Moderate Photoinhibition of Photosystem II Protects Photosystem I from Photodamage at Chilling Stress in Tobacco Leaves

Abstract

It has been indicated that photosystem I (PSI) is susceptible to chilling-light stress in tobacco leaves, but the effect of growth light intensity on chilling-induced PSI photoinhibition in tobacco is unclear. We examined the effects of chilling temperature (4°C) associated with moderate light intensity (300 μmol photons m(-2) s(-1)) on the activities of PSI and photosystem II (PSII) in leaves from sun- and shade-grown plants of tobacco (Nicotiana tabacum cv. k326). The sun leaves had a higher activity of alternative electron flow than the shade leaves. After 4 h chilling treatment, the sun leaves showed significantly a higher PSI photoinhibition than the shade leaves. At chilling temperature the sun leaves showed a greater electron flow from PSII to PSI, accompanying with a lower P700 oxidation ratio. When leaves were pre-treated with lincomycin, PSII activity decreased by 42% (sun leaves) and 47% (shade leaves) after 2 h exposure to the chilling-light stress, but PSI activity remained stable during the chilling-light treatment, because the electron flow from PSII to PSI was remarkably depressed. These results indicated that the stronger chilling-induced PSI photoinhibition in the sun leaves was resulted from a greater electron flow from PSII to PSI. Furthermore, moderate PSII photoinhibition depressed electron flow to PSI and then protected PSI activity against further photodamage in chilled tobacco leaves.

Keywords: chilling temperature; electron transfer; growth light intensity; photoprotection; photosystem I; photosystem II.

FIGURE 1

Light response changes in photosynthetic electron flow in the sun and shade leaves of tobacco. (A) PSII electron transport rate (J_F) calculated based on measurements of PSII quantum yields, assuming the equal distribution of absorbed light between PSI and PSII. (B) Rate of electron transport consumed by carboxylation plus oxygenation of RuBP (J_G), calculated by the data from gas exchange measurements. (C) The difference between J_F and J_G, J_F – J_G represents a common way to estimate alternative electron flow besides the Calvin cycle and photorespiration. Values are means ± SE (n = 6).

FIGURE 2

Relationship between photoinhibition of PSII (B,D) and PSI (A,C) in sun and shade tobacco leaves. Detached leaves incubated in the presence or absence of lincomycin (1 mM) overnight in darkness were exposed to 4°C and 300 μmol photons m^-2 s^-1 for 2, 4, or 6 h. F_m was measured after dark adaptation to estimate the amount of active PSII centers. P_m was measured after dark adaptation to estimate the amount of active PSI centers. All values are expressed relative to the controls before chilling-light treatment, and shown as means ± SE (n = 6). Asterisks indicate significant differences between the sun and shade leaves.

FIGURE 3

Changes in relative Q_A reduction (qL) (A,C) and PSI oxidation ratio [Y(ND)] (B,D) during exposure to 4°C and 300 μmol photons m^-2 s^-1. After measuring qL and Y(ND) at 24°C and 297 μmol photons ^-2 s^-1, leaves were incubated in the presence or absence of lincomycin (1 mM) overnight in darkness and subsequently exposed to 4°C and 300 μmol photons m^-2 s^-1 for 2, 4, or 6 h. During chilling-light treatment, qL and Y(ND) were measured at 4°C and 297 μmol photons m^-2 s^-1. Values are means ± SE (n = 6).