Root Xylem Influence on the Water Relations and Drought Resistance of Rice

In order to determine the importance of root axial resistanceto water flow for drought resistance of rice (Oryza sativa L.)aseries of glasshouse and growth chamber studies was conductedfrom 1985 to 1986. A preliminary study surveyed root distributionand histological characteristics of six cultivars grown in aerobicsoil (20x20x90cm boxes) under well–watered ormoisturedeficit conditions. Subsequently, four experiments were conductedwith plants grown in culture solution. Our results demonstratethat plant breeders can use root thickness as a selection indexfor xylem size for root diameters up to about 1–2 mm.Usingthe Poiseuille–Hagen Law for water movement in capillaries,rice root axial resistance explained differences in leaf waterpotential and transpiration when only one cultivar was used,but did not explain differences among cultivars. Thus, increasingroot xylem vessel radii probably will not directly increasedrought resistance. Key words: Rice (Oryza sativa), roots, xylem characteristics, drought resistance     

Evaluating the resistance of six rice cultivars to drought: restriction of deep rooting and the use of raised beds

Soil water deficits reduce rice (Oryza sativa L.) productivity under upland field conditions. In this study, we constructed screening facilities to evaluate the performance of rice cultivars under drought conditions and to assess the roles of deep roots. Two experiments were conducted with six rice cultivars, including drought-tolerant and drought-susceptible cultivars, grown in two root environments: a root-restricted treatment that restricted rooting depth with water-permeable sheets, and a raised bed that reduced water availability in the surface soil by inserting a gravel layer between the topsoil and subsoil layers to interrupt capillary transport of water. In the root-restricted treatment, in which root growth was restricted to the surface 25-cm layer, leaf water potential decreased faster in cultivars with a large canopy during drought stress, and there was little difference in panicle weight among cultivars. With a normal (unrestricted) root environment, the deepest-rooting cultivar (‘IRAT109’) maintained higher leaf water potential during drought, although panicle weight under drought stress was affected by yield potential as well as by deep rooting. Under the intermittent drought stress in the raised bed, deep-rooting cultivars accumulated more nitrogen and produced more biomass, and the difference in panicle weight between deep-rooting drought-tolerant and shallow-rooting drought-susceptible cultivars was magnified by the raised bed compared with the yield differences under drought in a normal root environment. These results demonstrate that the drought screening facilities we developed can help to identify superior cultivars under upland field conditions without time-consuming measurement of deep root systems.     

Proteomic analysis of rice leaves during drought stress and recovery

Three-week old plants of rice (Oryza sativa L. cv CT9993 and cv IR62266) developed gradual water stress over 23 days of transpiration without watering, during which period the mid-day leaf water potential declined to approximately -2.4 MPa, compared with approximately -1.0 MPa in well-watered controls. More than 1000 protein spots that were detected in leaf extracts by proteomic analysis showed reproducible abundance within replications. Of these proteins, 42 spots showed a significant change in abundance under stress, with 27 of them exhibiting a different response pattern in the two cultivars. However, only one protein (chloroplast Cu-Zn superoxide dismutase) changed significantly in opposite directions in the two cultivars in response to drought. The most common difference was for proteins to be up-regulated by drought in CT9993 and unaffected in IR62266; or down-regulated by drought in IR62266 and unaffected in CT9993. By 10 days after rewatering, all proteins had returned completely or largely to the abundance of the well-watered control. Mass spectrometry helped to identify 16 of the drought-responsive proteins, including an actin depolymerizing factor, which was one of three proteins detectable under stress in both cultivars but undetectable in well-watered plants or in plants 10 days after rewatering. The most abundant protein up-regulated by drought in CT9993 and IR62266 was identified only after cloning of the corresponding cDNA. It was found to be an S-like RNase homologue but it lacked the two active site histidines required for RNase activity. Four novel drought-responsive mechanisms were revealed by this work: up-regulation of S-like RNase homologue, actin depolymerizing factor and rubisco activase, and down-regulation of isoflavone reductase-like protein.     

Genotypic Variation in Root Growth Angle in Rice (Oryza sativa L.) and its Association with Deep Root Development in Upland Fields with Different Water Regimes

Deep root development, which is important for the drought resistance in rice (Oryza sativa L.), is a complex trait combining various root morphologies. The objective of this study was to elucidate genotypic variation in deep root development in relation to morphological indicators such as vertical root distribution and root growth angle. Two experiments were conducted: one on upland fields, and one in pots and fields. In experiment 1, the root systems of six rice cultivars on upland fields were physio-morphologically analyzed under different water regimes (irrigated and intermittent drought conditions during panicle development). In experiment 2, cultivar differences in root growth angles were evaluated with 12 cultivars using the basket method under irrigated conditions. No cultivar × environment interactions were found for total root length or deep root length between irrigated and drought conditions in experiment 1. This suggests that constitutive root growth, which is genetically determined, is important for deep root development under intermittent drought conditions during reproductive stage. Among root traits, the deep root ratio (i.e., deep root weight divided by total root weight) was most closely related to deep root length under both water regimes. This suggested that vertical root distribution constitutively affects deep root length. Significant genotypic variation existed in the nodal root diameter and root growth angle of upland rice in experiment 2. It was considered that genotypes with thick roots allocated more assimilates to deep roots through root growth angles higher to the horizontal plane on upland fields. This is the first report on genotypic variation in the root growth angle of rice on upland fields. It should prove useful for rough estimations of genotypic variation in the vertical root distribution of upland rice because root growth angle is rapidly and easily measured.     

干旱复水对柑橘幼苗叶片光合、叶绿素荧光和根系构型的影响

干旱严重影响柑橘的生长和发育.为探索柑橘对干旱胁迫的响应机制,本试验以抗旱性不同的三湖红橘和三湖化红为材料,通过盆栽控水进行干旱胁迫和复水处理,研究处理后植株叶片光合、叶绿素荧光和根系构型的变化.结果表明: 干旱显著降低了两种柑橘幼苗的净光合作用速率、气孔导度、蒸腾速率和胞间CO₂浓度,而三湖红橘的下降幅度更小;复水后,光合参数均有所恢复,但仍低于对照.三湖红橘水分利用率在干旱15 d后开始显著高于对照,而三湖化红除干旱15 d外的其他处理时间均低于对照.干旱提高了两种幼苗的PSII最大光合效率,但抑制了三湖化红的PSII实际光合效率.干旱到一定程度后,两种幼苗的PSII电子传递速率和光化学淬灭均下降,干旱和复水后非光化学淬灭在三湖红橘中下降,但在三湖化红中上升.根系构型分析表明,干旱导致两种幼苗的根表面积和根体积下降,同时抑制了三湖化红的总根长,但能够提高三湖红橘的总根长和总根尖数.进一步分析不同直径的侧根长度发现,三湖红橘的一类侧根长度在干旱胁迫10 d后开始增加,而三湖化红的一类侧根长度在干旱前期没有变化,干旱20 d时显著下降;除三湖红橘的三类侧根外,两种幼苗其余直径等级侧根的生长均受干旱抑制.除总根尖数外,复水后根系生长各参数均没有恢复.干旱对三湖红橘光合性能的影响小于三湖化红,并且前者能够维持更高的水分和光能利用率.干旱后三湖红橘根尖数和细根长度增加,可能有助于提高其对水分的吸收能力.     

东乡普通野生稻与栽培稻苗期抗旱性的比较

干旱影响水稻生长发育,不论什么时期发生最终都导致产量损失。研究水稻资源抗旱性有助于水稻抗旱改良和稳定干旱胁迫下水稻的产量。东乡普通野生稻被公认为是栽培稻的祖先,对增强水稻抗旱性可能十分重要。对4份来自3个仅存的居群的东乡野生稻与15份栽培稻进行苗期抗旱性比较,考察了3次重复的盆栽土培试验中8个抗旱指标。表明东乡普通野生稻比栽培稻更为抗旱,表现在最大根长、茎长、根干重、根鲜重、根干鲜重比及抗旱指数等6个性状,而不表现在根数及根茎长比;其中茎长、最长根长、根干重、根鲜重及根系相对含水量对水稻苗期抗旱性影响更大。采用抗旱指数和抗旱总级别值法对水稻抗旱性进行评定,结果表明4份东乡野生稻材料间的抗旱性存在很大差异,且来水桃树下居群的抗性最高,东乡野生稻抗旱性可能与其原生境状况有关。结果认为东乡普通野生稻可作为栽培稻抗旱改良的遗传资源。     

Differential responses of antioxidative system to chilling and drought in four rice cultivars differing in sensitivity

Responses of antioxidative defense systems to chilling and drought stresses were comparatively studied in four cultivars of rice (Oryza sativa L.) differing in sensitivity, two of them (Xiangnuo no. 1 and Zimanuo) are tolerant to chilling but sensitive to drought and the other two (Xiangzhongxian no. 2 and IR50) are tolerant to drought but sensitive to chilling. The seedlings of rice were transferred into growth chamber for 5 d at 8 degrees C as chilling treatment, or at 28 degrees C as control, or at 28 degrees C but cultured in 23% PEG-6000 solution as drought stress treatment. Under drought stress the elevated levels of electrolyte leakage, contents of H(2)O(2) and total thiobarbituric acid-reacting substances (TBARS) in Xiangzhongxian no. 2 and IR50 are lower than those in Xiangnuo no. 1 and Zimanuo. On the contrary, Xiangnuo no. 1 and Zimanuo have much lower level of electrolyte leakage, H(2)O(2) and TBARS than Xiangzhongxian no. 2 and IR50 under chilling stress. Activities of antioxidant enzymes (superoxide dismutase (SOD), catalase, and ascorbate-peroxidase (APX)) and contents of antioxidants (ascorbaic acid and reduced glutathione) were measured during the stress treatments. All of them were enhanced greatly until 3 d after drought stress in the two drought-tolerant cultivars, or after chilling stress in the two chilling-tolerant cultivars. They all were decreased at 5 d after stress treatments. On the other hand, activities of antioxidant enzymes and contents of antioxidants were decreased greatly in the drought-sensitive cultivars after drought stress, or in the chilling-sensitive cultivars after chilling stress. The results indicated that tolerance to drought or chilling in rice is well associated with the enhanced capacity of antioxidative system under drought or chilling condition, and that the sensitivity of rice to drought or chilling is linear correlated to the decreased capacity of antioxidative system.     

Involvement of polyamines in the drought resistance of rice

This study investigated whether and how polyamines (PAs) in rice (Oryza sativa L.) plants are involved in drought resistance. Six rice cultivars differing in drought resistance were used and subjected to well-watered and water-stressed treatments during their reproductive period. The activities of arginine decarboxylase, S-adenosyl-L-methionine decarboxylase, and spermidine (Spd) synthase in the leaves were significantly enhanced by water stress, in good agreement with the increase in putrescine (Put), Spd, and spermine (Spm) contents there. The increased contents of free Spd, free Spm, and insoluble-conjugated Put under water stress were significantly correlated with the yield maintenance ratio (the ratio of grain yield under water-stressed conditions to grain yield under well-watered conditions) of the cultivars. Free Put at an early stage of water stress positively, whereas at a later stage negatively, correlated with the yield maintenance ratio. No significant differences were observed in soluble-conjugated PAs and insoluble-conjugated Spd and Spm among the cultivars. Free PAs showed significant accumulation when leaf water potentials reached -0.51 MPa to -0.62 MPa for the drought-resistant cultivars and -0.70 MPa to -0.84 MPa for the drought-susceptible ones. The results suggest that rice has a large capacity to enhance PA biosynthesis in leaves in response to water stress. The role of PAs in plant defence to water stress varies with PA forms and stress stages. In adapting to drought it would be good for rice to have the physiological traits of higher levels of free Spd/free Spm and insoluble-conjugated Put, as well as early accumulation of free PAs, under water stress.     

Better performance of germination in hyperosmotic solutions in conspecific weedy rice than cultivated rice

Weeds and crops that grow together often confront similar types of environmental stress, especially drought stress. Weedy rice (Oryza sativa f. spontanea) and cultivated rice (O. sativa L.) provide a unique pair consisting of a weed and a conspecific model crop that can be used to study the drought tolerance of plants across a large distributional range. The investigation on weedy rice's damage to paddy fields showed that it was more serious in dry direct seeding than water direct seeding. Compared with water direct seeding, the seeds of cultivated rice and weedy rice in dry direct seeding will absorb water and germinate under the condition of insufficient soil moisture. Our hypothesis is that weedy rice seeds have evolved stronger germination ability than coexisting cultivated rice under water stress, so that they can obtain more growth space in the early stage in dry direct seeding and thus obtain higher fitness. Seeds of weedy rice populations and coexisting rice cultivars were collected from 61 sites across China and were germinated with 20% polyethylene glycol‐6000 to simulate drought stress. Two drought response indices, which assessed germination rate and germination index, plus one germination stress tolerance index, indicated significantly greater drought tolerance in weedy rice populations than in coexisting rice cultivars (P < 0.01). Drought tolerance for the three indexes were indica weedy rice > indica rice cultivars, japonica weedy rice > japonica rice cultivars, and indica weedy rice > japonica rice cultivars. These results indicate that weedy rice populations show stronger drought stress tolerance than coexisting rice cultivars at various sites, specifically during the seed germination period. Furthermore, Pearson's correlation found that drought response of weedy rice populations and coexisting rice cultivars were significantly different with these environmental factors: latitude, altitude, annual mean precipitation, mean annual temperature, mean precipitation in the sowing month, mean temperature in the sowing month, and sowing methods. Weedy rice shows different patterns of drought tolerance variation across geographical (latitude and altitude) and environmental (precipitation) gradients compared to coexisting rice cultivars. This study suggests that weedy rice might have evolved new drought tolerance and could provide a useful source of genetic resources for improving drought tolerance of crop cultivars and breeding direct seeded cultivars to reduce the usage of seeds in direct seeding.     

Expression profiling of rice cultivars differing in their tolerance to long-term drought stress

Understanding the molecular basis of plant performance under water-limiting conditions will help to breed crop plants with a lower water demand. We investigated the physiological and gene expression response of drought-tolerant (IR57311 and LC-93-4) and drought-sensitive (Nipponbare and Taipei 309) rice (Oryza sativa L.) cultivars to 18 days of drought stress in climate chamber experiments. Drought stressed plants grew significantly slower than the controls. Gene expression profiles were measured in leaf samples with the 20 K NSF oligonucleotide microarray. A linear model was fitted to the data to identify genes that were significantly regulated under drought stress. In all drought stressed cultivars, 245 genes were significantly repressed and 413 genes induced. Genes differing in their expression pattern under drought stress between tolerant and sensitive cultivars were identified by the genotype x environment (G x E) interaction term. More genes were significantly drought regulated in the sensitive than in the tolerant cultivars. Localizing all expressed genes on the rice genome map, we checked which genes with a significant G x E interaction co-localized with published quantitative trait loci regions for drought tolerance. These genes are more likely to be important for drought tolerance in an agricultural environment. To identify the metabolic processes with a significant G x E effect, we adapted the analysis software MapMan for rice. We found a drought stress induced shift toward senescence related degradation processes that was more pronounced in the sensitive than in the tolerant cultivars. In spite of higher growth rates and water use, more photosynthesis related genes were down-regulated in the tolerant than in the sensitive cultivars.     

转反义OsRACK1 基因增强水稻抗旱能力

RACK1是一种多功能支架蛋白, 广泛参与植物生长发育过程的调节。利用反义RNA技术抑制水稻(Oryz a sativa)RACK1基因的表达, 分析了RACK1基因在响应干旱胁迫中的功能。采用实时定量PCR对获得的转基因植株的RACK1基因表达进行分析, 结果表明转基因水稻RACK1基因表达受抑制程度达到50%左右。与非转基因水稻(对照)相比, 转基因水稻耐干旱能力强, 膜脂过氧化程度低且丙二醛的含量少, SOD活性高。这些结果表明, RACK1蛋白负调节水稻对干旱胁迫的耐受过程, 并且这种调节作用在很大程度上与植株体内的氧化还原系统有关。     

Photosynthetic responses and acclimation of two castor bean cultivars to repeated drying–wetting cycles

To investigate the responses of castor bean to repeated drying–wetting cycles (RDWC), morpho-physiological parameters of two cultivars (Jiaxiang 2 and Hangbi 8) were determined by a pot experiment under well-watered control and RDWC. RDWC inhibited plant growth and leaf development, decreased water loss rate (WLR), and enhanced leaf mass per area (LMA) and chlorophyll content as indicated by spectral reflectance indices for both cultivars. Photosynthesis was inhibited by progressive drought stress but quickly recovered after rewatering for each cycle. Both cultivars exhibit a similar pattern of acclimation to RDWC: (1) higher LMA and lower WLR, (2) increased photosynthetic capacity under drought stress with increasing cycle numbers, (3) quick recovery and over-compensation for photosynthesis after rewatering, and (4) increased chlorophyll content. Jiaxiang 2 shows a high capacity for water preservation under drought stress and an over-compensation for photosynthesis after rewatering compared with Hangbi 8.     

Water stress mediated root trait dynamics and identification of microsatellite markers associated with root traits in rice ( Oryza sativa L.)

To identify microsatellite markers associated with root traits for drought tolerance in rice (Oryza sativa L.) a study was conducted at Department of Plant Physiology, College of Agriculture, Trivandrum, Kerala Agricultural University. A set of thirty-five rice genotypes were exposed to water stress and evaluated for physio-morphological components as indices of water stress tolerance. Observations were made on leaf rolling score and root traits, especially the root length, root dry weight, root volume and root shoot ratio at booting stage. As of the data obtained, ten tolerant and ten susceptible varieties were selected for bulk line analysis to identify the DNA markers linked with target gene conferring drought tolerance. Out of 150 SSR primers screened, RM474 showed polymorphism between the tolerant and susceptible bulks. Individual genotypes of the bulks also showed the same product size of the respective tolerant and susceptible bulks.     

Abscisic Acid Accumulation in Detached Leaves of Rice (Oryza sativa L.) in Response to Water Stress: A Correlation with Leaf Size

When water stress was imposed on detached leaves of two rice(Oryza sativa L.) cultivars, more ABA per unit fresh weightaccumulated in IR20, a small-leaved cultivar, than in 63–83,a large-leaved cultivar; the difference being up to threefold.In an F₂ population of a cross between the two cultivars ABAaccumulation was found to be significantly negatively correlatedwith leaf fresh weight. This correlation persisted in the F₃generation. Such a correlation was not evident, however, whena number of rice cultivars, which varied widely in leaf size,were examined. The difference in ABA accumulation between IR20 and 63–83was not accounted for by different spatial patterns of waterloss or ABA accumulation within a leaf, and cultivar differencesin ABA content were maintained both across, and at various positionsalong the leaf. No major differences in leaf anatomy were observed between thetwo cultivars. Differences found in leaf water relations characteristicswere few and generally minor. It therefore seems unlikely thatthese properties account for the difference between the cultivarsin the ability to accumulate ABA or for the correlation withleaf size. Oryza sativa L, rice, water stress, abscisic acid, leaf size     

转反义OsRACK1基因增强水稻抗旱能力

RAC1是一种多功能支架蛋白,广泛参与植物生长发育过程的调节。利用反义RNA技术抑制水稻（Oryzasativa）RACK1基因的表达,分析了RACK1基因在响应干旱胁迫中的功能。采用实时定量PCR对获得的转基因植株的RACK1基因表达进行分析,结果表明转基因水稻RACK1基因表达受抑制程度达到50％左右。与非转基因水稻（对照）相比,转基因水稻耐干旱能力强,膜脂过氧化程度低且丙二醛的含量少,SOD活性高。这些结果表明,RACK1蛋白负调节水稻对干旱胁迫的耐受过程,并且这种调节作用在很大程度上与植株体内的氧化还原系统有关。     

Effects of water stress on plant growth and root proteins in three cultivars of rice (Oryza sativa) with different levels of drought tolerance

Rice ( Oryza sativa L.) is considered a drought-sensitive crop species; however, within this species, there are considerable varietal differences in sensitivity to this environmental stress. In the present work, the effect of water stress on germination, plant growth and root proteins in three rice cultivars (Sinaloa, IR10120 and Chiapas) was analyzed. Seed germination and plant growth were found to be significantly inhibited by polyethylene glycol (PEG)-imposed water deficit in cv. Sinaloa; cvs IR10120 and Chiapas were more tolerant to water stress. Fluorographs of two-dimensional electropherograms of in vivo-labeled polypeptides were analyzed to identify changes in the root protein patterns that resulted when plants were grown in the presence of 10% PEG for 10 days. The treatment induced or increased the synthesis of eight polypeptides or groups of polypeptides in cv. Sinaloa, seven in cv, IR10120 and four in cv. Chiapas. The synthesis of several polypeptides was decreased by the PEG treatment in cv. Sinaloa and cv. IR10120. Most of these PEG-induced changes in the root protein patterns were cultivar-specific and only one 26-kDa protein with a pI of 6.0 was induced by water deficit in the two cultivars Sinaloa and IR10120.     

Enhanced tolerance to drought stress in transgenic rice plants overexpressing a small heat-shock protein,sHSP17.7

Exposure of rice (Oryza sativa L.) seedlings to a high temperature (42°C) for 24 h resulted in a significant increase in tolerance to drought stress. To try to determine the mechanisms of acquisition of tolerance to drought stress by heat shock, the rice small heat-shock protein gene, sHSP17.7, the product of which was shown to act as molecular chaperones in vitro and in vivo in our previous study, was overexpressed in the rice cultivar “Hoshinoyume”. Western and Northern blot analyses showed higher expression levels of sHSP17.7 protein in three transgenic lines than in one transgenic line. Drought tolerance was assessed in these transgenic lines and wild-type plants by withholding water for 6 days for evaluation of the ability of plants to continue growth after water-stress treatments. Although no significant difference was found in water potential of seedlings between transgenic lines and wild-type plants at the end of drought treatments, only transgenic seedlings with higher expression levels of sHSP17.7 protein could regrow after rewatering. Similar results were observed in survival rates after treatments with 30% polyethylene glycol (PEG) 3640 for 3 days. These results suggest that overproduction of sHSP17.7 could increase drought tolerance in transgenic rice seedlings.     

Identification of Cytokinins in Root Exudate of the Rice Plant

Cytokinins, cis-zeatin and cis- and (trans-ribosylzeatin, wereidentified in the root exudate of the rice plant (Oryza sativa,indica cultivar IR-24) after several chromatographic separationsand combined gas-liquid chromatography-selected ion monitoring(GC-SIM) analysis. The presence of trans-zeatin ribotide wassuggested by enzyme hydrolysis, subsequent chromatographic separationand GC-SIM. The comparatively high content of the ribotide inthe root exudate suggests the form of cytokinins to be transportedfrom roots to other parts in the rice plant. (Received July 22, 1982; Accepted November 25, 1982)