比较了Avicenniagerminans对叶片气体交换和水关系的季节性影响,真正的红树林,和直立果果,一个红树林合伙人,在委内瑞拉北部的沿海地区。在Chichiriviche的CiénegaelOthtional上,无植被冲积沙平原向海一侧的泻湖周围最丰富。直立树是内陆社区的主要灌木,但是这两个物种共同出现在冲积平原陆地边缘的植被岛上。在CiénegaelOstional的植被岛上,与雨季相比,在旱季,德国A.(一种具有叶面盐腺的物种)的气体交换比直立人C.(一种缺乏盐腺的物种)的气体交换少。在干旱季节,接近饱和的光照强度和昼夜总CO2吸收的平均光合作用速率降低到69%和61%,分别,他们的价值观在下雨的德国,但是对于48%和30%,分别,直立人的雨季值。同样,在旱季,直立木的气孔导度和蒸腾失水比德国的减少更多,结果表明,与雨季相比,直立木在旱季的水分利用效率提高了3.4倍。土壤环境在确定植物气体交换模式中的重要性得到了这两种物种黎明木质部张力的大季节性变化的证明(从雨季的1.34MPa增加到旱季的5.50MPa。对于直立木,则为0.40至5.78MPa)。这些值反映了由于雨季淡水淹没上层土壤层以及旱季土壤蒸发导致盐浓度逐渐增加(几乎是海水中的两倍)而引起的土壤环境变化。在各个昼夜周期中,两种物种的木质部张力都发生了很大的变化,A.germinans达到最大2.36MPa。对于直立人,这些昼夜变化的幅度在旱季大大减少,与每年这个时候非常低的蒸腾速率一致。叶细胞的渗透压也倾向于高于C.直立人(在旱季,A.germinans的最大渗透压为8.3MPa)。与冲积平原上真正的红树林向海分布有关。而A.germinans的叶子没有表现出任何多汁的变化,直立树的叶片肉质随叶片的增加而增加,在旱季略高于雨季。多汁的叶子也具有较高的细胞汁液渗透压和NaCl浓度。观察到直立树最多汁的叶子是在海岸线上生长的裸露灌木。在旱季,这些海岸线植物显示出高的气体交换率和低值的黎明木质部张力(0.89兆帕),表明他们可以从浅水表中获得相对非盐水。在个别植物上,暴露的枝条比遮蔽的叶具有更多的肉质叶和更高的渗透压和NaCl浓度,证明了叶面吸收海雾中盐系对直立人的离子关系的重要性。因此,尽管直立树的分布集中在咸水区,该物种显然可以从永久进入浅层水位的栖息地延伸到季节性暴露于低土壤水势和高盐浓度的区域。
Seasonal effects on leaf gas exchange and water relations were compared for Avicennia germinans, a true mangrove, and Conocarpus erectus, a mangrove associate, at coastal sites in northern Venezuela. On the Ciénega el Ostional at Chichiriviche, A. germinans was most abundant around lagoons on the seaward side of the vegetation-free alluvial sand plain. C. erectus was the dominant shrub in inland communities, but the two species co-occurred on vegetation islands at the landward edge of the alluvial plain. On the vegetation islands of the Ciénega el Ostional, gas exchange in A. germinans (a species with foliar salt glands) was less severely curtailed in the dry season compared with the rainy season than was gas exchange in C. erectus (a species lacking salt glands). Average rates of photosynthesis at near-saturating light intensities and total diurnal CO2 uptake were reduced in the dry season to 69 and 61%, respectively, of their values in the rainy for A. germinans, but to 48 and 30%, respectively, of their rainy-season values for C. erectus. Similarly, stomatal conductance and transpirational water loss were more reduced in the dry season for C. erectus than for A. germinans, with the result that C. erectus showed a 3.4-fold increase in water-use efficiency in the dry season compared with the rainy season. The importance of the soil environment in determining plant gas-exchange Patterns was evidenced by large seasonal shifts in dawn xylem tension for the two species (which increased from 1.34 MPa in the rainy season to 5.50 MPa in the dry season for A. germinans, and from 0.40 to 5.78 MPa for C. erectus). These values reflected changes in the soil environment caused by inundation of the upper soil layers by fresh water in the rainy season and a progressive increase in salt concentrations (to almost twice those in sea water) by evaporation from the soil in the dry season. Large changes in xylem tension were observed for both species during individual day-night cycles, reaching a maximum of 2.36 MPa for A. germinans. For C. erectus, the magnitude of these day-night changes was greatly reduced in the dry season, consistent with its very low transpiration rates at this time of year. Leaf-cell osmotic pressures also tended to be higher in A. germinans than C. erectus (attaining a maximum of 8.3 MPa for A. germinans in the dry season), and were related to the more seaward distribution of the true mangrove on the alluvial plain. Whereas leaves of A. germinans did not show any changes in succulence, leaf succulence in C. erectus increased with leafage and was slightly higher in the dry season than the rainy season. The more succulent leaves also had higher cell-sap osmotic pressures and NaCl concentrations. The most succulent leaves of C. erectus were observed for exposed shrubs growing on the shoreline. During the dry season, these shoreline plants showed high rates of gas exchange and low values for dawn xylem tension (0.89 MPa), indicating that they had access to relatively non-saline water from the shallow water table. On individual plants, exposed shoots had more succulent leaves and higher osmotic pressure and NaCl concentrations than sheltered leaves, demonstrating the importance of foliar absorption of salt borne in sea spray for the ionic relations of C. erectus. Thus, although the distribution of C. erectus is centred on brackish-water zones, this species can apparently extend from habitats with permanent access to a shallow water table through to areas where it is seasonally exposed to low soil water potentials and high salt concentrations in the substratum.