Germination and Photosynthetic Responses to Salinity and Alkalinity in Avicenna marina Propagules

Avicenna marina(Forssk.)Vierh is a halophytic mangrove.The reproductive unit is green and has photosynthetic propagules.Mangroves are naturally exposed to fluctuations in some abiotic factors at the soil surface,including salinity and alkalinity.The objective of this study was to determine the effects of two salts including NaCl and NaHCO_(3)on germination processes and discuss the relationships between cotyledon photosynthesis and embryo axis growth in A.marina propagules.These propagules came from Al Birk,located on the shoreline of the Saudi Red Sea.The results showed that the studied salts did not affect neither the final germination percentage nor the embryo axis growth.However,rooting and root growth were delayed by both salts at 300 mM and were strongly inhibited by 600 mM NaHCO_(3).Both NaCl and NaHCO_(3)reduced the photosynthetic activity.These two salts did not affect the other photosynthetic parameters,including stomatal conductance,net transpiration,and intercellular CO_(2).Thereafter,the reduction in net photosynthesis was not related to any limitation of stomatal conductance.The early germination phase was independent of cotyledon photosynthesis,whereas rooting and root growth may be limited by reduced photosynthesis under NaCl and NaHCO_(3).

Ecophysiological aspects and photosynthetic pathways in 105 plants species in saline and arid environments of Tunisia

In Tunisian arid regions, plant life forms, ecotypes, physiological types and photosynthetic pathways （03, 04 or CAM） remain unclear. Understanding the characters of these plant species could be important for land restoration. A literature survey was conducted for 105 plants species in arid regions of Tunisia. These plant species belong to several ecotypes： halophytes, xerophytes, gypsophytes, psamophytes, xero-halophytes, gypso-halophytes, psamo-halophytes, psamo-xerophytes, xero-gypsophytes and hygro-halophytes. The variation of photosynthetic pathway types in the 105 studied species shows that 56.2% were C3, 41.0% were C4, 1.9% were CAM and 1.0% were C3-CAM. The C3 pathway was more abundant in the halophytes, whereas the C4 one was more common in the xerophytes, gypsophytes, gyp- so-halophytes and psamo-halophytes. The ratio of C3 to C4 species （C3/C4 ratio） was 0.2 in the psamo-halophytes, 0.8 in the gypso-halophytes, 1.1 in the xerophytes, 1.6 in the xero-halophytes, 1.8 in the hygrohalophytes, 2.0 in the psamo- phytes and 3.8 in the halophytes. The annuals were mainly C3 plants whereas most of perennials were C4 ones. The C3/C4 ratio was 1.3 in succulent species and 1.4 in non-succulent species. Thus, succulence seems not to affect the distribution of C3 and C4 pathways within the studied plants. This investigation shows high percentages of C4 plants occurred in Tuni- sian arid regions. However, there were significant differences in their abundance among ecotypes. Basing on C3/C4 ratio, we can find that the abundance of the C4 pathway was in the following order： psamo-halophytes, gypso-halophytes, xe- rophytes, xero-halophytes, hygro-halophytes, psamophytes and halophytes. In Tunisian arid regions, C4 species were most abundant in xerophytes and less abundant in halophytes. In the Chenopodiaceae the number of C3 plants was 13 and the number of C4 species was 20 and in the Poaceae the number of C3 was 23 and the number of C4 was 19 species. Thus, the most C4 proportion was in the Chenopodiaceae and Poaceae species. This confirms the fact that the Cheno- podiaceae and the Poaceae were the leading families that tolerate salinity and aridity.