Photosynthetic oxygen evolution at low water potential in leaf discs lacking an epidermis.

Land plants encountering low water potentials (low psiw) close their stomata, restricting CO2 entry and potentially photosynthesis. To determine the impact of stomatal closure, photosynthetic O2 evolution was investigated in leaf discs from sunflower (Helianthus annuus L.) plants after removing the lower epidermis at low psiw. Wounding was minimal as evidenced by O2 evolution nearly as rapid as that in intact discs. O2 evolution was maximal in 1% CO2 in the peeled discs and was markedly inhibited when psiw was below -1.1 MPa. CO2 entered readily at all psiw, as demonstrated by varying the CO2 concentration. Results were the same whether the epidermis was removed before or after low psiw was imposed. Due to the lack of an epidermis and ready movement or CO2 through the mesophyll, the loss in O2 evolving activity was attributed entirely to photosynthetic metabolism. Intact leaf discs showed a similar loss in activity when measured at a CO2 concentration of 5%, which supported maximum O2 evolution at low psiw. In 1% CO2, however, O2 evolution at low psiw was below the maximum, presumably because stomatal closure restricted CO2 uptake. The inhibition was larger than in peeled discs at psiw between -1 and -1.5 MPa but became the same as in peeled discs at lower psiw. Therefore. as photosynthesis began to be inhibited by metabolism at low psiw, stomatal closure added to the inhibition. As psiw became more negative, the inhibition became entirely metabolic.

Molecular cloning and functional characterization of two kinds of betaine-aldehyde dehydrogenase in betaine-accumulating mangrove Avicennia marina (Forsk.) Vierh.

Glycinebetaine is an important osmoprotectant in bacteria, plants, and animals, but only little information is available on the synthesis of glycinebetaine in tree plants. Among four mangrove species, glycinebetaine could be detected only in Avicennia marina. Pinitol was the main osmoprotectant in the other three species. The level of glycinebetaine in A. marina increased under high salinity. Betaine-aldehyde dehydrogenase (BADH) was detected in all four species, but choline monooxygenase could not be detected. A cDNA library was constructed from the leaves of A. marina. Two kinds of BADH cDNAs were isolated, one homologous to the spinach chloroplast BADH, and the other with unique residues SKL at the end of C-terminus. The BADH transcription levels of the former were higher than those of the latter. The levels of the former BADH increased at high salinity whereas those of the latter were independent of salinity. BADHs were expressed in Escherichia coli and purified. Two kinds of A. marina BADHs exhibited similar kinetic and stability properties, but were significantly different from those of spinach BADH. A. marina BADHs efficiently catalyzed the oxidation of betainealdehyde, but not the oxidation of omega-aminoaldehydes and were more stable at high temperature than the spinach BADH.

Photosynthesis during desiccation in an intertidal alga and a land plant.

This study was undertaken to determine how photosynthesis tolerates desiccation in an intertidal alga Fucus vesiculosus L. and a terrestrial sunflower Helianthus annuus L. Photosynthetic O2 evolution generally was inhibited at low water potentials (psiw) but more in sunflower leaves than in Fucus fronds at the same psiw. As psiw decreased, less carbon accumulated in an organic carbon store in Fucus. The inhibition of photosynthesis appeared to be mostly biochemical because it could not be prevented by supplying additional CO2 or by supplying CO2 from the internal organic carbon store. The inhibition of photosynthesis and carbon storage occurred after turgor disappeared and thus when solute concentrations were increasing in the cells. Solute concentrations were much higher in Fucus than in sunflower. After desiccation to the air-dry state (psiw below - 10 MPa), photosynthesis could not recover in sunflower but it recovered rapidly when Fucus was exposed to seawater. The lack of recovery in sunflower was associated with inability to recover turgor probably because of breaks in cell membranes. The ability to recover in Fucus was gradually lost during 1.5 d of desiccation at 45% relative humidity. At lower humidities, recovery was lost sooner as small amounts of water were removed. We conclude that photosynthesis tolerated desiccation more in Fucus than in sunflower because of differences in the molecular environment around the photosynthetic enzymes. Important aspects of this environment were features that prevented membrane breakage but promoted the retention of small amounts of water that were critical for viability.

A system for measuring leaf gas exchange based on regulating vapour pressure difference.

A system for measurement of leaf gas exchange while regulating leaf to air vapour pressure difference has been developed; it comprises an assimilation chamber, leaf temperature controller, mass flow controller, dew point controller and personal computer. A relative humidity sensor and air and leaf temperature sensors, which are all used for regulating the vapour pressure difference, are mounted into the chamber. During the experiments, the computer continuously monitored the photosynthetic parameters and measurement conditions, so that accurate and intenstive measurements could be made.When measuring the light-response curve of CO2 assimilation for single leaves, in order to regulate the vapour pressure difference, the leaf temperature and relative humidity in the chamber were separately and simultaneously controlled by changing the air temperature around the leaf and varying the air flow rate through the chamber, respectively. When the vapour pressure difference was regulated, net CO2 assimilation, transpiration and leaf conductance for leaves of rice plant increased at high quantum flux density as compared with those values obtained when it was not regulated.When measuring the temperature-response curve of CO2 assimilation, the regulation of vapour pressure difference was manipulated by the feed-forward control of the dew point temperature in the inlet air stream. As the vapour pressure difference was regulated at 12 mbar, the maximum rate of and the optimum temperature for CO2 assimilation in rice leaves increased 5 µmolCO2 m(-2) s(-1) and 5°C, respectively, as compared with those values obtained when the vapour pressure difference took its own course. This was reasoned to be due to the increase in leaf conductance and the decrease in transpiration rate. In addition, these results confirmed that stomatal conductance essentially increases with increasing leaf temperature under constant vapour pressure difference conditions, in other words, when the influence of the vapour pressure difference is removed.This system may be used successfully to measure inter- and intra-specific differences and characteristics of leaf gas exchange in plants with a high degree of accuracy.