Changes in Root Hydraulic Conductivity During Wheat Evolution

A better understanding of the mechanisms of water uptake by plant roots should be vital for improving drought resistance and water use efficiency (WUE). In the present study, we have demonstrated correlations between root system hydraulic conductivity and root characteristics during evolution using six wheat evolution genotypes (solution culture) with different ploidy chromosome sets (Triticum boeoticum Bioss., T. monococcum L.: 2n = 2x = 14; T. dicoccides Koern., T. dicoccon (Schrank) Schuebl.: 2n = 4x = 28;T. vulgare Vill., T. aestivum L. cv. Xiaoyan No. 6: 2n = 6x = 42). The experimental results showed that significant correlations were found between root system hydraulic conductivity and root characteristics of the materials with the increase in ploidy chromosomes (2x→6x) during wheat evolution. Hydraulic conductivity of the wheat root system at the whole-plant level was increased with chromosome ploidy during evolution, which was positively correlated with hydraulic conductivity of single roots, whole plant biomass,root average diameter, and root growth (length, area), whereas the root/shoot ratio had an inverse correlation with the hydraulic conductivity of root system with increasing chromosome ploidy during wheat evolution. Therefore, it is concluded that that the water uptake ability of wheat roots was strengthened from wild to modern cultivated species during evolution, which will provide scientific evidence for genetic breeding to improve the WUE of wheat by genetic engineering.     

根径向水流导度测定技术的改进

根系径向流的水力学性质主要是根的径向水流导度,它取决于径向水流通道的状况。利用改进的现有原位的测定根系径向水流导度的蒸腾计技术,设计了一个简便的４室吸水测定装置,可一次性获得根本质部水势和根径向水流导度,缩短测定时间１０ｍｉｎ,确保测定精度。然后用改进的装置测定了生长在不同水分条件下冬小麦（Ｔｒｉｔｉｃｕｍ ａｅｓｔｉｖｕｍＬ．）根系的径向水流导度,结果显示根系的平均径向水流导度为４．６３＊１０＾     

Aerenchyma Formed Under Phosphorus Deficiency Contributes to the Reduced Root Hydraulic Conductivity in Maize Roots

Root hydraulic conductivity has been shown to decrease under phosphorus （P） deficiency. This study Investigated how the formation of aerenchyma is related to this change. Root anatomy, as well as root hydraulic conductivity was studied In maize （Zea mays L.） roots under different phosphorus nutrition conditions. Plant roots under P stress showed enhanced degradation of cortical cells and the aerenchyma formation was associated with their reduced root hydraulic conductivity, supporting our hypothesis that air spaces that form in the cortex of phosphorusstressed roots Impede the radial transport of water in a root cylinder. Further evidence came from the variation In aerenchyma formation due to genotypic differences. Five maize inbred lines with different porosity in their root cortex showed a significant negative correlation with their root hydraulic conductivity. Shoot relative water content was also found lower In P-deficient maize plants than that in P-sufficient ones when such treatment was prolonged enough, suggesting a limitation of water transport due to lowered root hydraulic conductivity of P-deficient plants.     

盐旱复合胁迫对小麦幼苗生长和水分吸收的影响

为明确盐害、干旱及盐旱复合胁迫对小麦幼苗生长和水分吸收的影响,从而为盐害和干旱胁迫下栽培调控提供理论依据。以2个抗旱性不同的小麦品种(扬麦16和耐旱型洛旱7号)为材料,采用水培试验,以NaCl和PEG模拟盐旱复合胁迫,研究了盐旱复合胁迫下小麦幼苗生长、根系形态、光合特性及水分吸收特性的变化。结果表明,盐、旱及复合胁迫下小麦幼苗的生物量、叶面积、总根长与根系表面积、叶绿素荧光和净光合速率均显著下降,但是复合胁迫处理的降幅却显著低于单一胁迫。盐旱复合胁迫下根系水导速率和根系伤流液强度显著大于单一胁迫,从而提高了小麦幼苗叶片水势和相对含水量。盐胁迫下小麦幼苗Na~+/K~+显著大于复合胁迫,但复合胁迫下ABA含量却显著小于单一的盐害和干旱胁迫。因此,盐旱复合胁迫可以通过增强根系水分吸收及降低根叶中ABA含量以维持较高光合能力,这是盐旱复合胁迫提高小麦适应性的重要原因。洛旱7号和扬麦16对盐及盐旱复合胁迫的响应基本一致,但在干旱胁迫下洛旱7号表现出明显的耐性。     

冬小麦幼苗根系适应土壤干旱的生理学变化

采用盆栽试验对冬小麦幼苗根系适应土壤干旱的生理学变化进行了初步研究。结果表明，随干旱胁迫的加剧，洛麦9133和济麦21幼苗根水势、根相对含水率和根系活力均降低，饱和亏、可溶性糖含量、脯氨酸含量、质膜透性以及SOD、POD活性均呈增加趋势。这说明，在干旱胁迫下，冬小麦幼苗根系通过降低水势、相对含水率和根系活力，增加渗透调节物质可溶性糖、脯氨酸含量和增强SOD、POD活性等生理上的变化以提高抗旱性，从而使冬小麦幼苗适应干旱逆境。     

EFFECTS OF ROOT PRUNING AT THE REGREENING STAGE ON DROUGHT TOLERANCE AND WATER USE EFFICIENCY OF WINTER WHEAT IN LATE GROWING STAGE

 通过盆栽试验研究了返青期根修剪对冬小麦(Triticum aestivum)后期耐旱性及水分利用效率的影响。在返青期设置了两个根修剪处理: 1)小剪根, 在植株一侧切去部分侧生根; 2)大剪根, 在主茎四周切去部分侧生根。不剪根者设为对照(CK)。研究结果显示, 两个根修剪处理均显著减少了小麦的根系, 但对根冠比没有显著影响。在花期, 两个根修剪处理的小麦旗叶的叶绿素荧光参数最大光化学效率(The maximum photochemical efficiency of PSⅡ, F_v/F_m)、 PSⅡ潜在活性 (PSⅡ potential activity, F_v/F_o)、实际光化学量子产量(Effective PSⅡ quantum yield, Φ_PSⅡ)、表观光合电子传递速率(Apparent rate of photosynthetic electron transport, ETR)、光化学淬灭系数( Coefficient of photochemical quenching, q_P)和非光化学淬灭系数(Coefficient of non-photochemical quenching, NPQ)值, 在停止供水7 d后, 均显著高于对照, 这表明根修剪小麦的耐旱性强于对照, 因此在干旱胁迫下有较高的光化学活性。小剪根处理在高水条件下对小麦产量无显著影响, 而在中度干旱条件下显著提高了小麦的产量, 因此, 小剪根处理显著提高了小麦的抗旱系数; 小剪根处理在高水分处理(土壤水分含量为田间持水量的85%)和中度干旱胁迫处理(土壤水分含量为田间持水量的55%)条件下, 均显著提高了小麦的水分利用效率。但大剪根处理由于严重影响了群体数量和产量, 水分利用效率和抗旱系数均没有提高。可见, 适当地减少根系有助于小麦的耐旱性和水分利用效率的提高。     

Effects of Phosphorus Nutrient on the Hydraulic Conductivity of Sorghum (Sorghum vulgare Pers.) Seedling Roots Under Water Deficiency

Hydroponic experiments were conducted in a growth chamber and changes in the hydraulic conductivity of sorghum (Sorghum vulgare Pers.) roots (Lpr) at the three-leaf stage were measured using the pressure chamber method. Water deficiency was imposed with polyethylene glycol (PEG) 6000 and the phosphorus (P) levels were controlled by complete Hoagland solution with and without P nutrient. The objective of this study was to investigate the effect of P nutrition on root Lpr under water deficiency. The results showed that the Lpr in P deficiency treatments decreased markedly, but the Lpr recovered to the same value as that of control when sufficient P was supplied for 4-24 h. Water deficiency decreased Lpr, but the hydraulic conductivity of the roots with sufficient P supply was still higher than that of plants without P supply. When resuming water supply, the Lpr of the water-deficient plants under P supply recovered faster than that of plants without P supply, which indicates that plants with sufficient P nutrient are more drought tolerant and have a greater ability to recover after drought. The treatment of HgCl2 indicated that P nutrient could regulate the Lpr by affecting the activity and the expression levels of aquaporins.     

Changes in Structure and Hydraulic Conductivity for Root Junctions of Desert Succulents as Soil Water Status Varies

Variations in hydraulic conductivity (L_P) and the underlying anatomical and morphological changes were investigated for main root-lateral root junctions of Agave deserti and Ferocactus acanthodes under wet, dry, and rewetted soil conditions. During 21 d of drying, L_P and radial conductivity (L_R) increased threefold to fivefold at junctions of both species. The increase in L_R was accompanied by the formation of an apoplastic pathway for radial water movement from the surface of the junction to the stele for A. deserti and by the rupture of periderm by emerging primordia of secondary lateral roots for F. acanthodes. During 7 d of rewetting, L_R decreased for junctions of A. deserti, as apoplastic water movement was not apparent, but L_R was unchanged for F. acanthodes. Axial conductance (K_h) decreased during drying for both species, largely because of embolism related to the degradation of unlignified cell wall areas in tracheary elements at the root junction. The resulting apertures in the cell walls of such elements would admit air bubbles at pressure differences of only 0.12-0.19 MPa. Rewetting restored K_h for both species, but not completely, due to blockage of xylem elements by tyloses. About 40% of the primary lateral roots of the monocotyledon A. deserti abscised during 21 d of drying. For the dicotyledon F. acanthodes, which can form new conduits in its secondary xylem, only 10% of the primary lateral roots abscised during 21 d of drying, consistent with the much greater frequency of lateral roots that persist during drought in the field compared with the case for the sympatric A. deserti.     

干旱胁迫下紫花苜蓿根系形态变化及与水分利用的关系

采用盆栽实验方法研究了紫花苜蓿(品种:陇东和阿尔冈金)根系形态、生物量、蒸腾耗水量等对持续干旱的反应及与水分利用效率(WUE)间的关系,以期揭示紫花苜蓿对干旱胁迫的适应机制。结果表明:干旱胁迫使得紫花苜蓿根系形态特征在年季间、茬次间和品种间发生了显著变化,主要表现为主根伸长生长受到抑制、主根直径变细、侧根和根系总长度伸长生长则被促进、根系表面积和直径≥1mm的侧根数目显著增加、根系生物量下降,这是紫花苜蓿对干旱逆境的适应策略,但这种适应性存在限度。另一方面,干旱胁迫条件下紫花苜蓿草产量和蒸腾耗水量也因生长年限、茬次和品种的不同而呈现不同程度的降低。紫花苜蓿根系形态性状(总根长、根系生物量与根冠比)与植株水分利用效率间具有显著的相关性,其中根重对水分效率的影响是第一位的。WUE在根系形态与冠层水分消耗的协同变化下得到有限提高。对干旱的耐性最终表现为第2年第1年、第1茬和第2茬第3茬、陇东阿尔冈金。     

小麦旗叶发育过程中光合效率的变化

作物的产量基本上取决于光合机构的大小和效率（Ｇａｒｄｎｅｒ等１９８５）。要不断提高作物的单位面积产量,除了尽可能满足水、肥供应以获得足够大的光合机构从而接受足够多的光能外,还必须提高叶片的光合效率。这是增加作物产量的一条必由之路。因此,很有必要深入开展光合效率调节控制机理的研究。关于叶片生长发育期间光合速率与叶片结构的变化,已经有大量的研究报告,并且有全面的综合评述（ˇＣａｔｓｋｙ和ˇＳｅｓｔàｋ１９９７）。然而,关于叶片生长发育期间光合效率特别是荧光参数如何变化的报告却很少。虽然ˇＣａｔｓｋｙ…     

镉胁迫下小麦根系分泌物的变化

通过水培和砂培两种方法 ,研究了镉胁迫下小麦 ( Triticum aestivm L.)根系分泌物变化。通过研究镉对小麦根系分泌氨基酸和糖 ,根系分泌其它一些次生代谢物质等的影响 ,以及根系分泌物对蔬菜种子萌发的影响 ,探讨镉胁迫下植物根系的生理生态效应。研究结果表明 :镉胁迫下 ,小麦根系分泌物无论在量上还是质上都有变化。镉胁迫下根系分泌的电解质、糖类、氨基酸以及其它一些次生代谢物质都有所变化 ,但变化情况有差异。电解质外渗率、糖类随 Cd2 + 浓度升高而增加 ,在低浓度 Cd2 + 作用下 ( 0 .5 mg/ L) ,随处理浓度的升高 ,氨基酸分泌量增加 ;当处理浓度高于相应浓度时 ,氨基酸分泌量随浓度升高而减少。随 Cd2 + 浓度升高 ,次生代谢物分泌种类减少。这说明小麦通过改变根分泌作用而缓解镉危害。由于根系分泌次生代谢物的活动比较复杂 ,因而根系的分泌作用有待于进一步研究。根系分泌物中可能含有某些物质抑制萝卜、白菜种子萌发 ,这是涉及到植物他感作用的问题 ,值得并有待于进一步研究。初步分析小麦根系在不同的环境中对镉胁迫的响应方式 ,为以后根分泌物的深入研究提供了一定的思路 ,也为镉污染区农作物的合理栽培提供新的参考资料。     

Effects of Phosphorus Nutrient on the Hydraulic Conductivity of Sorghum （Sorghum vulgare Pers.） Seedling Roots Under Water Deficiency

Hydroponic experiments were conducted in a growth chamber and changes in the hydraulic conductivity of sorghum (Sorghum vulgare Pers.) roots (Lpr) at the three-leaf stage were measured using the pressure chamber method. Water deficiency was imposed with polyethylene glycol (PEG) 6000 and the phosphorus (P) levels were controlled by complete Hoagland solution with and without P nutrient. The objective of this study was to investigate the effect of P nutrition on root Lpr under water deficiency. The results showed that the Lpr in P deficiency treatments decreased markedly, but the Lpr recovered to the same value as that of control when sufficient P was supplied for 4-24 h. Water deficiency decreased Lpr, but the hydraulic conductivity of the roots with sufficient P supply was still higher than that of plants without P supply. When resuming water supply, the Lpr of the water-deficient plants under P supply recovered faster than that of plants without P supply, which indicates that plants with sufficient P nutrient are more drought tolerant and have a greater ability to recover after drought. The treatment of HgCl2 indicated that P nutrient could regulate the Lpr by affecting the activity and the expression levels of aquaporins.     

Spatio-temporal Variations in Axial Conductance of Primary and First-order Lateral Roots of a Maize Crop as Predicted by a Model of the Hydraulic Architecture of Root Systems

Rates at which water can be transported along plant roots (axial pathway) vary through time, in part depending on xylem maturation. Because of experimental constraints, the dynamics of root functional heterogeneity under field conditions remains mostly uncharted territory. Recent advances in mechanistic modelling offer opportunities to bypass such experimental limitations. This paper examines the dynamics of local variations in axial conductance of primary and first-order lateral roots of a maize crop using the architecture-based modelling approach developed by Doussan et al. (Annals of Botany: 81, 213–223, 1998). Specifically, we hypothesised that points of major resistance to long distance water transfers could arise from discrepancies between the hydraulic maturity (or water carrying capacity) of main axes and branch roots. To test this assumption, spatial distributions of root axial conductance were tested after 30, 60 and 100 days at soil depths of 10, 50 and 100 cm under a maize (Zea mays L.) crop sown at a density of 8 plants m⁻². As the crop developed, the corresponding root populations encompassed ever increasing amounts of hydraulically mature first-order laterals (branch roots): after a 100-day growth period, the vast majority of laterals had reached their maximum axial conductance at all soil depths down to 100 cm. In contrast, the axial conductance of a large proportion of main axes (primary roots) remained low, even at shallow soil depths and after 100 days of growth. The imbalance between the hydraulic maturity of primary and lateral roots was most conspicuous at soil depths of 100 cm, where ~10% only of the former compared to ~80% of the latter, had reached their maximum axial conductance after a 100-day growth period.     

Contribution of Different Mechanisms to Zinc Efficiency in Bread Wheat During Early Vegetative Stage

Zinc (Zn) has a vast number of functions in plant metabolism and consequently Zn deficiency has a range of effects on plant growth. There are a number of different possible mechanisms by which plants tolerate Zn deficiency (generally expressed as Zn efficiency), such as Zn uptake, translocation to the shoot and physiological efficiency. However, there have been no direct comparisons of the relative importance of these possible mechanisms of Zn efficiency in a large set of genotypes of contrasting Zn efficiency. Soil and solution culture studies were conducted to examine the relative contribution of different mechanisms of Zn efficiency at the whole plant level in bread and durum wheat during early vegetative stage. Zn treatments were 0, 0.05, 0.1 and 1 mg/kg soil in the soil culture, and nil in the solution culture. Visual symptoms of Zn deficiency, dry matter production, Zn uptake, Zn distribution between roots and shoots, Zn utilization in roots and shoots and Zn remobilisation from the seed into growing parts were examined. Significant genotypic differences were observed in most criteria and responses differed with external Zn supply. The results of the present study suggest that while there are a number of different mechanisms contributing to Zn efficiency, uptake is the major mechanism and the effect of this is modified by the physiological efficiency within the shoot. Root:shoot partitioning was not strongly associated with Zn efficiency and seed Zn remobilisation was not linked to Zn efficiency. Visual symptoms of the severity of Zn deficiency was a good predictor of Zn efficiency and was correlated with Zn uptake.     

Hydrogen Peroxide Appears to Mediate a Decrease in Hydraulic Conductivity in Wheat Roots under Salt Stress

Changes in the root diameter were measured in wheat, Triticum aestivum L., using a sensor of small displacements in osmotic experiments, and the hydraulic conductivity (L _p) of the root was calculated by the method of initial flows. The L _p decreased by two to three times during 30-min exposure in 0.1–9 mM H₂O₂. In 150 mM NaCl, changes in L _p exhibited two-phase kinetics: an initial increase in L _p by two and a half to three times, followed by a decrease to 1.6–50% of the control level within the subsequent 24 h. After one-day-long exposure of the roots in a solution containing 150 mM NaCl and 100 mg/l catalase, L _p did not differ from the control level. Root pretreatment with salicylate, which promotes the formation of H₂O₂, prevented the initial increase in L _p in 150 mM NaCl, probably, due to a drastic L _p decrease. The activity of guaiacol peroxidase in the roots increased by three times within the 20-min exposure of the roots to the salt solution. The data suggest that hydrogen peroxide can mediate the L _p decrease during the first day of salinity stress.     

Root hydraulic conductivity of Larrea tridentata and Helianthus annuus under elevated CO2

While investigations into shoot responses to elevated atmospheric CO₂ are extensive, few studies have focused on how an elevated atmospheric CO₂ environment might impact root functions such as water uptake and transport. Knowledge of functional root responses may be particularly important in ecosystems where water is limiting if predictions about global climate change are true. In this study we investigated the effect of elevated CO₂ on the root hydraulic conductivity (L_p) of a C₃ perennial, Larrea tridentata, and a C₃ annual, Helianthus annuus. The plants were grown in a glasshouse under ambient (360 μmol mol^–1) and elevated (700 μmol mol^–1) CO₂. The L_p through intact root systems was measured using a hydrostatic pressure-induced flow system. Leaf gas exchange was also determined for both species and leaf water potential (ψ_leaf) was determined in L. tridentata. The L_p of L. tridentata roots was unchanged by an elevated CO₂ growth environment. Stomatal conductance (g_s) and transpiration (E) decreased and photosynthetic rate (A_net) and Ψ_leaf increased in L. tridentata. There were no changes in biomass, leaf area, stem diameter or root : shoot (R : S) ratio for L. tridentata. In H. annuus, elevated CO₂ induced a nearly two-fold decrease in root L_p. There was no effect of growth under elevated CO₂ on A_net, g_s, E, above- and below-ground dry mass, R : S ratio, leaf area, root length or stem diameter in this species. The results demonstrate that rising atmospheric CO₂ can impact water uptake and transport in roots in a species-specific manner. Possible mechanisms for the observed decrease in root L_p in H. annuus under elevated CO₂ are currently under investigation and may relate to either axial or radial components of root L_p.     

不同年代推出的冬小麦品种叶片气体交换日变化的差异

选择60年来北京地区广泛种植的3个冬小麦( Triticum aestivum L.)品种,在相同的环境条件下种植.为了研究它们的产量与单位叶面积的净光合速率( Pn ) 的关系,测定了不同生育期 Pn 、蒸腾速率( Tr )的日变化,并用 Pn / Tr 计算叶片瞬时的水分利用率( WUE ).结果表明:单位叶面积净光合速率与产量之间的关系随生育期不同而变化.在拔节期高产品种"京冬8号"(九十年代推出)的光合速率和蒸腾速率在一天中总是最高,一天中差异最大时,分别比低产品种"燕大1817"(四十年代推出)高77%和69%.而其水分利用率却小于低产品种.这种差异随小麦的生长发育而变化,一般上午10:00前"京冬8号"的光合速率较高,而10:00后"燕大1817"的光合速率较高.到腊熟期,低产品种"燕大1817"的光合速率在一天中始终最高.蒸腾速率的变化规律与光合速率相似,然而"燕大1817"叶片的水分利用率一般最高.与现代推出的品种不同,老品种"燕大1817"叶片的光合作用午休现象不明显,说明它可能具有一定的抗光氧化性.我们认为,在品种改良的过程中,叶片光合作用的潜力可能有所提高,但它的抗光氧化性可能减弱.     

不同年代推出的冬小麦品种叶片气体交换日变化的差异

选择 6 0年来北京地区广泛种植的 3个冬小麦 (TriticumaestivumL .)品种 ,在相同的环境条件下种植。为了研究它们的产量与单位叶面积的净光合速率 (Pn)的关系 ,测定了不同生育期Pn、蒸腾速率 (Tr)的日变化 ,并用Pn/Tr计算叶片瞬时的水分利用率 (WUE)。结果表明 :单位叶面积净光合速率与产量之间的关系随生育期不同而变化。在拔节期高产品种“京冬 8号”(九十年代推出 )的光合速率和蒸腾速率在一天中总是最高 ,一天中差异最大时 ,分别比低产品种“燕大 1817”(四十年代推出 )高 77%和 6 9%。而其水分利用率却小于低产品种。这种差异随小麦的生长发育而变化 ,一般上午 10 :0 0前“京冬 8号”的光合速率较高 ,而 10 :0 0后“燕大 1817”的光合速率较高。到腊熟期 ,低产品种“燕大 1817”的光合速率在一天中始终最高。蒸腾速率的变化规律与光合速率相似 ,然而“燕大1817”叶片的水分利用率一般最高。与现代推出的品种不同 ,老品种“燕大 1817”叶片的光合作用午休现象不明显 ,说明它可能具有一定的抗光氧化性。我们认为 ,在品种改良的过程中 ,叶片光合作用的潜力可能有所提高 ,但它的抗光氧化性可能减弱。     

Root clumping may affect the root water potential and the resistance to soil-root water transport

We have appraised for clumped root systems the widely-accepted view that the resistance to water flux from soil to roots (‘soil resistance’) is low under most field conditions, so that root water potential would closely follow the mean soil water potential. Three root spatial arrangements were studied, simulating either the regular pattern generally assumed in models, or two degrees of root clumping frequently observed in the field. We used a numerical 2-dimensional model of water transfer which assumes a control of evapotranspiration by root signalling. Calculations were carried out at two evaporative demands and for two contrasting soil hydraulic properties. The rate of soil depletion, the timing of the reduction in evapotranspiration and the difference between root water potential and mean soil water potential were all affected by the root spatial arrangement, with a greater effect at high evaporative demand and low soil hydraulic conductivity. Almost all the soil water reserve was available to plants without reduction in evapotranspiration in the regular case, while only a part of it was available in clumped cases. In the regular case, calculated ‘soil resistances’ were similar to those calculated using Newman's (1969) method. Conversely they were higher by up to two orders of magnitude in clumped root spatial arrangements. These results place doubt on the generality of the view that ‘soil resistance’ is low under common field conditions. They are consistent with the results of field experiments, especially with recent data dealing with root-to-shoot communication.