Spanish Researchers Study Salt Stress on Olive Trees

Summary

Spanish researchers have pub­lished a study on the effects of salt stress on olive trees, high­light­ing the grow­ing issue of soil salin­iza­tion in the Mediterranean basin and pre­sent­ing poten­tial solu­tions. The study found that olive trees exhibit vary­ing degrees of salt tol­er­ance, with graft­ing salt-tol­er­ant root­stocks onto sen­si­tive cul­ti­vars being a rec­om­mended method to enhance resilience in salin­ized soils.

Spanish researchers have pub­lished a first-of-its-kind study into the effects of salt stress on olive trees. 

The study, pub­lished in the jour­nal Biology, presents a com­pre­hen­sive review of the impli­ca­tions of, and pos­si­ble solu­tions to, soil salin­iza­tion, a grow­ing prob­lem glob­ally and one of par­tic­u­lar con­cern in the Mediterranean basin.

The Mediterranean basin is highly sus­cep­ti­ble to salin­ity due pri­mar­ily to low rain­fall, mil­len­nia of agri­cul­tural irri­ga­tion and sea­wa­ter intru­sion.

Agricultural irri­ga­tion con­tributes heav­ily to soil salin­iza­tion because irri­ga­tion water the plants do not absorb evap­o­rates, leav­ing behind a pro­gres­sive salt accu­mu­la­tion. 

An annual irri­ga­tion of 1,000 mil­lime­ters with water hav­ing a salt con­tent as low as 300 mil­ligrams per liter is esti­mated to add 300 kilo­grams of salts per hectare. This is fur­ther exac­er­bated by the ions con­tained in fer­til­iz­ers.

Seawater intru­sion is a com­plex phe­nom­e­non that results from the over­ex­ploita­tion of coastal aquifers for human water con­sump­tion and agri­cul­tural and live­stock uses com­bined with reduced recharge of these aquifers, which is asso­ci­ated with increased demand for water in river basins.

This phe­nom­e­non is com­pounded by cli­mate change, which leads to ris­ing sea lev­els and dis­rupted pre­cip­i­ta­tion pat­terns. 

Rivers that expe­ri­ence reduc­tions in their basins con­tribute less water to coastal aquifers, which in turn are sub­ject to a greater inflow of salt­wa­ter due to sea level rise and increased storm surges. 

This leads to the salin­iza­tion of aquifers and, sub­se­quently, of their asso­ci­ated ecosys­tems and estu­ar­ies.

Olive trees are well-known to be salt-tol­er­ant, with saline irri­ga­tion fre­quently employed in olive-grow­ing regions of var­i­ous Mediterranean coun­tries, such as Spain, Israel and Tunisia, where water short­age is one of the major bar­ri­ers to sus­tain­able agri­cul­ture.

Olive trees dis­play both struc­tural and bio­chem­i­cal strate­gies to man­age salt stress. These include thicker root cell walls, increased pro­duc­tion of osmo­pro­tec­tants, such as pro­line and man­ni­tol, and enhanced antiox­i­dant sys­tems to com­bat reac­tive oxy­gen species.

The researchers found, how­ever, that the abil­ity of the olive tree to tol­er­ate salin­ity varies sig­nif­i­cantly between cul­ti­vars.

Cultivars such as Royal de Cazorla and Kalamata were found to exhibit the most con­sis­tent salt tol­er­ance, whilst Leccino and Shiraz were among those classed as salt-sen­si­tive and unsuit­able for salin­ized soils unless grafted onto a salt-tol­er­ant root­stock.

Grafting sen­si­tive cul­ti­vars onto tol­er­ant root­stocks, often derived from wild olives, can enhance resilience. 

As with other fruit trees, olive tree behav­ior is affected by the root­stock used, and graft­ing wild tree root­stocks is a tra­di­tional method for pro­duc­ing stronger trees with improved fruit qual­ity. 

Unlike their domes­ti­cated rel­a­tives, wild olive trees exhibit high genetic vari­abil­ity and are a valu­able source of genes resis­tant to abi­otic stresses.

Already a proven tech­nique for reduc­ing the adverse effects of salin­ity in grapevines, researchers expect that salt-tol­er­ant root­stocks will sim­i­larly mit­i­gate salt stress in olives.

Therefore, they rec­om­mend using salt-tol­er­ant cul­ti­vars or root­stocks in salin­ized soils in the short and medium term. In con­trast, the time-con­sum­ing process of breed­ing salt-tol­er­ant cul­ti­vars is car­ried out.

This tech­nique may be increas­ingly impor­tant as mod­ern cul­ti­va­tion shifts towards high-den­sity, irri­gated sys­tems, which demand higher water use and increase salin­ity risk.

Multi-omics approaches, com­bin­ing genomics, tran­scrip­tomics, pro­teomics and metabolomics, are pro­posed as the future of olive stress research. 

Integrating data from these domains with arti­fi­cial intel­li­gence and machine learn­ing tools could lead to pre­dic­tive mod­els for cul­ti­var per­for­mance under stress. These could be used, for exam­ple, to select promis­ing cul­ti­vars or root­stocks.

Such approaches could also be used to develop chem­i­cal prim­ing strate­gies. Priming is the mech­a­nism through which plants can per­ceive a mild stim­u­lus that induces pro­tein post-trans­la­tional mod­i­fi­ca­tions, such as phos­pho­ry­la­tion and car­bony­la­tion. 

These can reg­u­late stress responses more effi­ciently than tra­di­tional gene expres­sion alone. Identifying suit­able post-trans­la­tional mod­i­fi­ca­tions could lead to prim­ing that enhances tol­er­ance to saline stress.

• Plant Stress
• Consejo Superior de Investigaciones Científicas