Heat Stress Effects are Stronger on Spikelets Than on Flag Leaves in Rice Due to Differences in Dissipation Capacity

Abstract

Rice is most sensitive to heat stress at the flowering stage, with different degrees of heat damage in spikelets and leaves. To investigate the heat damage in spikelets and flag leaves, two rice genotypes, N22 (heat‐tolerant) and GT937 (heat‐sensitive), were subjected to a heat‐stress treatment (40 °C for 15 days). The results showed that more damaging was found in spikelets than in flag leaves and the heat stress significantly decreased the seed‐setting rate by 12.41 % in N22 and by 65.02 % in GT937. However, the difference in the net photosynthetic rate of the flag leaf between heat‐stressed and control was not significant. Moreover, the difference of temperatures in spikelets and flag leaves was attributed to the differences in heat dissipating. Under heat stress, the transpiration rate was significantly higher in flag leaves than in spikelets, and the temperature in flag leaves were at least 4 °C cooler than in spikelets. Although the spikelet temperatures did not differ significantly between the two genotypes under heat stress, spikelets of GT937 were more severely damaged than those of N22, which might result from the differences in the antioxidant capacities between genotypes. Results showed that little difference of superoxide dismutase, peroxidase and catalase activities of spikelets was found in N22, while significant reduction was found in GT937 under heat stress, compared with control. These results suggest that organ temperature is controlled mainly by transpirational cooling, and that heat stress is an indirect result of oxidative stress, rather than a direct result of heat damage.

Resilience of Pollen and Post‐Flowering Response in Diverse Sorghum Genotypes Exposed to Heat Stress under Field Conditions

“…Enhanced leaf thylakoid membrane damage, coupled with reduction in chlorophyll content and Fv/Fm, resulted in significant reductions in net CO 2 assimilation rate of up to 48% in sorghum genotypes exposed to heat stress compared with the control. The reduction in transpiration rate (up to 60%) under heat stress in all the tested 24 sorghum genotypes ( Supplemental Table 1) is opposite to the response documented in other cereals like rice (Zhang et al, 2016) where, on exposure to heat stress, plants enhance their transpiration to maintain canopy and tissue temperature. However, the tested sorghum genotypes seem to employ the limited transpiration trait (Riar et al, 2015) quite effectively, with conserving water taking precedence over canopy cooling.…”

Heat Stress Is More Damaging to Superior Spikelets than Inferiors of Rice (Oryza sativa L.) due to Their Different Organ Temperatures

“…Because there was no obvious difference in the transpiration rates between the superior and inferior spikelets under both natural and stress conditions ( Figure 7A ), which played an important role in reducing the plant organ temperatures (Zhang et al, 2016), we inferred the panicle type might be another important factor contributing to this temperature difference. As shown in Figure 5 , higher spikelet temperatures were observed in location A than B, which had the same canopy height, while no obvious difference was shown among locations B, C, and D at different canopy heights, indicating the panicle type, rather than the canopy height, mainly contributed to the temperature differences without leaves.…”

“…Many factors, such as the organ size, shape, location within the canopy and cooling ability as well as the environment, i.e., the air temperature, relative humidity, wind speed and radiation, are involved in mediating the spikelet temperature (Sheehy et al, 1998; Yan et al, 2008, 2010; Zhang et al, 2016). These factors can lead to different surface temperatures for identical objects in the same air temperature (Zhang et al, 2016).…”

“…These factors can lead to different surface temperatures for identical objects in the same air temperature (Zhang et al, 2016). It has been reported that rice panicles can attain temperatures that are more than 6°C lower than the ambient temperature when atmospheric evaporative demand is high, whereas the panicle temperature can exceed the air temperature by 4°C under humid conditions (Matsui et al, 2007; Tian et al, 2010).…”

“…Rice plant temperatures are not only depending on the organ position, size, shape, surface area, and hull morphology but also influenced by the air temperature, air humidity, radiation, and plant type (Sheehy et al, 1998; Yan et al, 2008, 2010). The results of Zhang et al (2016) indicated that moderate heat stress of 40°C significantly decreased the spikelet fertility, whereas it caused little damage to the photosynthesis in flag leaves at anthesis because when the panicle temperature was above 38°C, the flag leaves were only approximately 34°C. This indicated that the damage degree induced by heat stress to rice plant organs mainly varied with their different temperatures.…”