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University of Tokyo (2015)

Analyses of the changes in stomatal and mesophyll CO2 diffusion conductances in response to the atmospheric CO2 concentration or soil water content

溝上, 祐介

Titre : Analyses of the changes in stomatal and mesophyll CO2 diffusion conductances in response to the atmospheric CO2 concentration or soil water content

Auteur : 溝上, 祐介

Université de soutenance : University of Tokyo

Grade : Doctoral Thesis 2015

Résumé
CO2 diffuses from ambient air to the chloroplast stroma. There are two large resistances in this diffusion pathway, stomatal resistance (rs) and mesophyll resistance (rm). rs is the resistance from the leaf surface to the intercellular air space through stomata. rm is the resistance from the intercellular air space to the chloroplast stroma. CO2 concentration is highest in the air (Ca), and lowered in the intercellular air space (Ci) and lowest in the chloroplast stroma (Cc), because of substantial rs and rm. These resistances are often expressed as conductances, inverse of resistances, gs and gm.
These conductances respond to various environmental changes. In this thesis, I conducted detailed analyses of CO2 diffusion conductances in response to drought and CO2 concentration. At first, I sought for factors that decrease gm under drought conditions. ABA was one of candidates that would cause the decrease in gm. Therefore, I used an ABA deficient mutant (aba1) and the wild type of Nicotiana plumbaginifolia. These plants were exposed to drought conditions to investigate whether the increase in ABA content in the leaves was needed for the decrease in gm. For the gm measurements, I constructed a special system to measure gm with high accuracy using the carbon isotope method that is considered as most reliable. Under drought conditions, aba1 did not show any decrease in gm whereas gm decreased in WT. Addition of ABA to aba1 leaves caused dramatic decreases in gm. I, thus, could demonstrate that the increase in ABA content in the leaf was necessary for the decrease in gm. However, the underlying mechanisms are still not clear.
In addition to this experiment, I investigated whether gm responded to high CO2 condition with these tobacco plants because some papers have reported rapid decrease in gm in response to high CO2. In both WT and aba1, gm decreased in response to high CO2. Therefore, ABA might not be necessary for decrease in gm in response to high CO2.
There are only a few papers reporting detailed analyses of responses of gm to elevated CO2. In particular, studies reporting responses of gm to long-tem elevation of CO2 are few. Because, when the stomata close, Rubisco tends to fix more CO2 evolved in the process of (photo)respiration than the CO2 directly from the ambient air, I used some stomatal mutants of Arabidopsis thaliana, which are insensitive to CO2, to uncouple the influence of gs on gm. To estimate gm, I also applied new methods that were proposed very recently. The plants were grown at 390 ppm and 780 ppm in growth chambers to investigate whether the responses of gm to elevated CO2 could be changed by growth CO2 concentration. In the short-term experiments, gm decreased in response to elevated CO2 regardless of gs responses and the calculation methods to estimate gm. In the long-term experiment, the responses of gm to elevated CO2 did not change with the growth CO2 concentration. However, nitrogen nutrition during the growth affected responses of gm to elevated CO2. The difference might be due to changes in chloroplast starch metabolism. With the decrease in CO2 concentration and/or nutritional N level, starch tended to accumulate, which would decrease gm. _ I investigated underlying mechanisms of the changes in gm in response to elevated CO2 and ABA. Recently, some studies have suggested that the PIP aquaporins could affect gm. Therefore, to clarify whether PIP aquaporins are involved in the changes in gm in response to elevated CO2 and ABA, I compared responses of gm to elevated CO2 and ABA among three T-DNA insertion lines of PIP aquaporins that are highly expressed in leaves (pip1 ;2, pip2 ;3 and pip2 ;6). The responses of gm to elevated CO2 were all the same among Col-0 and all T-DNA insertion lines. However, in pip2 ;6, gm was insensitive to ABA. As PIP2 ;6 was mainly expressed around the vascular tissue, PIP2 ;6 would not play roles in mesophyll cells as CO2 facilitators. Previous reports have demonstrated that the relationships between leaf water relations and PIP aquaporins. Then the changes in the water relations would affect gm. _ Clarification of the relationships between leaf water relations and CO2 diffusion in the leaves will be prerequisite to improve plant performance in semiarid and arid areas. Also, detailed analyses of responses of CO2 diffusion conductances to high CO2 will be helpful to improve plant performance in the high CO2 world. The results are discussed in the light of these future perspectives.

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