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The relationship between leaf photosynthetic capacity (p n, max), net canopy CO2- and H2O-exchange rate (NCER and E t, respectively) and canopy dry-matter production was examined in Lollium perenne L. cv. Vigor in ambient (363±30 μl· l(-1)) and elevated (631±43 μl·l(-1)) CO2 concentrations. An open system for continuous and simultaneous regulation of atmospheric CO2 concentration and NCER and E t measurement was designed and used over an entire growth cycle to calculate a carbon and a water balance. While NCERmax of full-grown canopies was 49% higher at elevated CO2 level, stimulation of p n, max was only 46% (in spite of a 50% rise in one-sided stomatal resistance for water-vapour diffusion), clearly indicating the effect of a higher leaf-area index under high CO2 (approx. 10% in one growing period examined). A larger amount of CO2-deficient leaves resulted in higher canopy dark-respiration rates and higher canopy light compensation points. The structural component of the high-CO2 effect was therefore a disadvantage at low irradiance, but a far greater benefit at high irradiance. Higher canopy darkrespiration rates under elevated CO2 level and low irradiance during the growing period are the primary causes for the increase in dry-matter production (19%) being much lower than expected merely based on the NCERmax difference. While total water use was the same under high and low CO2 levels, water-use efficiency increased 25% on the canopy level and 87% on a leaf basis. In the course of canopy development, allocation towards the root system became greater, while stimulation of shoot dry-matter accumulation was inversely affected. Over an entire growing season the root/shoot production ratio was 22% higher under high CO2 concentration.

Effects of rising atmospheric CO2 concentrations on gas exchange, growth and productivity were investigated on an important grassland species, Trifolium repens L. cv. Blanca. Pure stands of this species were cultivated over an entire growing season in small acrylic greenhouses with an artificial atmosphere of ±367 or ±620 ppm CO2, respectively. Effects on growth and development were examined in a functional growth analysis, while consequences for gas exchange were determined by photosynthesis and transpiration measurements on canopy level. The stands were regularly clipped for production assessment. Canopies grown at high CO2 levels showed an average increase in productivity of almost 75%. Growth analysis indicated development of a larger foliage area as the major cause, particularly in the first days of regrowth after cutting. The growth advantage that began in this stage was maintained or bettered during the following weeks. The difference between gas exchange measurements expressed per unit leaf area and per unit ground area suggested that changes in net photosynthesis and respiration did not contribute to the increase in total yield. Transpiration declined under high CO2 if expressed on a leaf area basis but total canopy transpiration was at least as large as in ambient CO2 due to the larger leaf area. Water-use efficiency calculations on the summer data indicated a 35% improvement with a doubling of CO2 concentration.

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Pearl millet (Pennisetum glaucum (L.) R. Br.) is the most important summer cereal crop in the arid regions of Tunisia. A total of 283 accessions from 18 landraces were collected during the growing season of 1997 and evaluated in vivo at IRA, Benguerdane (during growing season 1998). Landrace (populations) grown by the farmers were shown to be mixtures of several types. Considerable variation was observed for all characters studied according to the descriptors developed by IBPGR and ICRISAT (1993). Time to flower varied from 30 to 66 days after sowing. Plant height ranged from 59 to 314 cm with a mean of 197.86 ± 0.43. Number of tillers varied between 0 and 19 per plant. Considerable variation in spike shapes was observed. Frequency distribution analysis of different spike shapes shows predominance of lanceolate (23.7%) followed by oblanceolate (23.5%) over cylindrical (18.3%), conical (15.7%), candle (8.7%), spindle (6.6%), club (3.1%) and globose (0.3%) types. The landraces collected mostly produced large, grey-coloured, obovate seeds, with partly corneous endosperm. Analysis of variance (ANOVA) was performed to evaluate variation amongst the different landraces. The differences are highly significant at (a = 0.05) for all characters studied. Duncan analysis and hierarchical clustering were carried out to (1) estimate resemblance between provenances; and (2) group landraces into homogeneous classes (clusters). The resulting dendrogram shows three principal clusters regrouping landraces according to their geographical origins or farming conditions. Pearl millet germplasm from Tunisia's arid region presents a wide range of genetic variability. It can be used as a starting basis for breeding programmes to select high yielding varieties tolerating adverse arid conditions.

Of fourteen steppic plant species, likely to be used in rehabilitation trials of degraded pastures in south Tunisia, germination capacities have been studied in different storage conditions : (1) ambient laboratory conditions without drying ; (2) ambient laboratory conditions after drying and hermetic sealing ; (3) at low temperatures (5°C) after drying and hermetic sealing. The seeds of Salsola vermiculata lose their germination capacity very quickly when stored in ambient laboratory conditions : a few months after seed dissemination. Seeds of one year old of Stipagrostis ciliata and Artemisia herba-alba still had some germination capacity. Of other species, seed viability even exceeded three years. Drying and hermetic sealing prolongs seed viability of all the studied species, as well in ambient laboratory conditions as at low temperatures. The advantage of storage at low temperatures is most evident for Salsola vermiculata, at least for the duration of this experiment. Taking into account the costs of seed conditioning, the results show that hermetic storage after drying is useful for most species studied and especially for Artemisia herba-alba and the four Poaceae studied. Storage at low temperatures should only be necessary for seeds of Salsola vermiculata or for the purpose of long term storage of plant genetic resources.

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