Tropical Lines Research Articles

Patterns of molecular evolution associated with two selective sweeps in the Tb1-Dwarf8 region in maize.

We focused on a region encompassing a major maize domestication locus, Tb1, and a locus involved in the flowering time variation, Dwarf8 (D8), to investigate the consequences of two closely linked selective sweeps on nucleotide variation and gain some insights into maize geographical diffusion, through climate adaptation. First, we physically mapped D8 at approximately 300 kb 3' of Tb1. Second, we analyzed patterns of nucleotide variation at Tb1, D8, and seven short regions (400-700 bp) located in the Tb1-D8 region sequenced on a 40 maize inbred lines panel encompassing early-flowering temperate and late-flowering tropical lines. The pattern of polymorphism along the region is characterized by two valleys of depleted polymorphism while the region in between exhibits an appreciable amount of diversity. Our results reveal that a region approximately 100 kb upstream of the D8 gene exhibits hallmarks of divergent selection between temperate and tropical lines and is likely closer than the D8 gene to the target of selection for climate adaptation. Selection in the tropical lines appears more recent than in the temperate lines, suggesting an initial domestication of early-flowering maize. Simulation results indicate that the polymorphism pattern is consistent with two interfering selective sweeps at Tb1 and D8.

A New Maize Heterotic Pattern between Temperate and Tropical Germplasms

New heterotic groups are needed for increasing genetic diversity and productivity in maize (Zea mays L.). A study was conducted with the following objectives: (i) to determine if exotic tropical maize germplasm (TRMG) can increase diversity of Chinese maize germplasm for grain yield and five yield component traits (YCTs), namely, ear length (EL), ear diameter (ED), number of kernel rows per ear (RE), number of kernels per row (KR), and 1000‐kernel weight (TKW), and (ii) to estimate general combining ability (GCA) and specific combining ability (SCA) effects to identify a possible new heterotic pattern. Twenty‐five temperate maize germplasm (TEMG) from both China and the United States were crossed with four TRMG from the International Maize and Wheat Improvement Center (CIMMYT). The 100 testcrosses were evaluated in field trials for grain yield and five YCTs at three locations in Yunnan, China. Tropical lines YML146 and YML145 showed, in crosses with most of the TEMG, significant positive SCA effects for grain yield and most of the YCTs, indicating that these lines had a different genetic base from that of TEMG and could increase genetic diversity of Chinese maize germplasm. The GCA effects were more important and reliable than SCA effects for heterotic pattern classification. Exotic line YML146 (derived from Suwan1) was identified as a new heterotic group, different from Reid, Lancaster, Tangsipingtou (TSPT), and Luda Red Cob (LDRC). A new heterotic pattern of temperate × Suwan1 was postulated, which could be as important as the widely accepted flint × dent heterotic pattern.

Community Resources and Strategies for Association Mapping in Sorghum

Association mapping is a powerful strategy for identifying genes underlying quantitative traits in plants. We have assembled and characterized genetic and phenotypic diversity of a sorghum [Sorghum bicolor (L.) Moench] panel suitable for association mapping, comprised of 377 accessions representing all major cultivated races (tropical lines from diverse geographic and climatic regions), and important U.S. breeding lines and their progenitors. Accessions were phenotyped for eight traits, and levels of population structure and familial relatedness were assessed with 47 simple sequence repeat (SSR) loci. The panel exhibited substantial morphological variation and little genotypic differentiation was observed between the converted tropical and breeding lines. The phenotypic and genotypic data were used to evaluate the performance of several association models in controlling for spurious associations. Our analysis indicated that association models that accounted for both population structure and kinship performed better than those that did not. In addition, we found that the optimal number of subpopulations used to correct for population structure was trait dependent. Although augmentation of the genotypic data with additional SSR loci may be necessary, the association models, genotypic data, and germplasm panel described here provide a starting point for sorghum researchers to begin association studies of traits and markers or candidate genes of interest.

A transgenic tropical maize line generated by the direct transformation of the embryo-scutellum by A. tumefaciens

An efficient Agrobacterium-mediated transformation system, from which transgenic tropical maize plants were directly generated without previous crosses with laboratory or temperate lines, was established. Experimental evaluations were focused on two main issues: (i) establishment of appropriate tissue culture conditions, which induced somatic embryogenesis from the scutellum-cells, and (ii) the delivery of T-DNA toward these cells. High rates of embryogenic-calli, mainly generated from the embryo-scutellum, were obtained when 15 mg l−1 AgNO3 were included into the N6-based induction medium; rates up to 19 plants per gram were regenerated from these induced calli. Regarding the Agrobacterium strains evaluated for their transformation capability on the tropical maize line LPC13 used here, best results were obtained from the EHA105 cells when applied at OD550 nm = 0.5–1.0. Physical microwounds before the Agro-infection proved to be an excellent way to promoting both the T-DNA transferring toward the embryo-scutellum and the increasing of rates of transient GUS expression. The highest frequencies of transient GUS expression corresponding to the scutellum-cells as well as the regeneration of whole transgenic plants emerged from them, were obtained using immature embryos wounded by bombarding at 80 lb/in2 followed for vacuum infiltration before and during the Agro-infection, respectively, or using embryos wounded by 5 s-sonication (without vacuum infiltration) before the Agro-infection. Transformation frequencies up to 5.41% and 6.82% were obtained from the Agro-infected embryos wounded by particle-bombardment and sonication, respectively. Analyses of the progenies confirmed the sexual transmission of the introduced genes and their stable expression.

Androgenetic haploids and SSR markers as tools for the development of tropical maize hybrids

The traditional process of obtaining maize hybrids involves the generation of inbred lines through successive generations of selfing and subsequent testcrosses in order to identify the best combining ability by allelic complementation. A fast alternative to obtain inbred lines is to induce the formation of haploids followed by chromosome doubling. However, even with the aid of haploid-inducing genetic sources, this strategy has not been widely used in maize breeding programs, partly due to difficulties inherent to haploid generation and identification. In order to evaluate the possibility of using dihaploids to generate homozygous maize tropical lines, we used the androgenetic haploid inducer line W23 as a female parent in crosses with the tropical single-cross hybrid BRS1010. Within the progeny of these crosses, 462 seeds were phenotypically selected as putative haploids by the purple-colored endosperm and colorless embryo conditioned by the R1-nj gene. Among these, only four individuals were confirmed as being haploids using SSR markers, chromosome counting and flow cytometry, showing that the phenotypic marker was not efficient in detecting haploids in the tropical maize genotype used. All four haploids as well as some diploid plants presented reduced size, corroborating the difficulties for haploid identification by phenotypic evaluation. Genetic diversity analysis revealed by SSR markers divided the haploids in two groups represented by flint and dent maize inbred lines, which could be helpful in identifying complementary dihaploid lines. The present article demonstrates that a combination of haploid production and SSR fingerprinting is a feasible strategy for maize hybrid development in tropical germplasm.

Screening tropical maize lines for the production and regeneration of friable and embryogenic type II callus

Twenty two maize lines popularly used by breeders in the Kenyan maize program were screened for their ability to form type II callus. The immature embryos were harvested two weeks after pollination. Two highland lines showed high embryogenic response and formed friable, type II calli. While 16 lines screened showed poor or retarded growth with subsequent death. There was successful plant regeneration from four lines collected from the highland breeding program that are genetically related. The results suggested that genetic factors may be responsible for the differences seen in the regenerability of the tropical lines. The two lines CMB7 and CMB8 are considered suitable for further evaluation using a transformation system

QTL mapping and genetic analysis of inhibitory effect of lysine on post-germination growth and seedling establishment of maize

Radicle elongation that allows rhizosphere colonization was used as a marker of seedling establishment. The goal of the present work was to determine the genetic basis of the effect of the aspartate family pathway, in particular lysine, on the radicle elongation of maize seedlings. For this purpose a population derived from an advanced backcross between a flint European line F2 and a highland tropical line F334 as the donor parent, was used for mapping QTLs related to the effect of lysine on radicle elongation. The parental lines showed contrasting germination efficiency and root elongation under both control conditions (imbibition on water at 20°C) and on medium supplied with lysine. Two QTLs for radicle elongation under control conditions were located on chromosome 2 (136 cM) near the marker bnlg1721 and on chromosome 5 (146 cM) near the marker umc1792. These QTLs explained 9.4% and 10.5%, respectively, of the phenotypic variability for radicle elongation. When germination was carried out on medium containing 5 mM lysine, three QTLs for radicle elongation were located on chromosome 7 (90 cM) near the marker umc1112, on chromosome 10 (42 cM) near the marker bnlg1037 and on chromosome 10 (68 cM) near the marker umc1053; these QTLs explained 12.0%, 12.3% and 12.4%, respectively, of the phenotypic variability for radicle elongation. Irrespective of the germination and post-germination medium, favourable alleles for all detected QTLs were associated with parental line F334.

Registration of 26 Tropical Maize Germplasm Lines with Resistance to Striga hermonthica

Registration of 26 Tropical Maize Germplasm Lines with Resistance to <i>Striga hermonthica</i>

Characterization of Root Systems with Respect to Morphological Traits and Nitrogen-Absorbing Ability in the New Plant Type of Tropical Rice Lines

ABSTRACT The objective of this study was to characterize the root systems of the new plant type (NPT) of tropical rice (Oryza sative (L)) lines compared with the root system of IR72, one of the elite modern varieties. Two field experiments were conducted to observe plant growth under canopy conditions in the 1997 and 1998 wet seasons. In addition, a pot experiment was conducted in the 1998 dry season to observe plant growth under isolated conditions. The root systems of the NPT lines were characterized by roots heavier than those of IR72. The larger root dry weight (DW) is attributed partly to a genetically determined high root/shoot ratio because, irrespective of growth conditions, the ratio was larger in the NPT lines than in IR72. An increase in root DW was accompanied by an increase in the length of the whole root system, including fine roots. However, despite the differences between the NPT lines and IR72 in root DW and root length, there were no significant differences in the amount of nitrogen (N) absorbed throughout the growth period. Therefore, the specific absorption rate of N per root DW (SARN, mg N g−1 root DW d−1) was lower in the NPT lines than in IR72. We concluded that, although the root systems of the NPT lines grew prolifically, they were not necessarily more efficient in terms of N absorption compared with the root system of IR72.

Root-shoot interaction as a limiting factor of biomass productivity in new tropical rice lines

The objective of the present study was to identify the limiting factors in biomass productivity of new tropical rice lines (panicle weight type) by analyzing the relationship between root and shoot activity. Five field experiments using three new lines, IR65598-112-2, IR65600-42-5-2, and IR68544-29-2-1-3-1-2, and one of the highest-yielding indica rice varieties (panicle number type), IR72, were conducted at the International Rice Research Institute from 1997 to 2002. Specific absorption rate of N (SARN , mg N g-1 root dry weight d-1), specific stem sap exudation rate (sap exudation rate, g exudate g-1 root dry weight 12 h-1) and specific root respiratory rate (root respiratory rate, μmol O2 g-1 root dry weight h-1) were determined as indices of root activity. Relative growth rate (RGR, mg g-1 d-1) was used as an index of shoot activity. Compared with IR72, the new lines showed lower RGR and SARN values during the early growth stages. In contrast, during the late growth stages, these activities were higher in the new lines than in IR72. The SARB and sap exudation rate showed a linear correlation with RGR at successive growth stages, regardless of the genotypes and growth conditions. These findings indicated that active root-shoot interaction was maintained throughout the growth period in high-yielding tropical rice, including the new lines and IR72. Therefore, it was considered that SARN and the sap exudation rate are useful indices of root activity regulating RGR in the new lines. However, the root respiratory rate could not be used for selection, because the relationship between RGR and the root respiratory rate changed with the growth conditions. Our findings support the hypothesis that root and shoot activities during maturation are important factors affecting the productivity of the new lines, which have not yet been able to attain the targeted yield.

Genetic Diversity, Specific Combining Ability, and Heterosis in Tropical Maize under Stress and Nonstress Environments

Estimation of genetic diversity and distance among tropical maize (Zea mays L.) lines and the correlation between genetic distance (GD) and hybrid performance would determine breeding strategies, classify inbred lines, define heterotic groups, and predict future hybrid performance. The objectives of this study were to estimate (i) heterosis and specific combining ability (SCA) for grain yield under stress and non‐stress environments; (ii) genetic diversity for restriction fragment length polymorphisms (RFLPs) within a set of tropical lines; (iii) GD and classify the lines according to their GD; and (iv) correlation between the GD and hybrid performance, heterosis, and SCA. Seventeen lowland, white tropical inbred lines were represented in a diallel study. Inbred lines and hybrids were evaluated in 12 stress and nonstress environments. The expression of heterosis was greater under drought stress and smaller under low N environments than under nonstress environments. A set of DNA markers identifying 81 loci was used to fingerprint the 17 lines. The level of genetic diversity was high, with 4.65 alleles/locus and polymorphism information content (PIC) values ranging from 0.11 to 0.82. Genomic regions with quantitative trait loci (QTL) for drought tolerance previously identified showed lower genetic diversity. Genetic distance based on RFLP marker data classified the inbred lines in accordance with their pedigree. Positive correlation was found between GD and F1 performance (F1), SCA, midparent heterosis (MPH) and high‐parent heterosis (HPH). Specific combining ability had the strongest correlation with GD. Environment significantly affected the correlations between F1, SCA, MPH, and HPH, with lower values of GD revealed in the more stressed conditions.

Genetic Diversity, Specific Combining Ability, and Heterosis in Tropical Maize under Stress and Nonstress Environments

Estimation of genetic diversity and distance among tropical maize (Zea mays L.) lines and the correlation between genetic distance (GD) and hybrid performance would determine breeding strategies, classify inbred lines, define heterotic groups, and predict future hybrid performance. The objectives of this study were to estimate (i) heterosis and specific combining ability (SCA) for grain yield under stress and non-stress environments; (ii) genetic diversity for restriction fragment length polymorphisms (RFLPs) within a set of tropical lines; (iii) GD and classify the lines according to their GD; and (iv) correlation between the GD and hybrid performance, heterosis, and SCA. Seventeen lowland, white tropical inbred lines were represented in a diallel study. Inbred lines and hybrids were evaluated in 12 stress and nonstress environments. The expression of heterosis was greater under drought stress and smaller under low N environments than under nonstress environments. A set of DNA markers identifying 81 loci was used to fingerprint the 17 lines. The level of genetic diversity was high, with 4.65 alleles/locus and polymorphism information content (PIC) values ranging from 0.11 to 0.82. Genomic regions with quantitative trait loci (QTL) for drought tolerance previously identified showed lower genetic diversity. Genetic distance based on RFLP marker data classified the inbred lines in accordance with their pedigree. Positive correlation was found between GD and F1 performance (F1), SCA, midparent heterosis (MPH) and high-parent heterosis (HPH). Specific combining ability had the strongest correlation with GD. Environment significantly affected the correlations between F1, SCA, MPH, and HPH, with lower values of GD revealed in the more stressed conditions.

ISOLATION OF PARTHENOCARPIC TROPICAL GYNOECIOUS LINES IN CUCUMBER (CUCUMIS SATIVUS)

Three tropical gynoecious cucumber lines were developed at first time in India at Indian Agricultural Research Institute, New Delhi by More and Seshadri (1988). Further efforts were diverted by the Senior author to develop the parthenocarpic tropical gynoecious cucumber lines with yellow skinned fruits similar to that of cv. Poona Khira primarily grown in Maharashtra state. Thirteen lines (F6) with yellow skinned fruits and six lines which produce green skinned fruits and true breeding for gynoecy were isolated from a complex cross [(Table Green 68 x SC3, F2) x Poona Khira] F3 x [(GY 14 x Table Green 68, F1) x Poona Khira] F3. These 19 lines were further evaluated for skin colour, gynoecy and parthenocarpic nature during Kharif, 1996 and Spring-Summer, 1997. Among the 13 yellow skinned parthenocarpic tropical gynoecious lines four namely, PKG-1-2, PKG- 1-11, PKG-1-12 and PKG-1-15 were found to be 100 per cent parthenocarpic. Similarly among the six green fruit skinned lines three viz., PKG-1-21, PKG-1-23 and PKG-1-24 showed 100 per cent parthenocarpic nature. In the remaining yellow/ green fruit skinned lines the parthenocapy ranged from 82.6 to 96.0 per cent. Thus seven parthenocarpic tropical gynoecious yellow and green skinned fruits which showed 100 per cent parthenocarpy were evaluated for their horticultural characters during Kharif, 1998 in plastic greenhouse. PKG-1-15, PKG-1-12 and PKG-1-24 were found to be promising for earliness while PKG-1-15 and PKG-1-21 exhibited high yield potential compared to the others. Further study is in progress on how this parthenocarpic gene operates in F1 hybrids when the pure parthenocarpic tropical gynoecious line is crossed with monoecious male parent.

NE-EDRsu1 and NE-EDRbt1, Disease-resistant Sweet Corn Populations with sugary1 and brittle1 Endosperms

perate and one-half tropical sweet corn, the latter derived from tropical field corns (Brewbaker, 1978). The tropical parents were chosen for their resistance to important diseases (Brewbaker, 1979). These include the seedling, stalk, and kernel rots caused by Fusarium moniliforme Sheld., and maize mosaic virus (MMV) and maize dwarf mosaic virus (MDMV). Tropical lines also incorporate racially nonspecific tolerance of common rust, southern rust, bacterial leaf blight, northern leaf blight, brown spot, and southern leaf blight. Causal agents are Puccinia sorghi Schw., Puccinia polysora Underw., Pseudomonas avenae Mains, Exserohilum turcicum (Pass.) Leonard & Suggs, Physoderma maydis

Novel synthetic Bacillus thuringiensiscry1B gene and the cry1B-cry1Ab translational fusion confer resistance to southwestern corn borer, sugarcane borer and fall armyworm in transgenic tropical maize

In order to develop a resistance management strategy to control tropical pests based on the co-expression of different toxins, a fully modified Bacillus thuringiensiscry1B gene and the translational fusion cry1B-cry1Ab gene have been developed. Both constructs were cloned under the control of a maize ubiquitin-1 or a rice actin-1 promoter and linked to the bar gene driven by the CaMV 35S promoter. Immature embryos from the tropical lines CML72, CML216, and their hybrids, were used as the target for transformation by microprojectile bombardment. Twenty five percent of the transformed maize plants with cry1B expressed a protein that is active against southwestern corn borer and sugarcane borer. Ten percent of the transgenic maize expressed single fusion proteins from the translational fusion gene cry1B-1Ab and showed resistance to these two pests as well as to the fall armyworm. Transgenic maize plants that carried the cry1B gene in T1 to T3 progenies transmitted trangenes with expected Mendelian segregation and conferred resistance to the two target insects. Molecular analyses confirmed the cry genes integration, the copy number, the size of protein(s) expressed in maize plants, the transmission, and the inheritance of the introduced cry gene. These new transgenic products will provide another recourse for reducing the build-up of resistance in pest populations.

Coincidence of flowering time and the productivity and quality of cauliflower hybrid seeds

The missing of flowering synchronization between the self-incompatible lines in a crop field of cauliflower hybrid seeds besides making the seed production smaller can compromise the genetic purity of them. The coincidence of the flowering time between two cauliflower lines was examined to study its effect on the productivity and quality of hybrid seeds. The treatments consisted of six different sowing dates, every fifteen days, using a self-incompatible tropical line pollinated by a winter line which does not present self-incompatibility. The following characteristics were evaluated: leaf average area and number of flowers per plant, number of siliques per plant, number and weight of seeds per plant, weight of thousand seeds and average number of seeds per silique. The germination standard test and genetic seed purity were determined for each treatment. The coincident flowering season between cauliflower lines affects directly the productivity and the genetic quality of the produced hybrid seeds. The closer the flowering time coincidence between the lines, the greater the number of seeds per silique and the smaller the percentage of non-hybrid seedlings. However, the coincidence of the flowering season between lines was found to influence physiological seed quality.

Nitrate Reductase Activity of Diverse Grain Sorghum Genotypes and Its Relationship to Nitrogen Use Efficiency

High crop N use efficiency is desirable for reducing the cost and reliance on fertilizer N, and it may help reduce ground water pollution. The objectives of this study were to (i) characterize and compare the seasonal trends in nitrate reductase activity (NRA) of seven grain sorghum [Sorghum bicolor (L.) Moench] genotypes of tropical and U.S. origin, (ii) determine how NRA is related to total plant N, dry matter, grain yield, and N use efficiency, and (iii) examine how NRA and plant N accumulation affect N use efficiency for biomass (NUE1) and grain production (NUE2). The genotypes were evaluated in field trials in 1994 and 1996 at applied N levels of 0 and 100 N kg ha−1 on a Sharpsburg silty clay loam soil (fine, smectitic, mesic Typic Argiudoll). Plant biomass, reduced N, NRA, NUE1, and NUE2 were determined at three growth stages (vegetative, anthesis, and physiological maturity). Genotypes did not differ for NRA at either N level; however, there was a definite tendency for the four tropical lines and two U.S. adapted lines to have greater NRA values at all growth stages than the one hybrid. Nitrate reductase activity decreased throughout the growing season, with the sharpest decline from anthesis to maturity. Nitrate reductase activity did not correlate with grain yield or shoot biomass, but did correlate with grain N concentration. Shoot and grain N concentration correlated with NUE1, while grain and shoot N contents and shoot N concentration were correlated with NUE2. Neither NUE1 nor NUE2 were significantly affected by the NRA level. Tropical lines had greater preanthesis N uptake accumulation (1.45 g plant−1) and greater NUE1 than the hybrid (0.90 g plant−1) and U.S. adapted lines (0.73 g plant−1). In terms of grain N use efficiency, the hybrid had a 14% greater value than U.S. adapted lines and a 17% greater value than the tropical lines. It appears that NRA is not a factor that limited N accumulation and use efficiency in grain sorghum.

Production of transgenic tropical maize with cryIAb and cryIAc genes via microprojectile bombardment of immature embryos

To enhance the level of resistance to insects in tropical maize germplasm we have developed techniques to successfully transform elite tropical maize inbred based on the activity of specific cryI proteins against four major maize pests - corn earworm, fall armyworm, southwestern corn borer and sugarcane borer. Constructs containing cryIAb or cryIAc synthetic genes were used. To generate transgenic plants we have established methods for biolistic bombardment and the selection and regeneration of immature embryos and calli from the elite tropical lines CML72, CML216, CML323, CML327 and hybrids. Transgenic plants resistant to the herbicide Basta(TM) contained the bands for the cry, bar and gus genes as detected by Southern blot analyses. A simple leaf bioassay presented varying levels of resistance to Southwestern corn borer of transgenic tropical maize carrying the cryIAc gene. Analyses of the progenies confirmed the sexual transmission of the introduced genes and their stable expression.

Effect of nitrate/ammonium ratio on biomass production, nitrogen accumulation, and use efficiency in sorghums of different origin

Plant nitrogen (N) uptake, growth, and N use efficiency may be affected by N form (NO3 ‐or NH4 +) available to the root. The objectives of this study were to determine the effect of mixed N form on dry matter production and partitioning, N uptake, and biomass N use efficiency defined as total dry matter produced per unit plant N (NUE1) in U.S. and tropical grain sorghums [Sorghum bicolor (L.) Moench]. The U.S. derived genotype CK 60 and three tropical genotypes, Malisor‐7, M 35–1, and S 34, were evaluated in a greenhouse trial using three nutrient solutions differing in their NO3 ‐/NH4 + ratio (100/0, 75/25, 50/50). Shoot and root biomass, N accumulation, and NUE, were determined at 10‐leaf and boot stages. Averaged over all genotypes, shoot and root biomass decreased when NH4 + concentration was increased in the solution. Shoot biomass was reduced by 11% for 75/25 and 26% for 50/50 ratios, as compared to 100/0 NO3 ‐/NH4 +. Similarly, root biomass reduction was about 34% and 45% for the same ratios, respectively. Increasing NH4 + concentration also altered biomass partitioning between shoot and root as indicated by decreasing root/shoot ratio. Total plant N content and NUE1 were also reduced by mixed N source. Marked genotypic variability was found for tolerance to higher rates of NH4 +. The tropical line M 35–1 was well adapted to either NO3 ‐ as a sole source, or to an N source containing high amounts of NH4 +. Such a characteristic may exist in some exotic lines and may be used to improve genotypes which do not do well in excessively wet soil conditions where N uptake can be reduced.

Transpiration efficiencies of indica and improved tropical japonica rice grown under irrigated conditions

There is limited information on the transpiration efficiency defined as the ratio of photosynthesis (A) to transpiration (T) of tropical japonica rice (Oryza sativa L.). In this study, transpiration efficiency (A/T) of seven tropical japonica lines developed by the International Rice Research Institute (IRRI) were compared with seven indica cultivars one week after flowering in 1993. The gas exchange rate and A/T of one genotype from each type were compared throughout the growing season in 1994. Both A and T were measured on topmost fully expanded leaves under saturating light with a portable photosynthesis system (LI-6200). Indica cultivars had higher T than the tropical japonica lines. The differences in A between the two types were relatively small and inconsistent across growth stages and years compared with the differences in T. The A/T was 25% and 30% higher for the tropical japonica than the indica type in 1993 and 1994, respectively. The differences in T and A/T between the two types were not related to the differences in leaf N content or leaf water content. A lower carbon isotope (13C) discrimination in a tropical japonica line than an indica cultivar confirmed that the improved tropical japonica lines had higher A/T than the indica cultivars.