Nutrient variation induced by rodent disturbance in Haloxylon ammodendron as a target transfer strategy

Nutrients form a link between herbivores and plant. This study explored the physiological and ecological response mechanism of Haloxylon ammodendron population to rodent disturbance in Gurbantunggut Desert from the perspective of nutrient cycle. Through field investigation, we quantified rodent disturbance intensity (DI) to H. ammodendron and analyzed the ecological response mechanism of H. ammodendron population to rodent disturbance from the perspective of plant and soil nutrient cycling and changes. The results indicated that moderate rodent DI (number of effective burrows = 3–6) was the maximum limit that can be tolerated by H. ammodendron; the threshold for optimal H. ammodendron response to rodent disturbance was mild (number of burrows = 1–3). Meanwhile, the rodent disturbance caused significant nutrient enrichment (e.g., organic carbon, available phosphorus, and available potassium) in the deeper soil (at 20–40 and 40–60 cm depth) and significantly reduced the soil total salt content (p < .05). Furthermore, as the DI increased, the branches of H. ammodendron showed significantly increased soluble total sugar, crude fiber, and total nitrogen contents (p < .05) but significantly decreased crude fat and crude protein contents (p < .05); these results are related to the nutritional target transfer strategy evolved by H. ammodendron for long‐term resistance to rodent disturbance. The current study clarified the optimal disturbance model for mutually beneficial H. ammodendron–great gerbil relationship, on the basis of which the ecological response mechanism of H. ammodendron population to rodent disturbance in deserts was illustrated. The current study provides a scientific basis for the protection mechanisms of desert plants to rodent disturbance.     

塔里木沙漠公路防护林地表凋落物分解对施肥的响应

为探讨沙漠公路防护林地表凋落物的分解速率和养分释放动态对施肥的响应,采用凋落物分解袋法,对塔里木沙漠公路防护林地乔木状沙拐枣(Calligonum arborescens)同化枝、梭梭(Haloxylon ammodendron)同化枝和多枝柽柳(Tamarix ramosissima)枝凋落物在施肥处理下的分解及养分释放特征进行研究。结果表明:经过420d的分解,3种凋落物质量残留率在对照(不施肥)、施用氮肥、施用磷钾复合肥处理间存在显著性差异(P0.05)。乔木状沙拐枣同化枝、梭梭同化枝和多枝柽柳枝在对照处理下的质量残留率分别为56.95%、31.32%和50.24%。施肥处理下3种凋落物均呈现出梭梭同化枝分解速率最快,多枝柽柳枝次之,乔木状沙拐枣同化枝分解最慢。施用磷钾复合肥极显著提高了3种凋落物的分解速率(P0.01);施用氮肥则促进多枝柽柳枝的分解,抑制乔木状沙拐枣和梭梭同化枝的分解。凋落物分解过程中,对照组3种植物凋落物的C、N、P和K元素均呈现净释放状态;施肥后凋落物的N、P和K元素呈现出富集-释放的模式。凋落物初始P含量和C/N、C/P比值是分解初期的主导因素,初始K、木质素、纤维素含量和C/N、木质素/N比值是分解后期的主要控制因素。研究表明,施肥显著影响沙漠公路防护林地表凋落物的分解,增加防护林地表凋落物的养分归还量,延后养分释放的时间,改善塔里木沙漠公路防护林地的土壤肥力。凋落物初始C/N比值是预测塔里木沙漠凋落物分解的重要因素,且不同分解时期影响凋落物分解的初始化学组成有所差异。     

准噶尔盆地东南缘梭梭种子雨特征

为了研究梭梭种子散布规律,通过布设种子雨收集器结合室内实验分析,对准噶尔盆地东南缘梭梭种子雨特征进行了研究。研究显示:(1)梭梭种子雨的累积密度达到平均189粒/m2,其中有活力种子占约80%;(2)种子散布的高峰集中在11月初到11月15日时间段,其落种量占整个种子雨的65%。其后种子雨密度随时间逐渐减小;(3)整个种子雨过程,不同时期散落的种子雨质量存在差异,表现为不同时期散落种子的萌发率呈现出先增大后减小的趋势;(4)变异函数分析表明,梭梭种子雨在8.12 m的有效变程内,种子雨具有明显的空间格局,其由空间自相关和随机因素引起的空间异质性各占50.0%。准噶尔盆地东南缘梭梭种子雨密度大且质量较高,同时其时空分布异质性较高,这些特征均将影响梭梭种群的分布格局和种群更新。     

梭梭幼苗的存活与地上地下生长的关系

近几十年来,全球气候变化以及人类活动的加剧,导致古尔班通古特南缘荒漠地区的降水与地下水位发生显著的改变,这些改变必然导致荒漠植物用水策略的适应性变化。以古尔班通古特沙漠南缘原始沙漠中建群种梭梭的1年生幼苗为研究对象,对气象因子、土壤含水率、梭梭幼苗死亡率、地上地下高度(深度)、面积、生物量状况进行了全生长期的连续监测,以期探明1年生梭梭的生长、生存规律。结果表明:梭梭幼苗在土壤水分较为充沛的4—6月生长迅速,但随着土壤水分的消耗,7月地上地下部分都出现了不同程度的萎缩,在8月又恢复生长。在全年生长期中,幼苗死亡率基本呈现逐月下降的趋势。这说明在干旱来临前,梭梭幼苗快速生长以抢占更多资源;当遭遇干旱时,幼苗通过同化枝凋落有效的维持了根系供水与地上部分需水之间的平衡,保证了存活同化器官的光合能力,从而降低了死亡率。同时,幼苗以牺牲地上部分生长为代价将更多的光合产物转向根系,使得幼苗能够获得更多的水分保证其生长、生存。     

Photosystem II activity in the leaves and assimilative branches of Alhagi sparsifolia Shap. under brief elevated temperature

Leaves and assimilative branches are crucial to the life cycle of Alhagi sparsifolia Shap. that is one of the main shrubs in the southern fringe of the Taklamakan desert. They reduce photosynthesis at midday during summer and exhibit different degrees of adaptability to severe environments. We investigated the changes in the PSII activity of leaves and assimilative branches of under elevated temperature and whether they have the same response and level of tolerance to different temperatures, thus to understand the roles of leaves and assimilative branches of A. sparsifolia in global warming. In this study, the kinetics of induced chlorophyll a fluorescence under heat-stress-induced inhibition of photosystem II in leaves and assimilative branches of A. sparsifolia after 15 min of treatment was investigated. The results show that both the activity and density of reaction centers of leaves are higher than assimilative branch at 35–48 °C, and both in the leaves and assimilative branches were decreased above 45 °C, the assimilative branches adapted better to the severe environment in terms of light energy transfer, light usage efficiency, and electron transport at 52 °C. Assimilative branches had better adaptability to elevated temperature than leaves of A. sparsifolia suggesting that assimilative branches might be more adaptive to severe environment.     

地下水埋深和季节性干旱对古尔班通古特沙漠南缘梭梭生理和生长的影响

干旱区因降水稀少,地下水成为荒漠植被重要且稳定的水源。选取古尔班通古特沙漠南缘建群种植物梭梭（Haloxylon ammodendron）为研究对象,通过测量不同地下水埋深（3.45、9.08、10.47、13.27 m和15.91 m）下生长季前期和后期同化枝生理生化指标（黎明水势、正午水势、含水量、氯离子、钠离子、脯氨酸和非结构性碳水化合物）和生长与形态特征（生长速率和胡伯尔值）,旨在认识荒漠植物对地下水埋深增加和季节性干旱的响应特征和调节适应机制。结果表明：（1）梭梭应对地下水埋深变化的生理调节对策,是采取先降低后升高黎明前同化枝水势、降低新枝形成期同化枝生长速率、增大胡伯尔值和积累非结构性碳水化合物的策略;（2）梭梭应对生长季大气干旱的生理调节对策,是通过降低黎明前同化枝水势、维持较高胡伯尔值、积累钠离子和消耗淀粉抵御季节性干旱;（3）在大气干旱与地下水水文干旱交互作用下,梭梭是采取降低正午同化枝水势、维持较高的同化枝含水量和积累可溶性糖的生态策略。综上所述,梭梭在响应地下水水文干旱和季节性大气干旱的生理特征间存在差异。研究结果丰富了水文和大气干旱对梭梭生理和生长影响的认知,可以为基于地下水资源管理的干旱区荒漠植被保育提供参考。     

Role of desert annuals in nutrient flow in arid area of Northwestern China: a nutrient reservoir and provider

Previous studies have tested the “vernal dam” hypothesis of spring ephemeral herbs in hardwood forests. The desert annual is a component of the desert ecosystem that takes advantage of water resources and temperature conditions during the rainy season to rapidly complete its life cycle within several months. To understand the role desert annual/ephemeral plants play in nutrient flow, we studied vegetation cover, nitrogen content and litter production of annual plants and litter decomposition rate in plant communities dominated by four shrubs (Haloxlon ammodendron, Hedysarum scoparium, Calligonum mongolicum, and Nitraria tangutorum) and two dominant annuals (Agriophyllum squarrosum and Halogeton arachnoideus Moq) in Minqin, northwestern China. Results indicate that over half of the total vegetation cover was provided by annuals. Annuals also took up a large amount of nitrogen (0.46–3.78 g N m⁻²) along the oasis–desert ecotone. Litter production and nutrient content were higher in areas dominated by annual plants than in areas dominated by shrubs. Furthermore, the litter decomposition rate of the annuals was higher than that of the shrubs, except for the shrub H. ammodendron, although almost all of the litter’s carbon (C) and nitrogen (N) remained after 6 months of decomposition. Without the annuals, more nutrients and rainwater might be lost through leaching or dust transfer caused by the wind erosion. In addition, green twigs of the annuals are the food for some animals, we found some green twigs and litter from annuals left in front of gerbil and rabbit burrows, sometimes even blocking these burrows. Thus, desert summer annuals, like nutrient reservoirs and providers, take up nutrients during the rainy season, providing some animals and microbes with food, and finally release these nutrients after death. Bao-Ming Chen and Gen-Xuan Wang contributed equally to this work.     

Canopy and leaf gas exchange of <Emphasis Type="Italic">Haloxylon ammodendron</Emphasis> under different soil moisture regimes

In order to reveal the drought resistance and adaptation of the C₄ desert plant Haloxylon ammodendron under artificially controlled soil moisture regimes, representative plants were selected to measure canopy photosynthesis using canopy photosynthetic measurement system. The results showed that appropriate soil moisture significantly enhances the canopy and leaf photosynthetic capacity, and extremely high soil moisture is not conducive to the photosynthesis of H. ammodendron.     

不同梭梭种群同化枝的解剖结构特征及其与生态因子的关系分析

该研究于中国西北干旱区按自然降水梯度从东向西选择6个梭梭种群,取其当年生同化枝作为试验材料,采用石蜡切片法,应用光学显微技术测定同化枝9项解剖结构指标,分析不同生境下梭梭同化枝解剖结构的变异特点以及与生态因子的关系,为干旱区生态环境保护和梭梭资源的保护与利用提供理论依据。结果表明:(1)干旱荒漠环境的梭梭同化枝解剖结构具有连续的栅栏组织、花环结构、含晶细胞和贮水组织等结构特征,但其表皮细胞、角质层厚度、气孔位置和气下室与旱生植物结构有差异,主要表现为表皮细胞和角质层较薄,气孔半下陷且气下室不发达。(2)梭梭同化枝各种群间的9项解剖结构指标都存在显著差异,其中变异系数较大的是维管束个数/直径和角质层厚度,变异系数分别为22.78%和15.20%;相关分析表明:同化枝直径、栅栏细胞切向长、贮水组织厚度和维管柱直径均与经度、纬度、海拔、一月均温、七月均温、年均温、年降水量和年相对湿度等8个生态因子有不同程度的相关性,其中与经度、海拔、七月均温和年平均相对湿度显著相关。(3)6个梭梭种群通过聚类分析可分为两大类,分析结果与所观察到的生境类型、地理分布的划分基本一致;随着自然降水量自东向西减少,梭梭的抗旱性相应逐渐增强。     

ACC deaminase-containing Arthrobacter protophormiae induces NaCl stress tolerance through reduced ACC oxidase activity and ethylene production resulting in improved nodulation and mycorrhization in Pisum sativum

Induction of stress ethylene production in the plant system is one of the consequences of salt stress which apart from being toxic to the plant also inhibits mycorrhizal colonization and rhizobial nodulation by oxidative damage. Tolerance to salinity in pea plants was assessed by reducing stress ethylene levels through ACC deaminase-containing rhizobacteria Arthrobacter protophormiae (SA3) and promoting plant growth through improved colonization of beneficial microbes like Rhizobium leguminosarum (R) and Glomus mosseae (G). The experiment comprised of treatments with combinations of SA3, G, and R under varying levels of salinity. The drop in plant biomass associated with salinity stress was significantly lesser in SA3 treated plants compared to non-treated plants. The triple interaction of SA3 + G + R performed synergistically to induce protective mechanism against salt stress and showed a new perspective of plant-microorganism interaction. This tripartite collaboration increased plant weight by 53%, reduced proline content, lipid peroxidation and increased pigment content under 200 mM salt condition. We detected that decreased ACC oxidase (ACO) activity induced by SA3 and reduced ACC synthase (ACS) activity in AMF (an observation not reported earlier as per our knowledge) inoculated plants simultaneously reduced the ACC content by 60% (responsible for generation of stress ethylene) in SA3 + G + R treated plants as compared to uninoculated control plants under 200 mM salt treatment. The results indicated that ACC deaminase-containing SA3 brought a putative protection mechanism (decrease in ACC content) under salt stress, apart from alleviating ethylene-induced damage, by enhancing nodulation and AMF colonization in the plants resulting in improved nutrient uptake and plant growth.     

梭梭木虱发生规律及其影响因子

在新疆古尔班通古特沙漠中的梭梭、白梭梭上发现由木虱引起的叶苞状虫瘿,发生率达到90%以上,虫瘿内有4种木虱:异色胖木虱(Caillardia robusta Loginova)、梭梭胖木虱(C. azurea Loginova)、矮胖木虱(C. nana Loginova)和显赫胖木虱(C. notata Loginova),后两种为中国新记录种。木虱在梭梭上1a发生两个高峰,丘间低地发生最为严重,沙丘边缘与砾石荒漠发生量相差不大,在5月中旬和6月中旬达到高峰期;木虱在白梭梭上1a发生两个高峰,低丘比高丘发生严重,阳坡比阴坡发生严重,在6月中旬和7月底8月初达到高峰期。梭梭和白梭梭上树龄越大虫瘿发生越严重。地势、树势、方位及地势、坡面对木虱虫瘿的发生都存在显著性影响,但是它们互作对木虱虫瘿的影响却不大。通过研究古尔班通古特沙漠梭梭和白梭梭上木虱虫瘿的发生规律及其与周边环境的关系,为保护荒漠生态环境提供依据。     

Alternanthera bettzickiana (Regel) G. Nicholson,a potential halophytic ornamental plant: Growth and physiological adaptations

Exploration and cultivation of salt tolerant plants is a very effective strategy for utilization of salt affected soils. In this investigation, physiological traits that are conducive for salt tolerance of the ornamental plant Alternanthera bettzickiana, Amaranthaceae, were explored. A. bettzickiana was grown on soil substrate having six salinity levels (2.86, 10, 20, 30, 40 and 50 dS m⁻¹). It was observed that this plant can grow even at a salinity level of 40 dS m⁻¹. The survival rate of this plant was 75, 42 and 0% at salinity levels of 30, 40 and 50 dS m⁻¹, respectively. A. bettzickiana plants produced 30.3% less biomass than controls at the salinity level of 20 dS m⁻¹ and even less under still higher salt stress. Photosynthesis continued even at the salinity level of 40 dS m⁻¹, though its rate was reduced to 59% in plants exposed to such salinity relative to plants not affected by salinity. Total soluble proteins values in leaf and stem showed a gradual increase when plants were exposed to increasing salt stress. Plants growing at the high salinity level showed highest decrease in leaf nitrate reductase activity. A. bettzickiana plants accumulated less Na⁺ in shoot as compared to root when grown under salt stress. It can be characterized as a salt-tolerant glycophyte that could be used for greening of salt affected soils.     

Water relations, gas exchange, photochemical efficiency, and peroxidative stress of four plant species in the Heihe drainage basin of northern China

Haloxylon ammodendron, Calligonum mongolicum, Elaeagnus angustifolia, and Populus hosiensis had different adaptations to limited water availability, high temperature, and high irradiance. C. mongolicum used water more efficiently than did the other species. Because of low transpiration rate (E) and low water potential, H. ammodendron had low water loss suitable for desert conditions. Water use efficiency (WUE) was high in E. angustifolia, but high E and larger leaf area made this species more suitable for mesic habitats; consequently, this species is important in tree shelterbelts. P. hosiensis had low WUE, E, and photosynthesis rates, and therefore, does not prosper in arid areas without irrigation. High irradiances caused photoinhibition of the four plants. The decrease of photochemical efficiency was a possible non-stomata factor for the midday depression of C. mongolicum. However, the species exhibited different protective mechanisms against high irradiance under drought stress. H. ammodendron and C. mongolicum possessed a more effective antioxidant defence system than E. angustifolia. These three species showed different means of coping with oxidative stress. Hence an enzymatic balance is maintained in these plants under adverse stress conditions, and the concerted action of both enzymatic and non-enzymatic reactive oxygen species scavenging mechanisms is vital to survive adverse conditions.     

古尔班通古特沙漠西部梭梭种群退化原因的对比分析

古尔班通古特沙漠植被的建群种梭梭在沙漠的低平地和小沙丘(高度<5 m)区域发生大面积退化死亡,而在大沙丘(高度>10 m)生境中梭梭却长势良好。对此分析了梭梭林枯死植株的空间分布、活株和死株的年龄结构,研究了地下水埋深以及对比低平地、小沙丘和大沙丘3种生境土壤理化性质对梭梭生存的影响。发现梭梭死亡植株在观测的19个样点中的15个呈显著的集群分布,表明梭梭植株死亡并非是种内自疏。梭梭各年龄级植株均在死亡,说明死亡与植株年龄关系不大。退化区地下水埋深过浅或过深均对梭梭生长不利,表明地下水埋深对梭梭生长产生显著影响。低平地和小沙丘区土壤电导率高,土壤水分入渗速率慢,且0-90 cm深度范围梭梭侧根数和成活的梭梭幼苗幼株数较少,与梭梭正常生长的大沙丘区明显不同。综合分析认为梭梭退化主要是由自身的根系分布特性,以及地下水位、土壤盐分、土壤水分入渗变化等综合作用,实质是梭梭水分利用受限和种群更新不良引起的。     

Effects of short- and long-term salinity on leaf water relations, gas exchange, and growth in Ipomoea pes-caprae

Ipomoea pes-caprae is widespread in pantropical coastal areas along the beach. The aim of this study was to investigate the salinity tolerance level and physiological mechanisms that allow I. pes-caprae to endure abrupt increases in salinity under brief or prolonged exposure to salinity variations. Xylem sap osmolality (X_osm), leaf water relations, gas exchange, and number of produced and dead leaves were measured at short- (1-7 d) and long- (22-46 d) term after a sudden increase in soil salinity of 0, 85, 170, and 255 mM NaCl. In the short-term, X_osm was not affected by salinity, but in the long-term there was a significant increase in plants grown in presence of salt compared with control plants. After salt addition, the plants showed osmotic stress with temporal cell turgor loss. However, the water potential gradient for water uptake was re-established at 4, 7 and 22 d after salt addition, at 85, 170 and 255 mM NaCl, respectively. In the short-term I. pes-caprae was able to tolerate salinities of up to 255 mM NaCl without significant reduction in carbon assimilation or growth. With the duration of stress, leaf ion concentration continued to increase and reached toxic levels at high salinity with a progressive decrease in photosynthetic rate, reduced leaf formation and accelerated senescence. Then, if high levels of soil salts from tidal inundation occur for short periods, the survival of I. pes-caprae is possible, but prolonged exposure to salinity may induce metabolic damage and reduce drastically the plant growth.     

Predicting the suitable habitats of parasitic desert species based on a niche model with Haloxylon ammodendron and Cistanche deserticola as examples

Haloxylon ammodendron, an excellent tree species for sand fixation and afforestation in the desert areas of western China, is threatened by climate change and anthropogenic activities. The suitable habitat of this species is shrinking at a remarkable rate, although conservation measures have been implemented. Cistanche deserticola is an entirely parasitic herb that occurs in deserts, is a source of “desert ginseng” worldwide, and has extremely high medicinal value. Little is known about using niche models to simulate habitat suitability and evaluate important environmental variables related to parasitic species. In this study, we modeled the current suitable habitat of H. ammodendron and C. deserticola by MaxEnt based on occurrence record data of the distributions of these two species in China. We grouped H. ammodendron and C. deserticola into three groups according to the characteristics of parasitic species and modeled them with environmental factors. The results showed that bioclimate was the most important environmental parameter affecting the H. ammodendron and C. deserticola distribution. Precipitations, such as annual precipitation, precipitation seasonality, and precipitation in the driest quarter, were identified as the most critical parameters. The slope, diurnal temperature range, water vapor pressure, ground‐frost frequency, and solar radiation also substantially contributed to the distribution of the two species. The proportions of the most suitable areas for Groups 1, 2, and 3 were 1.2%, 1.3%, and 1.7%, respectively, in China. When combined with cultural geography, five hot spot conservation areas were determined within the distribution of H. ammodendron and C. deserticola. The comprehensive analysis indicated that by using MaxEnt to model the suitable habitat of parasitic species, we further improved the accuracy of the prediction and coupled the error of the distribution of a single species. This study provides a useful reference for the protection of H. ammodendron forests and the management of C. deserticola plantations.     

Morphological characteristics of annual Zostera marina shoots at various germination temperatures

The timing of the transition from seed, seedlings and development into flowering is paramount importance in annual-type Zostera marina, because flowering is the first step of sexual reproduction. A majority of plants use environmental cues to regulate the transition to their developmental stages because plants must flower synchronously for successful outcrossing and must complete their sexual reproduction under favorable external conditions. The morphological characteristics (seeds and lateral shoot production, branch number, and inflorescence length) of reproductive shoots of Z. marina L. were examined in outdoor mesocosms to better understand the reproductive strategies of annual populations. Seeds in the germination experiment were divided into two groups: those exposed to cold (7 °C; vernalized group) and those left untreated (25-21 °C; non-vernalized group). All 600 seeds (300 from each group) were cultured for 2 months at 7, 10, 15, 20, and 25 °C in an indoor incubator. In the vernalized group, the germination rates were almost significantly higher than in the non-vernalized group. However, germination rates were not significantly affected by germination temperature. In outdoor mesocosms, production of vegetative shoots was observed in plants germinated at 15 and 20 °C in the vernalized group and at 10, 15 and 20 °C in the non-vernalized group. The highest number of vegetative shoots produced (35) was observed in plants germinated at 20 °C in the vernalized group, whereas seeds of either group failed to produce vegetative shoots when germinated at a low temperature (7 °C).In the flowering phase, the number of branches per shoot in the vernalized group was significantly higher than in the non-vernalized group. The total number of spadices on the 1st branches of plants in the vernalized group (germination at 20 °C) was significantly lower than that in the non-vernalized group at the same germination temperature. The total number of spadices per reproductive shoot in the vernalized group (germination at 10 °C) was also higher than in the non-vernalized group. Thus, both low temperature (vernalization) and seed germination temperature have implications for the sexual and asexual propagation of annual Z. marina populations.     

Soil water utilization characteristics of Haloxylon ammodendron plantation with different age during summer

Haloxylon ammodendron is one of the main shrubs that were used in desertification control project in China. Large area (2700 km²) of H. ammodendron plantation, especially more than 10 years plantation, has degraded outside Minqin oasis, northwest China. It is hard for H. ammodendron plantation to utilize ground water deeper than 20 m and to use precipitation with only mean annual 116.2 mm and uneven distribution in growing season. Thus, soil water might be the main water source of H. ammodendron plantation. Moreover, following H. ammodendron grows up, more soil water will be needed. In this study, it is hypothesized that H. ammodendron plantation would utilize deeper soil water as its age increased. Water use characteristics of different ages of H. ammodendron plantation (2 years, 5 years, 10 years, 20 years and 30 years) were examined by stable oxygen isotope technology outside Minqin oasis in July 2009. The δ¹⁸O values of water in H. ammodendron xylem, six different depths of soil (20 cm, 30 cm, 50 cm, 100 cm, 150 cm and 200 cm), rain and ground water (replaced by well water) were compared to determine major water source of H. ammodendron plantation. Meanwhile, the density, height, length and width of individuals were measured, and soil water contents were examined at the six depths in these plantation. The results showed that in 5 years H. ammodendron plantation, soil water content in 50 cm was significantly lower than moving sand dune and other ages, and soil water content in 100–200 cm was lower than moving sand dune and 2 years H. ammodendron plantation. The main depth that H. ammodendron used soil water increased as the age increased; 2 years H. ammodendron mainly used 50–200 cm soil water; 5 years H. ammodendron used 100–200 cm soil water and started to use ground water; 10 years H. ammodendron used 150–200 cm soil water and ground water, plant density of H. ammodendron declined and soil water content recovered gradually; 20 years and 30 years H. ammodendron mainly used ground water. Therefore, excessive consumption to soil water of high plant density might be one of the reasons for the degradation of H. ammodendron plantation. The critical age of H. ammodendron plantation is 5 years outside Minqin oasis because it consumed excessive soil water. If the density of 5 years H. ammodendron plantation was not decreased by selective cutting at present, it would degrade as the stand age increased later. It is suggested that initial density of new H. ammodendron plantation outside Minqin oasis must be reduced to slow down its consumption of soil water in future, and then the stability of H. ammodendron plantation could be sustained for a longer time.     

Drought tolerance and antioxidant enzymatic activity in transgenic ‘Swingle’ citrumelo plants over-accumulating proline

In this study we investigated the effects of the high endogenous proline level on water relations, gas exchange and antioxidant enzymatic activity in leaves of transgenic ‘Swingle’ citrumelo rootstocks transformed with the P5CSF129A gene coding for the key-enzyme for proline synthesis, under water deficit. Leaf total water, osmotic and pressure potentials, stomatal conductance, photosynthetic rates and xylem sap flow were evaluated in non-transformed control and transgenic plants during water deficit treatment. Malondialdehyde (MDA) content, catalase (CAT; EC 1.11.1.6), superoxide dismutase (SOD; EC 1.15.1.1) and ascorbate peroxidase (APX; EC 1.11.1.11) activities were quantified in leaves collected based on their total water potential, representing the following conditions: irrigated (Ψ_w = −1.3 MPa), moderate stress (Ψ_w = −2.3 to −2.5 MPa), severe stress (Ψ_w = −3.8 to −3.9 MPa) and recovery (24 h after re-irrigation: Ψ_w = −1.3 to −1.9 MPa). Osmotic adjustment was observed in transgenic plants until 11 days after withholding water, while pressure potential in non-transformed controls was close to zero after nine days of water deprivation. This superior maintenance of turgor pressure in leaves of transgenic plants led to higher stomatal conductance, photosynthetic and transpiration rates when compared to non-transgenic plants. Drought caused a significant decrease in APX and SOD activities in control plants, followed by an increase after re-watering. On the other hand, CAT was more active in control than in transgenic plants under irrigated condition and both stress levels. Our results suggest that transgenic plants were able to cope with water deficit better than non-transformed controls since the high endogenous proline level acted not only by mediating osmotic adjustment, but also by contributing to gas exchange parameters and ameliorating deleterious effects of drought-induced oxidative stress.