Partitioning Phosphorus and Potassium in Pecan Trees during High- and Low-crop Seasons

Potassium (K) and phosphorus (P) partitioning were characterized in bearing pecan [Carya illinoinensis (Wangenh.) K. Koch] trees at selected times of the year during three successive years. The first-year trees had little to no crop, followed by a small crop the second year, and a near optimal to excess crop during the third year. Trees bearing a large crop accumulated more K and P in July than trees with little or no crop. K and P content in trees increased from budbreak until July, and then decreased until budbreak the following year. Allocation patterns of K and P among tree components changed during the growing season, with the greatest changes occurring in the annual plant parts. Results indicate that leaves were the principal source of labile K and P for developing fruit. A rapid accumulation of K in the fruit with a concurrent loss from leaves started in July when fruit began a rapid volume increase that continued during carbohydrate deposition in the cotyledon. At first, detectable shuck split K transported to the fruit ceased, but leaf K was partially replenished, presumably from K in perennial tissue because total tree K was static or decreased slightly. In contrast, rapid P transport to fruit began after fruit expansion while cotyledons were developing, presumably associated with oil synthesis and after initial shuck split for storage. Leaf P content continued to decline until leaves were killed by freezing temperatures in the fall. Data indicate that potentially large crops signal additional early season K and P absorption and accumulation in leaves and other tissue long before the fruit are strong K and P sinks. This suggests a complex signaling mechanism, essentially telegraphing a copious demand during the latter part of the growing season. Pecan fruit production is irregular, typified by high production one year followed by one year or more of low production (Sparks, 1986). This may best be characterized as alternate bearing with irregular symmetry. Alternate bearing is typically associated with a lack of return bloom rather than flower or fruit abortion. Certain cultural practices, such as nutrition, light and water management, fruit thinning, vegetation control, and others, reduce alternate bearing intensity, but no management program eliminates irregular bearing. In central Oklahoma, pecan budbreak is typically the second week in April (Fig. 1). Pecans are heterodichogamous, with pollination during midto late May, depending on cultivar. Catkin (staminate flower) differentiation for next year’s crop can be detected about 3 weeks after budbreak (Wetzstein and Sparks, 1984). Following pollination, fruit growth is slow and then becomes rapid about the first of July (Diver et al., 1984; McKay, 1947). During fruit enlargement, the endosperm is noncellular. In midto late August (for most cultivars), the endosperm becomes cellular (gel stage) and the pericarp begins lignification, ending fruit enlargement. Early August is also when pistillate flower induction occurs for next year’s crop (Amling and Amling, 1983); however, differentiation is delayed until bud swell in March (Wetzstein and Sparks, 1983). Deposition of cotyledonary materials (dough stage) immediately follows the gel stage. Fruit ripen (shuck split) in central Oklahoma between early September and early November, depending on cultivar. There are cultivars that produce earlierand laterripening fruit, but they are not grown in Oklahoma. ‘Maramec’, the cultivar used in this study, ripens the last week in October. The last average spring freeze is 1 Apr. and the first killing (–2 C) fall freeze is 15 Nov. in central Oklahoma (Board of Regents of the University of Oklahoma, 2003). Potassium and P are phloem-mobile macronutrients (Marschner et al., 1996) that are readily translocated from leaves to developing pecan fruit (Sparks, 1977, 1988). Potassium functions in osmoregulation, carbohydrate translocation, protein synthesis, enzyme activation, cell expansion, and stomatal regulation (Pallardy, 2008). Potassium is not known to occur in any organic forms. Phosphorus is a constituent of nucleotides, phospholipids, and high-energy phosphate compounds used to transfer energy (Pallardy, 2008). Phosphorous occurs in organic and inorganic forms in the plant and is probably transported in both forms. Alternate-bearing pecan trees show distinct differences in leaf K and P accumulation and depletion between large (ON) and small (OFF) crop loads (Krezdorn, 1955). ON year trees had higher concentrations of leaf K and P in April, shortly after budbreak, probably resulting from greater stored reserves following an OFF year. Potassium and P increased in leaves of OFF year trees faster than ON year trees, resulting in similar leaf K and P concentrations within 2 weeks after budbreak, and the concentrations remained comparable between fruit production levels, although decreasing, for several weeks. Potassium concentration in leaves of ON trees dropped below that of OFF trees by the end of June and stayed lower throughout the remainder of the growing season. Similarly, Diver et al. (1984) reported that K concentration in leaves on bearing shoots was less than those on vegetative shoots from late June through September. Diminution of leaf K during this time period was presumably associated with carbohydrate transport (Haeder, 1977; Mengel, 1980; Mengel and Haeder, 1977; Vreugdenhil, 1985) to the rapidly expanding fruit and later in the growing season to developing cotyledons (Diver et al., 1984). Received for publication 11 May 2009. Accepted for publication 28 June 2009. Funding for this study was provided by the Oklahoma Agricultural Experiment Station, by the Samuel Roberts Noble Foundation, and by the Oklahoma Pecan Growers’ Association. Corresponding author. E-mail: [email protected]. J. AMER. SOC. HORT. SCI. 134(4):399–404. 2009. 399 Phosphorus concentration in leaves of ON trees exceeds that of OFF trees from late June through mid-August (Krezdorn, 1955). By late August, the leaf P concentration of ON trees dropped below OFF trees, about 6 weeks later than when K was depleted from leaves of ON trees. The loss of leaf P coincided with the onset of cotyledon deposition, but not fruit expansion. In another study, leaf P concentration was similar between fruiting and vegetative shoots throughout the growing season (Diver et al., 1984). However, there was a large accumulation of P in the kernel during the last 4 weeks before fruit maturity, with the majority accumulating during the final 2 weeks. Sparks (1977) reported that prolific fruit production frequently resulted in scorched (necrotic leaf margins) leaves in late summer that led to partial defoliation. The concentration of leaf P in late September was 15% lower and leaf K was 25% lower on shoots where fruit were retained versus those shoots where fruit were removed in July, implicating a critical shortage of one or both of these elements during fruit maturation. The largest accumulation of K in mature fruit was in the shuck, while the largest P accumulation was in the kernel. Later work by Sparks (1988) found that a shortage of P created by a strong cotyledon sink resulted in leaf scorch. Nutrient demand created by developing fruit affects nutrient partitioning in pecan (Acuña-Maldonado et al., 2003; Diver et al., 1984; Smith et al., 2007; Sparks, 1977, 1988) and other orchard crops (Brown et al., 1995; Millard, 1995; Oland, 1959; Picchioni et al., 1997; Stassen et al., 1981; Taylor 1967; Titus and Kang, 1982; Tromp, 1983; Weinbaum et al., 1994a, 1994b, 1994c). Alternate bearing is a significant problem in pecan that may be mitigated by nutrient management (Wells and Wood, 2007). The objective of this study was to elucidate allocation patterns of P and K at selected times during lowand high-crop years. Materials and Methods Ten 15-year-old ‘Maramec’ trees growing in a Teller sandy loam (fine-loamy, mixed, active, thermic Udic Argiustoll) at the Cimarron Valley Research Station near Perkins, OK, were selected based on uniformity of size, vigor, and location within the orchard. Trees were spaced 12.2 · 12.2 m and were 9.3 ± 1.2 m tall with 29 ± 3-cm diameter trunks measured 1.4 m above the ground at the start of the study. A 7.3-m-wide vegetation-free area was maintained the entire row length with selected herbicides. Trees were drip irrigated based on the model developed by Worthington (Worthington and Stein, 1993). Pest management was according to Oklahoma Cooperative Extension Service recommendations (Smith and McCraw, 1998; von Broembsen et al., 1999). Trees received nitrogen (N) application annually as a single application the second week of March or as a split application with 60% applied in March and 40% applied the first week in October. The annual N application was 125 kg ha whether applied as a single or split application. Nitrogen treatment did not affect K or P allocation, therefore, N treatments were pooled. Production substantially differed among years. In 1998, there was no harvestable yield, probably the result of abundant production in 1997 (19.1 kg/tree). Yield in 1999 was low (8.8 kg/tree), and in 2000, the trees were overloaded (22 kg/tree). An exceptionally early fall freeze (7–10 Oct.) in 2000 (Smith, 2002) damaged the crop, preventing normal shuck opening. However, yield was measured by harvesting and weighing the fruit (shucks attached) and then correcting the yield by deducting the shuck weight. TISSUE SAMPLES. Samples included roots, <1 cm diameter and $1 cm diameter, that were excavated with a backhoe. The root samples were collected from each tree in an area 1 m wide, 2 m long, and 1.5 m deep. A new location was chosen each time roots were sampled. Root samples were washed to remove adhering soil, dried at 70 C, and then segments of the roots were cut for grinding. Samples were ground to pass through a 20-mesh screen, and then 10to 20-g aliquots were stored in jars until analysis. The trunk was sampled by boring about 10 2.5-cm holes with a spade bit 5 cm deep. The dead outer bark was discarded, and it was then divided into inner bark and wood. The samples were collected in a vertical line on the trunk to reduce damage, and the holes were then filled with mortar mix. Samples were dried and prepared for analysis as described above. Ten current season shoots per tree were sampled on each date. Shoots were dried, ground, and an aliquot was stored for analysis. Leaf samples consisted of all leaves (leaflets and rachis) on five current season shoots. Five fruit clusters were collected during each sample date, with the whole pistillate flower or fruit used for analysis, except in 1999 when the mature fruit was divided into shuck (involucre), shell (pericarp), and kernel (embryo, including cotyledons). Samples were prepared as described earlier, with an aliquot stored for analysis. SAMPLE TIME. Sample times were at stage 4 budbreak (Wetzstein and Sparks, 1983) (second to third week of April), pollination (last week of May), the third week of July (beginning of rapid fruit enlargement), first shuck split detected (<1% of fruit; first week of October), and immediately after the first killing freeze in the fall (12 Nov. 1998, 18 Nov. 1999, and 10 Oct. 2000) (Fig. 1). Fig. 1. Annual cycle of pecan phenology and reproductive development. Simultaneous events for the current season crop (outside circle) and next year’s crop (inside circle) are identified. The timeline is representative of central Oklahoma. Trees with type 1 catkin development are protandrous and those with type 2 are protogynous. 400 J. AMER. SOC. HORT. SCI. 134(4):399–404. 2009. SAMPLE ANALYSIS. Samples were analyzed for P colorimetrically and for K using atomic absorption spectroscopy. Samples were redried at 70 C for 24 h before weighing a 1-g sample for digestion (dry ash), extraction, and analysis. TOTAL P AND K ESTIMATION. Trunk diameter was measured annually 1.4 m above the ground while trees were dormant. Biomass of the various components was estimated from annual trunk diameter measurements using equations developed for pecan (Smith and Wood, 2006). Total P and K in each tree component were calculated by multiplying the estimated weight by the P or K concentration, except for the fruit. Fruit mass was determined by calculating the number of fruit at harvest for each tree based on the mean fruit weight of duplicate 20-fruit samples per tree and total tree yield. The number of fruit per tree was then used to calculate fruit weight for each tree during the various sample times based on the mean fruit weight of the five fruit clusters ( 20 fruit/tree) sampled on each date. Total P or K in the fruit was calculated from the fruit weight and the P or K concentration. Fruit harvested in 1999 were divided into shuck, shell, and kernel, each part was analyzed for P and K, and the total P and K was calculated for each fruit part. In 2000, the early freeze prevented analysis of the fruit parts because the shuck adhered to the shell and the kernel was incompletely developed. STATISTICAL ANALYSIS. Data were analyzed for each tree component using a mixed model as a split-plot design with year (crop load) nested within sample date. The interaction (sample time · crop load) had 10 single-tree replications. Means were compared using the protected least significant difference (LSD) test at the 5% level. In addition, the gain or loss in the concentration of K or P from one sampling date to the next was calculated using the elemental concentration as % change = ðy2 y1=y1Þ3 100 where y1 is the elemental concentration at the earlier sample date and y2 is the elemental concentration on the subsequent sample date.