Effects of low temperature stress on excitation energy partitioning and photoprotection in Zea mays.

Analysis of the partitioning of absorbed light energy within PSII into fractions utilised by PSII photochemistry (Φ _PSII ), thermally dissipated via ΔpH- and zeaxanthin-dependent energy quenching (Φ _NPQ ) and constitutive non-photochemical energy losses (Φ _f,D ) was performed in control and cold-stressed maize (Zea mays L.) leaves. The estimated energy partitioning of absorbed light to various pathways indicated that the fraction of Φ _PSII was twofold lower, whereas the proportion of thermally dissipated energy through Φ _NPQ was only 30% higher, in cold-stressed plants compared with control plants. In contrast, Φ _f,D , the fraction of absorbed light energy dissipated by additional quenching mechanism(s), was twofold higher in cold-stressed leaves. Thermoluminescence measurements revealed that the changes in energy partitioning were accompanied by narrowing of the temperature gap (ΔT _M ) between S _2/3 Q _B ^- and S ₂ Q _A ^- charge recombinations in cold-stressed leaves to 8°C compared with 14.4°C in control maize plants. These observations suggest an increased probability for an alternative non-radiative P680 ⁺ Q _A ^- radical pair recombination pathway for energy dissipation within the reaction centre of PSII in cold-stressed maize plants. This additional quenching mechanism might play an important role in thermal energy dissipation and photoprotection when the capacity for the primary, photochemical (Φ _PSII ) and zeaxanthin-dependent non-photochemical quenching (Φ _NPQ ) pathways are thermodynamically restricted in maize leaves exposed to cold temperatures.