Oxidation of the reaction center chlorophyll of photosystem I is induced via close cooperation of photosystems II and I with progress of drought stress in soybean seedlings

Abstract

ABSTRACT Drought is a serious abiotic stress that impairs plant productivity. We previously suggested that in rice (Oryza sativa L.) plants, the reaction center chlorophyll of photosystem I (PSI) (P700) is oxidized in response to drought stress via close cooperation of photosystem II (PSII) and PSI to protect PSI, whose fitness is crucial for photosynthesis and plant growth. It has rarely been reported whether such a defense system functions in other plant species. To collect another example, this subject was examined using soybean [Glycine max (L.)]. Irrigation of soil-cultured soybean seedlings was restricted to gradually impose drought stress. A decrease in the quantum efficiency of PSII [Y(II)], induction of non-photochemical quenching (NPQ) of excess light energy at PSII, reduction of the primary quinone electron acceptor of PSII (QA), and P700 oxidation proceeded before severe drought stress led to severe leaf wilting and photoinhibition of PSII and PSI. P700 oxidation, Y(II), and reduction of QA were strongly correlated in this period. These results are in accordance with those observed in rice. In contrast, such correlations disappeared when severely drought-stressed seedlings were included, as P700 oxidation was hampered while Y(II) further decreased in such seedlings. These results suggest that close cooperation of PSII and PSI leading to P700 oxidation functions to protect PSI against drought stress in soybean, and that similar defense system functions in soybean and rice unless drought stress are severe. The drought-stress responses of PSII and PSI are suggested to be regulated by acidification of thylakoid lumen in soybean, according to the changing patterns in the PSII and PSI parameters. Based on the similarity of the photochemistry of soybean and rice in response to drought stress, the effectiveness of P700 oxidation as an index for drought stress diagnosis is discussed. P700 oxidation is affected by factors other than drought stress such as N nutrition levels. However, effects of N nutrition levels are expected to be canceled by using another parameter obtained in P700 measurement as reference, suggesting the suitability of P700 oxidation for drought stress diagnosis.