Genetic diversity analysis of Cynodon dactylon (bermudagrass) accessions and cultivars from different countries based on ISSR and SSR markers

ISSR and SSR markers were used to evaluate genetic diversity among 33 Cynodon dactylon accessions and 22 cultivars from four different countries in order to provide information on how to improve the utilization of bermudagrass germplasms. Eighty eight bands were amplified by nine SSR primer combinations and 236 bands were observed from 23 ISSR primers. The results showed that 97.7% of the SSR primers and 86.9% of the ISSR primers were polymorphic. The genetic similarity coefficients (GSC), gene diversity (He) and Shannon index (I) were 0.58–0.97, 0.27 and 0.41, respectively, for ISSR and 0.52–0.97, 0.29, and 0.43 for SSR. The UPGMA analysis clustered the 55 accessions (cultivars) into three groups. The cluster results produced by the ISSR data were close to the SSR data results. Analysis based on the combined ISSR and SSR data was more closely related to the geographical distribution of the tested germplasm.     

Assessment of genetic diversity in Colletotrichum falcatum Went accessions based on RAPD and ISSR markers

Sugarcane is susceptible to red rot disease caused by phytopathogenic fungus Colletotrichum falcatum Went which ultimately affect the economy of farmers as well as sugar based industry. One of the various ways to control this devastating disease is to develop disease resistance sugarcane cultivar and this requires the complete understanding of genetic makeup of pathogen. Although South Gujarat is well known sugarcane cultivating area, less published data can be found about PCR-based genetic diversity in prevalent C. falcatum accessions. So, present investigation aims at finding molecular variation among the ten accessions of C. falcatum using RAPD and ISSR molecular markers. A total of 35 RAPD and 39 ISSR primers were screened across 10 C. falcatum accessions, of which 15 RAPD and 21 ISSR primers have showed consistent amplification. Statistics related to genetic variation were estimated using NTSYS-PC by means of Dice’s coefficient. The results revealed 80.6% and 68.07% polymorphism and similarity coefficient ranged from 0.43 to 0.91 and 0.73 to 0.93 in RPAD and ISSR analysis respectively. The dendrogram generated using RAPD, ISSR and combined RAPD-ISSR grouped accessions into different clusters which reveal considerable level molecular variation among the C. falcatum accessions. It is also evident from PCA plots that accessions are rather dispersed with tested marker systems indicating good genetic base. So, in nut shell, we found considerable genetic variation and relatedness within C. falcatum accessions collected from different areas of south Gujarat, India using RAPD and ISSR markers.     

Genetic characterization of Iranian safflower (Carthamus tinctorius) using inter simple sequence repeats (ISSR) markers

Safflower (Carthamus tinctorious L.) is valued as a source of high quality vegetable oil. 20 ISSR primers were used to assess the genetic diversity of 18 accessions of safflower collected from different geographical regions of Iran. The ISSR primers combinations revealed 57.6 % polymorphism, among 338 genetic loci amplified from the accessions. The sum of effective number of alleles and observed number of alleles were 29.76 and 36.77, respectively. To understand genetic relationships among these cultivars, Jacquards’ similarity coefficient and UPGMA clustering algorithm were applied to the ISSR marker data set. ISSR markers grouped accessions into two main clusters and four sub clusters. Also, the principal coordinate analysis (PCoA) supported the cluster analysis results. The results showed these genotypes have high genetic diversity, and can be used for alternative safflower breeding program.     

Congruence between morphological and molecular markers inferred from the analysis of the intra-morphotype genetic diversity and the spatial structure of Oxalis tuberosa Mol.

Oxalis tuberosa is an important crop cultivated in the highest Andean zones. A germplasm collection is maintained ex situ by CIP, which has developed a morphological markers system to classify the accessions into morphotypes, i.e. groups of morphologically identical accessions. However, their genetic uniformity is currently unknown. The ISSR technique was used in two experiments to determine the relationships between both morphological and molecular markers systems. The intra-morphotype genetic diversity, the spatial structures of the diversity and the congruence between both markers systems were determined. In the first experience, 44 accessions representing five morphotypes, clearly distinct from each other, were analyzed. At the molecular level, the accessions exactly clustered according to their morphotypes. However, a genetic variability was observed inside each morphotype. In the second experiment, 34 accessions gradually differing from each other on morphological base were analyzed. The morphological clustering showed no geographical structure. On the opposite, the molecular analysis showed that the genetic structure was slightly related to the collection site. The correlation between both markers systems was weak but significant. The lack of perfect congruence between morphological and molecular data suggests that the morphological system may be useful for the morphotypes management but is not appropriate to study the genetic structure of the oca. The spatial structure of the genetic diversity can be related to the evolution of the species and the discordance between the morphological and molecular structures may result from similar selection pressures at different places leading to similar forms with a different genetic background.     

Genetic diversity and relationship of global faba bean (<Emphasis Type="Italic">Vicia faba</Emphasis> L.) germplasm revealed by ISSR markers

Genetic diversity and relationships of 802 faba bean (Vicia faba L.) landraces and varieties from different geographical locations of China and abroad were examined using ISSR markers. A total of 212 repeatable amplified bands were generated with 11 ISSR primers, of which 209 were polymorphic. Accessions from North China showed highest genetic diversity, while accessions from central China showed low level of diversity. Chinese spring faba bean germplasm was clearly separated from Chinese winter faba bean, based on principal component analysis and UPGMA clustering analysis. Winter accessions from Zhejiang (East China), Jiangxi (East China), Sichuan (Southwest China) and Guizhou (Southwest China) were quite distinct to that from other provinces in China. Great differentiation between Chinese accessions and those from rest of the world was shown with a UPGMA dendrogram. AMOVA analyses demonstrated large variation and differentiation within and among groups of accessions from China. As a continental geographic group, accessions from Europe were genetically closer to those from North Africa. Based on ISSR data, grouping results of accessions from Asia, Europe and Africa were obviously associated with their geographical origin. The overall results indicated that the genetic relationship of faba bean germplasm was closely associated with their geographical origin and their ecological habit.     

Molecular diversity and genetic relationships among Sri Lankan pomegranate Punica granatum landraces assessed with inter simple sequence repeat (ISSR) regions

Pomegranate Punica granatum was first introduced to Sri Lanka, possibly through ancient trade routes, thousands of years ago. However, there is no information about the diversity of the pomegranate germplasm in the country, which is important both for breeding new varieties and for conservation efforts. We used inter‐simple sequence repeat (ISSR) regions to investigate the genetic diversity and population structure of pomegranate on the island of Sri Lanka. Hundred and twenty accessions representing seven populations from all pomegranate growing regions of the country were analyzed using 20 ISSR primers. A total of 107 loci were amplified with an average polymorphism information content of 0.3. While the average inter‐population genetic distance was 0.141, it was 0.149 between populations, indicating moderate genetic diversity both within and among populations. Analysis of molecular variance and Nei's genetic diversity revealed higher genetic variation within populations than among populations, and low genetic differentiation (G_ST) in pair‐wise comparison of populations also suggested limited population differentiation. A considerable level of among‐population gene flow (Nm) was indicated, irrespective of geographical structure and distances. The results of cluster analysis was also in agreement with above analysis and suggest human mediated gene flow and migration patterns. Cluster analysis revealed two main population clusters with several sub‐clusters. While these clusters did not show any correlation with geography, all red peeled accessions clustered into a small sub‐cluster. The results indicate that analysis of ISSR variability is sufficiently informative and powerful to assess the genetic diversity of P. granatum landraces in Sri Lanka.     

Evaluation of genetic relationships in the genus Houttuynia Thunb. in China based on RAPD and ISSR markers

The genetic relationships among 70 accessions of Houttuynia Thunb. from Sichuan, Chongqing, Guizhou and Jiangsu provinces in China were tested using RAPD and ISSR markers. The results showed that the polymorphism of Houttuynia germplasm was high at the DNA level. ISSR markers are more efficient than RAPD markers at uncovering the polymorphism of the genus Houttuynia. The genetic variation between the cultivated and the wild Houttuynia cordata accessions was insignificant according to RAPD and ISSR markers. The results of cluster analysis by using UPGMA method showed that the groups based on ISSR GS was correlated with chromosome numbers and many accessions with the same chromosome numbers could be classified together. Analysis based on RAPD GS was more related to geographic distribution. Furthermore, the cluster analysis based on RAPD and ISSR markers also showed that the genetic diversity in mountainous and margin areas of Sichuan Basin was more plentiful than that at the bottom of the Basin and its surrounding highlands or hills. Houttuynia emeiensis accession could not be separated completely from H. cordata accessions, it was closely related to H. cordata cytotype A with the chromosome number of 36. Within H. cordata, the genetic similarities between each pair of cytotypes C, D, E, F, G, H, I, J, K and L were higher, but the genetic similarities between each of them to the cytotype A were relatively lower. The phylogeny of the germplasm resources of the genus Houttuynia was also discussed.     

用RAPD和ISSR检测的橡胶树野生种质和栽培品种的遗传多样性

用19个RAPD引物和12个ISSR引物对14份野牛橡胶树种质和我国的37份栽培品种进行了遗传多样性分析。RAPD引物共产生132条带,多态性带占88．6％,相似系数变化范围在0．432—0．947。ISSR引物其产生101条带,多态性带占87．1％,相似系数为0．505—0．941。平均基因杂合度分析表明野生种质比栽培品种具有较高的遗传多样性。根据UPGMA法对51份材料进行聚类分析,结果表明,ISSR分析中所有材料可分为2类：第一类为野生种质,第二类为栽培品种：而RAPD分析中野牛种质和栽培品种不能被分为明显的两人类。虽然ISSR和RAPD的聚类分析结果存在差异,但对两种方法进行的相关分析表明,他们之间仍存在极显著相关性,相关系数为0．574。品种PR107、热研217等一些栽培品种可以通过特异带在51份供试材料中被区分开。这些结果可以对橡胶树的育种上作起到一定的指导作用,同时RAPD和ISSR技术也是进行橡胶树品种鉴定和遗传多样性研究的有效手段。     

ISSR-based clustering of cultivated flax germplasm is statistically correlated to thousand seed mass

Inter-simple sequence repeat (ISSR)-polymerase chain reaction (PCR) polymorphism was generated to provide useful markers for assessment of genetic diversity within flax germplasm collections. We used nine previously selected anchored ISSR primers for fingerprinting of 53 flax cultivars or genotypes and obtained 62 scorable bands, from which 45 bands (72.6%) were polymorphic. An efficient separation of 53 flax accessions into four groups and eight subgroups was achieved using unweighted pair group method with arithmetic means (UPGMA) clustering procedure based on genetic similarity expressed by the Jaccard similarity coefficient (JSC). Clustering procedure within both groups and subgroups successfully produced smaller homogenous clusters, whereas clustering between the main four groups of flax accessions displayed only a continuous decrease of similarity with a weak clustering effect. Statistical significance of grouping and subgrouping within a cluster dendrogram was estimated by calculation of the error flag and cophenetic correlation parameter for each branch. Principal coordinates (PCO) analysis mostly confirmed the separation by UPGMA clustering. We observed a statistically significant correlation between the number of total vs polymorphic bands in ISSR patterns. A one-way analysis of variance (ANOVA) test confirmed statistically significant differences in the average thousand seed mass (TSM) between eight subclusters of flax accessions from an ISSR-PCR-based UPGMA dendrogram, which indicate statistical correlation between flax ISSR polymorphism (the structure of ISSR-based clustering) TSM.     

Identification and genetic relationships among nine Albizzia species based on morphological and molecular markers

Abstract

Identifying germplasm is an important component for efficient and effective management of plant genetic resources. This investigation was undertaken for the identification and analysis of genetic variation within 9 species of Albizzia through 33 morphological parameters, and 15 Random Amplified Polymorphic DNA (RAPD) and 17 Inter Simple Sequence Repeat (ISSR) primers. The use of selected RAPD and ISSR primers generated a total of 163 and 201 amplified DNA fragments, respectively. High frequencies of polymorphism, 95.05% for RAPD and 96.02% for ISSR, were detected. Statistical approaches were employed to construct genetic relationships by RAPD, ISSR and morphological analysis. Cluster analysis by the unweighted pair-group method (UPGMA) of Nei's similarity generated dendograms with similar topology that gave a better reflection of the diversity and affinities between species. These molecular results were comparable to main morphological characteristics. The correlation matrices generated by RAPD and ISSR markers were highly correlated (r = 0.843 at p = 1.0), thereby indicating congruence between these two marker systems. Both morphometric data and molecular markers have the potential to analyse genetic variation among the nine species of Albizzia, thus providing a major input for management strategy of plant genetic resources.     

ISSR Markers as a Tool for the Assessment of Genetic Diversity in Passiflora

Genetic variation among sweet, purple, and yellow passion fruit accessions was assessed using inter-simple sequence repeat (ISSR) markers. Eighteen ISSR primers were used to evaluate 45 accessions. The number of polymorphic bands per primer varied from 4 to 22, with 12.4 bands per primer on average. Nei's genetic distance between accessions ranged from 0.04 to 0.35. Clustering using the neighbor-joining method resulted in the formation of 11 major clusters. It was not possible to classify the accessions according to their geographic origin, showing that there is no structure in the gene bank. The overall mean Shannon-Weaver diversity index was 0.32, indicating good resolution of genetic diversity in passion fruit germplasm using ISSR markers. Our results indicate that ISSR can be useful for genetic diversity studies, to provide practical information for parental selection and to assist breeding and conservation strategies.     

Genetic differentiation among Sri Lankan traditional rice (Oryza sativa) varieties and wild rice species by AFLP markers

Traditional rice varieties are one important component of the biodiversity of Sri Lanka. However, no proper studies have been performed on genetic diversity of the Sri Lankan traditional rice varieties used in our breeding programs. In the present study, the genetic diversity of 46 traditional rice (Oryza sativa) varieties and 5 wild rice species is assesed using Amplified Fragment Length Polymorphism (AFLP) markers. Ten primer combinations generated a total of 784 fragments. Of these, 772 fragments were polymorphic (98.4%). UPGMA analysis based on Jaccard's similarity separated the accessions into four major clusters. A cophenetic correlation with r=0.786 strongly supported the clustering pattern of UPGMA dendrogram. A principal coordinate analysis (PCoA) also confirmed the UPGMA clusters. Accessions referred to the same cluster showed similar morphological characteristics (e.g. height, grain colour etc.) while accessions which are known to be morphologically distinct appeared genetically separated. In addition, the clustering pattern distinctly separated lowland and upland rice varieties. This genetic diversity assessment at the molecular level provides reliable information for selection of germplasm in the development of new rice varieties and in conservation of traditional rice genetic resources.     

Genetic diversity assessment of a germplasm collection of Salvia miltiorrhiza Bunge. based on morphology,ISSR and SRAP markers

Salvia miltiorrhiza is one of the most important traditional Chinese medicinal plants for its therapeutic effects. In the present study, morphological traits, ISSR (inter-simple sequence related) and SRAP (sequence-related amplified polymorphism) markers were used to analyze the genetic diversity of 59 S. miltiorrhiza phenotypes. Out of the 100 ISSR primers and 100 SRAP primer combinations screened, 13 ISSRs and 7 SRAPs were exploited to evaluate the level of polymorphism and discriminating capacity. The results showed that the 13 ISSRs generated 190 repeatable amplified bands, of which 177 (93.2%) were polymorphic, with an average of 13.6 polymorphic fragments per primer. The 7 SRAPs produced 286 repeatable amplified bands, of which 266 (93.4%) were polymorphic, with an average of 38.1 polymorphic fragments per primer. Cluster analysis readily separated different morphological accessions, wild and cultivated controls based on morphological traits, ISSR and SRAP markers. The study indicated that morphological traits, ISSR and SRAP markers were reliable and effective for assessing the genetic diversity of phenotypic S. miltiorrhiza accessions. The overall results suggested that the introduction of genetic variation from morphology-based germplasms enlarged the genetic base for the collection, conservation and further breeding program of S. miltiorrhiza germplasm.     

烟草种质资源遗传多样性的IRAP和ISSR标记比较分析

采用IRAP和ISSR两种分子标记方法,对国内外28份烟草种质资源进行聚类分析,并对两种方法的结果予以比较。结果如下：（1）用IRAP方法对供试材料进行PCR扩增,共得到214条条带,其中187条具有多态性,多态性比率达87．4％。当L1取值0．495时,可将28份材料划分为3个类群;L2取值0．34时,又可将第3类群划分为3个亚类群和3个单一品种组成的独立个类;（2）用ISSR方法进行PCR扩增时,共得到334条条带,其中250条有多态性,多态性比率为75％。当L1取值0．41时,可将28份烟草种质划分为2个类群和2个独立个类Coker176、BaSmaUovina,第一类群（Ⅰ）为来自美国的两个品种RG11和K730,其它24个品种为第二类群（Ⅱ）;（3）IRAP和ISSR两种方法的相关系数为0．823（r0．01=0．478）,说明这两种方法的结果高度相关,均可用来进行烟草种质资源遗传多样性的分析。用IRAP方法得到的品种间遗传相异系数（GD）平均为0．423,高于ISSR方法,而且条带更稳定、多态性比率更高,更易揭示不同品种材料的遗传差异。     

Estimation of genetic variation in Cynodon dactylon accessions using the ISSR technique

Genetic variation in 55 accessions of Cynodon dactylon was estimated using inter-simple sequence repeat (ISSR) markers. The plant materials used in this study originated from 17 countries. A total of 236 ISSR fragments were generated with 14 primers. Fragment sizes ranged from 200 to 3000 bp. All scorable bands were polymorphic in nature and none of the primers used produced monomorphic bands, indicating a high level of genetic variation in this grass. The accessions were found to be clustered into eight major groups through the unweighted pair-group method with arithmetic averages. Genetic similarity coefficients (GSC) among the 55 accessions ranged from 0.52 to 0.95. The results clearly indicate that a high level of variation exists in Cynodon accessions. This study shows that the ISSR technique is a reliable tool for differentiating Cynodon accessions and for determining the genetic relationships among them.     

Genetic diversity of mango cultivars estimated using SCoT and ISSR markers

Two molecular marker systems, SCoT and ISSR were used for identification and genetic comparison analysis of 23 mango germplasm accessions collected within Guangxi province of China. Using 18 selected SCoT primers 158 bands were generated, of which 104 (65.82%) were polymorphic. Eighteen selected ISSR primers amplified 156 bands with 87 (55.77%) being polymorphic. The cultivars of Xiang Ya Mango type and their progeny have high genetic similarity with each other. The 23 cultivars were clustered into two major groups based on the SCoT analysis and three major groups based on the ISSR analysis with UPGMA. These clusters are in accordance with their known origins and main phenotypic characteristics. Our results indicated that the SCoT analysis better represents the actual relationships than ISSR analysis, although both analyses give similar results. The results also demonstrate that the SCoT marker system is useful for identification and genetic diversity analysis of mango cultivars.     

Selection of parents for crossing based on genotyping and phenotyping for stripe rust (<Emphasis Type="Italic">Puccinia striiformis</Emphasis>) resistance and agronomic traits in bread wheat breeding

Bread wheat (Triticum aestivum L.) germplasm consisting of 45 genotypes were clustered phenotypically using ten morphological traits and Area Under Disease Progress Curve (AUDPC) as measure of stripe rust resistance. The clustering was ratified by using twenty three molecular markers (SSR, EST and STS) linked to stripe rust (Puccinia striiformis f. sp. tritici) resistant QTLs. The aim was to asses the extent of genetic variability among the genotypes in order to select the parents for crossing between the resistant and susceptible genotypes with respect to stripe rust. The Euclidian dissimilarity values resulted from phenotypic data regarding morphological traits and AUDPC were used to construct a dendrogram for clustering the accessions. Using un-weighted pair group method with arithmetic means, another dendrogram resulted from the similarity coefficient values was used to distinguish the genotypes with respect to stripe rust. Clustering based on phenotypic data produced two major groups and five clusters (with Euclidian dissimilarity ranging from 2.44 to 16.16) whereas genotypic data yielded two major groups and four clusters (with percent similarity coefficient values ranging from 0.1 to 46.0) to separate the gene pool into highly resistant, resistant, moderately resistant, moderately susceptible and susceptible genotypes. With few exceptions, the outcome of both type of clustering was almost similar and resistant as well as susceptible genotypes came in the same clusters of molecular genotyping as yielded by phenotypic clustering. As a result seven genotypes (Bakhtawar-92, Frontana, Saleem 2000, Tatara, Inqilab-91, Fakhre Sarhad and Karwan) of diverse genetic background were selected for pyramiding stripe rust lesistant genes as well as some other agronomic traits after hybridization.     

Genetic Relationships Among Wild and Cultivated Accessions of Curry Leaf Plant (Murraya koenigii (L.) Spreng.), as Revealed by DNA Fingerprinting Methods

Murraya koenigii (L.) Spreng. (Rutaceae), is an aromatic plant and much valued for its flavor, nutritive and medicinal properties. In this study, three DNA fingerprinting methods viz., random amplification of polymorphic DNA (RAPD), directed amplification of minisatellite DNA (DAMD), and inter-simple sequence repeat (ISSR), were used to unravel the genetic variability and relationships across 92 wild and cultivated M. koenigii accessions. A total of 310, 102, and 184, DNA fragments were amplified using 20 RAPD, 5 DAMD, and 13 ISSR primers, revealing 95.80, 96.07, and 96.73% polymorphism, respectively, across all accessions. The average polymorphic information content value obtained with RAPD, DAMD, and ISSR markers was 0.244, 0.250, and 0.281, respectively. The UPGMA tree, based on Jaccard’s similarity coefficient generated from the cumulative (RAPD, DAMD, and ISSR) band data showed two distinct clusters, clearly separating wild and cultivated accessions in the dendrogram. Percentage polymorphism, gene diversity (H), and Shannon information index (I) estimates were higher in cultivated accessions compared to wild accessions. The overall high level of polymorphism and varied range of genetic distances revealed a wide genetic base in M. koenigii accessions. The study suggests that RAPD, DAMD, and ISSR markers are highly useful to unravel the genetic variability in wild and cultivated accessions of M. koenigii.     

Classification of genetic diversity in Abelmoschus tuberculatus germplasm collection using morphometric data

Abelmoschus tuberculatus Pal & Singh is endemic to India and is the close relative of cultivated okra, A. esculentus. Data on 49 selected accessions of A. tuberculatus were used to study genetic diversity for a range of morphological characters through the use of multi-variate statistics. Epicalyx segments, shape, size and persistence; petal colour and petal blotch; stem and fruit pubescence; pigmentation of various plant parts; days to flowering; plant height; and the fruit yield/plant were the important components of variation and contributed significantly to the total variation in germplasm accessions under study. The accessions could be grouped into four distinct clusters. The grouping pattern, however, did not reflect relationship between geographic origin and genetic diversity. The potential use of the variability in okra improvement programmes is discussed.     

Evaluation of Diversity Based on Morphological Variabilities and ISSR Molecular Markers in Iranian <Emphasis Type="Italic">Cynodon dactylon</Emphasis> (L.) Pers. Accessions to Select and Introduce Cold-Tolerant Genotypes

The main goals of the present study were to screen Iranian common bermudagrasses to find cold-tolerant accessions and evaluate their genetic and morphological variabilities. In this study, 49 accessions were collected from 18 provinces of Iran. One foreign cultivar of common bermudagrass was used as control. Morphological variation was evaluated based on 14 morphological traits to give information about taxonomic position of Iranian common bermudagrass. Data from morphological traits were evaluated to categorize all accessions as either cold sensitive or tolerant using hierarchical clustering with Ward’s method in SPSS software. Inter-Simple Sequence Repeat (ISSR) primers were employed to evaluate genetic variability of accessions. The results of our taxonomic investigation support the existence of two varieties of Cynodon dactylon in Iran: var. dactylon (hairless plant) and var. villosous (plant with hairs at leaf underside and/or upper side surfaces or exterior surfaces of sheath). All 15 primers amplified and gave clear and highly reproducible DNA fragments. In total, 152 fragments were produced, of which 144 (94.73%) being polymorphic. The polymorphic information content (PIC) values ranged from 0.700 to 0.928. The average PIC value obtained with 15 ISSR primers was 0.800, which shows that all primers were informative. Probability identity (PI) and discriminating power between all primers ranged from 0.029 to 0.185 and 0.815 to 0.971, respectively. Genetic data were converted into a binary data matrix. NTSYS software was used for data analysis. Clustering was done by the unweighted pair-group method with arithmetic averages and principle coordinate analysis, separated the accessions into six main clusters. According to both morphological and genetic diversity investigations of accessions, they can be clustered into three groups: cold sensitive, cold semi-tolerant, and cold tolerant. The most cold-tolerant accessions were: Taft, Malayear, Gorgan, Safashahr, Naein, Aligoudarz, and the foreign cultivar. This study may provide useful information for further breeding programs on common bermudagrass. Selected genotypes can be evaluated for other abiotic stresses such as drought and salinity.