Some relationships between the gas exchange, biochemistry and molecular biology of photosynthesis during leaf development of tomato plants after transfer to different carbon dioxide concentrations

Abstract

ABSTRACT Tomato plants were exposed to four concentrations of CO 2 (350, 700, 1050 or 1400 μmol CO 2 mol −1 ) for 31 d. The light‐saturated rate of photosynthesis (A) of the unshaded fifth leaf was measured at either an ambient CO 2 concentration of 350 μmol CO 2 mol −1 [ A (350)] or at the level of CO 2 at which the plants were grown. The chloroplast protein composition and the level of transcripts of nuclear or plastid photosynthesis‐associated genes (PAGs), as well as the main carbohydrate content of the fifth leaf maintained horizontal and unshaded, were also measured during leaf development. At 60 and 95% leaf expansion, the A of high‐CO 2 ‐grown plants measured at growth CO 2 was higher than the A (350) of the plants grown at ambient CO 2 . However, in the fully mature leaves, A (growth CO 2 ) declined linearly as growth CO 2 concentration increased. The A (350) of plants exposed to elevated CO 2 up to 60% leaf expanion had not acclimated to high CO 2 . At 95% leaf expansion, A (350) was lower in plants grown at high CO 2 . A versus CO 2 ( C i ) for mature leaves showed that A of the plants grown at high CO 2 was lower over the entire range than that for plants grown at present ambient CO 2 concentration. Lines fitted to the linear part of the A / C i curves were concurrent at a C i of 49μmol CO 2 mol −1 and A =−1.21μmol CO 2 m −2 s −1 . This C t value is close to Λ * (46 μmol CO 2 mol −1 ), the compensation point at 27°C calculated from the equation described in Brooks & Farquhar (1985, Planta 165, 397–406). This A is an estimate of respiration in the light ( R 1 ) and was not affected by acclimation to elevated CO 2 . Thylakoid proteins (photo‐system I core protein, D 1 and D 2 of the photosystem II core complex, cytochrome f ) were all reduced by elevated CO 2 only in the fully mature leaves (31d exposure), whereas the large and small subunits of Rubisco and Rubisco activase proteins had already declined after 22 d exposure. Transcript levels of the plastid‐encoded PAG ( rbcL, psbA, psaA‐B ) were reduced in the mature leaves by elevated CO 2 when expressed on a total RNA basis, but they were not sensitive to elevated CO 2 when expressed on a chloroplast 16S rRNA basis. However, rbcS, rca and cab mRNA transcripts were lower in the plants grown at high CO 2 than in control plants after 22 d exposure when expressed on a nuclear rRNA basis. The loss of these nuclear PAGs was correlated with an accumulation of soluble sugars and starch.