The indeterminate phenology of peanut makes digging decisions a challenge because of the wide range of pod maturity associated with each plant (Sanders et al., 1982). Virginia market type peanut in North Carolina require between 142 and 160 d from emergence to optimum pod maturity (Jordan, 2008a). Negative ramifications associated with abiotic or biotic stress can occur in some years (Wynne and Coffelt, 1980). Cooler temperatures in September and October can prevent pods from reaching full maturity (Jordan, 2008a). The balance between digging prior to optimum pod maturity but before pod shed due to inclement weather or disease is often difficult to realize (Jordan, 2008a). Timing of digging to obtain optimum yield and quality varies annually due to differences in environmental and edaphic conditions, cultivar selection, and plant health (Jordan, 2008a; Roberson, 2008; Shew, 2008; Young et al., 1982). Delaying digging often improves market grade characteristics even though pod yield is compromised due to pod shed (Jordan, 2008a). Increasing pod retention would be advantageous when environmental conditions require digging past the optimum timing (Beam et al., 2002).

The plant growth hormone ethylene plays a key role in the maturation process of peanut and is involved in several metabolic processes in most plant tissues (Tiaz and Zieger, 2006) at every stage of plant development (Tso, 1990). Ethylene is involved in physiological processes associated with senescence and ripening of leaves and fruit (Mattoo and Aharoni, 1988). Ethylene increases the sensitivity of the signaling pathway promoting senescence and ripening that is initiated by other age-related factors (Buchanan-Wollaston, 1997). Plants often respond to external stresses caused by biotic and abiotic agents by shedding non-essential vegetative and/or reproductive structures such as peanut pods (Buchanan-Wollaston, 1997; Mattoo and Aharoni, 1998).

Toole et al. (1964) reported that Virginia market type peanut seeds displaying dormancy were induced to germinate by ethylene. Seeds of other crops displaying non-dormancy exhibited better germination and growth when subjected to ethylene (Balls and Hale, 1940; Haber, 1926) where ethylene is responsible for the mobilization and utilization of stored carbohydrates and other constituent reserves (Ketring and Morgan, 1969). Gynophore development of peanut is accompanied by ethylene production (Ketring et al., 1982) with increased concentrations during initial stages of peg growth (Lee et al., 1972). Ethylene also affects gynophores by determining size and growth of peanut pods (Shlamovitz et al., 1995). Foliar applications of ethylene converted prostrate growth habit of peanut to a more upright growth habit (Ziv et al., 1976) with physical stunting noted at concentrations exceeding 1.0% (Ketring, 1977). Ketring and Schubert (1980) found that applications of ethylene to peanut resulted in either no effect or a decrease in yield and quality, with increased peg formation at the expense of pod formation. However, Azu (1979) reported that ethylene had no effect on peanut pod yield.

Several plant growth regulators have been previously evaluated in peanut. Prohexadione calcium (Beam et al., 2002), the auxin transport inhibitor cyclanilide (Jordan et al., 2004), the growth regulators Early Harvest® (Beasley et al., 2004) and PGR-IV® (York et al., 1996), the nitrophenolic plant growth regulator Chaperone® (Faircloth et al., 2007), and the harpin protein product Messenger® (Jordan et al., 2005). Most growth regulating materials have had minimal effects on pod yield or market grade characteristics.

The ethylene binding inhibition of 1-MCP was discovered in the late 1980's (Prange and Delong, 2003), and by 1999 was approved by the EPA for use in ornamental crops (Watkins, 2006). When binding between 1-MCP and the ethylene binding receptor occurs, signal transduction by ethylene is inactivated and physiological effects of endogenous and exogenous ethylene are prevented (Blankenship and Dole, 2003; Serek et al., 2006). The proposed limiting factor of 1-MCP efficacy is restricted absorption through leaf and fruit material where tissue structure and cuticle resistance limit gas diffusion of 1-MCP (Nanthachai, 2007).

Previous research has documented response of several vegetables, fruits, and flowers to 1-MCP (Zhang et al., 2007; Saftner et al., 2007; Jones et al., 2001; Chope et al., 2007; Sisler and Serek, 1997). Effects of 1-MCP on peanut and other agronomic row crops have not been evaluated or published in the scientific literature. Theoretically, 1-MCP may reduce the senescence and abscission of mature pods by inhibiting the ethylene cascade. Reducing senescence and abscission may provide greater harvest flexibility in peanut. Therefore, one objective of this research was to determine the effects of 1-MCP on pod yield and market grade characteristics when applied at 7 or 14 d before projected optimum digging date when peanut was dug on this date or when digging was delayed. A second objective was to determine the effects of 1-MCP on pod shed when 1-MCP was applied 10 d before projected optimum digging date when peanut was dug on this date or when digging was delayed.

Three separate experiments were conducted in North Carolina to evaluate the effects of 1-MCP on pod yield, market grade characteristics, and pod retention of peanut. In the first experiment, the cultivar NC-V 11 (Wynne et al., 1991) was evaluated during 2005 at the Peanut Belt Research Station near Lewiston-Woodville, NC on a Goldsboro sandy loam soil (fine-loamy, siliceous subactive, thermic, Aquic Paleudults) and in 2007 at the Upper Coastal Plain Research Station near Rocky Mount, NC on a Goldsboro sandy loam soil (fine-loamy, mixed, semiactive, thermic Typic Hapludults). In the second experiment, the cultivar Phillips (Isleib et al., 2006) was evaluated during 2006 and 2007 at the Upper Coastal Plain Research Station on a Goldsboro sandy loam soil (fine-loamy, mixed, semiactive, thermic Typic Hapludults). The final experiment was conducted during 2008 in four separate fields at the Peanut Belt Research Station with the cultivar Perry (Isleib et al., 2003). Soil in three fields was a Rains sandy loam (fine-loamy siliceous, semiactive, thermic Typic Paleaquults) while soil in the fourth field was a Pantego loam (fine-loamy, siliceous, semiactive, thermic Umbric Paleaquults). Experiments were established in conventional tillage systems on raised seed beds with a plot size of 2 rows (91-cm spacing) by 9-m long. Peanut was seeded at a rate to obtain a final in-row population of 13 plants/m.

Treatments consisted of 1-MCP at 26 g/ha applied 7 and 14 d prior to the projected optimum pod maturity as determined by the hull scrape method (Williams and Drexler, 1981) in experiments with NC-V 11 and Phillips. In the experiment with Perry, 1-MCP was applied at 26 g/ha at 10 d prior to the projected optimum pod yield as determined by the hull scrape method (Williams and Drexeler, 1981). Peanut was dug at projected optimum pod maturity and 7 d after projected optimum pod maturity in experiments with NC-V 11 and Phillips. Experiments with the cultivar Perry were dug at projected optimum pod maturity and 20 d after projected optimum pod maturity. A no-1-MCP control was included for all digging dates in all experiments. Crop oil concentrate at 1.0% (v/v) (Agri-Dex®, Helena Chemical Co., Memphis TN) was included immediately after mixing 1-MCP in distilled water. Treatments were applied with a CO₂-pressurized backpack sprayer calibrated to deliver 281 L/ha using regular flat fan nozzles (Spraying System Co., Wheaton, IL).

The treatment structure consisted of either a factorial arrangement of three 1-MCP treatments (experiments with NC-V 11 and Phillips) or two 1-MCP treatments (experiment with Perry) and two digging dates. Agronomic and pest management practices other than applications of 1-MCP and digging dates were held constant over the entire experiment and were based on Cooperative Extension Service recommendations specific to each location (Brandenburg, 2008; Jordan, 2008a, 2008b; Shew, 2008).

Peanut was allowed to air dry for 5 to 8 d after digging prior to threshing and final pod yield was adjusted to 8% moisture. A 500-g sample of pods from each plot was removed and used to determine percentages of sound mature kernels (%SMK), sound splits (%SS), total sound mature kernels (%TSMK), extra large kernels (%ELK), other kernels (%OK), and fancy pods (%FP) in experiments with Phillips (2006 and 2007) and Perry (2008). Pod yield only was determined in experiments with NC-V 11 (2005 and 2007); market grades were not recorded for this cultivar. The effect of 1-MCP on pod retention, measured by counting the number of pods from five randomly selected plants in each plot within 5 d after digging was determined in the experiment with the cultivar Perry (2008). It is postulated that a higher number of pods remaining on plants reflects pod retention, assuming pod set occurred prior to 1-MCP application.

The experimental design for all experiments was a randomized complete block with treatments replicated four times. Data for pod yield, %SMK, %SS, %TSMK, %ELK, %OK, %FP, and pod number and mass from five plants were subjected to analysis of variance appropriate for the factorial arrangement of treatments for individual experiments. Means for significant main effects and interactions were separated using Fisher's protected LSD test at P ≤ 0.05.