Performance evaluation of five Mediterranean species to optimize ecosystem services of green roofs under water-limited conditions.

Rapid urban growth in Mediterranean cities has become a serious environmental concern. Due to this expansion, which covers adjacent horizontal ground, a critical deficit of green areas has been increasing. Moreover, irrigation is considered an important issue since water is one of the most limiting natural resources all over the world. The main objective of this study was to perform a long-term experiment to assess five Mediterranean species for extensive green roof implementation in Mediterranean-climate conditions. Brachypodium phoenicoides, Crithmum maritimum, Limonium virgatum, Sedum sediforme and Sporobolus pungens were grown in experimental modules under well-watered and water-limited conditions (irrigation at 50% and 25% ET0, respectively). Plant growth and cover, relative appearance, color evolution and water use were determined periodically for two years. Shoot and root biomass were quantified at the end of the experimental period. The effects of the irrigation treatments and seasonal changes were assessed to identify the advantages and disadvantages of each species according to their environmental performance. All species survived and showed adequate esthetic performance and plant cover during the experiment. S. sediforme registered the lowest variation of relative appearance along the experiment, the highest biomass production and the lowest water consumption. Nevertheless, B. phoenicoides appeared to be an interesting alternative to S. sediforme, showing high esthetic performance and water consumption throughout the rainy season, suggesting a potential role of this species in stormwater regulation related with runoff reduction. S. pungens performed well in summer but presented poor esthetics during winter.

Mesophyll conductance to CO2 and Rubisco as targets for improving intrinsic water use efficiency in C3 plants.

Water limitation is a major global constraint for plant productivity that is likely to be exacerbated by climate change. Hence, improving plant water use efficiency (WUE) has become a major goal for the near future. At the leaf level, WUE is the ratio between photosynthesis and transpiration. Maintaining high photosynthesis under water stress, while improving WUE requires either increasing mesophyll conductance (gm ) and/or improving the biochemical capacity for CO2 assimilation-in which Rubisco properties play a key role, especially in C3 plants at current atmospheric CO2 . The goals of the present analysis are: (1) to summarize the evidence that improving gm and/or Rubisco can result in increased WUE; (2) to review the degree of success of early attempts to genetically manipulate gm or Rubisco; (3) to analyse how gm , gsw and the Rubisco's maximum velocity (Vcmax ) co-vary across different plant species in well-watered and drought-stressed conditions; (4) to examine how these variations cause differences in WUE and what is the overall extent of variation in individual determinants of WUE; and finally, (5) to use simulation analysis to provide a theoretical framework for the possible control of WUE by gm and Rubisco catalytic constants vis-à-vis gsw under water limitations.

Mesophyll conductance to CO2 in Arabidopsis thaliana.

The close rosette growth form, short petioles and small leaves of Arabidopsis thaliana make measurements with commercial gas exchange cuvettes difficult. This difficulty can be overcome by growing A. thaliana plants in 'ice-cream cone-like' soil pots. This design permitted simultaneous gas exchange and chlorophyll fluorescence measurements from which the first estimates of mesophyll conductance to CO(2) (g(m)) in Arabidopsis were obtained and used to determine photosynthetic limitations during plant ageing from c. 30-45 d. Estimations of g(m) showed maximum values of 0.2 mol CO(2) m(-2) s(-1) bar(-1), lower than expected for a thin-leaved annual species. The parameterization of the response of net photosynthesis (A(N)) to chloroplast CO(2) concentrations (C(c)) yielded estimations of the maximum velocity of carboxylation (V(c,max_Cc)) which were also lower than those reported for other annual species. As A. thaliana plants aged from 30 to 45 d, there was a 40% decline of A(N) that was entirely the result of increased diffusional limitations to CO(2) transfer, with g(m) being the largest. The results suggest that in A. thaliana A(N) is limited by low g(m) and low capacity for carboxylation. Decreased g(m) is the main factor involved in early age-induced photosynthetic decline.

Analysis of leakage in IRGA's leaf chambers of open gas exchange systems: quantification and its effects in photosynthesis parameterization.

The measurement of the response of net photosynthesis to leaf internal CO2 (i.e. A-Ci curves) is widely used for ecophysiological studies. Most studies did not consider CO2 exchange between the chamber and the surrounding air, especially at the two extremes of A-Ci curves, where large CO2 gradients are created, leading to erroneous estimations of A and Ci. A quantitative analysis of CO2 leakage in the chamber of a portable open gas exchange system (Li-6400, LI-COR Inc., NE, USA) was performed. In an empty chamber, the measured CO2 leakage was similar to that calculated using the manufacturer's equations. However, in the presence of a photosynthetically inactive leaf, the magnitude of leakage was substantially decreased, although still significant. These results, together with the analysis of the effects of chamber size, tightness, flow rate, and gasket material, suggest that the leakage is larger at the interface between the gaskets than through the gaskets. This differential leakage rate affects the parameterization by photosynthesis models. The magnitude of these errors was assessed in tobacco plants. The results showed that leakage results in a 10% overestimation of the leaf maximum capacity for carboxylation (Vc,max) and a 40% overestimation of day respiration (Rl). Using the manufacturer's equations resulted in larger, non-realistic corrections of the true values. The photosynthetic response to CO2 concentrations at the chloroplast (i.e. A-Cc curves) was significantly less affected by leakage than A-Ci curves. Therefore, photosynthetic parameterization can be improved by: (i) correcting A and Ci values for chamber leakage estimated using a photosynthetically inactive leaf; and (ii) using A-Cc instead of A-Ci curves.

Decreased Rubisco activity during water stress is not induced by decreased relative water content but related to conditions of low stomatal conductance and chloroplast CO2 concentration.

Rubisco activity decreases under water stress, for reasons as yet unclear. Here, the covariation of stomatal conductance (gs) and relative water content (RWC), often observed during water stress, was impaired to assess the separate effects of these factors on Rubisco activity. Three different treatments were applied to soybean (Glycine max) and tobacco (Nicotiana tabacum): leaf desiccation (LD), in which stomatal closure was accompanied by large decreases of RWC; water stress (WS), in which minor decreases of RWC were observed along with stomatal closure; and exogenous application of abscisic acid (ABA), which triggered stomatal closure without changing RWC. Decreased RWC did not induce decreased initial Rubisco activity, which was impaired only in soybean by 40% when the gs dropped below 50 mmol m(-2) s(-1), regardless of the treatment. The mechanism for decreased activity differed among treatments, owing to decreased activation in LD and to total activity and protein content in WS and ABA. Despite the occurrence of Rubisco regulation, CO2 availability in the chloroplast, not impairment of Rubisco activity, limits photosynthesis during WS.

Analysis of the virus-induced inhibition of photosynthesis in malmsey grapevines.

• Virus infections decrease photosynthesis in plants, but the mechanistic basis is poorly understood. This was analysed in Banyalbufar malmsey, a grapevine (Vitis vinifera) variety of Mallorca (Spain). • The aim of this study was to analyse the mechanisms by which virus infection affect photosynthesis. Gas exchange (limitation analysis), chlorophyll fluorescence and Rubisco activity were compared in potted virus-infected and virus-free potted plants, and in field-grown young lowly infected and older highly infected plants. • Virus infection resulted in decreased photosynthesis (c. 50%). Stomatal limitation was unaffected in virus-infected plants, demonstrating that stomatal closure was not causing photosynthesis decreases. Chlorophyll fluorescence and limitation analysis suggested that the inhibition of primary light reactions was only a minor effect of virus infection. By contrast, mesophyll conductance to CO2 and Rubisco activity substantially decreased in virus-infected plants, corresponding to increases in the limitations to photosynthesis imposed by mesophyll conductance and carboxylation. • It is concluded that decreases in carboxylation and, possibly, in mesophyll conductance are the primary mechanisms by which virus infection impairs photosynthesis in Banyalbufar malmsey.

Drought-inhibition of photosynthesis in C3 plants: stomatal and non-stomatal limitations revisited.

There is a long-standing controversy as to whether drought limits photosynthetic CO2 assimilation through stomatal closure or by metabolic impairment in C3 plants. Comparing results from different studies is difficult due to interspecific differences in the response of photosynthesis to leaf water potential and/or relative water content (RWC), the most commonly used parameters to assess the severity of drought. Therefore, we have used stomatal conductance (g) as a basis for comparison of metabolic processes in different studies. The logic is that, as there is a strong link between g and photosynthesis (perhaps co-regulation between them), so different relationships between RWC or water potential and photosynthetic rate and changes in metabolism in different species and studies may be 'normalized' by relating them to g. Re-analysing data from the literature using light-saturated g as a parameter indicative of water deficits in plants shows that there is good correspondence between the onset of drought-induced inhibition of different photosynthetic sub-processes and g. Contents of ribulose bisphosphate (RuBP) and adenosine triphosphate (ATP) decrease early in drought development, at still relatively high g (higher than 150 mmol H20 m(-2) s(-1)). This suggests that RuBP regeneration and ATP synthesis are impaired. Decreased photochemistry and Rubisco activity typically occur at lower g (<100 mmol H20 m(-2) s(-1)), whereas permanent photoinhibition is only occasional, occurring at very low g (<50 mmol H20 m(-2) s(-1)). Sub-stomatal CO2 concentration decreases as g becomes smaller, but increases again at small g. The analysis suggests that stomatal closure is the earliest response to drought and the dominant limitation to photosynthesis at mild to moderate drought. However, in parallel, progressive down-regulation or inhibition of metabolic processes leads to decreased RuBP content, which becomes the dominant limitation at severe drought, and thereby inhibits photosynthetic CO2 assimilation.

Regulation of photosynthesis of C3 plants in response to progressive drought: stomatal conductance as a reference parameter.

We review the photosynthetic responses to drought in field-grown grapevines and other species. As in other plant species, the relationship between photosynthesis and leaf water potential and/or relative water content in field-grown grapevines depends on conditions during plant growth and measurements. However, when light-saturated stomatal conductance was used as the reference parameter to reflect drought intensity, a common response pattern was observed that was much less dependent on the species and conditions. Many photosynthetic parameters (e.g. electron transport rate, carboxylation efficiency, intrinsic water-use efficiency, respiration rate in the light, etc.) were also more strongly correlated with stomatal conductance than with water status itself. Moreover, steady-state chlorophyll fluorescence also showed a high dependency on stomatal conductance. This is discussed in terms of an integrated down-regulation of the whole photosynthetic process by CO2 availability in the mesophyll. A study with six Mediterranean shrubs revealed that, in spite of some marked interspecific differences, all followed the same pattern of dependence of photosynthetic processes on stomatal conductance, and this pattern was quite similar to that of grapevines. Further analysis of the available literature suggests that the above-mentioned pattern is general for C3 plants. Even though the patterns described do not necessarily imply a cause and effect relationship, they can help our understanding of the apparent contradictions concerning stomatal vs. non-stomatal limitations to photosynthesis under drought. The significance of these findings for the improvement of water-use efficiency of crops is discussed.

Analysis of the relative increase in photosynthetic O(2) uptake when photosynthesis in grapevine leaves is inhibited following low night temperatures and/or water stress

We found similarities between the effects of low night temperatures (5 degrees C-10 degrees C) and slowly imposed water stress on photosynthesis in grapevine (Vitis vinifera L.) leaves. Exposure of plants growing outdoors to successive chilling nights caused light- and CO(2)-saturated photosynthetic O(2) evolution to decline to zero within 5 d. Plants recovered after four warm nights. These photosynthetic responses were confirmed in potted plants, even when roots were heated. The inhibitory effects of chilling were greater after a period of illumination, probably because transpiration induced higher water deficit. Stomatal closure only accounted for part of the inhibition of photosynthesis. Fluorescence measurements showed no evidence of photoinhibition, but nonphotochemical quenching increased in stressed plants. The most characteristic response to both stresses was an increase in the ratio of electron transport to net O(2) evolution, even at high external CO(2) concentrations. Oxygen isotope exchange revealed that this imbalance was due to increased O(2) uptake, which probably has two components: photorespiration and the Mehler reaction. Chilling- and drought-induced water stress enhanced both O(2) uptake processes, and both processes maintained relatively high rates of electron flow as CO(2) exchange approached zero in stressed leaves. Presumably, high electron transport associated with O(2) uptake processes also maintained a high DeltapH, thus affording photoprotection.

Leaf Photosynthesis and Respiration of High CO(2)-Grown Tobacco Plants Selected for Survival under CO(2) Compensation Point Conditions.

Four self-pollinated, doubled-haploid tobacco, (Nicotiana tabacum L.) lines (SP422, SP432, SP435, and SP451), selected as haploids by survival in a low CO(2) atmosphere, and the parental cv Wisconsin-38 were grown from seed in a growth room kept at high CO(2) levels (600-700 parts per million). The selected plants were much larger (especially SP422, SP432, and SP451) than Wisconsin-38 nine weeks after planting. The specific leaf dry weight and the carbon (but not nitrogen and sulfur) content per unit area were also higher in the selected plants. However, the chlorophyll, carotenoid, and alkaloid contents and the chlorophyll a/b ratio varied little. The net CO(2) assimilation rate per unit area measured in the growth room at high CO(2) was not higher in the selected plants. The CO(2) assimilation rate versus intercellular CO(2) curve and the CO(2) compensation point showed no substantial differences among the different lines, even though these plants were selected for survival under CO(2) compensation point conditions. Adult leaf respiration rates were similar when expressed per unit area but were lower in the selected lines when expressed per unit dry weight. Leaf respiration rates were negatively correlated with specific leaf dry weight and with the carbon content per unit area and were positively correlated with nitrogen and sulfur content of the dry matter. The alternative pathway was not involved in respiration in the dark in these leaves. The better carbon economy of tobacco lines selected for low CO(2) survival was not apparently related to an improvement of photosynthesis rate but could be related, at least partially, to a significantly reduced respiration (mainly cytochrome pathway) rate per unit carbon.

Selection of Nicotiana tabacum Haploids of High Photosynthetic Efficiency.

Anthers of Nicotiana tabacum cultivar Wisconsin 38 were treated with the mutagen ethyl methane sulfonate and cultured to obtain populations of haploid plants of high genetic variability. The selection of these populations by their photosynthetic efficiency was carried out in a hydroponic culture chamber with a CO(2) atmosphere concentration close to the compensation point. Plants that survived 45 days of treatment were grown in pots in a greenhouse and their performance was compared to a population of unselected haploids. The growth characteristics, net photosynthesis, and chlorophyll content were measured and the haploid character verified. Selected plants were propagated by in vitro culture of buds and then diploidized to obtain seeds. Growth and other characteristics of the plants obtained were compared with those of the parental variety (Wisconsin 38) in a field assay. Growth, dry weight, leaf area, and net photosynthesis of selected plants were higher than in control plants.

La sanidad en la República Dominicana

También publicado en la OSP.Publicación No. 190, Julio,1942

Public health in the Dominican Republic

Also Publisher in PSB.Publication No.177, June 1942