Location: Fruit and Nut Research
Title: 'Late winter/early spring' xylem sap characteristics influence pecan crop load Author
Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 29, 2014
Publication Date: January 7, 2015
Citation: Wood, B.W. 2015. 'Late winter/early spring' xylem sap characteristics influence pecan crop load. HortScience. 49(7):886-890. Interpretive Summary: Alternate bearing is the most important biological problem of most pecan farmers. This research implicate the amount of sucrose, and possibly other carbohydrates, moving toward buds of shoots during ‘late winter’ as a major factor contributing to the final phase of floral development in early spring. Sap flow and sucrose concentration therefore appear to be thighly linked to subsequent year nut production by individual trees. This finding indicates that it is important that pecan farmers manage trees to ensure high carbohydrate reserves as they go into the dormant season if nut yields are to be high the following year.
Technical Abstract: Alternate bearing by individual pecan [Carya illinoinensis (Wangenh.) K. Koch] trees is problematic for nut producers and processors. There are many unknowns regarding alternate bearing physiology, such as the relationship between xylem sap flow, and its constituent sugars, with subsequent crop-load. Multiyear field studies of trees in either the ‘On’ or ‘Off’ phase of alternate bearing were evaluated for the relationship between ‘late winter’ sap flow volume and sugar concentration, and that growing season’s in-shell nut yield. Sap flowing in 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, comprising 55-75% of the total on a molar basis, depending on tree bearing status and time of sampling. Both sap flow volume and sugar concentrations were much greater in ‘On’ phase than ‘Off’ phase trees. In the case of sucrose, the dominant carbohydrate, excreted sap from ‘On’ phase trees contained 19.9-fold greater concentration of sucrose than from ‘Off’ phase trees. Sap concentration of all simple carbohydrates was much greater at inception of sap flow than a few days later; thus, sap carbohydrate concentration declined over time. 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 of both ‘Cheyenne’ and ‘Moneymaker’ trees increase in a hyperbolic curvilinear manner as ‘late winter’ sap flow increases. Sap flow from ˜ 25 year-old ‘Cheyenne’ and ˜ 100 year-old ‘Moneymaker’ trees produced a near maximum nut yield at ˜ 10 L/day/tee and ˜ 15 L/day/tree, respectably. These findings implicate the amount of sucrose, and possibly other carbohydrates, moving acropetally toward axillary bud meristems of shoots during ‘late winter’ as a major factor contributing to the final phase of floral development in early spring.