Sap flow, gas exchange, and hydraulic conductance of young apricot trees growing under a shading net and different water supplies

The experiment was carried out in a research field near Murcia, Spain, over a 3-week period between September 26 and October 16, 2000. Sixteen trees were used in the experiment, eight of which were placed under a rectangular shading net, while the other eight were maintained in the open air. Trees were irrigated once per day and, after October 5th, water was witheld from eight trees (four shaded and four unprotected for 5 days). The leaf stomatal conductance and the photosynthesis rates were higher in the shaded trees than in the exposed plants, probably because the leaf water potential was lower in the unshaded plants. This higher leaf conductance partially compensated for the effect of low radiation on transpiration, and the reduction of daily sap flow registered in shaded trees was only around 10-20%. The net also affected trunk diameter changes, with the shaded trees showing lower values of maximum daily shrinkage. Soil water deficit and high radiation had a similar effect on plant water parameters, lowering leaf water potential, leaf stomatal conductance, and the photosynthesis rate. The effects of both conditions were accumulative and so the exposed water-stressed plants showed the lowest values of total hydraulic resistance and water use efficiency, while the shaded well-irrigated trees registered the highest values for both parameters. For this reason, we think that net shading could be extended to apricot culture in many areas in which irrigation water is scarce and insolation is high.     

Improved growth and water use efficiency of cherry saplings under reduced light intensity

Cherry (Prunus avium L.) saplings were grown under natural sunlight (controls) or moderate shading (up to 30%, depending on the incident light intensity and the hour of the day). Reduced light intensity increased the dry mass of each of the plant components studied. Consequently, the total dry mass of shaded plants was significantly greater than that of controls at the end of the growing season. However, the diurnal trend in the level of photosynthesis (per unit of leaf area) of shaded plants was similar to the controls in August, but lower in September. As the growing season proceeded, reduced photosynthetic rates, thinner mesophyll and larger specific leaf area in the shaded plants indicated that leaf development had adapted to shaded conditions throughout the growing season. It is suggested that increased growth of shaded plants was caused by a higher initial relative growth rate and a greater whole-plant photosynthesis. Shading consistently reduced transpiration over the season, therefore improving water use efficiency of shaded leaves. Our results suggest that a moderate reduction in light intensity can be a useful method for improving growth and saving water in hot and dry environments.     

Influence of light intensity,temperature and humidity on photosynthesis and transpiration ofSasa nipponica andArundinaria pygmaea

Photosynthesis and transpiration were simultaneously measured under different light intensity, temperature and humidity conditions inSasa nipponica andArundinaria pygmaea grown in exposed and shaded habitats. Both species showed a saturated light curve for photosynthetic rate. The saturation point was lower in shaded plants. The apparent quantum yields were larger inS. nipponica and in shaded plants, while the maximum photosynthesis was higher inA. pygmaea and exposed plants. The temperature response of photosynthesis showed an optimum curve in both species. The optinum temperatures were 20 C inS. nipponica and 25 C inA. pygmaea. The influence of humidity on photosynthesis was insignificant for both species. The responses of transpiration to light intensity and relative humidity showed a saturated curve and an optimal one, respectively. There was a significant relationship between transpiration and stomatal frequency, both of which were higher inS. nipponica, while water use efficiency was higher inA. pygmaea. These results suggest thatS. nipponica adapts itself better to shaded, low temperature and less water stress habitats as compared withA. pygmaea.     

Response of citrus trees to modified radiation regime in semi-arid conditions

Citrus trees are characterized by a large canopy and low hydraulicconductivity. In Israel's semi-arid summer climate this couldcause transpiration to exceed water uptake and cause temporaryexcessive water deficits. It was hypothesized that reductionof radiative load would reduce transpiration and thus reducedeficits. Net radiation of lemon trees in the hottest season was reducedby shading hedgerows with reflective nets for approximatelyone month in both 1994 and 1995. Stem sap flow and climate variableswere measured continuously. Daily courses of leaf conductanceand leaf water potentials were measured on selected days. Midday net radiation below the dense and sparse shade net treatmentswas 47% and 73% of that above the control trees. Midday ‘sunlit’leaf temperatures below the nets were reduced by 2.7 and 1.6C,respectively. The reduction in net radiation caused large changes in leafconductance. Average midday sunlit leaf conductance measuredin 1995 under the dense and sparse treatments and control were4.1, 2.9 and 1.8mm s^–1, respectively (significantly differentat P <0.01). Similar differences in sunlit leaf conductancewere found in 1994. Shade leaf conductance was not affectedby the treatments. Daily total and midday sap flow under the dense net were reducedby 6–7% and 10–11%, respectively. Sap flow underthe sparse net did not change significantly in 1994, but in1995 daily and midday sap flows were reduced by 6% and 7%, respectively.Midday leaf water potentials increased by 0.2 and 0.1 MPa underdense shade in 1994 and 1995, respectively. Under sparse shademidday leaf water potentials increased by 0.1 MPa in 1994, butdid not change significantly in 1995. A modified Penman-Monteith model evaluated transpiration ifleaf conductance were constant in the different radiation environments.At leaf conductance levels found in the unshaded trees, denseshade was estimated to cause a 25% reduction in transpiration,while leaf conductance values found in trees under the denseshade would lead to an increase in transpiration of more than35% in unshaded trees. The ability of the tree to maintain almost constant transpirationin different radiation environments and thus avoid water deficitby adjusting the conductance of sunlit leaves is discussed interms of environmental influences and significance to the plant'swater balance. Key words: Tree transpiration, stomatal closure, climate modification, citrus     

Does shading explain variation in morphophysiological traits of tropical epiphytic orchids grown in artificial conditions?

Light is one of the main factors of physical environment and it controls plant growth and development by interfering with photosynthesis, especially concerning CO₂ assimilation. Photosynthetic characteristics and growth of C3 epiphytic orchids Miltonia flavescens and Miltonia spectabilis var. moreliana were analyzed under four radiation regimens (25, 50 and 75?% of global radiation and full sunlight). Anatomical characterizations were performed on plants grown at 25?% shade. Artificial shading was obtained using different shading nylon nets. The highest values of light-saturated photosynthetic, dark respiration, net photosynthetic and leaf transpiration rates, stomatal conductance and intercellular to atmospheric CO₂ concentration ratio were observed at full sunlight and 25?% shade. Moreover, both species allocated greater amount of leaf dry weight in those treatments. On the other hand, it was observed a greater investment in pseudobulb biomass in more shaded conditions (50 and 75?%), corroborating with the highest values of intrinsic water-use efficiency observed in those treatments. It was found a significant effect of shading on leaf area and specific leaf area. The anatomical features reflected strategies to save water. The phenotypic plasticity and principal component analysis suggested that the physiological traits were more responsive to light levels than the morphological traits. The results indicate that those species appear to be adapted to high irradiances conditions and are capable of adjusting, via morphophysiological changes, to light availability.     

弱光下生长的葡萄叶片蒸腾速率和气孔结构的变化

 植物能够对生长环境产生生态适应性,这种适应性可从气孔导度、光合速率、水分利用效率等生态指标上反映出来。为了研究葡萄蒸腾特性对弱光环境的适应性变化,本试验以‘京玉’葡萄幼苗（Vitis vinefera cv. Jingyu）为试验材料,通过遮光处理（2个处理,分别遮光65%和85%）营造弱光环境,测定了在弱光环境下生长的葡萄叶片蒸腾速率、气孔导度、水分利用效率对光照强度的响应,同时用扫描电镜技术观察了气孔的发育。结果表明,弱光环境下生长的葡萄幼苗,叶片的水势较高,但水分利用效率较低,叶片蒸腾速率和气孔导度变化对光照强度的响应缓慢,而自然光下生长的葡萄叶片则反应较迅速。通过对气孔结构的研究发现,与自然光照环境下生长的植株相比,在弱光环境下生长的葡萄幼苗,叶片下表皮的气孔横轴变宽,大小气孔之间差异减少,气孔外突,表皮细胞变大甚至扭曲,角质层变薄。说明葡萄幼苗能够对弱光环境产生适应性变化,其蒸腾特性的变化与其气孔结构的变化相关,具有一致性。     

Stomatal response of engelmann spruce to humidity, light, and water stress

Stomatal response of Engelmann spruce (Picea engelmannii Engelm.) to environmental conditions was studied in the natural subalpine environment and under controlled laboratory conditions. Stomata of naturally occurring trees responded to the difference in absolute humidity from leaf to air. When foliage was exposed to full sunlight, stomatal conductance decreased as the absolute humidity difference increased. In the shade, where photosynthetically active radiation was 10% of that in full sunlight, stomatal closure at large absolute humidity differences was much more complete. No effect of soil or air temperatures on stomatal aperture was observed in the field, nor were differences among three contrasting sites detected. Under growth chamber conditions, stomata responded to photosynthetically active radiation, but conductances were influenced by leaf-to-air differences in absolute humidity. Leaf water potentials below - 15 bars resulted in lower conductances over a range of humidity and light conditions. Because net photosynthesis under shaded conditions in the natural environment must be very low, stomatal closure could result in considerable savings in water while having a minimum effect on net photosynthesis.     

Interspecific Transition Among Caragana microphylla,C. davazamcii and C. korshinskii Along Geographic Gradient. Ⅱ. Characteristics of Photosynthesis and Water Metabolism

The characteristics of photosynthesis and water metabolism of Caragana microphylla Lam., C. davazamcii Sancz. and C. korshinskii Kom. populations in different sites (117.6o-105.7o E, 44.6o-38.8o N) were studied. (1) From the east to the west, the responses of the three species to photosynthetically available radiation (PAR) in net photosynthesis rate increased, the relative humidity of the air which corresponded to the occurrence of maximum photosynthesis rate decreased, and the corresponding air temperature increased. Along the same gradient, the before-noon superiority of the photosynthesis became evident, and the photosynthesis rate and the light use efficiency (LUE ) increased, while the transpiration rate decreased, thus the water use efficiency (WUE ) increased notably, and the leaf water content decreased gradually. From the east to the west, the plants took a water-saving strategy step by step with higher photosynthesis rate and lower transpiration rate. These physiological changes in the plants were adaptable to the conditions of light, temperature and humidity in the habitat of the plants, and might be the biological foundation for the geographical transition among C. microphylla , C. davazamcii and C. korshinskii. (2) The adaptation of photosynthetic system of C. microphylla , C. davazamcii and C. korshinskii to PAR, air humidity and temperature exhibited the interspecific continuity, which was consistent with the environmental gradient. In different species and different sites, the diurnal changes of net photosynthesis rate, the daily cumulative value of net photosynthesis, the diurnal changes of transpiration rate, the daily cumulative value of transpiration, the water use efficiency and the diurnal changes of leaf water content varied with longitudinal descent (from the east to the west). The characteristics of photosynthesis and water metabolism indicated that the geographical transition among C. microphylla , C. davazamcii and C. korshinskii was in gradual change, and these three species formed a geographical cline.     

Local weak light induces the improvement of photosynthesis in adjacent illuminated leaves in maize seedlings

To copy with highly heterogeneous light environment, plants can regulate photosynthesis locally and systemically, thus, maximizing the photosynthesis of individual plants. Therefore, we speculated that local weak light may induce the improvement of photosynthesis in adjacent illuminated leaves in plants. In order to test this hypothesis, maize seedlings were partially shaded, and gas exchange, chlorophyll a fluorescence and biochemical analysis were carefully assessed. It was shown that local shading exacerbated the declines in the photosynthetic rates, chlorophyll contents, electron transport and carbon assimilation‐related enzyme activities in shaded leaves as plants growth progressed. While, the decreases of these parameters in adjacent illuminated leaves of shaded plants were considerably alleviated compared to the corresponding leaves of control plants. Obviously, the photosynthesis in adjacent illuminated leaves in shaded plants was improved by local shading, and the improvement in adjacent lower leaves was larger than that in adjacent upper ones. As growth progressed, local shading induced higher abscisic acid contents in shaded leaves, but it alleviated the increase in the abscisic acid contents in adjacent leaves in shaded plants. Moreover, the difference in sugar content between shaded leaves and adjacent illuminated ones was gradually increased. Consequently, local weak light suppressed the photosynthesis in shaded leaves, while it markedly improved the photosynthesis of adjacent illuminated ones. Sugar gradient between shaded leaves and adjacent illuminated ones might play a key role in photosynthetic regulation of adjacent illuminated leaves.     

Urban tree root systems and their survival near houses analyzed using ground penetrating radar and sap flow techniques

Root systems of two mature Field maple trees (Acer campestre L.) growing in both shaded and non-shaded sites, on clay soil in an urban environment, were analyzed by ground penetrating radar (GPR), light microscope and sap flow techniques. The ground surface above the root systems was covered by asphalt. However, a small piece of garden existed near the non-shaded tree, and root area of roots growing in this direction increased significantly, due to a presumed increase in available water and nutrients. However, no garden was present near the shaded tree, therefore roots remaining under the asphalt surface did not increase in area in any particular direction. Maximum rooting depth of shaded and exposed trees, as determined by GPR, was approximately 1.4 and 1.7 m, respectively. The trees utilized relatively large amounts of water for transpiration, i.e. 65–140 l per fine summer day and in average 10 m³ per growing season. However, transpiration expressed per root surface area (and/or whole root system enveloping area) was practically the same in both trees, i.e. 1 dm³ m^-2 d^-1 or almost 100 dm³ m^-2 per growing season. These figures represented about 50% of potential evapotranspiration when considering projected crown areas. Increased transpiration under long-term high evaporation demands may cause occasional local drying of soil around roots, associated with soil shrinking in clay, which can be followed by serious damage to buildings.