Insight into Alternate Bearing of Pecan

The fundamental cause of alternate bearing in pecan [Carya illinoinensis (Wangenh.) K. Koch] is unknown, but is closely linked to the size of the dormant season carbohydrate pool. Nut yields (over a period of up to 78 years) were evaluated, for 66 cultivars, in regards to alternate bearing intensity (I). Best-fit regression analysis indicates no association between I and fruit ripening date (FRD) or nut volume; although, there was moderate association with post-ripening foliation periods (PRFP) in that I tends to decrease as the length of the PRFP decreases. Multiple regression models indicated that FRD and nut volume were poor predictors of I, however PRFP possessed significant inverse predictive power. Late-season canopy health, as measured by percentage of leaflet retention, decreased as FRD approached early-season ripening. Late-season photoassimilation rate was higher on foliage of trees with late FRDs than those with midor early-season ripening dates. Sprays of either Alar or ProGibb to the canopy of either “On” or “Off” trees altered return bloom the following year. These data provide new insight into the complex nature of alternate bearing in pecan and are interpreted to indicate that alternate bearing in pecan is regulated by; a) gibberellins during the time of fruit development, and b) the size of the carbohydrate pool at about the time of bud break. INTRODUCTION Alternate bearing is regarded as the economically most important biological problem of pecan husbandry in the U. S. and is largely due to the trees’ nature to alternately vary, from year to year, in regards to flowers produced and/or set (Amling and Amling, 1983). Its limited study has produced two generalized theories –the “Carbohydrate Theory” (CT) and the “Phytohormone Theory” (PT) (Barnet and Mielke, 1981). Neither theory has been clearly defined in terms of specific floral evocation, induction, or developmental processes. Of the two, the CT has been most commonly advocated (Wood, 1991). It purports that flowering, and subsequent cropping, is fundamentally controlled by the tree’s dormant season carbohydrate pool. Conversely, the PT purports that flowering, and subsequent cropping, are fundamentally controlled by flower suppressing or promoting phytohormones-like substances produced by fruit and /or leaf tissues. Pecan differs in a fundamental manner from many alternate bearing crop species in that individual trees sometime bear two or three consecutive “On” crops of moderate to high yields before exhibiting a pronounced “Off” year (Sparks, 1974). Thus, alternate bearing by pecan trees can display pronounced “irregularity”. Understanding the alternate bearing phenomenon is, therefore, fundamentally important to the optimization of husbandry strategies. The general CT predicts that fruit-set is proportional to the dormant season carbohydrate pool and implies that pool size is proportional to late-season net photoassimilation capacity of the tree’s canopy. Thus it is commonly assumed that cultivars ripening fruit relatively early in the growing season, or possessing a longer post-ripening foliation period, will accumulate a larger carbohydrate pool and, therefore, exhibit a lower intensity of alternate bearing than those ripening relatively late in the season. Because of ramifications pertaining to breeding strategies, cultivar selection, and husbandry strategies, this putative relationship merits validation. We report here an estimation of the relationship between alternate bearing intensity (I) and fruit ripening date (FRD), nut volume, and length of the post-ripening foliation period (PRFP). We also report on the influence of foliar sprays Proc. XXVI IHC – Deciduous Fruit and Nut Trees