Photosynthetic Responses of Leaves to Water Stress, Expressed by Photoacoustics and Related Methods

Abstract

The effect of rapid dehydration of detached tobacco leaves (Nicotiana tabacum L.) on the photochemical apparatus of photosynthesis was studied in vivo by a combination of methods: photoacoustics, chlorophyll a fluorescence, and cytochrome fdifference spectroscopy. It was shown that the inhibition of gross 02 evolution was mainly caused by inactivation of PSII: (a) The saturation curve of cytochrome-fphotooxidation by farred (>710 nanometers) light was resistant to the stress, leading to the conclusion that photosystem I (PSI) was largely unaffected by the stress. (b) The extent of the chlorophyll a variable fluorescence arising from photosystem II (PSII) decreased with the progression of the stress, but was largely unaffected when the leaf was preincubated with electron donors to PSII, such as hydroxylamine. It is concluded that the drought damage to PSII occurred on the photooxidative side. Despite the extensive inhibition of PSII and the relative preservation of PSI, the apparent PSII/PSI activity balance was somewhat larger in stressed leaves than in the control, as indicated by photoacoustic measurements of Emerson enhancement. These measurements were performed continuously under conditions which favor transitions to either state 1 or 2, showing that the transition to state 2 was considerably inhibited. Simultaneous measurements of chlorophyll fluorescence induction at 680 and 730 nm at room temperature were also used to probe changes in energy distribution between PSII and PSI and indicated that the transition from a dark adapted state to state 2 was also affected in water-stressed leaves. The saturation curve of the far-red light effect in Emerson enhancement was not changed by the stress, giving another independent evidence for the drought resistance of PSI activity. This apparent preservation of the imbalance in photochemical activities in favor of PSII, despite the fact that PSII is strongly inhibited, and PSI is not, supports a previous suggestion that the electron transfer between the two photosystems is not random but that a large extent of PSII and PSI units are specifically linked.A photoacoustic study, reported in the first paper of this series (16), has confirmed that desiccation of tobacco leaves results in a substantial reduction of the in vivo quantum yield of gross 02 evolution and indicated a direct effect on whole chain electron transfer, independent of stomatal aperture. Work with algae and isolated chloroplasts pinpointed some partial reactions of electron transport to be more sensitive to water stress than others. ' Supported by the United States-Israel Binational Agricultural Research and Development Fund (BARD). Grant No. I-388-8 1.For example, in algae undergoing desiccation, both electron transport between PSII and PSI and water splitting were most sensitive to desiccation (31). Block of electron transport on the donor and acceptor side of PSII has also been reported in dried chloroplast films (23). In intact leaves exposed to water stress conditions, the peak P in the Chl a...

Thermoluminescence Study of Photosystem II Activity in Haberlea rhodopensis and Spinach Leaves During Desiccation

“…Their chloroplast structure and composition and the photochemical reactions, particularly PS II activity, have been shown to be directly affected. The harmful effects of severe dehydration were manifested in considerable changes of electron transport capacity and in reduction of O 2 evolution (Govindjee et al, 1981;Havaux et al, 1986;Chen and Hsu, 1995;He et al, 1995), but the primary target of PS II drought inhibition is still controversial. Damage to electron donation from the water oxidizing system (Canaani et al, 1986), considerable depletion of the PS II core and structural reorganisation of the remaining centres (Giardi et al, 1996), changes in PS II membrane protein metabolism (He et al, 1995), and inhibition of both the oxygen-evolving complex and the acceptor side of PS II (Skotnica et al, 2000) have been considered as some possible reasons for PS II impairment in water-stressed plants.…”

Increased Tolerance to Photoinhibitory Light in Paraquat-Resistant Conyza bonariensis Measured by Photoacoustic Spectroscopy and 14CO2-Fixation

“…We have generally observed that under many different stress conditions there is no obligatory linear relation between the inhibition of oxygen evolution and energy storage. Energy storage is usually much less sensitive to many stresses than oxygen evolution (4,10). Energy storage reflects total photoactivity, including those photoactivities that are not related directly to oxygen evolution.…”

Utilizable water in leaves of 8 arid species as derived from pressure‐volume curves and chlorophyll fluorescence

“…While these 3 phases have not been recognized before, they could easily be superimposed on some data sets that compared F v /F m with environmental variables (e.g. Havaux et al 1986a; Larcher et al 1990), while the middle phase seems to be missing from some (e.g. Epron and Dreyer 1992; Boorse et al 1998) and the third phase was not reached by others (e.g.…”

“…4), indicating that TLP has no physiological significance in setting the limits of overall UWC. We chose WC at F c as setting the preferred lowest limit of utilizable water on grounds that (a) it is sharply defined; (b) it loosely corresponded to the critical F v /F m used for setting the limits to chilling and heat tolerance in other studies ( Larcher et al 1990, <0.7; Boorse et al 1998, 50% drop in activity); (c) it does not exceed the limits at which full photochemical recovery is possible ( Havaux et al 1986a,b; Valladares and Pearcy 1997; de Mattos et al 1999, <0.65); (d) it corresponded to a RWC for our most sclerophyllous species (29%), similar to the point (25% RWC) at which the Mediterranean oak, Quercus petraea , first showed a significant effect of water stress on F v /F m , representing the final stage of ‘dehydration‐induced dysfunctions in photosynthesis’ ( Epron and Dreyer 1992); and (e) it was very close to B, representing the theoretical limit of water availability.…”