The Effect of Temperature on the Conversion of Protochlorophyll to Chlorophyll a in Etiolated Barley Leaves.

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Abstract
The effect of temperatures from -195 to +55[degree]C on the photochemical transformation of protochlorophyll to chlorophyll a in etiolated barley leaves has been studied. At -195[degree]C no transformation occurs. At -70[degree]C fairly rapid and extensive conversion takes place. The conversion increases in rate and extent with increase in temp. up to 40[degree]C. Freezing the leaves at -10 or -20[degree]C appears to damage the transformation system. This damage can be partially avoided by freezing the leaves quickly at -77[degree]C before raising them to -10 or -20[degree]C for irradiation. The extent of the transformation, which is limited by a given low temp., can be increased by illumination of the frozen leaf at a higher temp., provided no thawing occurs. Thawing of the leaf completely prevents the transformation. Leaves heated to 40[degree]C begin to lose their capacity for bringing about the phototransformation of protochlorophyll to chlorophyll a. The longer the period of heating the greater becomes the loss of transformation capacity, and the higher the temperature, the more rapidly the loss takes place. Heating the leaves at 55[degree]C for 5 minutes almost completely destroys their transformation capacity. These expts. indicate that the protochlorophyll holochrome is a pigment-protein complex. Kinetic studies on the transformation in etiolated barley leaves at wave lengths 589, 579/577, and 546 m[mu] show the progress of the reaction to be rigorously a second-order process with respect to the protochlorophyll; but at 436 m[mu] the second-order rate law is not strictly obeyed. The cause of this is not known. This kinetic behavior coupled with the progressive lowering of the transformation limit with lowering of the temperature suggests that the reaction is not strictly a photochemical, intramolecular process but involves intermolecular interactions. Because the reaction depends on the first power of the light intensity, it is unlikely that the two photoactivated holochromes have to react.