Assessment of changes in photosystem II structure and function as affected by water deficit in Amaranthus hypochondriacus L. and Amaranthus hybridus L.

The present study describes the behaviour of photosystem II (PSII) in Amaranthus hypochondriacus and Amaranthus hybridus under water stress conditions, assessed by the analyses of the polyphasic rise in chlorophyll a fluorescence (O-J-I-P). We determined the adaptive behaviour in relation to the regulation of the different functional and structural parameters of PSII, which was a direct and rapid response due to changes in soil water status indicated by a decrease in leaf water potential and relative water content. It allows for the identification of specific key or limiting chlorophyll fluorescence parameters which could be used to identify traits conveying tolerance. For the above partial processes of PSII function studied, it seems that A. hybridus remained the more stable upon water stress (after 17 days of withholding water), concerning the specific energy fluxes of absorption/reaction centre (ABS/RC) apparent (antenna size) and trapping/reaction centre (TR/RC) (maximum trapping flux), as well as the density of the reaction centres/cross section (RC/CS) and the phenomenological trapping flux/cross section (TR(o)/CS). It was clear that amaranth adjusts the non-photochemical (k(n)) deactivation constant of PSII and to a less extend also the photochemical (k(p)) deactivation constant by means of photoregulation, which forms the basis of the quenching of chlorophyll a fluorescence. Although drought stress caused the deactivation of RCs leading to a decrease in the density of active RCs, the plants compensated by increasing the efficiency of the conversion of trapped excitation energy to electron transport beyond Q(A) (efficiency of exciton trapping/reaction centre: ET(o)/TR(o)). Subsequent damage to PSII might be the reason for the slow, or lack of recovery, for most of the parameters measured.