2017 ASHS Annual Conference

In the pecan [Carya illinoinensis (Wangenh.) K. Koch] industry, alternate bearing is the most important horticultural problem. Alternate bearing in perennial tree crops is a propensity for years of high yield interspersed with years of lower yield. Alternate bearing occurs at the shoot level in pecans, but little is known about the internal cues that trigger shoots to become reproductive. This study approaches the mysteries of alternate bearing in pecan by determining whether three genes known to control flowering 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. The flowering genes of interest were pecan homologs of LEAFY (LFY), APETALA1 (AP1), and FLOWERING LOCUS T (FT). The tissues selected for this two-year study included leaves and buds, analyzed individually, sampled from fruiting and nonfruiting shoots on mature ‘Western’ cultivar pecan trees. Three PGR applications of ethephon (as Ethepon2^® [100 mg∙L^–1 a.i.]), aminoethoxyvinylglycine (AVG; as ReTain^® [88 mg∙L^–1 a.i.]), and two gibberellic acid rates (GA_3; as ProGibb^® [50 & 100 mg∙L^–1 a.i.]) were made at the shoot level one week before tissue sampling, RNA extraction, and qrtPCR analyses of normalized gene expression. CpFT expression was higher in leaves than in bud tissues. Alternatively, CpLFY and CpAP1 both had higher expression levels in buds than in leaf tissues. No differences due to PGR treatment were detected in leaf CpLFY expression in either year of the study. However, bud CpLFY expression levels were significantly impacted by sampling date in 2014, and by both sampling date and PGR treatment in 2015. Following PGR treatment in 2015, larger differences were observed due to tissue sampling date than treatment for leaf CpAP1 (70.5% higher lsmeans on one date than another). In 2014 bud CpAP1 expression there was a significant treatment by date interaction in which ethephon increased CpAP1 expression by 18.3%, but only on one date. In 2015 bud CpAP1 expression was significantly higher in fruiting versus nonfruiting shoots by 25.2%, but, again, only on one date. Results reveal differential expression of these key flowering genes based on tissue type, sampling date, and fruiting status of the shoot. Furthermore, these results indicate that more research on effects of PGRs is necessary not only for understanding flowering behavior in pecan, but also mitigating the intensity of alternate bearing.