Researchers Study How Lack of Chill Hours Impacts Olive Development, Oil Quality

Rising annual aver­age tem­per­a­tures across much of the olive-grow­ing world have cre­ated uncer­tainty that olive trees will receive the nec­es­sary 200 to 600 chill hours between 2 ºC and 10 ºC required to enable ver­nal­iza­tion.

Researchers from the Andalusian Institute of Agricultural, Fisheries, Food and Organic Production Research and Training (IFAPA) and the Canary Islands are work­ing on an ongo­ing study to deter­mine which of the world’s most pop­u­lar olive vari­eties may be best pre­pared for ris­ing win­ter tem­per­a­tures in the most pro­duc­tive olive oil-pro­duc­ing region on Earth.

“We are very wor­ried about cli­mate change, and there were a lot of papers or mod­els fore­cast­ing what will hap­pen with cli­mate change in the future in the Mediterranean with olives,” Raúl de la Rosa, a senior researcher at IFAPA, told Olive Oil Times.

“However, all the mod­els were based on the­o­ret­i­cal assump­tion, and no prac­ti­cal tri­als had been done about what hap­pens when you plant olive trees in a place where there is no win­ter,” he added.

On the island of Tenerife, the largest and most pop­u­lous of the Canary Islands, com­mer­cial olive grow­ing began in 2005.

According to data from Aemet, the state-run mete­o­ro­log­i­cal agency, aver­age win­ter tem­per­a­tures on the arable parts of the island range from 12.5 ºC to 17.5 ºC, with aver­age daily lows between 10 ºC and 12.5 ºC. In Andalusia, aver­age win­ter tem­per­a­tures gen­er­ally sit within the 2 ºC to 10 ºC win­dow.

Some cli­mate mod­els pre­dict that the cur­rent warm­ing trend will result in Andalusia’s win­ter tem­per­a­tures more closely match­ing those of the Canary Islands in the next 30 years.

Over the past six years, the IFAPA researchers and their local part­ners planted Picual, Hojiblanca, Cornicabra, Arbequina, Coratina, Korneiki and Martina – a cross of Picual (also known as Marteño) and Arbequina – near Córdoba and Málaga, in Andalusia, and on Tenerife.

Each of the three grove loca­tions has markedly dif­fer­ent cli­mates, with the groves near Málaga expe­ri­enc­ing slightly higher aver­age win­ter tem­per­a­tures than those in Córdoba.

This helped the researchers deter­mine that tem­per­a­ture was caus­ing the main dif­fer­ences between olive devel­op­ment in Tenerife and Córdoba instead of other fac­tors such as soil com­po­si­tion.

Since the study began, the researchers have observed sev­eral marked dif­fer­ences in how olives behave in Tenerife com­pared to Andalusia, with the most notice­able dif­fer­ences involv­ing the olive tree flow­er­ing period, oil accu­mu­la­tion and chem­i­cal com­po­si­tion.

“The the­o­ret­i­cal mod­els that were pro­duced here in the penin­sula fore­cast that when there is no win­ter chill, the olive will not flower,” de la Rosa said. ​“But when we went to the Canary Islands, we found that what actu­ally hap­pens when there is no win­ter chill is very dif­fer­ent.”

Instead, Guacimara Medina, an agri­cul­tural exten­sion tech­ni­cian and Ph.D. researcher involved in the study, told Olive Oil Times that olives planted in the Canary Islands have two flow­er­ing peri­ods from January to May.

“When there’s no win­ter chill, the olive trees always flower, but for much longer than in Andalusia, and there are sev­eral flow­er­ing peri­ods from January to May,” she said. ​“The flow­er­ing period is longer, and the olive trees flower for three months, some­times more.”

As a result of the extended flow­er­ing peri­ods and their non-endemic nature on Tenerife, Medina added that the trees were increas­ingly sus­cep­ti­ble to dam­age from pests, requir­ing more inten­sive phy­tosan­i­tary inter­ven­tion.

Along with the flow­er­ing period, Medina said that oil accu­mu­la­tion was much higher in olives grown on the island.

“Due to the mild tem­per­a­tures, the oil accu­mu­la­tion does­n’t stop,” she said. ​“In Mediterranean con­di­tions, the oil accom­mo­da­tion fin­ishes when the tem­per­a­tures decrease.”

However, Tenerife’s sub­trop­i­cal cli­mate means that oil accu­mu­la­tion can reach up to 60 per­cent, com­pared to 20 per­cent in the most effi­cient vari­eties in Andalusia.

Early flow­er­ing and the con­tin­ued oil accu­mu­la­tion also mean many olives are ready to be har­vested by the end of July or early August in the Canary Islands, mak­ing it the first palace to pro­duce olive oil each crop year in the European Union.

As a result, Medina said that local pro­duc­ers on the island are seek­ing a pro­tected geo­graph­i­cal cer­ti­fi­ca­tion, such as a Protected Designation of Origin (PDO) or Protected Geographical Indication (PGI), to help pro­mote the island’s oils.

In terms of chem­i­cal com­po­si­tion, Medina said olives grown on Tenerife have a dis­tinct polyphe­nol pro­file com­pared to their coun­ter­parts in Andalusia.

“The polyphe­nols are dif­fer­ent and higher than in Andalusia,” she said. Specifically, there are more toco­pherols, an organic chem­i­cal com­pound with vit­a­min E activ­ity, in Canarian oils than in Andalusian oils.

However, higher year-round tem­per­a­tures also sig­nif­i­cantly lower the mar­gin for error dur­ing the trans­for­ma­tion process to ensure that all the oil yielded meets the extra vir­gin olive oil stan­dards.

De la Rosa said other stud­ies have shown that when oil accu­mu­la­tion in Arbequina olives hap­pens at higher tem­per­a­tures, oleic acid con­cen­tra­tions decrease while linoleic acid con­cen­tra­tions rise.

“So the sta­bil­ity of the oil is very low,” he said. ​“This is a big prob­lem. In the future, if the oil accu­mu­la­tion occurs at a higher tem­per­a­ture, it will neg­a­tively affect the olive qual­ity.”

“Our job is to look for cul­ti­vars with the genet­i­cally higher sta­bil­ity of the oil,” he added.

For exam­ple, de la Rosa said Martina – the cross between Picual and Arbequina – has shown promise. ​“So maybe in a future cli­mate sce­nario, we will use mainly cul­ti­vars like those,” he said.

However, the research tri­als – the most recent of which started six months ago – must con­tinue before con­crete con­clu­sions can be reached.