Excessive Pi application declines the Rubisco-limited photosynthesis
Because the onset of P toxicity symptoms requires illumination, the photosynthetic metabolism should provide an insight into the cause of P toxicity under illumination conditions. We measured the CO2 assimilation rate (A ), and quantum yield of photosystem (PS) II [Y(II)], for dissipation by down-regulation [Y(NPQ)], and for other non-photochemical losses [Y(NO)] (Kramer et al., 2004) as a function of the internal partial pressures of CO2 in the leaves (Ci) of rice plants grown under different Pi conditions. The low-Pi plants showed lower A than the control-Pi plants, especially at high Ci levels (Figure 2a). The Y(II) was lower in the low-Pi plants than in the control-Pi plants at high Ci levels (Figure 2b). In contrast, the low-Pi plants showed higher Y(NPQ) than the control-Pi plants at high Ci levels (Figure 2c). However, the low-Pi and control-Pi plants showed similar Y(NO) (Figure 2d). These responses showed a typical Pi limitation in photosynthesis (Sharkey, 1985; Fabre et al., 2019), and lowering Pi decreased Aby limiting the Pi turnover, and NPQ was stimulated to protect the photosynthetic electron transport (PET) system at high Ci levels.
On the other hand, A decreased with an increase in Pi application above 1.2 mM Pi (Figure 2e). In addition, the response to Ci became more linear. The initial slope of A: Ci was the highest in the control-Pi plants and decreased with increasing Pi application (Figure 2i). Because the initial slope of A :Ci suggested Rubisco activity (von Caemmerer & Farquhar, 1981), these results indicated that the Rubisco activity decreased with increasing in Pi application. An increase in Pi application lowered Y(II) in the wide range of Ci, and the response of Y(II) to Ci also became linear in a manner similar to that of A (Figure 2f). Y(NPQ) increased with an increase in Pi application, but was suppressed at low Ci levels in the 3.0 mM Pi plants compared to the control-Pi plants (Figure 2g). Y(NO) was similar in all the Pi applications except for the 3.0 mM Pi conditions (Figure 2h). This implied that although NPQ is stimulated with increasing Pi accumulation in the leaves, excessive Pi accumulation disturbs the redox-state of the PET chain. In fact, the Fv/Fm value was significantly decreased in the 3.0 mM Pi treatments (Figure 2j).