Why the UCBI Pistachio Rootstock is Salinity Tolerant

Why the UCBI Pistachio Rootstock is Salinity Tolerant

Louise Ferguson, Extension Specialist
Department of Plant Sciences, University of California Davis

In the late 1990s Sanden, Grattan, Kallsen and I did a series of experiments demonstrating the relative salinity tolerance of the primary California pistachio rootstocks.  We did a single greenhouse experiment with budded non-bearing trees using growth rate as a parameter and two field trials, one using a five year old established orchard and a second in which we established a new orchard under saline conditions. In both filed trials yield and quality were the indicators of salinity tolerance.  Collectively these trials demonstrated that, based on growth and yield, the single species rootstock , Pistacia atlantica, Atlantica, by growth was the most saline tolerant rootstock and that P. integerrima, PGI, was the most saline sensitive rootstock.  The hybrids of these two species; P. atlantica X P. integerrima,  UCBI  and the reciprocal cross, P integerrima x P. atlantica, PG II, based on growth and yield, were intermediate in salinity tolerance.

The Atlantica and PGII seedling rootstocks subsequently demonstrated erratic susceptibility to the soil borne fungus, Verticillium dahliae, Verticillium, and were dropped from commercial production.  Currently, of the four seeding rootstocks tested, the single species, PGI,  and the hybrid, UCBI hybrid have emerged as the two major industry rootstocks, both available as seedling rootstocks and the latter UCBI rootstock now available as a clonally propagated rootstock.  The  PGII hybrid, also a commercially important rootstock now available as the clonally propagated Platinum®  is not included in the following discussion as it was dropped from the second, longer term, field trial.  The results discussed here will are for seedling UCBI and PGI rootstocks.

The collective field results of the two field trials demonstrated ‘Kerman’ pistachio scions on both UCBI and PGI rootstocks tolerated a root zone ECe of 5-6 dS/m.  Above that, for every 1 dS/m increase in ECe trees on a UCBI seedling rootstock had a yield decline of 1.4%. Trees on PGI rootstocks had a yield decline of 3.2% (Sanden et al. 2014)

The nursery trial using budded non-bearing UCBI and PGI seedling rootstocks demonstrated there was a difference in how the rootstocks absorbed, translocated and stored sodium, Na+,  and chloride, Cl- (Ferguson et al. 2004).  This is demonstrated in Figures 1 and 2 below.  Figure 1 demonstrates that, as soil ECe rises, the UCBI rootstock, (blue bar), clearly takes up less Na+ than the Atlantica rootstock (green bar) or Integerrima (brown bar) and transports a far smaller proportion to the scion leaves than the other two rootstocks.  Similarly, as shown in Figure 2 the UCBI rootstock takes up less Cl- than the other two rootstocks.  Collectively this data suggests the UCBI hybrid rootstock takes up less Na+ and Cl- and sequesters both in the rootstock.  This suggests a difference in dealing with these ions in uptake at the root level, and in transport at the trunk level. 

Figure 1.  As soil ECe rises the UCBI rootstock, (blue bar), clearly takes up less Na+ than the Atlantica rootstock (green bar) or Integerrima (brown bar) and transports a far smaller proportion to the scion leaves than the other two rootstocks.  This suggests exclusion at the root level and a sequestering mechanism within the rootstock.

Figure 2. As soil ECe rises the UCBI rootstock, (blue bar), clearly takes up less Cl- than the Atlantica rootstock (green bar) or Integerrima (brown bar).  As with Na+, this pattern again suggests exclusion of  Cl- at the root level and a sequestering mechanism within the rootstock.

It was these findings that precipitated the 2012-2021 investigations of Drakakaki and Godfrey discussed below.

Drakakaki demonstrated the UCBI rootstock was excluding Na+  and Cl- at the root level, and sequestering Na+  in the vacuoles (Zhang et al. in press 2021).  They  found a correlation between vacuolar sequestration of Na+ in the root cortex and suberization of the exodermis and endodermis which decreased  Na+  uptake.  Both parameters increased with increasing salinity stress.

Godfrey et al. (2019, in press 2021) demonstrated the UCBI rootstock was intercepting Na+  in the transport stream and storing it in the xylem parenchyma and recirculating Cl- in the phloem. 

Collectively, the results of Godfrey and Zhang confirm and explain the earlier findings of Ferguson and Sanden demonstrating that the higher salt tolerance of the UCBI hybrid seedling root versus the seedling PGI rootstock.  The UCBI seedling rootstock is more salinity tolerant because it excludes Na+  and Cl- at the root level, stores Na+  in the vacuoles of the root’s cortex, retrieves Na+  from the xylem stream, storing it in the xylem parenchyma and recirculates Cl- in the rootstock trunk phloem.  What is interesting is that none of these mechanisms are present in the UCBI parents even though the seedling rootstocks Drakakai and Godfrey used were almost certainly from the original UCBI parents. These results were produced with seedling rootstocks.  However, there is no physiological reason to think these mechanisms of salinity tolerance would be different in clonally propagated rootstocks.

Currently, Dr. Pat J. Brown is investigating the genetics of salinity tolerance in UCBI and has located two quantitative trait loci (QTLs), or locations in the  UCB-1 genome, one on a P. atlantica chromosome and one on a P. integerrima chromosome, that control the amount of sodium and chloride in leaves of ungrafted UCBI seedling rootstocks.  The implication is that some UCBI seedlings may be more salinity tolerant than others. The original UCBI clonal selection carries the “good” (salinity tolerant) allele, a form of the gene on the chromosome, at both these QTL, locations, and thus may be a good choice for saline sites.

So, how does this information impact pistachio growers now, in the short term?  Except for determining the relative salinity tolerance of the current commercially available rootstocks our trials did not change what growers have been doing for decades.  Salinity cannot be eliminated; it can only be managed.  Below are the principle management strategies, as presented by Mae Culumber, Fresno County Farm Advisor, at multiple Advances in Pistachio Production Short Courses Statewide Pistachio Days.

In saline situations:

·       use a UCBI rootstock

·       maintain the soil Ece below 4.5 dS/m

·       sample your soil and irrigation water

o   preplant

o   before and after the growing season

·       address sodicity first and salinity second

o   apply gypsum before winter rains or leaching

·       leach during the dormant period

o   when Et is lowest

o   when ECe is highest

o   and before the spring root flush

However, in the mid and long term the results discussed above are of great significance and impact.  In the midterm the phenotypic and physiological biomarkers identified by Godfrey and Drakakaki can be used to identify seedling rootstocks for clonal propagation. Field studies by Dr. Patrick J. Brown, geneticist in the Department of Plant Sciences at UC Davis are now ongoing to determine whether the salinity tolerance QTL detected in ungrafted rootstocks translates into yield differences in mature orchards, and molecular work is underway to try to connect the genetic and physiological observations in these different studies. In the long term, understanding the molecular mechanisms behind these differences in rootstock performance can be used to further improve the salinity tolerance of pistachio rootstocks.

The researchers cited above gratefully acknowledge the sustained support of the California Pistachio Research Board and special thanks to CPRB Research Subcommittee Chair Tom Coleman who took a chance in supporting these basic investigations of salinity tolerance mechanisms in pistachios.

References:

Ferguson, L., J.A. Poss, S.R. Grattan, G.M. Grieve, D. Wang, C. Wilson, and T.J. Donovan, and C.-T. Chao. 2002. Pistachio rootstocks influence scion growth and ion relations under salinity and boron stress. J. Amer. Soc. Hort. Sci. 127(2):194-199. https://pdfs.semanticscholar.org/4394/68445601acf03e5e3c6a6cc2bae86cf6c6b4.pdf

Godfrey, J.M.,  L. Ferguson and M. A. Zwieniecki. 2020.  Sodium retrieval from sap may permit maintenance of carbohydrate reserves in mature xylem tissues of a salt-tolerant hybrid pistachio rootstock exposed to 100mM NaCl.  In Press; Jour. Amer. Soc. Hort. Sci. March 2021.

Godfrey, J.M.,  L. Ferguson,  B L Sanden,  A. Tixier,  O. Sperling,  S. R, Grattan and M. A. Zwieniecki. 2019. Sodium interception by xylem parenchyma and chloride recirculation in phloem may augment exclusion in the salt tolerant Pistacia genus: context for salinity studies on tree crops. Tree Physiology, Volume 39, Issue 8, August 2019, Pages 1484–1498. https://doi.org/10.1093/treephys/tpz054

https://academic.oup.com/treephys/article-abstract/39/8/1484/5489906/?redirectedFrom=fulltext

Sanden, B.L., L. Ferguson and D.L Corwin. 2014. Development and long term salt tolerance of pistachio rootstocks using saline groundwater. https://www.researchgate.net/publication/286646321_Development_and_long-term_salt_tolerance_of_pistachios_from_planting_to_maturity_using_saline_groundwater View this Article

­­Zhang, S.,  A. Quartararo, O. K. Betz, S. Madahhosseini, A. S.Heringer,  T. Le, Y. Shao,  T. Caruso, L. Ferguson, J. Jernstedt, T. Wilkop, G. Drakakaki. 2021. Root vacuolar sequestration and suberization are prominent responses of Pistacia spp. rootstocks to salinity stress. In press: Plant, Cell, Environ. March 2021