水分胁迫下春小麦根系吸水功能效率的研究

对水分胁迫下不同根系大小的春小麦籽粒产量与水分利用效率、相对生长速度和根系功能效率之间的关系进行了研究。实验分为3个水分处理和3个根系大小处理,3个水分处理分别是土壤含水量保持在田间持水量的80％-90％(H),50％-60％(M)和30％-40％(L)。根系大小的处理是(1)大根系处理(B),(2)中根系处理(M)和(3)小根系处理(S)。实验结果表明,在中度和重度干旱条件下小根系处理(MS和LS)的作物具有较高的WEUg,WUEdm、相对生长速度、根系功能效率和籽粒产量;在高水分处理中,上述参数的数值在大根系处理中相对较高。根呼吸耗C量在作物的整个生育期中都占有十分重要的地位,尽管根呼吸耗C比例随作物的生长而呈同步增加的趋势,但实际耗C量却呈逐步下降的趋势,同一时期作物的根呼吸速率与土壤含水量之间存在正相关关系,说明适当降低土壤含水量可以有效地减少根呼吸耗C量,有利于提高作物的存活率。在干旱半干旱地区,春小麦的根功能效率尚未达到最高值,作物产量仍有潜力可挖,通过适时补灌和减少同化物向根系分配的比例和根系对同化产物的消耗量来达到提高春小麦籽粒产量的方法是可行的,但不是长久之计。     

旱农区春小麦根系生长冗余与碳平衡关系的研究

利用McCree(1986)测定整株植物碳平衡的方法,研究了半干旱区3个春小麦品种的籽实产量、根量、根冠比与碳平衡的关系。结果表明在开花期地方品种和尚头较之现代品种有更大的根量和根冠比。同一品种不同水分条件下根和茎叶CO2交换率(CER)波动幅度与水分条件正相关;在相同水分条件下不同品种间,品种和尚头波动幅度最大,品种定西-24次之,品种陇春-8139最小。从根和茎叶呼吸及日净碳获得量随水分供给条件的变化来看,品种和尚头和品种定西-24与品种陇春-8139相比在资源获取器官上存在着生长冗余,而且这种冗余也表现在生理代谢上,随水分条件的改善而增加。说明大根系品种在根系上存在着冗余,减少根系冗余可望成为半干旱区小麦高产育种的一条有效途径。     

黄土旱塬冬小麦返青期断根对根冠比、水分利用及产量的影响

采用适当的农艺措施来影响根系生长以提高作物水分利用效率是节水农业研究的一项重要内容。通过田间试验研究了旱作冬小麦‘长武135’(Triticum aestivum cv. Changwu135)返青期切断部分侧生根对根冠比、水分利用效率及产量的影响。与不断根处理相比,冬小麦切断部分侧根后,极显著地减少了表层的根量,花期时断根和不断根小麦在0~20 cm土层根量分别249.70和307.52 g·m^-2,100 cm以上总根量分别为305.53和368.73 g·m^-2。断根比不断根处理根呼吸速率下降了25.57%。断根也抑制了小麦的群体数量,断根和不断根处理单位面积的穗数分别为590.33和646.33 m^-2,但断根显著增加了千粒重,断根和不断根分别为45.99和41.47 g,收获指数也有一定提高。断根对籽粒产量没有显著影响,但断根后土壤含水量显著增加,水分消耗减少。以生物量计算的水分利用效率和以产量计算的水分利用效率分别提高了32.52%和29.98%。因此,在旱地农业中,通过返青期人工断根措施削减根系降低根系对同化产物的消耗和减少耗水量来达到提高冬小麦水分利用效率的方法,是可行的。但今后还需对断根措施作进一步研究,以期实现产量和水分利用效率的同步提高。     

灌溉对干旱沙区紫花苜蓿生物学特性的影响

研究了干旱沙区不同水分处理下紫花苜蓿的 (Medicago sativa L.)生物学特性。结果表明 ,不同水分处理对紫花苜蓿植株高度、根系伸长生长和地上生物量形成的影响是不同的。在生长季内 ,灌水量最少的喷灌处理 W3植株高度最低 ,灌水量较多的漫灌处理 W1和灌水量居中的喷灌处理 W2植株高度相对较高 ,表明较多的灌水有利于植株高度的生长 ;对于根系长度则相反 ,灌水量最少的喷灌处理 W3根系伸长生长较快 ,到结实后期根系长度达到最大值 1 0 7.60 cm,说明适当的干旱可以促进紫花苜蓿根系伸长生长 ;地上生物量则是灌水量居中的喷灌处理 W2最高 ,表明在干旱沙区这种特殊的环境条件下 ,采取 W2这种灌溉方式种植紫花苜蓿 ,既可以获得较高的地上生物量 ,又可以节约利用水分     

单播与混播对两个冬小麦品种产量和水分利用效率的影响

选取黄土旱塬两个冬小麦品种长武135和平凉40,采用生态替代法连续两年在田间条件下研究了单播和混播方式对冬小麦产量和水分利用效率的影响.结果表明:单播条件下平凉40单位面积根量为367.60 g·m-2,显著大于长武135的297.31 g·m-2,且其根系在各土层中分布较均匀,表明与长武135相比,平凉40吸收土壤水分的能力较强,但其在单播条件下的产量和水分利用效率较低.混播增加了平凉40和长武135根系在较深土层中的根量分布,两品种根量分别比单播条件下多13.36和8.50 g·m-2,混播增强了小麦根系对深层土壤水分的吸收利用,提高了单位面积总产量,增加了群体水分利用效率.与平凉40相比,长武135分配了更多的干物质到繁殖器官,使其根量和地上营养器官的生物量减小,从而提高了籽粒产量和收获指数,表现出较高的水分利用效率.     

红砂幼苗根系形态特征和水分利用效率对土壤水分变化的响应

为探讨干旱与半干旱区受损红砂种群幼苗适宜生长的土壤水分条件,采用盆栽方法,研究了红砂幼苗在充分灌溉(FI)、适度灌溉(MI)、干旱处理(DT)3个水分处理下根系形态和水分利用效率的变化特征。结果表明:(1)红砂幼苗根系形态因水分条件和根序的不同而各异;随灌溉量的减少红砂幼苗根系直径和根体积均表现为FIMIDT,但干旱处理促进了根系的伸长生长和比表面积和比根长增加,根系形态的可塑性是红砂幼苗获取水分适应干旱环境的重要策略之一。(2)随根序的升高,各处理水平下红砂幼苗根长、比根长均显著减少,而其根直径和体积却显著增加,表明红砂幼苗根系内部具有高度的形态异质性。(3)与FI处理相比,MI和DT处理下红砂幼苗根系总生物量分别增加了50.00%、19.23%,但MI和DT处理却显著降低了红砂幼苗地上生物量,特别是叶片生物量下降幅度最大,分别降低了62.15%、83.28%,导致根冠比随灌溉量的减少而逐渐增加。(4)干旱处理显著提高了红砂幼苗的水分利用效率。研究认为,在灌溉量减少的情况下,红砂幼苗可通过根长、根系表面积和体积、直径等形态变化来优化其空间分布构型,以调节植株对水分的利用,提高水分利用效率。     

测墒补灌对不同穗型小麦品种耗水特性和干物质积累与分配的影响

在田间试验条件下, 以中穗型小麦(Triticum aestivum)品种‘山农15’和大穗型品种‘山农8355’为供试材料, 设置3个0-140 cm土层土壤相对含水量处理: W0 (拔节期65%, 开花期60%)、W1 (拔节期70%, 开花期70%)、W2 (拔节后8天70%, 开花后8天70%), 采用测墒补灌的方法补充土壤水分达到目标相对含水量, 对两个不同穗型小麦品种的耗水特性和干物质积累与分配进行了研究。结果表明: (1)两品种籽粒产量均以W0处理最低, ‘山农15’ W1和W2处理无显著差异, ‘山农8355’ W1处理显著高于W2处理; 两品种W1处理的水分利用效率和灌溉水利用效率均显著高于W2处理。‘山农15’ W1处理的籽粒产量和灌溉水利用效率分别显著低于和高于‘山农8355’的W1处理, 水分利用效率无显著差异; 两品种W2处理的籽粒产量、水分利用效率和灌溉水利用效率均无显著差异。(2)两品种总耗水量以W0处理最低, ‘山农15’ W1处理显著低于W2处理, ‘山农8355’两处理无显著差异; 两品种W1处理的土壤供水量及其占总耗水量的比例显著高于W2处理。‘山农15’ W1处理的总耗水量和灌水量占总耗水量的比例显著低于‘山农8355’, 土壤供水量占总耗水量的比例显著高于‘山农8355’; 两品种W2处理总耗水量, 土壤供水量及其占总耗水量的比例无显著差异。(3)两品种W1处理成熟期干物质积累量显著高于其他处理, W1处理提高了‘山农8355’开花后干物质积累量及其对籽粒的贡献率, 对‘山农15’无显著影响。‘山农15’ W1和W2处理成熟期干物质积累量显著低于‘山农8355’, 开花前贮藏同化物向籽粒的转运量和转运率、对籽粒的贡献率均显著高于‘山农8355’, 开花后干物质积累量及其对籽粒的贡献率低于‘山农8355’。综合考虑干物质积累与分配、籽粒产量、水分利用效率和灌溉水利用效率, W1处理是两品种节水高产的最佳土壤相对含水量处理。     

土壤水分对两个冬小麦品种产量和竞争能力的影响

研究了黄土塬区两个旱作冬小麦品种(长武135和平凉40,前者是后者的换代品种)在不同土壤水分条件下竞争能力和产量形成的关系。研究设2种土壤水分条件(土壤含水量为田间最大持水量的75%～80%和40%～45%),采用生态替代法设计了同一播种密度的6个播种比例组合的盆栽试验。低水分单播条件下,长武135具有较高的产量(长武135为27.59g/pot,平凉40为24.91g/pot),而混播条件下平凉40在产量和相对产量上较长武135品种具有明显的竞争优势。高水分条件下,平凉40产量随播种比例的减小下降较快(长武135产量曲线斜率35.468,平凉40为36.237)。平凉40低水分单播时花后干物质积累量较少(长武135为0.67g/pot,平凉40为0.55g/pot),而混播时较多(长武135各比例均值0.58g/pot,平凉40为0.71g/pot),导致了平凉40品种单播时产量较低和混播时产量较高。平凉40地下生物量显著大于长武135(高水分条件下,长武135为10.03g/pot、平凉40为11.51g/pot;低水分条件下二者分别为8.41g/pot和10.69g/pot),且耗水量大(高水分条件下平凉40多耗水2.72kg/pot,低水分条件下多耗0.98kg/pot),而耗水量/地下生物量的比值平凉40低于长武135,从而平凉40品种单位根量消耗的水分低于长武135。总之,在两个生长属性接近的冬小麦品种中,老品种以较大的根系生物量赢得了较高的竞争能力,消耗了较多的水分,而新品种虽然根系生物量较低,耗水量较少,但籽粒产量却较高。亦即,小麦新品种籽粒产量的提高是与根系的减少和对水资源竞争能力的下降相伴随的。     

底墒与磷肥互作对春小麦产量形成的影响

研究了浇底墒水和施磷肥对春小麦产量形成的影响.实验设4个处理对照(CK);耕作层施磷(P);浇30 nm底墒水(W);施磷加浇30mm底墒水(PW).测定土壤含水量、根系生物量、地上生物量和测产等.结果表明,WP处理根重和中下层分配比例均较高.与W处理相比,WP处理利用底墒并没有增加,但水分利用效率和产量均增加.决定产量的主要因素是小麦早期小穗和小花形成的数量和质量.W和WP两个浇底墒水的处理,其土壤水分利用量平均为34.6 mm,略高于所浇的底墒水的量(30 mm).CK、P、W、WP等4个处理的籽粒产量分别为91 7.7,1191.1,2516.5,2734.1 kg/hrm2,水分利用效率分别为0.186,0.192,0.172,0.207 g/(mm.m2).     

根修剪对黄土旱塬冬小麦(Triticum aestivum)根系分布、根系效率及产量形成的影响

通过田间试验研究了不同时期根修剪处理对冬小麦（Triticum aestivum）根系大小与分布、根系效率、水分利用效率及产量形成的影响。设置4个根修剪处理：越冬期小剪根（WS）、越冬期大剪根（WB）,返青期小剪根（GS）、返青期大剪根（GB）,未剪根小麦作为对照（CK）。结果表明,到花期时,各根修剪处理小麦的在0～120cm总根量均显著小于对照。与对照相比各根修剪处理主要是显著地减少了上层土壤中的根量。但WS和GS两小剪根处理和对照相比在中层土壤中有较大的根量;花后各处理小麦旗叶的气孔导度和蒸腾速率均显著大于对照。这说明根修剪处理减少了小麦表层的根量,从而削弱了表土干旱信号对作物与外界气体交换的抑制作用。花期时各根修剪小麦的净光合速率均显著高于对照,而单位面积上的根呼吸速率均显著小于对照,根修剪处理提高了小麦的根系效率,使更多的光合产物用于籽粒生产,从而提高了小麦的收获指数。根修剪还提高了小麦的水分利用效率,其中WS、WB、GS处理的水分利用效率显著高于对照。但是GB处理的水分利用效率却没有显著提高。因此,本研究进一步证明了由不同年代品种得到的推测,认为在旱地农业中,通过遗传育种或采用适当农艺措施优化根系分布,既可以减少生长前期作物对水分的过度消耗,又能够削弱花后表土过度干旱对作物生长抑制作用,同时降低根系对同化产物的消耗,对作物产量及水分利用效率的提高具有积极的作用。     

Response of root respiration and root exudation to alterations in root C supply and demand in wheat

Rising atmospheric CO₂ concentrations have highlighted the importance of being able to understand and predict C fluxes in plant-soil systems. We investigated the responses of the two fluxes contributing to below-ground efflux of plant root-dependent CO₂, root respiration and rhizomicrobial respiration of root exudates. Wheat (Triticum aestivum L., var. Consort) plants were grown in hydroponics at 20°C, pulse-labelled with ¹⁴CO₂ and subjected to two regimes of temperature and light (12 h photoperiod or darkness at either 15°C or 25°C), to alter plant C supply and demand. Root respiration was increased by temperature with a Q ₁₀ of 1.6. Root exudation was, in itself, unaltered by temperature, however, it was reduced when C supply to the roots was reduced and demand for C for respiration was increased by elevated temperature. The rate of exudation responded much more rapidly to the restriction of C input than did respiration and was approximately four times more sensitive to the decline in C supply than respiration. Although temporal responses of exudation and respiration were treatment dependent, at the end of the experimental period (2 days) the relative proportion of C lost by the two processes was conserved despite differences in the magnitude of total root C loss. Approximately 77% of total C and 67% of ¹⁴C lost from roots was accounted for by root respiration. The ratio of exudate specific activity to CO₂ specific activity converged to a common value for all treatments of 2, suggesting that exudates and respired CO₂were not composed of C of the same age. The results suggest that the contributions of root and rhizomicrobial respiration to root-dependent below-ground respiration are conserved and highlight the dangers in estimating short-term respiration and exudation only from measurements of labelled C. The differences in responses over time and in the age of C lost may ultimately prove useful in improving estimates of root and rhizomicrobial respiration.     

Characterization of Root Exudates at Different Growth Stages of Ten Rice (Oryza sativa L.) Cultivars

Abstract: Plant root exudates play important roles in the rhizosphere. We tested three media (nutrient solution, deionized water and CaSO₄ solution) for three periods of time (2, 4 and 6 h) for collecting root exudates of soil‐grown rice plants. Nutrient culture solution created complications in the analyses of exudates for total organic C (TOC) by the wet digestion method and of organic acids by HPLC due to the interference by its components. Deionized water excluded such interference in analytical analyses but affected the turgor of root cells; roots of four widely different rice cultivars excreted 20 to 60 % more TOC in deionized water than in 0.01 M CaSO₄. Furthermore, the proportion of carbohydrates in TOC was also enhanced. Calcium sulfate solution maintained the osmotic environment for root cells and did not interfere in analytical procedures. Collection for 2 h avoided under‐estimation of TOC and its components exuded by rice roots, which occurred during prolonged exposure. By placing plants in 0.01 M CaSO₄ for 2 h, root exudates of soil‐grown traditional, tall rice cultivars (Dular, B40 and Intan), high‐yielding dwarf cultivars (IR72, IR52, IR64 and PSBRc 20), new plant type cultivars (IR65598 and IR65600) and a hybrid (Magat) were collected at seedling, panicle initiation, flowering and maturity and characterized for TOC and organic acids. The exudation rates were, in general, lowest at seedling stage, increased until flowering but decreased at maturity. Among organic acids, malic acid showed the highest concentration followed by tartaric, succinic, citric and lactic acids. With advancing plant growth, exudation of organic acids substituted exudation of sugars. Root and shoot biomass were positively correlated with carbon exudation suggesting that it is driven by plant biomass. As root exudates provide substrates for methanogenesis in rice fields, large variations in root exudation by cultivars and at different growth stages could greatly influence CH₄ emissions. Therefore, the use of high‐yielding cultivars with lowest root excretions, for example IR65598 and IR65600, would mediate low exudate‐induced CH₄ production. The screening of exciting rice cultivars and breeding of new cultivars with low exudation rates could offer an important option for mitigation of CH₄ emission from rice agriculture to the atmosphere.     

Low-level herbivory by root-knot nematodes (Meloidogyne incognita) modifies root hair morphology and rhizodeposition in host plants (Hordeum vulgare)

Low amounts of root infestation by plant parasitic nematodes are suggested to increase nutrient supply and in turn enhance microbial activity and net mineralization rate in the rhizosphere. These effects are generally related to “leakage” of plant-derived metabolites from damaged roots. Besides leakage, the present study examines other nematode–host interactions such as alterations in root exudation and morphology, which were almost not considered yet. This includes undamaged root parts in order to assess systemic plant response. The root-knot nematode Meloidogyne incognita (Kofoid and White 1919; Chitwood 1949) and barley (Hordeum vulgare L. cv. Europa) was used as model system. Host plants were grown in mini-rhizotrons inoculated with 0, 2,000, 4,000 or 8,000 M. incognita for 4 weeks. Root morphology, rhizodeposition (sugars, carboxylates, amino acids), and rhizosphere microbial communities (PLFAs) were assessed. In treatments with 4,000 nematodes, shoot biomass, total N and P content increased by the end of the experiment. Generally, an enhanced release of plant metabolites (sugars, carboxylates, amino acids) from the apical root zone occurred 1 week after inoculation with 4,000 and 8,000 M. incognita, indicating root leakage. Low levels of root herbivory stimulated root hair elongation in both infected and uninfected roots. These systemic changes in root morphology likely contributed to the increased sugar exudation in uninfected roots in all nematode treatments at 3 weeks after inoculation. Root-knots formed a separate microhabitat within the root-system. They were characterised by decreased rhizodeposition and increased fungal to bacterial ratio in the adhering rhizosphere soil. The present study provides the first evidence that, apart from leakage, nematode root herbivory at background levels induces local and systemic effects on root morphology and exudation, which in turn may affect plant performance.     

模拟喀斯特异质性小生境下三叶鬼针草地上地下协同生长对策

模拟在喀斯特异质生境下,通过随机区组实验,研究三叶鬼针草（Bidens pilosa L.）在两种土壤生境（浅而宽、深而窄）和3种水分处理（对照、减水50%、减水70%）下植物的地上和地下生长关系及生物量分配格局。结果显示：（1）两种生境中三叶鬼针草的地上生长（株高、地径、叶面积、叶生物量）与地下根系生长（根长、根表面积、根体积、根生物量）均随着施水量的减少而降低;叶面积比率随着施水量的减少而增加;根质量比在浅而宽土壤生境中呈先增后减的趋势,而在深而窄土壤生境中呈增加趋势。（2）两种生境中三叶鬼针草的地上生物量与地下根系生物量、叶面积与根长、叶面积与各层根系生长均呈显著正相关关系。但在浅而宽土壤生境中,三叶鬼针草的地上生物量与各土层根系生物量均呈显著正相关,而在深而窄土壤生境中,地上生物量仅与中上土层根系生物量呈显著正相关。研究表明三叶鬼针草在不同生境中均具有较好的地上地下协同生长对策,在增强对地下资源获取的同时也增强了对地上资源的获取。在浅而宽土壤生境中,三叶鬼针草通过协调根系的横向拓展能力与植物叶片的生长来应对快速的干旱缺水;在深而窄土壤生境中,植株能较好地协调根系向下拓展能力与地上叶面积的生长,更好地利用土壤深层的水分资源。     

Effect of cadmium on growth and respiration of barley plants grown under two temperature regimes

We studied cadmium effect on the respiratory pathways ratio in the organs of barley (Hordeum distichum L., cv. Novichok) plants grown in water culture at two temperature regimes. Mineral nutrients were supplied daily in exponentially increasing amounts in order to provide for steady-state growth. CdSO₄ (30, 60, or 100 μmol/l) was added to nutrient solution at a single time in the beginning of the exponential growth period (19 days after germination). In further 6 days, the relative growth rate and biomass accumulation declined stronger with the increase in the cadmium concentration in plants grown at 13/8°C (day/night) than at 21/17°C (day/night). Cadmium suppressed root respiration (down to 60% of control) stronger than leaf respiration, and the roots manifested a higher sensitivity to the inhibitor of alternative oxidase, salicylhydroxamic acid. The respiratory pathways ratio in the roots occurred against the background of activated lipid peroxidation (POL). The highest POL activity and the highest proportion of alternative respiration pathway (AP) (up to 46% of total respiration) were observed in the roots in the presence of the highest cadmium concentration (100 μM) under lower temperature (13/8°C). Thus, high cadmium concentrations affected strongly the total rate of respiration and respiratory pathways ratio. Growth temperature modulated Cd effects on respiration. AP activation is one of the mechanisms for maintenance of root cell homeostasis under cadmium-induced stress.     

乌兰布和沙区紫花苜蓿根系生长及吸水规律的研究

研究了不同水分处理下,乌兰布和沙区紫花苜蓿系生长发育规律及根系吸水速度,结果表明：不同水分处理的根系生长规律最基本一致的,在生长季风均呈增加的趋势;但适度干旱可促进根系的伸长生长。在当地土壤类型条件下,根系主要分布在０－３０cm土层;根重密度在土壤剖面上的分布遵循对数规律,并随深度的增加呈降低趋势。运用一维土壤水分运动方程,计算得到了不同水分处理根系吸水速度在土壤剖面上的分布状况;根系吸水速率与土壤含水量和根密度密切相关。     

Growth Depression in Mycorrhizal Citrus at High-Phosphorus Supply (Analysis of Carbon Costs)

Mycorrhizal-induced growth depression of plants in high-P soil has been reported in many species. The carbon costs of factors contributing to this growth depression were analyzed in Volkamer lemon (Citrus volkameriana Tan. & Pasq.) colonized by the mycorrhizal (M) fungus Glomus intraradices Schenck and Smith. M and nonmycorrhizal (NM) plants were each grown at two P-supply rates. Carbon budgets of M and NM plants were determined by measuring whole-plant carbon assimilation and respiration rates using gas-exchange techniques. Biomass, M colonization, tissue-P concentration, and total fatty acid concentration in the fibrous roots were determined. Construction costs of the fibrous roots were estimated from heat of combustion, N, and ash content. Root-growth respiration was derived from daily root growth and root-construction cost. M and NM plants grown in high-P soil were similar in P concentration, daily shoot carbon assimilation, and daily shoot dark respiration. At 52 d after transplanting (DAT), however, combined daily root plus soil respiration was 37% higher for M than for NM plants, resulting in a 20% higher daily specific carbon gain (mmol CO2 [mmol carbon]-1 d-1) in NM than M plants. Estimates of specific carbon gain from specific growth rates indicated about a 10% difference between M and NM plants. Absolute values of specific carbon gain estimated by whole-plant gas exchange and by growth analysis were in general agreement. At 52 DAT, M and NM plants at high P had nearly identical whole-plant growth rates, but M plants had 19% higher root dry weight with 10% higher daily rates of root growth. These allocation differences at high P accounted for about 51% of the differences in root/soil respiration between M and NM plants. Significantly higher fatty acid concentrations in M than NM fibrous roots were correlated with differences in construction costs of the fibrous roots. Of the 37% difference in daily total root/soil respiration observed between high-P M and NM plants at 52 DAT, estimated daily growth respiration accounted for only about 16%, two-thirds of which was associated with construction of lipid-rich roots, and the remaining one-third with greater M root growth rates. Thus, of the 37% more root/soil respiration associated with M colonization of high-P plants, 10% was directly attributable to building lipid-rich roots, 51% to greater M root biomass allocation, and the remaining 39% could have been used for maintenance of the fungal tissue in the root and growth and maintenance of the extramatrical hyphae.     

Physiological adaptations to phosphorus deficiency during proteoid root development in white lupin

Release of large amounts of citric acid from specialized root clusters (proteoid roots) of phosphorus (P)-deficient white lupin (Lupinus albus L.) is an efficient strategy for chemical mobilization of sparingly available P sources in the rhizosphere. The present study demonstrates that increased accumulation and exudation of citric acid and a concomitant release of protons were predominantly restricted to mature root clusters in the later stages of P deficiency. Inhibition of citrate exudation by exogenous application of anion-channel blockers such as ethacrynic- and anthracene-9-carboxylic acids may indicate involvement of an anion channel. Phosphorus-deficiency-induced accumulation and subsequent exudation of citric acid seem to be a consequence of both increased biosynthesis and reduced metabolization of citric acid in the proteoid root tissue, indicated by increased in-vitro activity and enzyme protein levels of phosphoenolpyruvate carboxylase (EC 4.1.1.31), and reduced activity of aconitase (EC 4.2.1.3) and root respiration. Similar to citric acid, acid phosphatase, which is secreted by roots and involved in the mobilization of the organic soil P fraction, was released predominantly from proteoid roots of P-deficient plants. Also ³³Pi uptake per unit root fresh-weight was increased by approximately 50% in juvenile and mature proteoid root clusters compared to apical segments of non-proteoid roots. Kinetic studies revealed a K _m of 30.7 μM for Pi uptake of non-proteoid root apices in P-sufficient plants, versus K _m values of 8.5–8.6 μM for non-proteoid and juvenile proteoid roots under P-deficient conditions, suggesting the induction of a high-affinity Pi-uptake system. Obviously, P-deficiency-induced adaptations of white lupin, involved in P acquisition and mobilization of sparingly available P sources, are predominantly confined to proteoid roots, and moreover to distinct stages during proteoid root development. Received: 10 September 1998 / Accepted: 22 December 1998     

Root Respiration,Photosynthesis and Grain Yield of Two Spring Wheat in Response to Soil Drying

The effects of soil water regime and wheat cultivar, differing in drought tolerance with respect to root respiration and grain yield, were investigated in a greenhouse experiment. Two spring wheat (Triticum aestivum) cultivars, a drought sensitive (Longchun 8139-2) and drought tolerant (Dingxi 24) were grown in PVC tubes (120 cm in length and 10 cm in diameter) under an automatic rain-shelter. Plants were subjected to three soil moisture regimes: (1) well-watered control (85% field water capacity, FWC); (2) moderate drought stress (50% FWC) and (3) severe drought stress (30% FWC). The aim was to study the influence of root respiration on grain yield under soil drying conditions. In the experiment, severe drought stress significantly (p < 0.05) reduced shoot and root biomass, photosynthesis and root respiration rate for both cultivars, but the extent of the decreases was greater for Dingxi 24 compared to that for Longchun 8139-2. Compared with Dingxi 24, 0.04 and 0.07 mg glucose m⁻² s⁻¹ of additional energy, equivalent to 0.78 and 1.43 J m⁻² s⁻¹, was used for water absorption by Longchun 8139-2 under moderate and severe drought stress, respectively. Although the grain yield of both cultivars decreased with declining soil moisture, loss was greater in Longchun 8139-2 than in Dingxi 24, especially under severe drought stress. The drought tolerance cultivar (Dingxi 24), had a higher biomass and metabolic activity under severe drought stress compared to the sensitive cultivar (Longchun 8139-2), which resulted in further limitation of grain yield. Results show that root respiration, carbohydrates allocation (root:shoot ratio) and grain yield were closely related to soil water status and wheat cultivar. Reductions in root respiration and root biomass under severe soil drying can improve drought tolerant wheat growth and physiological activity during soil drying and improve grain yield, and hence should be advantageous over a drought sensitive cultivar in arid regions.     

Herbivore-induced changes in plant carbon allocation: assessment of below-ground C fluxes using carbon-14

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with¹⁴CO₂. During the next 8 h,¹⁴C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and¹⁴C recovered from soil. While herbivory increased root and soil respiration, the peak time for¹⁴CO₂ evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.