Diallel analysis of photosynthetic rate and related leaf characters among contrasting genotypes of Lolium perenne

Combining ability, heritability and genotypic relations of different physiological traits in cacao hybrids

Selecting parents and evaluating progenies is a very important step in breeding programs and involves approaches such as understanding the initial stages of growth and characterizing the variability among genotypes for different parameters, such as physiological, growth, biomass partitioning and nutrient translocation to the aerial part. In these cases, facilitating tools can be used to understand the involved gene dynamics, such as diallel crosses and genetic and phenotypic correlations. Our main hypothesis is that the contrasting phenotypes of these parental genotypes of cocoa used are due to genetic factors, and progenies derived from crosses of these parental genotypes are useful for breeding programs related to plant architecture, physiological parameters and translocation of mineral nutrients. We aimed to evaluate the combining abilities in progenies of cacao (Theobroma cacao L) originating from contrasting parents for canopy vigor. Emphasis was given to the evaluation of morphological and physiological parameters and the phenotypic and genotypic correlations to understand the dynamics of the action of the genes involved, as well as in expression profile from genes of gibberellins biosynthesis pathway in the parents. Fifteen F1 progenies were obtained from crosses of six clones (IMC 67, P4B, PUCALA, SCA 6, SCA 24 and SJ 02) that were evaluated in a randomized complete block design with four replicates of 12 plants per progeny, in a balanced half table diallel scheme. It is possible to identify and select plants and progenies of low, medium and high height, as there is expressive genetic variability for the evaluated parameters, some of these on higher additive effects, others on larger nonadditive effects and others under a balance of these effects. Most physiological parameters evaluated show that for selection of plants with the desired performance, no complex breeding methods would be necessary due to the high and medium heritability observed. Strong genetic components were observed from many of the correlations, which indicate the possibility to formulate selection indices for multi-traits, such as dwarfism or semidwarfism, tolerance to increase of leaf sodium concentrations and maintenance of the photosynthetic apparatus integrity under these conditions. Additionally, plants with higher carbon fixation, better water use, higher carboxylation efficiency and greater magnesium accumulation in leaves can be selected.

Genetic analysis of photosynthesis and productivity in corn

The genetic relationships among different inbred stocks of corn with respect to different indices of photosynthetic efficiency and plant productivity was assessed by means of diallel graphs. The salient feature of the current study was "apparent overdominance" for some important indices of photosynthetic efficiency, viz. photosynthetic rate/unit leaf area (at the silking stage), total chlorophyll content and chlorophyll a content, as well as for total dry matter production and economic yield. The results could be explained in terms of complementary gene action and the multiplicative effects of the sub-components that comprise these traits. However, complete dominance was recorded for photosynthetic rate (at the grain filling period), leaf area/plant, number of leaves/plant, number of leaves above the ear, ratio of chlorophyll a/b and harvest index. In the majority of cases graphic analysis was possible only when a number of epistatic parents were omitted from the analysis. Although the position of the array points for different indices of photosynthetic efficiency was consistent across the years, the ontogenetic and seasonal differences in the genetic behaviour of parents, meaning thereby, different loci being active at different stages and seasons, was apparent. The same was true for the epistatic parents. Non-allelic interaction of the genes rather than the non-random distribution of the genes among the parents seemed to be the most common cause of disturbance in the Wr/Vr relationship.Inbred stock MG 115 was identified to be the most promising because it embodied an efficient photosynthetic machinery by virtue of increasing the number of alleles for a majority of the indices of photosynthetic efficiency, thereby enabling it to register the highest biological productivity and economic yield. Further, inbred stocks MG 138, MG 121, and MG 125 were also promising for different photosynthetic parameters.

A genetic analysis of the photosynthetic properties of populations of Danthonia spicata that have different growth responses to light level

Three populations of the grass Danthonia spicata were observed to have different rates of biomass accumulation when grown in common environment treatments. The populations were native to adjacent sites of different successional age and different levels of shading. Twelve individuals from each population were clonally replicated and two replicates were grown in each of two light treatments, 100% and 22% of unshaded sunlight. Following growth in the treatments the populations all exhibited the same mean light-saturated photosynthetic rate of 11.7 μmol m^-2s^-1. This rate is intermediate for published values of sun and shade species and for species from along a successional gradient. There was no difference in photosynthetic rate among treatments. There was significant genetic variation for lightsaturated photosynthetic rate within populations but no significant differences among populations. The populations had similar leaf water potential values of-1.12 MPa in all treatments. There were significant differences among treatments and genotypes for specific leaf weight which resulted in significant differences among treatments and no significant differences among genotypes in light-saturated photosynthetic rate expressed on a leaf weight basis. Lightsaturated photosynthetic rate had a high heritability and low plasticity. We postulate that photosynthetic rate is under strong selection and that the observed rates permit populations of D. spicata to grow in a wide range of habitat light levels.

Cytogenetics of Lolium perenne : Part 1: Chiasma frequency variation in inbred lines

A number of inbred lines of Lolium perenne have been developed as far as the fifth inbred generation and are being used for basic studies on the cytogenetics of the species. An analysis on variation in the pattern of chiasma frequency and distribution, including the parent plants and all generations down to the S5, reveals that the effect of inbreeding is to reduce chiasma frequency and to increase both the cell and bivalent variances. Evidence is presented for a genetic basis of polygenic control of chiasma formation and distribution, and a model is suggested for control over the three related components of variation which involves a two-stage level of regulation.

A 9 × 9 diallel analysis in peanuts (A. hypogaea L.): Flowering time, tops' weight, pod yield per plant and pod weight

Four quantitative traits were studied by analysing F2 data derived from a 9 × 9 diallel cross utilizing widely divergent, inbred, erect cultivars of peanuts, A. hypogaea. Bidirectional dominance was found in the traits total pod yield per plant and number of days from planting to first flower; in pod size, the alleles giving small pods were consistently dominant and for high tops' weight, dominance and overdominance were found. The high heritability of pod yield/plant (0.79) indicates that breeding for higher yield/plant can succeed if large F2 populations are grown and rigorous visual selection combined with progeny testing are employed. The genetic correlations of pod yield/plant with other traits were low. Breeding for plants with large (jumbo) pods can be aided by the fact that they are homozygous recessive, or nearly so. Simultaneous breeding for high yields and large pods is possible: there was a positive (but low) genetic correlation between the two (0.16). A modification by which less biased estimates of the number of effective factors can be obtained and a possible relationship between bidirectional dominance and genic interaction were proposed.

DIALLEL AND PATH COEFFICIENT ANALYSIS OF TALL FESCUE (FESTUCA ARUNDINACEA) REGROWTH UNDER CONTROLLED CONDITIONS

Relationships among leaf elongation rate, leaf width, leaf area expansion rate, number of tillers/plant, and net CO2 exchange (NCE) with total forage yield/plant of tall fescue, Festuca arundinacea Schreb., were studied in a controlled environmental chamber. Among progeny of a 6 × 6 diallel, number of tillers/plant was correlated highly significantly (1% level) with total forage yield/plant (r = 0.697). Path coefficient analyses showed that both tillers/plant and yield/tiller had large positive direct effects on total yield/plant. Utilizing a diallel analysis it was found that additive genetic effects were most important for all traits except for NCE. Progenies did not differ significantly for NCE. Nonadditive genetic effects, although of less magnitude than additive effects, were also found to be important for number of tillers/plant and yield/tiller. Highest broad sense heritabilities were found for number of tillers/plant, yield/tiller, and yield/plant with values of 69, 66, and 66%, respectively