The influence of metribuzin [4-ammo-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] on net photosynthesis of goosegrass [Eleusine indica (L.) Gaertn. ♯3 ELEIN] and six cultivars of bermudagrass [Cynodon dactylon (L.) Pers. ♯ CYNDA cvs. ‘Common’, ‘Tufcote’, ‘Vamont’, and ‘Midiron’, Cynodon transvaalensis Davy., and Cynodon dactylon X Cynodon transvaalensis ‘Tifway’] was evaluated. Metribuzin rapidly inhibited carbon dioxide (CO2) uptake within 1 h after application in both goosegeass and bermudagrasses. Very often, net photosynthesis was completely inhibited. Net photosynthesis in goosegrass was inhibited by lower rates of metribuzin than those required for similar reduction in bermudagrasses. Common, Tufcote, Tifway, and C. transvaalensis bermudagrasses were less sensitive to metribuzin than Midiron and Vamont based upon the degree of inhibition of net photosynthesis, the rate of its recovery, and the level of foliar injury. Recovery from inhibition of net photosynthesis was observed in both species 24 h after treatment with low metribuzin rates. High rates caused several days lag period before net photosynthesis resumed. After 10 days, 95% of the goosegrass foliage was injured (either chlorosis or necrosis) from 2 kg ai/ha metribuzin treatment. The threshold concentration for inducing leaf injury in bermudagrasses was 1 kg ai/ha. Inhibition of photosynthesis was initially rapid in both grasses; however, recovery was evident earlier and there was less foliar injury in bermudagrasses than in goosegrass. Selective control of goosegrass in bermudagrass may therefore be partially dependent upon rapid metabolism and/or inactivation of metribuzin in the bermudagrass.

Photosynthetic responses of rewetted Nostoc flagelliforme to CO2, desiccation, light and temperature were investigated under emersed conditions in order to characterize its ecophysiological behaviour in nature. Net photosynthesis increased to a maximum rate at about 30% water loss, then decreased, while dark respiration always decreased with the progress of desiccation. Light-saturated photosynthesis and dark respiration were significantly reduced at 8 °C, but remained little affected by changes of temperature within the range of 15–35 °C. Photosynthetic efficiency (α) was maximal at the beginning of desiccation and then reduced with increased water loss. Saturating irradiance for photosynthesis was about 194–439 μmol quanta m−2 s−1, being maximal at about 30% water loss. No photoinhibition was observed at irradiances up to 1140 μmol m−2 s−1. Light compensation points were about 41–93 μmol m−2 s−1. Photosynthesis of N. flagelliforme was CO2-limited at the present atmospheric CO2 level. The CO2-saturated photosynthesis increased with increase of irradiance (190–1140 μmol m−2 s−1) and temperature (8–25 °C) and decreased significantly with water loss (0–75%). Photosynthetic affinity for CO2 was sensitive to temperature and irradiance. The CO2 compensation point (Γ) increased significantly with increased temperature and was insensitive to irradiance. Desiccation did not affect Γ values before water loss exceeded 70%. Photorespiratory CO2 release did not occur in N. flagelliforme at the current atmospheric CO2 level.

The physiological potential for acquisition of atmospheric ammonia (NH3) was investigated in three European meadow grasses (Arrhenatherum elatius, Bromus erectus and Brachypodium pinnatum) competing in chalk grasslands. Experiments were carried out with plants cultivated for about three months on a soil–sand mixture at high root nitrogen supply or in nutrient solutions at low root nitrogen supply. Two different root nitrogen regimes were applied to the solution-grown plants: 130 μmol NO3 plant−1 wk−1 (approx. 50 kg nitrogen ha−1 in three months); or 130 μmol NO3− plus 130 μmol NH4+ plant−1 wk−1. Each regime was combined with two levels of NH3 fumigation (0 and 70 nmol mol−1 air for 24 d). Uptake of gaseous NH3 in the shoots was investigated under controlled environmental conditions including NH3 concentrations ranging from 0 to 30 nmol mol−1 air. Concurrently, photosynthesis, glutamine synthetase activity, nitrogen allocation, biomass allocation and apoplastic cation composition were measured. For A. elatius, the influence of photorespiration on NH3 acquisition was also assessed. Independently of plant nitrogen status, ammonia compensation points in A. elatius and B. erectus plants were <0.5 nmol mol−1. The total leaf conductance to NH3 absorption remained constant at increasing NH3 concentrations, showing that the capacity for assimilation was unaltered. Whereas internal factors in the leaves did not cause differences in the potential for NH3 acquisition between the species, other factors of NH3 acquisition were quite different: B. erectus had higher stomatal conductance and, thus, higher NH3 uptake rates per unit leaf area compared to A. elatius and B. pinnatum; higher stomatal conductances of B. erectus were to a large extent offset by a lower leaf area per plant, resulting from a lower growth rate and thicker leaves than in the two other species. The rate of photorespiration in Arrhenatherum constituted at least 15% of the net photosynthetic rate. Surprisingly, suppression of photorespiration indicated that NH3 uptake was supported by photorespiration. Bromus responded to fumigation with 70 nmol NH3 mol−1 air for 24 d by lowering the root[ratio ]shoot ratio and increasing the nitrogen concentration in the stem dry matter. The total leaf conductance to NH3 uptake decreased in all three species upon exposure to NH3, while the stomatal conductance was unaffected. The NH3 exposure caused lower apoplastic concentrations of H+, Mg2+ and Ca2+ in A. elatius and B. erectus.

Responses of gas exchange and photosynthesis to changes in CO2 concentration and PPFD were examined in well watered plants of Delosperma tradescantioides Bgr. to establish the relative importance of these environmental changes on the photosynthetic machinery in this CAM-cycling species which grows naturally in both exposed and partly shaded environments. Plants were grown at two PPFDs (220 [LL] and 550 [HL] μmol m−2 s−1). HL plants had larger leaves with higher specific weight, water content and diurnal malic acid fluctuation. Photosynthetic PPFD responses were typically those of sun and shade species for HL and LL plants, both under 21% O2 and non-photorespiratory (2% O2) conditions. The CO2 compensation point in the absence of non-photorespirational CO2 evolution in the light (Γ∗) was c. 30 μmol mol−1. Irradiation reduced mitochondrial respiration by >50%. Comparison of the PPFD responses of linear electron flow rates derived from gas exchange measurements and from fluorescence analysis ([Jscr ]F) indicated effective photosynthetic control. [Jscr ]F was always larger than electron flow rates calculated from gas exchange, indicating that processes other than carboxylation and oxygenation were consistently important in energy consumption under all sampled environmental conditions. Regardless of PPFD during growth, electron flow to carboxylation and [Jscr ]F were linearly correlated, demonstrating that the photosynthetic apparatus was well adapted to PPFD during growth. In HL plants, non-photochemical quenching increased, and photochemical quenching and the quantum yield of linear electron transport through PS II decreased more slowly with increasing PPFD than in LL plants. In plants of both treatments non-photochemical energy dissipation seemed to be exhausted when the proportion of photons not utilizable by photochemistry exceeded 0·7. Results illustrate a pronounced ability of D. tradescantioides to acclimate to a 100% change in the prevailing PPFD and lend support to the hypothesis that CAM cycling might act as a photoprotective process.

Polypeptides of 21, 36 and 37 kDa are induced in the unicellular green alga Chlamydomonas reinhardtii when cells are transferred from high (5%) to low (0·03%) CO2 concentrations. The synthesis of these polypeptides is correlated with the induction of the CO2-concentrating mechanism. Interaction between the induction of low-CO2-inducible polypeptides, the CO2-concentrating mechanism and photorespiration has been studied in wild-type C. reinhardtii with the aim of clarifying whether the glycolate pathway is involved in algal acclimation to limiting CO2 conditions. Our results showed that the induction of the 37 kDa periplasmic carbonic anhydrase and 21 kDa polypeptide under low-CO2 conditions was not observed in the presence of aminooxyacetate, an inhibitor which completely blocks glycolate metabolism. However, the induction of the 36 kDa polypeptide was not affected by this inhibitor. The presence of aminooxyacetate during the acclimation to low CO2 conditions also inhibited the increase in the photosynthetic affinity for inorganic carbon shown by low-CO2-acclimated Chlamydomonas cells without the inhibitor. Our results suggest that the induction of the CO2-concentrating mechanism may require the function of the glycolate pathway. In addition, our results also indicate that there is differential regulation of the induction of these three low-CO2-inducible polypeptides in Chlamydomonas reinhardtii.