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Researchers in Israel stud­ied the effects of high envi­ron­men­tal tem­per­a­tures on olive trees, deter­min­ing that intense heat reduced the quan­tity and qual­ity of olive oil and ulti­mately led to a reduced yield. Further research could develop olive vari­eties resilient to high tem­per­a­tures that would secure farm­ers’ crops against unsea­son­able warm weather and cli­mate change.

Olive trees are known for their resilience to extreme weather vari­a­tions. Global warm­ing, how­ever, has posed poten­tially great risks in many olive oil pro­duc­ing ter­ri­to­ries around the world where alter­nat­ing waves of extremes extend beyond their nor­mal weather patterns.

Higher-than-nor­mal heat brings early blos­som­ing. Colder days can freeze the blos­soms and pre­vent flow­er­ing and fruit devel­op­ment. Productivity is dimin­ished and the olive oil yield is reduced.

To test how the olive trees behave in hot weather, the researchers placed five-year-old pot­ted trees of five dif­fer­ent olive vari­eties in two loca­tions: one with hot sum­mers with tem­per­a­tures often exceed­ing 40°C (104°F) and another with rel­a­tively mild sum­mers with tem­per­a­tures around 30°C (86°F).

The loca­tions were specif­i­cally selected so the trees expe­ri­enced tem­per­a­tures beyond the fluc­tu­a­tions nat­u­rally occur­ring in their usual olive oil pro­duc­ing territories.

The cul­ti­vars used were the Barnea, Coratina, Suri, Picholine and Koroneiki.

The exper­i­ment spanned two har­vest­ing sea­sons and the trees were irri­gated. Each month olive dru­pes were sam­pled from the trees to undergo his­to­log­i­cal and phys­i­o­log­i­cal analy­ses and an oil accu­mu­la­tion evaluation. 

At the end of each sea­son, a cer­tain num­ber of dru­pes from all five vari­eties were har­vested and processed using a lab­o­ra­tory-scale cen­trifu­gal system.

The results showed that higher-than-usual tem­per­a­tures affected the olive fruit growth and weight, the oil accu­mu­la­tion in the fruits and the oil com­po­si­tion. The effects were found to be geno­type-depen­dent with each cul­ti­var exhibit­ing dif­fer­ent responses to peri­ods of intense heat.

“In order to under­stand the mech­a­nism of the sen­si­tiv­ity and the resis­tance of olive cul­ti­vars to high tem­per­a­tures, we ana­lyzed the tran­scrip­tome [the RNA tran­scripts], ” researcher Giora Ben-Ari told Olive Oil Times.

“We decoded the gene expres­sion pat­tern of all genes involved in the olive oil biosyn­the­sis. It seems like the main mech­a­nism of high tem­per­a­ture envi­ron­ment resis­tant is the abil­ity to delay fruit devel­op­ment and oil accumulation.”

In high tem­per­a­tures, the trees of the Barnea and the Koroneiki cul­ti­vars delayed the devel­op­ment of the olive fruits until milder tem­per­a­tures pre­vailed, while the olive fruit weight was reduced in all cul­ti­vars except the Barnea.

In terms of olive oil quan­tity, among the cul­ti­vars tested the Barnea exhib­ited sta­bil­ity against high envi­ron­men­tal tem­per­a­tures with fruit weight and con­tent of olive oil almost iden­ti­cal in olive fruits sam­pled from both loca­tions. The Koroneiki and the Suri cul­ti­vars were affected by warmer tem­per­a­tures with fruit weight and olive oil accu­mu­la­tion dimin­ish­ing in trees at the high tem­per­a­ture site com­pared to the trees at the mod­er­ate tem­per­a­ture site.

On the other hand, the qual­ity of the olive oil dete­ri­o­rated in all five cul­ti­vars in the high-tem­per­a­ture group. Oleic acid and polyphe­nol con­tent in the dru­pes sam­pled from the high-tem­per­a­ture site mea­sured lower than those sam­pled from the mod­er­ate-tem­per­a­ture site.

In con­clu­sion, the Koroneiki cul­ti­var appeared the most sus­cep­ti­ble to hot envi­ron­ments in all ana­lyzed para­me­ters among the cul­ti­vars tested. The Coratina and the Picholine were also affected but man­aged to retain the amount of oil in the olive dru­pes when exposed to warmer tem­per­a­tures, while the olive oil pro­duced from the Souri cul­ti­var retained to some extent its organolep­tic char­ac­ter­is­tics. The Barnea cul­ti­var exhib­ited some loss in the olive oil qual­ity, but retained the con­cen­tra­tion of oil and the weight of the olive fruits under heat.

Ben-Ari explained that apart from spec­i­fy­ing which cul­ti­vars are bet­ter suited to deal with intense heat, another objec­tive of their research is to cre­ate a ​‘breed­ing pro­gram’ to develop new vari­eties that exhibit increased tol­er­ance to higher temperatures.

“Future research will look at the gene expres­sion in order to under­stand the dif­fer­ences between sen­si­tive and resis­tant cul­ti­vars,” he said. ​“This will help in breed­ing pro­grams to develop resis­tant cul­ti­vars. In addi­tion, last year we planted 100 cul­ti­vars in both loca­tions, and in the next sev­eral years we will screen these cul­ti­vars in order to iden­tify the resis­tant ones.”

Their results should be treated cau­tiously due to the rel­a­tively short time period and lim­ited sam­pling, the researchers noted. Nevertheless, it could pave the way for more research on the sub­ject to yield use­ful data for grow­ers fac­ing increas­ing cli­matic extremes.

• PLoS ONE


• Epicurious