The Photostability of Different Chlorophyll Forms in Dark Grown Leaves of Wheat

The effect of denaturing treatments on the stability against high intensity irradiation (red light, 700 W m^?2) was investigated in vivo for various chlorophyll forms in wheat. Three pigment forms were investigated: the 650-form (protochlorophyllide) present in dark grown leaves; the 684-form (chlorophyllide) formed within 5 s after photoreduction of the 650-form; and the 673-form (chlorophyll), into which the 684-form has been transformed 25 min after photoreduction of the 650-form. (The pigment forms are denoted by their absorption maxima in the red region before denaturation.) Two denaturing treatments were used: heat treatment (water of 55°C for 2 min) and freezing and thawing (freezing in liquid nitrogen followed by thawing in water of 25°C). Heat treatment as well as freezing and thawing caused a shift in the absorption peak of the two nonesterified pigment forms. The peak of of the chlorophyllide 684-form shifted to 673 nm and that of the protochlorophyllide 650-form to 636 nm. The absorption maximum of the chlorophyll 673-form was not affected by the above treatments. Heat treatment as well as freezing and thawing had profound effects on the structural organization of the plastid pigments, as shown by a decrease in the photostability. For the 684-form, heat treatment reduced the photostability by a factor of about 14 (half-life in strong light changed from 170 s to 12 s). Freezing and thawing also reduced the photostability, although the effect was less pronounced (c. 3–4 times decrease in half-life). Upon transformation of the chlorophyllide 684-form into the chlorophyll 673-form (the Shibata-shift) the pigments became less sensitive to light, and were no longer “aggregated” by heat treatment. The “aggregating” effect of freezing and thawing was still present after the Shibata shift. The results thus verify a clear difference in structural organization of the 684-form and the 673-form, since the two pigment forms were differently affected by heat treatment. The 650-form behaved similarly to the 684-form, although it appeared to be slightly less aggregated by heat treatment. — The decrease in photostability, caused by heat treatment of the 684-form, changed the kinetics for the photodecomposition from a first towards a second order reaction.