Study On The Mechanisms Underlying The Effects Of Drought On Leaf Photosynthesis And Carbon Dioxide Diffusion Efficiency

Drought,as the most dangerous natural natural disasters,had seriously threatened the sustained development of irrigated agriculture in China due to its rapid increasing frequency,intensity and duration.Promoting leaf photosynthesis was recognized as one of the most effective ways to increase crop productivity.However,the mechanisms of drought’s impact on photosynthesis are still unclear,and clarification the response mechanism of photosynthetic substrate CO₂ diffusion efficiency in response to drought is the key to understading the effects of drought on phtosynthesis.Therefore,two different photosynthetic functional crops,tomato and maize,cultivated in the controlled climate chamber,were taken as the research objects.By setting up short-term soil progressive drought（short-term drought）and long-term drought-rewatering（long-term drought）treatments,well-watered plants as the control group（CK）,to quantitatively analysis the the three limiting photosynthesis factors,thus to determine the main factors affecting photosynthesis under drought;Additional,to explore the real-time signals in regulation of stomatal conductance and mesophyll conductance under drought,as well as the effect of long-term drought on the characteristics of stomatal structure and mesophyll anatomical structure,thus to determine the regulation mechanisms affecting CO₂ diffusion efficiency under short-term and long-term drought;Finally,the influences of plant water transport capacity on CO₂ diffusion efficiency under drought were discussed,to reveal that there was a coordination mechanism between water transport and CO₂ diffusion acclimation to drought.Combined with the above series of studies,we could have a more comprehensive and in-depth understanding of the mechanism of drought affecting photosynthesis and CO₂ diffusion.The main findings were as follows:（1）Soil drought inhibited the net photosynthetic rate of tomato and maize leaves,but the mechanisms on photosynthesis were differed in different drought intensity.Under mild drought conditions,lower Rubisco enzyme activity was the main factor limiting photosynthesis of tomato and maize leaves.Under the short-term drought,as the drought progressed,whenΨ_soil≤-0.83 MPa,the limitation of stomatal conductance（g_s）,mesophyll conductance（g_m）,and photosynthetic biochemical capacity contributed to36.93%,41.99%,and 21.08%,respectively,indicating that the decrease of g_s and g_m under drought was the main factor restricting tomato leaf photosynthesis.The g_s in maize decreased from 0.19 mol·H₂O·m^-2·s^-1 in CK to 0.02 mol·H₂O·m^-2·s^-1 in drought,the lower g_s was the main limitation to maize leaf photosynthesis.（2）Drought was beneficial to increase Rubisco enzyme activity and soluble protein（Cpr）content of tomato and maize.Compared with severe drought treatment,Rubisco enzyme activity and Cpr content decreased after rehydration.The response of chlorophyll content to drought was inconsistent among different crops.The content of chlorophyll a and b of tomato plants under drought treatment was higher than that of CK treatment,while chlorophyll a and b of maize under drought were generally lower than that of CK treatment.Drought significantly reduced the assimilation rate of tomato and maize leaves under the same light intensity,and the light saturation point,light compensation point and dark respiration rate decreased with the increase of drought,but rewatering had a positive compensation effect on these indexes.Drought resulted in photoinhibition by mainly reducing the photochemical quenching and increasing the non-photochemical quenching,thus reduced the reduction of photochemical efficiency,and then affected the photosynthetic efficiency of leaves.（3）Under the progressive soil drought,g_s decreased firstly with the decrease of hydraulic signal(Ψ_leaf),while g_m and chemical signal（ABA）were not affected,indicating that g_s was more sensitive to drought than g_m.As the drought progressed,whenΨ_soil≤-1.01 MPa,Ψ_leaf and g_s continued to decrease.At this time,drought significantly reduced g_m and increased ABA content.These showed that the decrease of g_s in the early stage of drought was mainly regulated byΨ_leaf,while under moderate or severe drought,the decrease of g_s and g_m was jointly regulated byΨ_leaf and ABA.In addition,long-term drought reduced the stomata aperture in tomato and maize.The highest distribution frequency of stomata aperture in the upper and lower epidermis of tomato leaves showed CK on upper epidermis>CK on lower epidermis>Drought on upper epidermis>Drought on lower epidermis;drought did not affect stomata density（SD）in the lower epidermis of tomato leaves,while significantly increased the SD on the upper epidermis.SD in maize leaves was only 0.34～0.36 times than that of in tomato.The response of SD in maize to drought was different from that of tomato plants.The specific manifestation was that drought significantly decreased SD in upper epidermis,while increased SD on the lower epidermis as compared with the CK treatment.（4）Long-term drought increased the liquid resistance limitation in tomato mesophyll structure up to99.3%,so g_m decreased affected by the mesophyll structure in response to long-term drought.The cell wall thickness(T_cw)in tomato when experienced long-term drought was 0.10μm,which was significantly increased by 25%compared with CK,and the cell wall resistance limitation was 8.53 times higher than that in CK treatment;Drought also reduced the thickness of mesophyll cells(T_mes),the area of mesophyll cells facing the intercellular space(S_mes/S)and the size and volume of chloroplasts,thereby reduced the contact area of CO₂ and chloroplasts.Drought did not significantly affect the leaf dry mass per area（LMA）,but significantly increased leaf thickness（LT）and the thickness of palisade tissue/sponge tissue thickness（PT/ST）,both of these two indexes were significantly negatively correlated with g_m.In addition,the maize mesophyll structure displayed a special"garland"structure,and long-term drought also increased the T_cw,reduced the length of chloroplasts and increased the diffusion distance of CO₂ to chloroplasts.The effect of drought on maize leaf structure（LMA,LT）was similar to that of tomato plants:drought reduced the thickness of mesophyll cells/leaf thickness(T_mes/LT),indicating that drought also reduced the proportion of mesophyll cells in leaves.（5）The lower water transport capacity in both plants under drought affected the CO₂ diffusion efficiency.Take tomato as an example.After a long-term drought treatment,the frequency of stem vessel between 40-60μm in the drought treatment distributed superlatively,while the distribution frequency of60-80μm was the highest under the CK treatment;the theoretical xylem hydraulic conductivity(K_xylem)of the droguht treatment was 18.59 mg·mm^-1·s^-1·KPa^-1,which was significantly lower than CK by 55.12%.Drought could also significantly affect the hydraulic water conductivity of plant branches(K_branch),leaf hydraulic conductivity(K_leaf),main vein diameter and small leaf vein density(VD_minor),but rewatering has a compensation effect on water hydraulic conductivity.Redundant analysis（RDA）results showed that K_leaf,main vein diameter,stomata aperture on adaxial leaf(SA_ada)and VD_minor were positively correlated with g_s and transpiration rate（T_r）,and these four factors explained 97.11%of the overall variation.These indexes were the main factors affecting g_s and Tr.Additionnally,tomato mesophyll anatomical structures,such as T_mes and S_mes/S,were significant positively correlated with hydraulic traits,such as VD_minor and K_leaf,and negatively related with PT/ST,indicating that the mesophyll anatomical traits were coordinated with leaf hydraulic and economic traits acclimation to long-term drought.