Heat-induced alterations in the pigment-protein complexes of the thylakoid membrane of C3 and C4 plants

Some thermally sensitive sites within the plant have been previously identified: the water-splitting apparatus is inactivated, and downsizing of photosystem II (PS II) occurs. The latter occurs via detachment of the peripheral light-harvesting complex from PS II, followed by lateral migration of the smaller PS II unit from grana to stroma lamellae. Because little else is known about how heat affects the plastid's photosynthetic pigment-protein complexes, this dissertation research was undertaken.; Detached leaves from various C{dollar}sb3{dollar} and C{dollar}sb4{dollar} plants were exposed to 46-47{dollar}spcirc{dollar}C for up to one hour under a light intensity of 45-55{dollar}mu{dollar}E m{dollar}sp{lcub}-2{rcub}{dollar}s{dollar}sp{lcub}-1{rcub}{dollar} and plastids were isolated. A combination of spectroscopic and biochemical techniques was used to detect any heat damage. A two-dimensional polyacrylamide gel electrophoretic procedure was perfected to analyze the photosystem I (PS I) and PS II pigment-protein complexes at any instant during the heat stress period. In support of other researchers' observations, PS I was shown to be the more thermally stable photosystem, whereas the electron donor side of PS II was particularly susceptible. Loss of some oxygen evolving enhancer proteins probably explains the decreased PS II activity. Further evidence for some PS II light-harvesting complex (LHC II) pigment-proteins forming a multimeric "connector" complex funneling energy between the bulk antenna and the core complex of PS II (CC II) was obtained. This "connector" as well as CC II and LHC II are progressively altered from multimeric to monomeric forms during heat. The thermally-induced lateral migration model was not supported by the data obtained. Chlorophyll bleaching was not a general response of a heat-stressed plant: chlorophyll was lost from C{dollar}sb3{dollar} dicots but not from C{dollar}sb3{dollar} and C{dollar}sb4{dollar} monocots. The steady state message levels for two nuclear genes, which encode some of the LHC II proteins, declined in heat stressed leaves. Carotenoid composition of membranes and pigment-proteins also changed during heat stress. The xanthophyll cycle was activated, and zeaxanthin was detected in the PS I and LHC II complexes of heated thylakoids. It was shown that light, albeit of low intensity, was necessary for this activation.