Non-photochemical quenching (NPQ) has been regarded as a safety valve to dissipate excess absorbed light energy not used for photochemistry. However, there exists no general consensus on the photo-protective role of NPQ. In the present study, we quantified the Photosystem II (PSII) photo-susceptibilities (m_pi) in the presence of lincomycin, under red light given to five shade-acclimated tree species grown in the field. Photosynthetic energy partitioning theory was applied to investigate the relationships between m_pi and each of the regulatory light-induced non-photochemical quenching [Y(NPQ)], the quantum yield of the constitutive non-regulatory non-photochemical quenching [Y(NO)], and the PSII photochemical yield in the light-adapted state [Y(PSII)] under different red irradiances. It was found that in the low to moderate irradiance range (50-800 μmol m^–2 s^–1) when the fraction of open reaction centers (qP) exceeded 0.4, m_pi exhibited no association with Y(NPQ), Y(NO), and Y(PSII) across species. However, when qP < 0.4 (1,500 μmol m^–2 s^–1), there existed positive relationships between m_pi and Y(NPQ) or Y(NO), but a negative relationship between m_pi and Y(PSII). It is postulated that both Y(NPQ) and Y(NO) contain protective and damage components, and that using only Y(NPQ) or Y(NO) metrics to identify the photo-susceptibility of a species is a risk. It seems that qP regulates the balance of the two components for each of Y(NPQ) and Y(NO). Under strong irradiance, when both protective Y(NPQ) and Y(NO) are saturated/depressed, the forward electron flow [i.e., Y(PSII)] acts as the last defense to resist photoinhibition.

Summary: Photosystem II (PSII), which splits water molecules at minimal excess photochemical potential, is inevitably photoinactivated during photosynthesis, resulting in compromised photosynthetic efficiency unless it is repaired. The energy cost of PSII repair is currently uncertain, despite attempts to calculate it. We experimentally determined the energy cost of repairing each photoinactivated PSII in cotton (Gossypium hirsutum) leaves, which are capable of repairing PSII in darkness. As an upper limit, 24 000 adenosine triphosphate (ATP) molecules (including any guanosine triphosphate synthesized at the expense of ATP) were required to repair one entire PSII complex. Further, over a 7‐h illumination period at 526–1953 μmol photons m⁻² s⁻¹, the ATP requirement for PSII repair was on average up to 4.6% of the ATP required for the gross carbon assimilation. Each of these two measures of ATP requirement for PSII repair is two‐ to three‐fold greater than the respective reported calculated value. Possible additional energy sinks in the PSII repair cycle are discussed.

Potassium ions enhance photosynthetic tolerance to salt stress. We hypothesized that potassium ions, by minimizing the trans‐thylakoid proton diffusion potential difference, can alleviate over‐reduction of the photosynthetic electron transport chain and maintain the functionality of the photosynthetic apparatus. This study investigated the effects of exogenous potassium on the transcription level and activity of proteins related to the photosynthetic electron‐transport chain of tobacco seedlings under salt stress. Salt stress retarded the growth of seedlings and caused an outflow of potassium ions from the chloroplast. It also lowered qP (indicator of the oxidation state of Q_A, the primary quinone electron acceptor in Photosystem II (PSII) and Y_PSII (average photochemical yield of PSII in the light‐adapted state) while increasing Y_NO+NF (nonregulatory energy dissipation in functional and nonfunctional PSII), accompanied by decreased expression of most light‐harvesting, energy‐transduction, and electron‐transport genes. However, exogenous potassium prevented these effects due to NaCl. Interestingly, lincomycin (an inhibitor of the synthesis of chloroplast‐encoded proteins in PSII) significantly diminished the alleviation effect of exogenous potassium on salt stress. We attribute the comprehensive NaCl‐induced downregulation of transcription and photosynthetic activities to retrograde signaling induced by reactive oxygen species. There probably exist at least two types of retrograde signaling induced by reactive oxygen species, distinguished by their sensitivity to lincomycin. Exogenous potassium appears to exert its primary effect by ameliorating the trans‐thylakoid proton diffusion potential difference via a potassium channel, thereby accelerating ATP synthesis and carbon assimilation, alleviating over‐reduction of the photosynthetic electron transport chain, and maintaining the functionality of photosynthetic proteins.

There are considerable variations in the percentage loss of hydraulic conductivity (PLC) at mid-day minimum water potential among and within species, but the underpinning mechanism(s) are poorly understood. This study tested the hypothesis that plants can regulate leaf specific hydraulic conductance (K_l) via precise control over PLC under variable ΔΨ (water potential differential between soil and leaf) conditions to maintain the −m/b constant (−m: the sensitivity of stomatal conductance to VPD; b: reference stomatal conductance at 1.0 kPa VPD), where VPD is vapor pressure deficit. We used Populus euphratica, a phreatophyte species distributed in the desert of Northwestern China, to test the hypothesis. Field measurements of VPD, stomatal conductance (g_s), g_s responses to VPD, mid-day minimum leaf water potential (Ψ_lmin), and branch hydraulic architecture were taken in late June at four sites along the downstream of Tarim River at the north edge of the Taklamakan desert. We have found that: 1) the −m/b ratio was almost constant (=0.6) across all the sites; 2) the average Ψ₅₀ (the xylem water potential with 50% loss of hydraulic conductivity) was −1.63 MPa, and mid-day PLC ranged from 62 to 83%; 3) there were tight correlations between Ψ₅₀ and wood density/leaf specific hydraulic conductivity (k_l) and between specific hydraulic conductance sensitivity to water potential [d(k_s)/dln(−Ψ)] and specific hydraulic conductivity (k_s). A modified hydraulic model was applied to investigate the relationship between g_s and VPD under variable ΔΨ and K_l conditions. It was concluded that P. euphratica was able to control PLC in order to maintain a relatively constant −m/b under different site conditions. This study demonstrated that branchlet hydraulic architecture and stomatal response to VPD were well coordinated in order to maintain relatively water homeostasis of P. euphratica in the desert. Model simulations could explain the wide variations of PLC across and within woody species that are often observed in the field.