Pecan [Carya illinoinensis (Wangenh.) K. Koch], a member of the hickory (Carya spp., Juglandaceae) genus, is native to North America. As a long-lived perennial tree crop, it has evolved from being a relatively minor wild component of riparian ecosystems to a significant silvicultural and horticultural crop (Wood et al., 1990), and it has been one of the most significant fruit crops in the United States (Wood, 2001). The large amount of benefits of pecans includes the rich taste of the kernel; a lack of bitterness; and a high content of fatty acids, proteins, phospholipids, and vitamins (Huang et al., 2019). These benefits have resulted in the provision of highly significant contributions to applications in the food industry, including baking; producing confections, ice cream, and gift packs; retailing; wholesaling; and serving various food service outlets. In addition to these features, it has the additional economic benefits of its uses in horticulture, the manufacture of wood, and as a source for the extraction of oil (Xue et al., 2018). As a tall arbor tree, the pecan tree also has significant ecological benefits for agroforestry systems (Guo et al., 2018a, 2018b). With the gradual realization of the potential value of the pecan, global demands for the nut have continued to increase annually. Countries outside the native range of the tree, including China, South Africa, Australia, Uruguay, Argentina, and Brazil, are now producing pecans, and their production is expected to increase over the next 30 years (Casales et al., 2018; Lazarotto et al., 2014; Wakeling et al., 2001; Zhang et al., 2015b). Therefore, further study on pecan propagation is merited.
To date, compared with other pecan cloning propagation techniques that include cutting and tissue culture, grafting is the most traditional and effective method to produce pecan cultivar trees (Zhang et al., 2015a). The growth of the grafted pecan tree is affected by its complex genetic system, which is determined by both the rootstock and scion. Although growers are still finding a more efficient propagation for the scion cultivar, instead of breeding or searching rootstocks, a better understanding of the impact of rootstocks on the growth of scion is necessary to the specific production goals. It is known that appropriate rootstocks not only provide scion resistance to environmental stresses but also increase growth vigor to reduce both the vegetative and reproductive times of the scion, therefore increasing market competition. In addition, a good rootstock can reduce orchard management costs and indirectly increase profits for the pecan growers.
A better understanding of the adaptation of the rootstocks to the specific constraints in certain orchards provides useful guides to monitor the performance of pecan trees to maximally increase the production of the orchard. Therefore, knowing the adaptation of different rootstocks to different environments becomes important for pecan breeders and growers, particularly for a long-term goal of the U.S. pecan breeding program.
A seedstock is the source of the seed planted to produce seedling rootstocks. Seedstock selections made by nurserymen have an apparent geographic distribution. The most critical limitation for a rootstock is its adaptation to the climate of the region. For example, the ‘Moore’ seedling was used as a rootstock in the eastern United States. It is a seedling selection from Florida, originating in Jefferson County near the town of Waukeenah, FL (Hammar and Hunter, 1949). Although pecans in Florida were introduced, the source of the seed that produced ‘Moore’ is unknown. ‘Moore’ carries the maternal haplotype most common in northern pecans that is also found in populations from Mexico and Texas (Grauke, 2010; Grauke et al., 2011). ‘Moore’ is a sib of the pecan cultivar Waukeenah, which is also used as a rootstock in the southeastern United States (Grauke and O’Barr, 1996; Grauke and Pratt, 1992). These trees are protandrous, with early pollen and midseason pistillate receptivity. ‘Moore’ was the most productive rootstock used in a test in Shreveport, LA, in the 1930s (Sitton and Dodge, 1938) and has produced the largest seedlings in tests that began in the 1980s (Carpenter et al., 1979). This rootstock produces vigorous and uniform seedlings that have some resistance to scab disease. Another example is ‘VC1-68’, a rootstock selection that originated near Phoenix, AZ, and is used as a popular rootstock in the western United States, particularly California (Grauke, 2010). The tree is protandrous with early pollen shed and midseason pistillate receptivity. The nuts are very large (41 nuts per pound) but are thick-shelled, resulting in a low kernel percentage (≈43%). The nuts germinate well, and the seedlings are uniform and vigorous, with excellent lateral root formation. The parent ‘Cape Fear’ is very susceptible to scab, but observations of the progeny suggest that they do not have this problem. Its freeze susceptibility may limit the use of this rootstock to the southern part of the southeastern and southwestern United States. However, different rootstocks have one or more contributions to tree growth, nut production, and/or nut quality, including consistent high quality, precociousness, early harvest, a high yield, a low degree of alternate bearing, late budbreak, scab resistance, and aphid tolerance for the mature pecan tree. Thus, understanding the impacts of various rootstocks on scion growth and nut production increases the interests of the breeders and growers. For example, physiological stress resistance is one of the major goals of rootstock breeding.
This study describes the drought stress resistance of the grafted young ‘Pawnee’ trees onto 12 different pecan seedling rootstocks. The objectives of this study were as follows: 1) to determine the effects of different rootstocks on the drought tolerance of the scion (‘Pawnee’) to provide guidance for the selection of the most drought-tolerant pecan rootstock, which would also support further pecan cultivation research in arid and semiarid areas; and 2) to analyze the differential water parameters of the same grafted ‘Pawnee’ scions to understand the interaction between the rootstock and scion, thus, providing a foundation for functional genomics research in pecan.
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