Effect of ploidy levels on the activities of Δ1-pyrroline-5-carboxylate synthetase,superoxide dismutase and peroxidase in Cenchrus species grown under water stress

The effects of ploidy levels on the activities of Δ¹-pyrroline-5-carboxylate synthetase (P5CS; EC not assigned), superoxide dismutase (SOD; EC 1.15.1.1) and guaiacol peroxidase (POX; EC 1.11.1.7), as well as malondialdehyde (MDA) and proline contents were studied in two months old plants of Cenchrus species. The Cenchrus species represent three ploidy levels: diploid, tetraploid, hexaploid and two life spans: annual and perennial. Plants were subjected to water stress for 2, 4, 6 and 8 d by withholding water under glasshouse conditions. Although the levels of proline increased with the magnitude of water stress, the P5CS activity did not show a corresponding increase in all species. Peroxidase and superoxide dismutase activities showed an increase or steady state in the early phase of drought and then declined with the further increase in the magnitude of water stress, indicating differing behaviors of species towards drought tolerance. Under drought, diploid Cenchrus species had a higher POX activity, MDA accumulation and lower proline content than tetraploid species. Lower POX and higher P5CS activities and proline contents, however, were observed in hexaploid and tetraploid species. Taken together, our findings suggest that diploid species have a less efficient antioxidant system to scavenge reactive oxygen species than tetra and hexaploid Cenchrus. This may result in a corresponding variability in growth and persistence under natural grasslands. The study also paves the way for investigations on the molecular events associated with drought in Cenchrus species differing in ploidy and life span.     

Dryland Wheat Domestication Changed the Development of Aboveground Architecture for a Well-Structured Canopy

We examined three different-ploidy wheat species to elucidate the development of aboveground architecture and its domesticated mechanism under environment-controlled field conditions. Architecture parameters including leaf, stem, spike and canopy morphology were measured together with biomass allocation, leaf net photosynthetic rate and instantaneous water use efficiency (WUE_i). Canopy biomass density was decreased from diploid to tetraploid wheat, but increased to maximum in hexaploid wheat. Population yield in hexaploid wheat was higher than in diploid wheat, but the population fitness and individual competition ability was higher in diploid wheats. Plant architecture was modified from a compact type in diploid wheats to an incompact type in tetraploid wheats, and then to a more compact type of hexaploid wheats. Biomass accumulation, population yield, harvest index and the seed to leaf ratio increased from diploid to tetraploid and hexaploid, associated with heavier specific internode weight and greater canopy biomass density in hexaploid and tetraploid than in diploid wheat. Leaf photosynthetic rate and WUE_i were decreased from diploid to tetraploid and increased from tetraploid to hexaploid due to more compact leaf type in hexaploid and diploid than in tetraploid. Grain yield formation and WUE_i were closely associated with spatial stance of leaves and stems. We conclude that the ideotype of dryland wheats could be based on spatial reconstruction of leaf type and further exertion of leaf photosynthetic rate.     

Pollen and stomata morphometrics and polyploidy in Eriotheca (Malvaceae‐Bombacoideae)

Approximately 70% of the angiosperm species are polyploid, an important phenomenon in the evolution of those plants. But ploidy estimates have often been hindered because of the small size and large number of chromosomes in many tropical groups. Since polyploidy affects cell size, morphometric analyses of pollen grains and stomata have been used to infer ploidy level. Polyploidy is present in many species of the Cerrado, the Neotropical savanna region in Central Brazil, and has been linked to apomixis in some taxa. Eriotheca gracilipes and Eriotheca pubescens are common tree species in this region, and present cytotypes that form reproductive mosaics. Hexaploid individuals (2n = 6x = 276) are polyembryonic and apomictic, while tetraploid and diploid individuals (2n = 2x = 92, 2n = 4x = 184) are sexual and monoembryonic. We tested whether morphometric analysis can be used to estimate ploidy levels in E. gracilipes and E. pubescens individuals. Pollen material from diploid and hexaploid individuals of E. gracilipes, and tetraploid and hexaploid individuals of E. pubescens, were fixed in 50% FAA, and expanded leaves were dried in silica gel. Pollen grains and stomata of at least five individuals from each population were measured. The results demonstrate that all measures were significantly different among cytotypes. Individuals with higher levels of ploidy (hexaploid) all presented measurements that were higher than those with lower levels (diploid and tetraploid). There was no overlap between ploidy levels in each species at 95% confidence interval. Thus, the size of the pollen grains and stomata are effective parameters for analysis of ploidy levels in E. gracilipes and E. pubescens.     

Photosynthetic response of tetraploid and hexaploid wheat to water stress

Photosynthetic characteristics of ear and flag leaves of wheat species, tetraploid Triticum dicoccoides Kom and hexaploid Bima1, were studied in plants grown under well-watered (WW) and water-stressed (WS) conditions. Compared to ears, flag leaves exhibited higher photosynthetic rate (P _N) at the filling stage, but more severe decrease under WS. P _N in the tetraploid wheat ear remained higher than that in the hexaploid wheat during the grain-filling stage. Water stress decreased PN in both the organs; this decline was caused by a reduction in Rubisco activity, not by drought-induced stomatal limitation. Tetraploid wheat ears exhibited higher relative water content and water-use efficiency than that of hexaploid wheat, under WS. The change in phosphoenolpyruvate carboxylase activity and carbon isotope composition indicated the absence of C₄ metabolism in the ears of both species under both conditions. The improved performance of the tetraploid wheat ears under WS was associated with better water relations.     

Photosynthetic characteristics in <Emphasis Type="Italic">Oryza</Emphasis> species

Photosynthetic rate (P _N), SPAD value, specific leaf area (SLA), flag leaf area (FLA), and nitrogen content (LN) of genus Oryza were investigated and their correlation was analyzed to assess some of the main photosynthetic traits among different species in the genus Oryza. The results revealed wide variation in these traits. The species O. rufipogon and O. australiensis exhibited maximum photosynthetic rate. Comparison of different types of genomes (diploid: 2n=2x=24; tetraploid: 2n=4x=48) and growth habit (shade- or sun-grown) showed the species of diploid (with genome symbol EE; 2n=2x=24) genomes, with perennial and sun-grown species, had high apparent photosynthesis compared to others. The species with BB/BBCC, shade-grown and the tetraploids showed high SPAD value, and the flag leaf in sun-grown species and diploids were thicker (low SLA) compared with others. However, no significant difference could be noticed among the different types of genomes. Higher leaf area was noticed among the species of CC/CCDD genome, perennial shade-grown species and tetraploids than in others. The variety IR 36 exhibited highest leaf nitrogen concentration. Correlation analysis showed a strong relationship between P _N and leaf nitrogen concentration while no marked relationships were observed among other characteristics. It implies that the species with thick and small leaves with high nitrogen concentration and high photosynthesis evolved better than others. O. rufipogon, with the same genome as O. sativa, could be one of the wild rice resources for elite crop improvement.     

Genetic differentiation and spatiotemporal history of diploidy and tetraploidy of Clintonia udensis

Polyploidy is an important factor shaping the geographic range of a species. Clintonia udensis (Clintonia) is a primary perennial herb widely distributed in China with two karyotypic characteristics—diploid and tetraploid and thereby used to understand the ploidy and distribution. This study unraveled the patterns of genetic variation and spatiotemporal history among the cytotypes of C. udensis using simple sequence repeat or microsatellites. The results showed that the diploids and tetraploids showed the medium level of genetic differentiation; tetraploid was slightly lower than diploid in genetic diversity; recurrent polyploidization seems to have opened new possibilities for the local genotype; the spatiotemporal history of C. udensis allows tracing the interplay of polyploidy evolution; isolated and different ecological surroundings could act as evolutionary capacitors, preserve distinct karyological, and genetic diversity. The approaches of integrating genetic differentiation and spatiotemporal history of diploidy and tetraploidy of Clintonia udens would possibly provide a powerful way to understand the ploidy and plant distribution and undertaken in similar studies in other plant species simultaneously contained the diploid and tetraploid.     

Relationship between ploidy level and genome size in strawberries

This study investigated the pattern of variation in nuclear DNA content at different ploidy levels in Fragaria (Strawberry, Rosaceae) using flow cytometry based on mean fluorescent intensity (MFI) reflected by propidium-iodide-stained nuclei. On average, MFI values were 237 for diploids F. vesca, F. viridis, and F. nubicola, 416.5 for tetraploid F. orientalis, 621.5 for hexaploid F. moschata, and 798 for octoploids F. × ananassa, F. virginiana, and F. chiloensis. Within diploids MFI ranged from 225.9 in F. vesca ssp. vesca to 255.4 in F. nubicola, and within octoploids varied from 766 in F. × ananassa to 808 in F. virginiana. The nuclear DNA variation was significant among diploid species (N = 21, P < 0.008), but not across octoploid species (N = 17, P>0.386). MFI values were also variable among different genotypes of a given species though not significant. The values of mean basic genome DNA (MFI divided by ploidy level) were 118.5, 104, 103.5, and 99.8, respectively, for diploids, tetraploid, hexaploid, and octoploid species. This indicates that relative genomic size decreases by increasing ploidy level, and that there is no direct proportional relationship between DNA content and ploidy levels in Fragaria, supporting the idea of genome downsizing during polyploidization in plants.     

Study on the response of diploid,tetraploid and hexaploid species of wheat to the elevated CO2

Study was done to compare the response of Triticum aestivum (hexaploid), Triticum durum (tetraploid) and Triticum monococcum (diploid) wheat species to the elevated CO₂ using Free Air CO₂ Enrichment (FACE) facility. It was demonstrated that the modern cultivar of wheat Triticum aestivum (hexaploid) was largely sink limited. It appeared to have less photosynthesis per unit leaf area than Triticum monococcum (diploid wheat). While leaf size, grain weight and amylase activity increased with the ploidy level from diploid to hexaploid wheat forms, the photosynthetic rate was reduced significantly. These wheat species responded differentially to the elevated CO₂. The larger leaf area and greater seed weight and presence of 38 KDa protein band caused by elevated CO₂ had additive effect in improving the productivity of hexaploid wheat by changing the source sink ratio. Whereas, such a source sink balance was not induced by elevated CO₂ in diploid wheat. The increasing CO₂ may present opportunities to breeders and possibly allow them to select for cultivars responsive to the elevated CO₂ with better sink potential.Key words: Elevated CO₂, FACE technology, Photosynthesis, Seed weight, Source sink ratio, Triticum     

Nuclear and cytoplasmic genome relationships in the genus Avena: Analysis by isoelectric focusing of ribulose biphosphate carboxylase subunits

Comparisons of the isoelectric points of small and large subunits of ribulose biphosphate carboxylase extracted from a number of diploid, tetraploid, and hexaploid Avena species have been used to obtain information on the nuclear and cytoplasmic genome relationships within the genus. All species tested had small subunits with similar isoelectric points, so their analysis provided no information of taxonomic value. Three types of large subunits could be distinguished by this method, and the distribution of each among the available species provides strong evidence against the involvement of a C genome diploid (such as A. ventricosa) as the maternal parent in the formation of either tetraploid or hexaploid species. One type of large subunit was confined to the perennial tetraploid, A. macrostachya, and its position in the genus and possible origin are discussed. The value of this approach in studying genome relationships within the genus Avena and related genera is assessed.     

Photosynthetic characteristics of diploid honeysuckle (<Emphasis Type="Italic">Lonicera japonica</Emphasis> Thunb.) and its autotetraploid cultivar subjected to elevated ozone exposure

In order to investigate the effect of chromosome doubling on ozone tolerance, we compared the physiological responses of a diploid honeysuckle (Lonicera japonica Thunb.) and its autotetraploid cultivar to elevated ozone (O₃) exposure (70 ng g⁻¹, 7 h d⁻¹ for 31 d). Net photosynthetic rate (P _N) of both cultivars were drastically (P<0.01) impaired by O₃. Although there were significantly positive correlation between P _N and stomatal conductance (g _s) in both cultivars under each treatment, the decreased g _s in O₃ might be the result rather than the cause of decreased P _N as indicated by stable or increasing the ratio of intercellular to ambient CO₂ concentration(C _i/C _a). P _N under saturating CO₂ concentration (P _Nsat) and carboxylation efficiency (CE) significantly decreased under O₃ fumigation, which indicated the Calvin cycle was impaired. O₃ also inhibited the maximum efficiency of photosystem II (PSII) photochemistry in the dark-adapted state (F_v/F_m), actual quantum yield of PSII photochemistry (Φ_PSII), electron transport rate (ETR), photochemical quenching coefficient (qP), non-photochemical quenching (NPQ), the maximum in vivo rate of Rubisco carboxylation (V_cmax) and the maximal photosynthetic electron transport rate (J_max) which demonstrated that the decrease in P _N of the honeysuckle exposed to elevated O₃ was probably not only due to impairment of Calvin cycle but also with respect to the light-harvesting and electron transport processes. Compared to the diploid, the tetraploid had higher relative loss in transpiration rate (E), (g _s), (P _Nsat), V_cmax and J_max. This result indicated that the Calvin cycle and electron transport in tetraploid was damaged more seriously than in diploid. A barely nonsignificant (P=0.086) interaction between O₃ and cultivar on P _N suggested a higher photosynthetic sensitivity of the tetraploid cultivar.     

Change in nuclear DNA content and pollen size with polyploidisation in the sweet potato (Ipomoea batatas,Convolvulaceae) complex

• Genome size evolution and its relationship with pollen grain size has been investigated in sweet potato (Ipomoea batatas), an economically important crop which is closely related to diploid and tetraploid species, assessing the nuclear DNA content of 22 accessions from five Ipomoea species, ten sweet potato varieties and two outgroup taxa.
• Nuclear DNA amounts were determined using flow cytometry. Pollen grains were studied using scanning and transmission electron microscopy.
• 2C DNA content of hexaploid I. batatas ranged between 3.12–3.29 pg; the mean monoploid genome size being 0.539 pg (527 Mbp), similar to the related diploid accessions. In tetraploid species I. trifida and I. tabascana, 2C DNA content was, respectively, 2.07 and 2.03 pg. In the diploid species closely related to sweet potato e.g. I. ×leucantha, I. tiliacea, I. trifida and I. triloba, 2C DNA content was 1.01–1.12 pg. However, two diploid outgroup species, I. setosa and I. purpurea, were clearly different from the other diploid species, with 2C of 1.47–1.49 pg; they also have larger chromosomes. The I. batatas genome presents 60.0% AT bases.
• DNA content and ploidy level were positively correlated within this complex. In I. batatas and the more closely related species I. trifida, the genome size and ploidy levels were correlated with pollen size. Our results allow us to propose alternative or complementary hypotheses to that currently proposed for the formation of hexaploid Ipomoea batatas.

     

Hybridization between diploid Centaurea pseudophrygia and tetraploid C. jacea (Asteraceae): the role of mixed pollination,unreduced gametes,and mentor effects

We studied hybridization between the diploid Centaurea pseudophrygia and the tetraploid C. jacea by performing crossing experiments and screening natural populations using flow cytometry. The experiments confirm that the studied species exhibit strong reproductive isolation. Interspecific hybrids were formed at a low frequency, including triploids (originating from reduced gametes) and tetraploids (involving unreduced gametes of the diploids). In contrast, hybrids were almost absent among seeds and adult plants of natural mixed populations and among the offspring from experimental pollinations with a mixture of pollen of both ploidy levels. We found that mixed pollination is an important mechanism for preventing hybridization between plants of different ploidy levels and sustaining the reproduction of the tetraploids. A mentor effect (induced selfing in the presence of pollen of different ploidy levels) was observed in both diploids and tetraploids, reinforcing the reproductive isolation between cytotypes. Higher ploidy levels (pentaploid, hexaploid) involving unreduced gametes of the tetraploid species were identified. Notably, pentaploids were discovered for the first time in Centaurea sect. Jacea. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 93–106.     

Cytogeography of Cenchrus ciliaris (Poaceae) in Tunisia

Belonging to the genus Cenchrus with 16–22 species, Cenchrus ciliaris L. (syn. Pennisetum ciliare (L.) Link, buffelgrass) is a perennial, common in warmer regions of both hemispheres, growing as a C4 grass in a wide range of habitats. In the present study we determined chromosome number and nuclear DNA content (2C DNA) for 28 natural populations collected from northern to southern Tunisia. Three ploidy levels were found: one tetraploid population (2n?=?4x?=?36), three pentaploid (2n?=?5x?=?45), and 24 hexaploid populations (2n?=?6x?=?54). The hexaploid chromosome number has already been reported for Tunisian populations of C. ciliaris but tetraploid and pentaploid (2n?=?45) are new for this area. The tetraploid population was found in the semi-arid north; pentaploids were mostly on the northern side of the arid region, while the hexaploids were located mainly in the arid southern Tunisian and Saharan region. 2C DNA values, assessed using flow cytometry, correlated with chromosome counts. Nuclear DNA content ranged from 2C?=?3.03 to 4.61 pg, revealing three ploidy levels corresponding to 4x, 5x, 6x, and mean 2C DNA amounts were of 3.03, 3.7 and 4.48 pg, respectively. Each cytotype produced viable pollen. Flow cytometric seed screening neither proved nor disproved apomixis. The most frequent hexaploid populations seem best adapted to arid conditions in southern Tunisia. The monoploid value, 1Cx, was constant. The existence of pentaploid cytotype suggests hybridization ability between tetraploids and hexaploids. It appears that polyploidization is the major evolutionary mechanism in the speciation of C. ciliaris.     

Polyploid evolution and biogeography in Chelone (Scrophulariaceae): morphological and isozyme evidence

Chelone is a genus of perennial herbs comprising three diploid species (C. cuthbertii, C. glabra, and C. lyonii) and a fourth species (C. obliqua) that occurs as tetraploid and hexaploid races. To assess patterns of isozyme and morphological variation, and to test hypotheses of hybridization and allopolyploidy, we analyzed variation among 16 isozyme loci from 61 populations and 16 morphological characters from 33 populations representing all taxa and ploidy levels. Based on morphological analyses using clustering (unweighted pair group method using an arithmetic average) and ordination (principal components analysis and canonical variance analysis) methods, we recognize three diploid species without infraspecific taxa. Polyploids in the C. obliqua complex were most similar morphologically to diploid populations of C. glabra and C. lyonii. Patterns of isozyme variation among polyploids, which included fixed heterozygosity and recombinant profiles of alleles present in diploids, suggested polytopic origins of tetraploids and hexaploids. Our data indicate independent origins of polyploids in or near the southern Blue Ridge, Interior Highlands and Plains, and Atlantic Coastal Plain regions from progenitors most similar to C. glabra and C. lyonii. Extant tetraploids were not implicated in evolution of hexaploids, and plants similar to C. cuthbertii appeared unlikely as diploid progenitors for polyploids. We propose multiple differentiation and hybridization/polyploidization cycles in different geographic regions to explain the pattern of allopatry and inferred polytopic origins among polyploids.     

Nucleoli and ploidy in Potorous cells (PTK2) in vitro

Most cells of the male PTK₂ cell line contain one nucleolus, and they are diploid. However, a proportion of cells have more than one nucleolus. Cells with two and three nucleoli were isolated and cloned into viable populations. Greater than 90% of the cells in these clonal populations maintained the abnormal nucleolar number of the originally isolated cell. Karyotype analysis of cells with two and three nucleoli demonstrated that the cells were respectively tetraploid and hexaploid. It was concluded that in PTK₂ cells, nucleolarity is a good index of ploidy even if the ploidy level is abnormal. Furthermore, long term monitoring of the tetraploid cells demonstrated virtually no tendency towards reversion to the diploid condition as suggested by other studies in Potorous.     

Influence of enhanced temperature on photosynthesis,photooxidative damage,and antioxidant strategies in <Emphasis Type="Italic">Ceratonia siliqua</Emphasis> L. seedlings subjected to water deficit and rewatering

Predicted future climatic changes for the Mediterranean region give additional importance to the study of photooxidative stress in local economic species subjected to combined drought and high-temperature conditions. Under this context, the impact of these stresses on photosynthesis, energy partitioning, and membrane lipids, as well as the potential ability to attenuate oxidative damage, were investigated in Ceratonia siliqua L. Two thermal regimes (LT: 25/18°C; HT: 32/21°C) and three soil water conditions (control, water stress, and rewetting) were considered. HT exacerbated the adverse effects of water shortage on photosynthetic rates (P _N) and PSII function. The decrease in P _N was 33% at LT whereas at HT it was 84%. In spite of this, the electron transport rate (ETR) was not affected, which points to an increased allocation of reductants to sinks other than CO₂ assimilation. Under LT conditions, water stress had no significant effects on yield of PSII photochemistry (Φ_PSII) and yields of regulated (Φ_NPQ) and nonregulated (Φ_NO) energy dissipation. Conversely, drought induced a significant decrease of Φ_PSII and a concomitant increase of Φ_NO in HT plants, thereby favouring the overproduction of reactive oxygen species (ROS). Moreover, signs of lipid peroxidation damage were detected in HT plants, in which drought caused an increase of 40% in malondialdehyde (MDA) content. Concurrently, a marked increase in proline content was observed, while the activities of catalase (CAT) and ascorbate peroxidase (APX) were unaffected. Despite the generation of a moderate oxidative stress response, C. siliqua revealed a great capability for photosynthetic recovery 36 h after rewatering, which suggests that the species can cope with predicted climate change.     

Evaluation of genus <Emphasis Type="Italic">Cenchrus</Emphasis> based on malondialdehyde,proline content,specific leaf area and carbon isotope discrimination for drought tolerance and divergence of species at DNA level

Cenchrus (family Poaceae) is an important component of major grass covers of the world. Largely it is apomictic and both annual and perennial species exist in nature. Variations in contents of malondialdehyde, proline, specific leaf area and carbon isotope discrimination for drought tolerance were estimated among eight prominent species of Cenchrus. Simultaneously, genetic variations were also estimated by employing 187 RAPD primers. Of these, 23 primers did not react, 2 performed poorly and 7 produced many non-scorable bands and one primer yielded a single monomorphic band. Rest of the 154 primers generated one or more unambiguously scorable fragments. Twelve hundred and four of the 1,296 putative loci were polymorphic (93%) between at least one pair-wise comparisons among eight species. Dice coefficient and neighbor-joining algorithm analyses showed clustering patterns that fit with the known habitat of the species except perennial, C. myosuroides which formed a node between two annuals species. When these species were subjected to water stress tolerance test, a correlation (r = 0.612) between specific leaf area (SLA) and carbon isotope discrimination (CID) and difference in levels of drought tolerance based parameters among eight species were observed. Of the eight species investigated two annuals viz., C. biflorus and C. echinatus showed highest level of genetic similarity which was also evident from the similar levels of SLA, MDA, proline contents and carbon isotope discrimination values observed in these two species.     

Influence of ploidy level on morphology,growth and drought susceptibility in <Emphasis Type="Italic">Spathiphyllum wallisii</Emphasis>

In this study, we analysed morphological, anatomical and physiological effects of polyploidisation in Spathiphyllum wallisii in order to evaluate possible interesting advantages of polyploids for ornamental breeding. Stomatal density was negatively correlated with increased ploidy level. Stomatal size increased in polyploids. Tetraploid Spathiphyllum plants had more ovate and thicker leaves. The inflorescence of tetraploids had a more ovate and thicker spathum, a more cylindrical spadix and a thicker but shorter flower stalk. Biomass production of the tetraploids was reduced, as expressed by lower total dry weights, and tetraploids produced fewer shoots and leaves compared with their diploid progenitors. Furthermore, tetraploid Spathiphyllum plants were more resistant to drought stress compared with diploid plants. After 15 days of drought stress, diploids showed symptoms of wilting, while the tetraploids showed almost no symptoms. Further, measurements of stomatal resistance, leaf water potential, relative water content and proline content indicated that the tetraploid genotypes were more resistant to drought stress compared with the diploids.     

Effects of water stress and nitrogen supply on leaf gas exchange and fluorescence parameters of <Emphasis Type="Italic">Sophora davidii</Emphasis> seedlings

Two-month-old seedlings of Sophora davidii were subjected to a randomized complete block design with three water (80, 40, and 20 % of water field capacity, i.e. FC₈₀, FC₄₀, and FC₂₀) and three N supply [N0: 0, Nl: 92 and Nh: 184 mg(N) kg⁻¹(soil)] regimes. Water stress produced decreased leaf area (LA) and photosynthetic pigment contents, inhibited photosynthetic efficiency, and induced photodamage in photosystem 2 (PS2), but increased specific leaf area (SLA). The decreased net photosynthetic rate (P _N) under medium water stress (FC₄₀) compared to control (FC₈₀) might result from stomatal limitations, but the decreased P _N under severe water deficit (FC₂₀) might be attributed to non-stomatal limitations. On the other hand, N supply could improve photosynthetic capacity by increasing LA and photosynthetic pigment contents, and enhancing photosynthetic efficiency under water deficit. Moreover, N supply did a little in alleviating photodamages to PS2 caused by water stress. Hence water stress was the primary limitation in photosynthetic processes of S. davidii seedlings, while the photosynthetic characters of seedlings exhibited positive responses to N supply. Appropriate N supply is recommended to improve photosynthetic efficiency and alleviate photodamage under water stress.     

Ploidy doubling by in vitro culture of excised protocorms or protocorm-like bodies in <Emphasis Type="Italic">Phalaenopsis</Emphasis> species

Endopolyploidy was observed in the protocorms of diploid Phalaenopsis aphrodite subsp. formosana with ploidy doubling achieved by in vitro regeneration of excised protocorms, or protocorm-like bodies (PLBs). Thirty-four per cent of the PLBs regenerated from the first cycle of sectioned protocorms were found to be polyploids with ploidy doubled once or twice as determined by flow-cytometry. The frequency of ploidy doubling increased as the sectioning cycles increased and was highest in diploid followed by the triploid and tetraploid. Regeneration of the endopolyploid cells in the tissue of the protocorms or PLBs is proposed as the source of the development of ploidy doubled plantlets. The frequency of ploidy doubling was similar in seven other Phalaenopsis species, although the rate of increase within cycles was genotype specific. In two species, a comparison of five parameters between 5-month-old diploid and tetraploid potted plants showed only the stomata density differed significantly. The flowers of the tetraploid plant were larger and heavier than those of the diploids. This ploidy doubling method is a simple and effective means to produce large number of polyploid Phalaenopsis species plants as well as their hybrids. The method will be beneficial to orchid breeding programs especially for the interspecific hybridization between varieties having different chromosome sizes and ploidy levels.