Heat Tolerant Photosynthesis in Soybean: Are There Beans That Can Beat the Heat?.

Climate predictions indicate that temperatures will increase and heat waves will occur more frequently. Increases in average growing temperatures have been shown to result in decreased yield of soybean and corn in the US. Research aimed at identifying genotypes with enhanced photosynthesis under elevated temperatures will be critical to improve soybean productivity. Thus, there is a need to study soybean heat tolerance and compare the photosynthetic responses of diverse genotypes to high temperatures.

The objectives of this research were to 1) characterize the photosynthetic responses of multiple soybean genotypes in controlled environments and under field conditions, and 2) identify genotypes with heat tolerant and susceptible photosynthetic responses. Light reactions of photosynthesis were assessed to discriminate between high temperature susceptibility and tolerant genotypes under the two experimental conditions.

To accomplish this ten soybean genotypes were grown in growth chambers for 28 days under optimal temperature conditions (28°C day and 22 °C night). After this, plants were exposed to mild (35 ⁰C) or extreme (45 ⁰C) day-time air temperatures for 4 hours for 7 days. During this heat treatment, chlorophyll fluorescence measurements and tissue samples for transmission electron microscopy were obtained from the second middle trifoliate. In the field, plants were initially grown under ambient conditions and then covered using heat tents, relying on the greenhouse effect to increase air temperatures. Air temperature was monitored continuously and chlorophyll fluorescence measurements obtained during the hottest times of the day.

Transmission electron micrographs revealed starch accumulation in chloroplasts under elevated temperatures. This buildup is accompanied by reduced efficiency of the light reactions. The main impact of heat stress was a reduction in maximum fluorescence and not an increase in minimum fluorescence as has been commonly described in other studies.