Figure 8

Putative sequence of events provoked by a 35 d S limitation on the electron transfer chain and ATP synthase in a young leaf chloroplast. Proteines processes (1 to 8) or biochemical compounds that are effectively repressed or induced by S restriction are indicated respectively in doted or plain blue lines, while those that are postulated to be repressed or induced are in black lines. In Control, the electron transfer chain produced NADPH required for CO₂ assimilation. After 35 d under Low S conditions, we observed: 1, a repression of ferredoxin-NADP reductase (FNR) and plastocyanin (PC) (Table 2) which could cause a perturbation of the electron transfer and a lower production of NADPH+H⁺; 2, the decline in CO₂ assimilation (Figure 3B), probably linked to NADPH+H⁺ depletion; 3, an intercellular CO₂ accumulation (Figure 3A) concomitant with the photosynthesis reduction; 4, the lower abundance of FNR may also result in the transfer of electrons to O₂ by Ferredoxin (Fdx), producing O₂^.-; 5, a higher abundance of Cu-Zn Superoxide Dismutase (SOD) (Table 3), which suggests that the of O₂^.-detoxification into H₂O₂ could be increased; 6, an accumulation of H₂O₂ (Figure 2D) probably due to ineffective detoxification process such as the repression of glutathione S-transferase, which leads to an oxidative stress; 7, an accumulation of Water Soluble Chlorophyll binding Protein (WSCP) while the chlorophyll content is maintained (Figure 2A), may signify that chlorophylls are protected against oxidative stress and that the photosystems remain efficient; 8, an accumulation of ATP synthase F1 complex, which in association with the H⁺ accumulation in the lumen due to proper functioning of photosystems and electron chains, suggests that ATP production is favoured. CF_O: membrane-embedded subunit of ATP synthase; CF₁: catalytic subunit of ATP synthase; Cyt b₆f: cytochrome b₆f complex; LHC: Light Harvesting Complex ; PQ: Plastoquinone.