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Marisa Y. Thompson, Jennifer Randall, Richard J. Heerema, and Dawn VanLeeuwen

Successful commercial pecan [Carya illinoinensis (Wangenh.) K. Koch] production relies on mitigation of alternate bearing, which is a function of pistillate flower production. Mechanisms of floral initiation in pecan are not well understood. Our objective was to assess the impact of select plant growth regulators (PGRs) on return bloom for commercial application in pecan trees grown in the Southwestern United States. A 2-year study evaluated effects of ethephon, aminoethoxyvinylglycine (AVG), and gibberellin GA3 (GA3) on subsequent season return bloom in fruiting and nonfruiting pecan shoots. Cultivars used were mature Western and immature Western and Pawnee. Effects of PGRs on return bloom of nonfruiting shoots were different from fruiting shoots. As compared with untreated control, a GA3 treatment on fruiting shoots of mature ‘Western’ trees increased the number of flowers per new shoot by 125%. For nonfruiting shoots on the mature ‘Western’ trees, the number of flowers per new shoot decreased significantly by all PGR treatments and as much as 93% for AVG. In previously nonfruiting shoots on the immature ‘Western’ trees, a GA3 treatment reduced the number of flowers per new shoot in the next season by 88.2%. Results from immature ‘Pawnee’ shoots did not show statistically significant differences. The effects of these PGRs on subsequent season flowering in pecan are complex. This study suggests that PGRs can be used to increase or decrease cropload through effects on return bloom and therefore have potential uses for mitigating alternate bearing.

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Marisa Y. Thompson, Jennifer J. Randall, Dawn VanLeeuwen, and Richard J. Heerema

Regarding pecan (Carya illinoinensis), alternate bearing, which is a biennial fluctuation of crop yield, is a major hindrance for the pecan industry. Little is known about the internal cues that trigger pecan shoots to become reproductive. This 2-year study approached the mysteries of alternate bearing of pecan by determining whether pecan homologs of three genes known to control floral initiation in other species are expressed differently at various times of the growing season or in distinct plant tissues, and whether expression of these genes can be manipulated by plant growth regulator (PGR) application when compared with an untreated control group. The flowering genes of interest were pecan homologs of leafy (CpLFY), apetala1 (CpAP1), and flowering locus t (CpFT). During year 1 (2014), PGRs ethephon and gibberellin GA3 were applied at the shoot level 1 week before each of three tissue sampling dates (13 June, 3 July, 29 July). During the following year (2015), two more PGRs were added to the study [a second double rate (2X) of gibberellin GA3 and ethylene inhibitor aminoethoxyvinylglycine (AVG)] for a total of four PGRs (applied on 10 June, 1 July, and 23 July) plus the untreated control. Experimental leaf and bud tissues were sampled from fruiting and nonfruiting shoots on mature ‘Western’ pecan trees and analyzed separately. Normalized expression levels of CpLFY and CpAP1 were significantly higher in buds than in leaves. Normalized expression of CpLFY in bud tissues differed statistically based on the sampling date in 2014, with the earliest date (13 June) having higher expression than the two later dates that year. In 2015, a treatment × date interaction revealed that, compared with the untreated control, CpLFY expression was significantly lower in shoots treated with both gibberellin GA3 dosages on 1 July. A few weeks later (23 July), CpLFY expression was lower in the 2X GA3 treatment group and higher in samples treated with AVG. In 2014, CpAP1 expression in buds was significant, with a treatment × date interaction in which ethephon increased CpAP1 expression, but only on one date (29 July). In 2015, bud CpAP1 expression was significantly higher in fruiting than in nonfruiting shoots; however, again, only on one date. The results reveal differential expression of these key flowering genes based on tissue type, sampling date, and fruiting status of the shoot and PGR treatment. Results suggest that more research of the effects of PGRs is necessary for understanding the flowering behavior of pecan and mitigating the intensity of alternate bearing.