Yummy! Raspberries carry the taste of warm summer days and sunshine. But with global warming, raspberry bushes may have to adapt to even warmer summers. What happens if these Danish red berries are exposed to more heat than they are accustomed to? Can they get heat stroke? Will above-average temperatures in the summer affect flower formation and thus lead to lower yields and poorer quality?

In his PhD study, Tek Prasad Gotame at Aarhus University examined how high temperatures affect raspberry plants in different ways. He found that a temperature above the physiologically optimum of the raspberry plant had a negative effect on a number of factors, including plant photosynthesis and the expression of certain genes.

He also found variation in how different raspberry varieties responded to the increased temperature. The results may have implications for how the raspberry plants are grown, and for which varieties farmers select.

- Raspberries are high-value crops because of their taste, nutritional quality and health-promoting properties. There is increasing interest across Europe in the longer growing season and the higher quality that can be achieved with a production of raspberries under glass or in plastic tunnels. To achieve a longer season, the control of flowering and fruit-setting is absolutely necessary. And to be able to control flowering and fruit-setting it is important to have a thorough knowledge of plant physiology and other factors, explains PhD student Tek Gotame.

He therefore set out to catalogue the response of different raspberry varieties to different temperatures. He also compared an organic production with a conventional production.

Photosynthetic rate drops

Tek Gotame examined three varieties of autumn raspberries and four varieties of summer raspberries over two consecutive years under Danish conditions. The first part of the study took place in a climate chamber and greenhouse, where the temperature was raised from 20°C to respectively 27, 32 or 37°C over a period of seven days during flower formation.

Photosynthesis is one of the main processes influenced by high-temperature stress. The process is affected at the molecular level as a result of up- and down-regulation of certain genes. It was precisely this up- and down-regulation that was impacted by the higher temperatures.

Tek Gotame found that when raspberry plants were exposed to temperatures above their physiological optimum, the up- and down-regulation of specific genes was affected, resulting in changes in plant morphology, anatomy, physiology and biochemistry, including photosynthesis. He identified the affected genes. In all varieties, 38 genes were down-regulated. Two additional genes were down-regulated only in the Autumn Bliss variety, but up-regulated in three other varieties.

All five of the raspberry varieties studied reacted negatively to heat stress in terms of the effectiveness of their photosynthesis in the midday heat. In the study, the higher temperatures reduced the photosynthetic efficiency of the plants by up to 14 percent. However, there were significant differences between the varieties in their level of chlorophyll content (the pigment that forms an essential component of photosynthesis) and flowering.

Flowering was also affected by the increased heat. Heat stress promoted early flowering in the Autumn Bliss variety and delayed it to a certain extent in the Autumn Treasure variety. Tek Gotame also measured the yield and quality of berries. The results showed that the Octavia and Glen Fyne varieties are promising candidates for summer production and organic farming, while the Autumn Bliss variety looks suitable for autumn production in tunnels under Danish conditions.

The study shows that there is a clear difference in the reactions of the varieties. This knowledge can be used in a commercial production to manipulate and optimise berry production in greenhouses and outdoors under warmer conditions. It is important to know the differences between the varieties, since some varieties are more suited to cultivation at elevated temperatures than others.

Further information: Senior researcher Lillie Andersen, Department of Food Science, email: lillie.andersen@agrsci.dk, telephone: +45 8715 8323, Mobile: +45 4029 2142