Effect Of Post-anthesis Heat Stress On Photosynthetic Characteristics Of Leaves In Wheat (Triticum Aestivum L.)

Wheat is one of the most important crops in the world.In China wheat plays a significant role in national food security and social economic development.With the intensification of global climate change,both the global average temperature and extreme high temperature events have been reported to increase in the past decades.High temperature is one of the primary stresses limiting the growth and productivity of wheat.Photosynthesis process is sensitive to high temperature and directly affects of dry matter accumulation and grain yield production.Therefore,understanding the responding of photosynthetic to heat stress will provide a theoretical support for wheat production in the future.To investigate the impacts of post-anthesis heat stress on the crop photosynthesis,experiments were carried out in phytotrons with two different heat-tolerant varieties?Yangmai 16?heat sensitive?and Xumai 30?heat tolerant??under four high temperature levels?T1?17/27??,T2?25/35??,T3?29/39 ??and T4?33/43 ???and three high temperature durations?3 days?D1?,6 days?D2?,9 days?D3??during anthesis and grain filling stages.The results showed that:1.During the treatments,leaf net photosynthetic rate?Pn?and chlorophyll content?Chl?a+b??showed the decreasing trend,while stomatal conductance?Cond?and intercellular CO₂ concentration?Ci?showed the increasing trend with the rising temperature levels and treatment durations.The variation of Pn and Cond were more obvious under heat stress during grain-filling stage than that during anthesis stage.However,the variation of Chi?a+b?and Ci were more obvious under heat stress during anthesis stage than that during grain-filling stage,which means the influence degree of heat stress on leaf photosynthesis parameters were different during different development stages.In addition,the effects of heat stress on the flag leaf were more serious than that on the second and third youngest fully expanded leaves,indicating the lower leaves were more sensitive to high temperature.The Pn and Chl?a+b?of Yangmai 16 decreased seriously than Xumai 30,while Cond and Ci increased seriously than Xumai 30.Moreover,the effect of high temperature levels on photosynthesis is more obvious than high temperature treatment durations.The reason for this phenomenon might be that,crop photosynthetic organs will damage if high temperature levels exceed the critical threshold,while the organs will not damage,even if extending the treatment durations when the temperature lower than the critical threshold.After the treatment,Pn under D2 and D3 treatments generally decreased except for Pn under D1 treatment,which means Pn could recover by adjusting the physical structure when short period of heat shock happened.2.During the treatments,maximal photochemical efficiency of PS?in darkness?F_v/F_m?,maximal fluorescence?F_m?and photochemical quenching co-efficient?qP?decreased,while minimal fluorescence?F₀?and non-photochemical quenching?NPQ?increased with the rising temperature levels and treatment durations.The variation of each chlorophyll fluorescence parameter for different temperature treatments were more obvious under heat stress during grain-filling stage than that during anthesis stage,which indicates that the impacts of heat stress on leaf photosynthesis parameters were more serious during grain-filling stage than anthesis stage.In addition,the effects of heat stress on the flag leaf were more obvious than that on the second youngest fully expanded leaf,indicating that the lower leaf PS?system under heat stress suffered more effects.The F_v/F_m and qP of Yangmai 16 decreased seriously than Xumai 30,while F₀ and NPQ increased seriously than Xumai 30,indicating that Yangmai 16 was more sensitive than Xumai 30 under post-anthes heat stress.Moreover,the effect of high temperature levels on each chlorophyll fluorescence parameter is obvious than high temperature treatment durations.The reason for this phenomenon might be that,the decreasing of PS?efficiency was mainly caused by the decreasing of qP and increasing of NPQ,while qP and NPQ are sensitive to high temperature levels than high temperature treatment durations.After the treatment,F_v/F_m under D2 and D3 treatments generally decreased except for F_v/F_m under D1 treatment.