The decay of tyrosine cation radical was found to be biphasic at 253 K. The fast phase corresponds to the YZ* component while the slow phase corresponds to the tyrosine D radical (YD*) component. At 253 K, the t1/2 value was ∼28.6 s for the fast phase and ∼190.7 s for the slow phase. The fast phase is attributed to the recombination of charges between YZ* and QA-. The activation energy for the reaction of YZ with QA- between 253 and 293 K was 48 kJ mol-1 in Cl--depleted photosystem 2 (PS2) membranes. Both the decay rate and the amplitude of the PAR-induced signal of YZ* were affected by addition of chloride anion. Change in the decay rate and the amplitude of the PAR-induced signal of YZ* was observed when other anions like Br-, I-, F, HCO3-, NO3-, PO43- were substituted in the Cl--depleted PS2. and A. Jajoo, S. Bharti, A. Kawamori.
Feeding K+ or Na+ nitrate salts in vivo enhanced the activity of phosphoenolpyruvate carboxylase (PEPC) in the leaf extracts of Alternanthera pungens (C4 plant) and A. sessilis (C3 species). The increase was more pronounced in A. pungens than in A. sessilis. Chloride salts increased the PEPC activity only marginally. However, the sulfate salts were either not effective or inhibitory. Feeding nitrate modulated the regulatory properties of PEPC in A. pungens, resulting in increased KI (malate) and decreased KA (glucose-6-P). The sensitivity of PEPC to malate, which gives a measure of phosphorylation status of the enzyme, indicated that feeding leaves with NO3- enhanced the phosphorylation status of the enzyme. The reduction in PEPC activity due to cycloheximide treatment suggested that increased synthesis of PEPC protein kinase may be one of the reasons for the enhancement in PEPC activity, after the nitrate feeding. We suggest that nitrate salts could be used as a tool to modulate and analyze the properties of PEPC in C3 and C4 plants. and A. V. Rajagopalan, R. M. Agarwal, A. S. Raghavendra.