Soil conditions, including sodicity, salinity, and poor drainage, limit distribution of pecan [Carya illinoinensis (Wangenh.) K. Koch] acreage in the southwestern United States. Multiple aspects of plant physiology and metabolism are affected by salinity stress (Tuteja, 2007). Low water potentials associated with high salt content in the soil solution make it more difficult for plant roots to acquire water. Sodic soils (soils with an exchangeable sodium percentage greater than 15%) are associated with unstable soil structure, poor drainage, and salt accumulation (Sumner, 1993). High levels of Na impact K uptake negatively in plants, causing a disturbance in the function of certain enzymes and stomata as well as in the osmotic balance (Tuteja, 2007).
Cell injury caused by ion excess in the leaves reduces growth and can lead to leaf senescence and plant mortality. Plants have been reported to have two phases of response to salinity stress: 1) young leaf growth is inhibited by osmotic stress and 2) senescence of mature leaves is accelerated by an ionic phase in which toxic levels of salts accumulate (Munns and Tester, 2008). Rajendran et al. (2009) suggested that the three main components of salinity stress tolerance in cereals are Na exclusion, Na tolerance in plant tissues, and osmotic stress tolerance. Munns and Tester (2008) described these mechanisms more broadly as Na or Cl exclusion, tolerance to accumulated Na or Cl, and tolerance to osmotic stress, and speculated that because plants use Na and Cl to maintain turgor pressure in leaves, the plant may need to develop a balance between these ions to avoid ion toxicity.
It is unclear which of the ions in saline water or soil cause salt injury in pecans. Faruque (1968) treated ‘Riverside’ pecan seedlings growing in sand culture with various salt solutions, expressed in terms of osmotic pressure. The results of that study determined that salt injury to leaves was caused by Cl rather than Na. Seedlings treated with NaCl solutions began to exhibit significant injuries (measured by percent necrosis of leaves) when growing in a 0.15-MPa osmotic pressure solution, and a 0.30-MPa solution resulted in death. Seedlings treated with CaCl2 began to show significant injuries at 0.20 MPa, but no injury was caused by Na2SO4 solutions with an osmotic pressure as great as 0.30 MPa, implicating Cl and not Na as the causal agent. Necrosis occurred in leaves of seedlings with a whole plant tissue content of 5959 mg⋅kg–1 Cl when trees were treated with either NaCl or CaCl2. Similarly, Harper (1946) found that leaf content of 6000 mg⋅kg–1 Cl caused severe damage to pecans. In contrast, Miyamoto et al. (1985) reported that Na content, but not Cl content, in pecan leaves demonstrated a strong negative correlation with leaf, stem, and root weight.
Although few studies have investigated pecan cultivar sensitivity to salts, Hanna (1972) found that seedlings produced by hand pollination of identical parentage exhibited a wide range of patterns of Cl absorption. Miyamoto et al. (1985) reported that ‘Riverside’ seedlings absorbed less Na and showed less salt damage than ‘Apache’ and ‘Burkett’, the two other cultivars tested.
The foliar K:Na ratio has been found to be a factor in salinity tolerance. Gorham (1990) reported that salinity tolerance in Aegilops (goatgrass) species was related to the ability to maintain high leaf K:Na ratios. Munns and Tester (2008) indicated that foliar K:Na ratios were a function of ion transporter genes and that salinity tolerance in plants may be related to increased leaf tissue K:Na ratios. Almeida et al. (2017) noted that Na inhibits K uptake by cells and likely inhibits K transporters. Wakeel (2013) pointed out that an optimal K:Na ratio is essential for the activation of enzymatic reactions in the cytoplasm required for maintaining plant growth.
Soil physical properties can affect salt accumulation in the soil profile. Clayey soils with low permeability and greater specific surface area tend to accumulate more salts than porous, well-drained soils (Warrence et al., 2002). Irrigated pecan orchards with low permeability (e.g., poorly drained alluvial soils) may exhibit salt accumulation (Miyamoto and Storey, 1995). Miyamoto et al. (1986) noted that ‘Western’ scions grafted to ‘Riverside’ rootstock grown in soils with a high clay content (silty clays and silty clay loams) were stunted and had a smaller trunk diameter than trees planted in coarser textured (loam) soil. Greater salt accumulation occurred in the clayey soils. In soils with a saturated paste extract electrical conductivity (ECe) in the upper 30 cm greater than 2.0 dS⋅m–1, trunk diameter was reduced. Branch dieback occurred when ECe exceeded 6.0 dS⋅m–1.
A major source of soil salts in irrigated systems is irrigation water. Deb et al. (2013) found that use of irrigation water with ECirr (irrigation water electrical conductivity) between 1.4 and 3.5 dS⋅m–1 resulted in soil EC1:1 (soil salinity of a 1:1 soil/water extract) of between 0.89 and 2.71 dS⋅m–1. In 1-year-old ‘Western’ grafted to ‘Riverside’ rootstock, this level of salinity resulted in budbreak delay and inhibition, as well as reduced seedling growth rate. Visible symptoms of salt injury occurred at an ECirr of 3.5 dS⋅m–1. Seedlings did not survive the 2-year test period when ECirr levels were 5.5 dS⋅m–1 or more.
Plant Zn content has been reported to be related to salinity tolerance (Cakmak, 2000). Zinc promotes the synthesis and activity of antioxidative enzymes that can help prevent damage from oxidative stress factors, including salinity (Cakmak, 2000). Therefore, improving the Zn nutritional status of plants grown in arid and semiarid regions with saline soils may be important for not only preventing Zn deficiency, but also for protecting plants against the damage caused by excess salinity. Norvell and Welch (1993) suggested that increased Zn may reduce plant accumulation of Na. Zinc has been shown to increase salinity tolerance in chickpea (Cicer arietinum L.), as evidenced by reduced levels of Na uptake and elevated levels of K in shoots (Saxena and Rewari, 1990). In soybeans (Glycine max L.) grown in two saline soils, one with primarily chloride salts, the other with sulfate salts, the uptake of Zn was suppressed in proportion to the salinity of the soil (Gupta and Gupta, 1984).
Long-term drought and expansion of pecan acreage in the semiarid southwestern United States have increased use of low-quality irrigation water in pecan production. This has accentuated the need for rootstocks that are tolerant of sodic and saline soil conditions. To this end, we conducted a field study to evaluate the effect of maternal genotype on pecan seedling tolerance to such conditions, and to evaluate the effect of the application of Zn–ethylenediaminetetraacetic acid (EDTA) on rootstock performance. We selected open-pollinated seeds from several cultivars originating from environmentally diverse parts of the pecan native range. We expected that seedlings with genetic origins that lie in lower precipitation regions where saline and sodic soils are more common would be more tolerant of these soil conditions than those with genetic origins that lie in higher precipitation areas.
Almeida, D.M., Oliveira, M.M. & Saibo, N.J.M. 2017 Regulation of Na+ and K+ homeostasis in plants: Towards improved salt stress tolerance in crop plants Genet. Mol. Biol. 40 1 326 345 doi: https://doi.org/10.1590/1678-4685-GMB-2016-0106
Deb, S.K., Sharma, P., Shukla, M.K. & Sammis, T.W. 2013 Drip-irrigated pecan seedlings response to irrigation water salinity HortScience 48 1545 1548
Faruque, A. H. M. 1968 The effect of salinity on phytotoxicity and ion uptake of pecan seedlings (Carya illinoensis wag, cv. Riverside) Texas A&M University College Station, TX PhD Diss
Gupta, V.K. & Gupta, S.P. 1984 Effect of zinc sources and levels on the growth and Zn nutrition of soybean (Glycine max. L.) in the presence of chloride and sulphate salinity Plant Soil 81 2 299 304
Hanna, J.D. 1972 Absorption and accumulation of chloride ions by pecan (Carya illinoensis KOCH) seedling rootstocks Texas A&M University College Station, TX PhD Diss
Harper, H.J. 1946 Effect of Cl on physical appearance and chemical composition of leaves on pecans and other native trees of Oklahoma Tech. Bul.
Heerema, R.J. 2013 Diagnosing nutrient disorders of New Mexico pecan trees New Mexico State University Guide H-658, College of Agricultural, Consumer and Environmental Sciences, New Mexico State University Las Cruces, NM
Miyamoto, S., Gobran, G.R. & Piela, K. 1985 Salt effects on seedling growth and ion uptake of three pecan rootstock cultivars Agron. J. 77 383 388
Miyamoto, S., Riley, T., Gobran, G. & Petticrew, J. 1986 Effects of saline water irrigation on soil salinity, pecan tree growth and nut production Irr. Sci. 7 83 95
Miyamoto, S. & Storey, J.B. 1995 Soil management in irrigated pecan orchards in the southwestern United States HortTechnology 5 219 222
Munns, R. & Tester, M. 2008 Mechanisms of salinity tolerance Annu. Rev. Plant Biol. 59 651 681 doi: https://doi.org/10.1146/annurev.arplant.59.032607.092911
Norvell, A.W. & Welch, R.M. 1993 Growth and nutrient uptake by barley (Hordeum vulgare L. cv. Herta): Studies using an N-(2hydroxyethyl) ethylenedinitrilotriacetic acid-buﬀered nutrient solution technique: I. Zinc ion requirements Plant Physiol. 101 619 625
Post, D.F., Hendricks, D.M. & Hart, J.M. 1977 Soils of the University of Arizona Experiment Station: Safford Agricultural Engineering and Soil Science 77-1 Report, University of Arizona
Rajendran, K., Tester, M. & Roy, S.J. 2009 Quantifying the three main components of salinity tolerance in cereals Plant Cell Environ. 32 3 237 249 doi: https://doi.org/10.1111/j.1365-3040.2008.01916
Saxena, A.K. & Rewari, R.B. 1990 Influence of zinc on nodulation and ion uptake by chickpea under saline conditions J. Indian Soc. Soil Sci. 38 2 363 364
Tuteja, N. 2007 Mechanisms of high salinity tolerance in plants Methods Enzymol. 428 419 438 doi: https://doi.org/10.1016/S0076-6879(07)28024-3
Wakeel, A. 2013 Potassium–sodium interactions in soil and plant under saline–sodic conditions J. Plant Nutr. Soil Sci. 176 3 344 354 doi: https://doi.org/10.1002/jpln.201200417
Warrence, J.N., Bander, J.W. & Pearson, K.E. 2002 Basics of salinity and sodicity effects on soil physical properties Department of Land Resources and Environmental Sciences, Montana State University Bozeman, MT
Zhang, H., Schroder, J.L., Pitman, J.J., Wang, J.J. & Payton, M.E. 2005 Soil salinity using saturated paste and 1:1 soil to water extracts Soil Sci. Soc. Amer. J. 69 1146 1151 doi: https://doi.org/10.2136/sssaj2004.0267