ASSOCIATION OF HIGH-TEMPERATURE INJURY WITH INCREASED SENSITIVITY OF PHOTOSYNTHESIS TO ABSCISIC-ACID IN WHEAT

Photosynthetic responses of intact seedlings and detached leaves of wh eat (Triticum aestivum L. cv. Len) exposed to 0 or 100 mu M (+/-)-ABA were investigated at 15, 25, or 35 degrees C. Photosynthetic gas excha nge of intact seedlings and Chl variable fluorescence of detached leav es were nearly constant at 15 degrees C but decreased steadily over ti me at 35 degrees C; (+/-)-ABA accentuated the decline and was increasi ngly efficacious as the temperature rose. Thylakoid Chi and protein co ntents and PSII activity changed slowly and responded slightly to (I)- ABA at 15 degrees C but declined rapidly at 35 degrees C with (+/-)-AB A. Analyses of Chi fluorescence spectra at 77K indicated that high tem perature and (+/-)-ABA altered the microenvironment of Chi beds in thy lakoid membranes, shifting emission by Chi-binding proteins of PSII an d causing preferential loss of PSI antenna pigments. Immunodetection l evels of Chi-binding proteins, such as LHCII, LHCI, and the P700 apopr oteins of PSI, declined after high temperature and (+/-)-ABA treatment s; Western blots detected single bands of Chi-binding proteins in inta ct seedlings and multiple aberrant bands in detached leaves. Their sim ilar effects and synergistic interactions indicate that ABA may be inv olved in high-temperature injury to photosynthetic processes.