Response of some Calvin cycle enzymes subjected to salinity shock in vitro.

Leaf enzyme extracts of 10 day old seedlings of horsegram were subjected to NaCl or Na2SO4 treatment in vitro. Salinity shock caused decline in the activities of RuBP carboxylase, R-5-P kinase, R-5-P isomerase and NADP-Gly-3-P dehydrogenase. At low concentrations, Na2SO4 did not alter the activities of R-5-P kinase, R-5-P isomerase and NADP-Gly-3-P dehydrogenase. RuBP carboxylase was found to be more sensitive to salt shock than the other enzymes studied. Further, NaCl was more toxic to the enzyme activities as compared to Na2SO4.

Action Spectra of Photosystems I and II in State 1 and State 2 in Intact Sugar Maple Leaves.

Photochemical activity, measured as energy storage of photosystems I (PSI) and II (PSII) together and individually, is studied in sugar maple (Acer saccharum Marsh.) leaves in the spectral range between 400 and 700 nm in state 1 and state 2. Total photochemical activity remains the same in both state 1 and state 2 between 580 and 700 nm, but it is lower in state 2 between 400 and 580 nm. Both PSI and PSII activities change significantly during the state transition due to the migration of light-harvesting chlorophyll a/b protein complex of PSII (LHCII). In the action spectra of PSI and PSII, peak positions vary depending on the association or dissociation of LHCII, except for the peak at 470 nm in the PSII spectrum. PSII activity is about 3 times higher than or equal to PSI in state 1 or state 2, respectively, over most of the spectrum except in the blue and far-red regions. At 470 nm, PSII activity is 8 or 1.6 times higher than PSI in state 1 or state 2, respectively. The amplitude of LHCII coupling-induced change is the same in both PSI and PSII between 580 and 700 nm, but it is less in PSI than in PSII between 400 and 580 nm, which explains the lower photochemical activity of the leaf in state 2 than in state 1. This may be due to a decrease in energy transfer efficiency of carotenoids to chlorophylls in LHCII when it is associated with PSI.

Sulfite inhibition of photochemical activity of intact pea leaves.

Sulfite treatment of pea leaf disks in light caused a significant decrease in the relative quantum yield of photosynthetic oxygen evolution and energy storage (ES) as measured by photoacoustic (PA) spectroscopy. The inhibition was concentration dependent and was less in darkness than in light, indicating light-dependent inhibitory site(s) on the photosynthetic electron transport chain. Further, in darksulfite-treated leaves, the energy storage was more affected than the relative quantum yield of oxygen evolution, suggesting that photophosphorylation and/or cyclic electron transport around PS I are sites of sulfite action in darkness. The Rfd values, the ratio of fluorescence decrease (fd) to the steady-state fluorescence (fs), decreased significantly in leaves treated with sulfite in light but were not affected in dark-treated ones, confirming the photoacoustic observations. Similarly, the ratio of variable fluorescence (Fv) to maximum fluorescence (Fm), a measure of PS II photochemical efficiency, was affected by sulfite treatment in light and not changed by treatment in darkness. An attempt was made to explain the mechanism of sulfite action on photosynthetic electron transport in light and in darkness.

Photoacoustic Study of Changes in the Energy Storage of Photosystems I and II during State 1-State 2 Transitions.

Using photoacoustic spectroscopy, state 1-state 2 transitions were demonstrated in vivo in intact sugar maple leaves (Acer saccharum Marsh.) by following the changes in energy storage of photosystems (PS) I and II. Energy storage measured with 650 nm modulated light (light 2) in the presence of background white light indicated the total energy stored by both photosystems (ES(t)), and in the presence of background far-red light showed the energy stored by PSI (ES(psi)). The difference between ES(t) and ES(psi) gave the energy stored by PSII (ES(psii)). While ES(t) remained nearly constant during state transitions, both ES(psi) and ES(psii) changed considerably. The ratio of ES(psii) to ES(psi), an indicator of the energy distribution between the two photosystems, decreased or increased during transition to state 2 or state 1, respectively. State transitions were completed in about 20 min and were fully reversible. During transition from state 1 to state 2, the fraction of excitation energy gained by PSI was nearly equal to that lost by PSII. This fraction of excitation energy transferred from PSII to PSI accounted for about 5% of the absorbed light (fluorescence is not considered), 19% of ES(t), 34% of ES(psii), and 43% of ES(psi) in state 2. NaF treatment inhibited the transition to state 1. Data in the present study confirm the concept of changes in absorption cross-section of photosystems during state transitions.

SO(2) Effect on Photosynthetic Activities of Intact Sugar Maple Leaves as Detected by Photoacoustic Spectroscopy.

Short-term (4 hours) effect of different concentrations of SO(2) fumigation on in vivo photochemical activities of sugar maple (Acer saccharum Marsh.) leaves was investigated using photoacoustic spectroscopy. The relative quantum yield of O(2) evolution (ratio of O(2) signal to the photothermal signal) and photochemical energy storage are increased by 0.05 microliter per liter of SO(2). This increase is more pronounced in 5 to 7 year old saplings than in 3 month old seedlings. Both oxygen-relative quantum yield and energy storage of seedlings are inhibited by increased concentrations of SO(2) and the inhibition is concentration dependent. The inhibition is greater in seedlings than in saplings at 2 microliters per liter of SO(2), indicating the more susceptible nature of seedlings. The present study indicates a concentration dependent differential effect of SO(2) on photochemical activities of sugar maple leaves.

Qualitative changes in the fluorescence spectra of intact pea leaves after photoinhibition.

In high light (1400 W m-2) treated, intact pea leaves, a decrease in the ratio of fluorescence emission at 685 to 730 nm and an increase in fluorescence intensity between 500 and 600 nm were observed. Furthermore, photoacoustically monitored heat emission increased slightly, and O2 evolution decreased significantly. These findings are interpreted as effects of a photoprotective mechanism separating the carotenoid pool from the chlorophylls. This is supported by fluorescence excitation measurements and the results of a study on the reversibility of the process.

Photosynthetic energy storage of Photosystems I and II in the spectral range of photosynthetically active radiation in intact sugar maple leaves.

The relative activity of Photosystems (PS) I and II in the spectral range between 400 and 720 nm was studied by measuring photosynthetic energy storage (ES) of an intact sugar maple leaf using photoacoustic spectroscopy. ES, determined with a modulated (80 Hz) monochromatic light beam in the presence of saturating intensity of background non-modulated white light, indicated the total energy stored by both photosystems (EST). Using background far-red light, ES of PS I (ESPS I) was quantified. ESPS II was derived from EST-ESPS I. EST dependence on intensity and wavelength of modulated light was studied at 470, 560, 640 and 680 nm. EST was maximum in red light and minimum in blue light. It decreased with an increase in modulated light intensity. The ratio ESPS II/ESPS I, measured at 640 nm, remained nearly constant with an increase in modulated light intensity. The relative quantum yield of EST spectrum showed two peaks around 610 and 660 nm, and declined sharply after 680 nm, revealing a clear red drop. ESPS I spectrum presented peaks around 610 and 670 nm, and a minimum between 440 and 470 nm. ESPS I was observed beyond 700 nm up to 720 nm, indicating the energy stored by cyclic electron transport. ESPS II spectrum showed broad peaks, around 460, 490, 600 and 660 nm, and a shoulder between 530 and 560 nm. ESPS II was always higher than ESPS I between 400 and 690 nm and reached zero around 700 nm.

Effect of sulfur dioxide and sulfite on photochemical energy storage of isolated chloroplasts--a photoacoustic study.

Photoacoustic spectroscopy was used to study the effect of sulfite and SO(2) on isolated corn mesophyll chloroplasts by monitoring the photochemical energy storage. Sulfite incubation of isolated chloroplasts, either in light or in darkness, caused a decrease in photochemical energy storage. The more pronounced decrease in light indicates a light-dependent sulfite inhibitory site(s) in chloroplasts. Also diphenylcarbazide caused a partial recovery of energy storage in sulfite treated chloroplasts indicating a possible site of damage at the water oxidizing system. Although the chloroplast membranes were found to be insensitive to high concentrations of SO(2) for relatively short exposure periods (10 min) in light, exposure of chloroplasts to 28.5 ng cm(-3) SO(2) for 10 min caused a decrease in energy storage. An attempt was made to explain the mechanism of action of sulfite and SO(2) in chloroplasts.