Effects Of Elevated CO₂ On Leaf Growth, Sucrose Transport And Grain Filling In Rice (Oryza Sativa L.) And Their Mechanism

The yields of C₃ crops such as rice and wheat, increased greatly at elevated CO₂condition. However, recent research was mainly focus on the yield, little attention hasbeen made to the factors affecting the yield forming. The vegetative growth stage is akey stage influencing the process of grain filling and final yield. Leaf growth anddevelopment is the most active metabolism at the vegetative growth stage. Afteranthesis, grain is becoming the predominant sink tissue, and its growth anddevelopment determine the yield and quality of rice. But the mechanism, by whichelevated CO₂ regulates the growth and development of leaf and grain is not clear now.One of the experiments was conducted to study the grain growth and developmentduring early stage of grain filling of rice (Oryza sativa L. sp. japonica, cvWuxiangjing-14) under the conditions of ambient atmosphere CO₂ concentration (CK)and FACE (Free Air CO₂ Enrichment, 200μmol·mol^-1 higher than ambient). The grainwidth, length, contents of soluble carbohydrate (reducing sugar and sucrose) andstarch, and activities of invertases including cell wall invertase (CWI), soluble acidinvertase (SAI) and neutral invertase (NI) in grain were measured. The filling rate onthe basis of dry weight of grain from anthesis till mature period with sampling every 3days and grain weight and size at maturity were tested. In the other experiment, weselected uniform rice seedlings at 6 leaf age, when the 6th leaf (L6, mature source leaf)just fully expanded and 7th leaf (L7, young developing sink leaf) began to develop,were treated respectively with elevated CO₂ concentration as 700μmol·mol^-1 andambient condition as 350μmol·mol^-1, and grown until the L7 fully expanded to studythe mechanism of leaf growth and development under different CO₂ concentrationconditions.The results showed that the growth process of grain was accelerated by FACE,and the grain size was significantly increased by FACE. The grain size andgrain-filling rate under FACE were about 3 days earlier to arrive the maxima thanthose under CK. The product of mature grain width and length under FACE wasincreased by 4.5ï¼…of CK. However, such obvious effect was not shown in the final grain weight. The contents of reducing sugar and sucrose, which were needed asmaterials and energy substance for the normal growth and development of grain, wereincreased during 1 week after anthesis, while the activities of CWI and NI enhancedduring 5 days after anthesis by FACE. However, the content of starch and activitie ofSAI in grain were not affected by elevated CO₂. In conclusion, higher contents ofsoluble carbohydrate and higher activities of CWI and NI under FACE supply morematerials for the growth of larger grain sink capacity. There is no significant effect onthe final grain dry weight resulting from a slight increase in sink activities and theearlier decrease in grain filling rate under FACE. A new balance between sink andsource needs to be established under elevated CO₂ condition in order to improve grainyield by means of increasing the grain weight on the basis of higher panicle grainnumber in rice.Elevated CO₂ significantly increased leaf area formation rate and biomassaccumulation rate during early growth process of L7. The net photosynthesis rates (A)of L6 and L7 were increased by elevated CO₂, and the response of net photosynthesisrate of L6 was stronger than L7. Lower ratio of chla to chlb(Cha/b), stable chlorophyllfluorescence (Fs), light compensation point (LCP), and higher practical quantum yieldof PSâ…¡(Yield), estimated electron transport rate (ETR), photochemical quenching(qP), non-photochemical quenching (qN), light saturation maximal A (A_max), apparentquantum yield (Φ) under elevated CO₂ condition, contributed to the increase of A forL6. While, the increase of A for L7 was mainly ascribed to the higher chlorophyllcontent, not the ratio of Chla/b, lower Yield, ETR, qP and higher qN. It is concludedthat the lower Chla/b, higher photochemical efficiency of PSâ…¡and higher light energyconversion efficiency were the reasons for higher A of L6 under elevated CO₂condition. The A of L7 was increased by the higher chlorophyll content. Theinhibition in photochemical efficiency of PSâ…¡was probably the reason for lower Aof L7 than that of L6.With leaf transition from sink to source, the contents of sucrose and starch wereincreased and the content of hexose was decreased, while the activities of both SPSand SS were increased gradually. Elevated CO₂ significantly increased the contents ofsucrose, hexose, starch and the activities of SPS, SS during the transition from sink tosource of rice leaf. The contents of TNC and the activities of SPS, SS were alsostimulated by elevated CO₂ in source leaf. Thus, it is suggested that elevated CO₂facilitated the assimilate synthesis and export in the source leaves and utilization in the sink leaves, as a result of up-regulation of photosynthesis and sucrose metabolismenzymes, but imbalance of source-sink relationship also occurred in the short-termCO₂ treatment.The mechanism of elevated CO₂ affecting the sucrose transport from source leafto sink leaf was studied by using ¹⁴C-sucrose as a tracer and the technique ofautoradiograph. From analyzing the picture of autoradiograph of developing leaf,acceleration of leaf growth and development under elevated CO₂ was furthersupported. Elevated CO₂ significantly increased the sucrose content in the phloem sapand the quantity of ¹⁴C-sucrose absorbed by source leaf and that partitioned into sinkleaf. The results suggested that elevated CO₂ stimulated the absorbability andtransport of sucrose in source leaf and the capacity to accept sucrose in sink leaf. Theabsorbability of exogenous sucrose in the source leaf could be triggered by sucroseand glucose or lower pH, inhibited by DNP. The same result was not found in the sinkleaf. It could be deduced that elevated CO₂ affected sucrose transport through theregulation of apoplastic loading in the source leaf and symplastic unloading in thesink leaf of rice seedling.The leaf morphology and structure of L6 and L7 on different DAT were alsoobserved. Results showed no significant effects of elevated CO₂ on the leaf thickness,vascular bundle area, phloem area, ratio of P/V (phloem area/vascular bundle area)and the diameter of plasmodesmata in L6 were found. But elevated CO₂ increasedthese parameters in L7. Therefore, there was a different response in leaf morphologyand structure between L6 and L7 to elevated CO₂. These results suggested thatelevated CO₂ made the L7 have larger capacity to accept the photosynthate at the sinkstage and export it at the mature stage.