Prof. Dr. Paul Struik

Heat tolerance is therefore a desirable, but complex, multi-faceted trait. The basic process of crop growth is photosynthesis: carbon dioxide from the air is fixed into sugars by the leaves making use of light energy from the sun. This process is sensitive for elevated temperatures, certainly in potato. The proportion of leafy material in the haulm becomes lower when temperature is higher, making the canopy more stemmy and therefore less efficient. At elevated temperatures, a smaller proportion of the dry matter produced ends up in the tubers, as more is invested in the continued branching of the canopy, but also because stolon and tuber formation are directly impeded by high air and soil temperatures. Also, quality of the tubers is affected: starch synthesis in the tubers is reduced when temperatures are high, whereas heat can result in various physiological tuber disorders.

But in December 2024, there was exciting news: the Realizing Increased Photosynthesis Efficiency (RIPE) research project led by the University of Illinois (USA) came with a remarkable paper on heat tolerance in potato. They genetically modified the potato to increase the efficiency of photosynthesis. Fixation of CO₂ during photosynthesis is catalyzed by the enzyme Rubisco. Rubisco, however, not only catalyzes CO₂ fixation but also shows a strong affinity for O₂, and then catalyzes photorespiration, a process that is very costly in terms of energy and involving loss of carbon. The problem is exacerbated by the fact that the relative affinity of Rubisco for oxygen is higher at higher temperatures. Therefore, photorespiration can reduce the efficiency of photosynthetic carbon fixation significantly, especially at elevated air temperature. The authors managed to increase the efficiency of photosynthesis by shortcutting photorespiration. This resulted in lower photorespiratory carbon losses and consequently in higher maximum carbon assimilation rates and higher maximum electron transport rates. These are the two main biochemical limiting factors determining the rate of photosynthesis. They planted transformed and non-transformed crops in two seasons; in one season the crop experienced an early heat wave and in the other season not. There was a tuber yield advantage of the transformed crop over the non-transformed crop in both seasons. It is noteworthy that only in the year with the heat wave they measured a positive effect of the genetic modification on the photosynthesis rate and the photosynthesis capacity, also later in the season, while the yield effect was also strongest in that same year. This effect is consistent with the larger carbon loss through photorespiration at higher temperatures that would normally occur without the photorespiratory shortcut. Quality analyses of the tubers indicated a similar composition of transformed and non-transformed tubers. The authors suggest that this transformation involving a shortcut in the photorespiration can protect the crop against heat wave stress and they surmise it might also future proof potato tuber quality as global warming will increase the frequency and intensity of heat waves.
Are we now closer to a heat-tolerant potato? Perhaps we should not be too optimistic and remain cautious. The effects on photosynthesis were not consistent across years, while yield advantages were visible in both years, albeit not of the same magnitude. As argued above, photosynthesis efficiency is only one of the many aspects of heat tolerance in potato and the link between the photosynthesis effects and yield effects in the warm year might have been indirect. As always: more research is needed. ●