illinoinensis (Wangenh.)  K. Koch] nuts.  The linkage between flowering, and subsequent crop-load, with early-season carbohydrate composition and flow volume of xylem sap is unknown.  Multiyear field studies of mature ‘Cheyenne’ and ‘Moneymaker’ pecan trees in either ‘On’ or ‘Off’ phases of AB were evaluated regarding this linkage.  Sap flowing from trunks of ‘Cheyenne’ trees just prior to, and at time of, budbreak consisted of hexoses (fructose and glucose), a disaccharide (sucrose), polysaccharides (raffinose and stachyose), and sugar alcohols (xylitol and sorbitol).  Sucrose is the overwhelmingly dominate simple carbohydrate at this growth stage, comprising 55-75% of the total molar composition, regardless of tree bearing status or sampling time during late winter and early spring as buds swell and break.  Both sap flow volume and concentration of individual simple carbohydrates were much greater in ‘On’ phase than ‘Off’ phase trees.  In the case of sucrose, the dominant carbohydrate, sap from ‘On’ phase trees average ≈ 20-fold more sucrose that of ‘Off’ phase trees.  Sap concentration of all carbohydrates was much greater at inception of sap flow than a few days later; thus, sap carbohydrate concentration declines over time during ‘late winter’ as buds transition to the early spring growth period as buds transition from ‘bud swell’ to ‘bud break’ and then ‘shoot growth’.  Depending on the crop year, individual ‘On’ phase ‘Cheyenne’ trees exhibited xylem sap flow volumes from 5.5 – 20.2-fold greater than that of ‘Off’ phase trees.  In-shell nut yield by both ‘Cheyenne’ and ‘Moneymaker’ trees increase in a hyperbolic manner as ‘late winter’ sap flow volume for individual trees increase.  Sap flow from ≈ 25 year-old ‘Cheyenne’ and ≈ 110 year-old ‘Moneymaker’ trees produced a near maximum nut yield when sap flow volume was at ≈ 10 L/tap/tree/season and ≈ 15 L/tap/tree/season, respectably.  These findings indicate that the amount of sucrose, and possibly other carbohydrates, moving toward axillary bud meristems of shoots during ‘late winter’ at about the time of ‘bud swelling’ is likely a major factor influencing  the final phase of floral development in early spring and therefore subsequent crop load.