Ioannis S. Tokatlidis

Density effects on environmental variance and expected response to selection in maize (Zea mays L.)

The study aimed to address the optimal plant population density in maize that maximizes phenotypic expression and differentiation, and lessens environmental effects on genotypic expression in terms of the response to selection. A set of seven short-season hybrids (Rom set) was tested under rainfed conditions (2006, 2007) in Romania, and a set of seven long-season hybrids (Gr set) was tested with irrigation (2007) in Greece. Experimentation was conducted under ultra-low (ULD), low (LD), middle (MD), and high (HD) densities (0.74, 2.51, 4.20, 8.40 plants/m2 for the Rom set, and 0.74, 3.13, 6.25, 8.33 plants/m2 for the Gr set). Phenotypic expression and differentiation for grain yield were highest at the ULD. Coefficient of variation (CV) for grain yield, ear length and kernel row number decreased as density decreased. Environmental conditions and hybrid plant-yield potential (i.e., maximum yield per plant) were crucial for the optimal density that achieved the lowest environmental variance. For the Rom set the lowest CV for grain yield was obtained at the LD in the unfavourable season and at the ULD in the favourable season. The less acquired variance was achieved at the ULD for the highest yielding hybrids and at the LD for the lowest yielding hybrids, revealing a negative association between plant-yield potential and optimal density. Concluding, a density proximal to the ULD approximates absence of competition in maize, and optimizes three determinant parameters for successful selection: selection intensity, heritability and phenotypic differentiation.

Development of crop cultivars by honeycomb breeding

The ability of agriculture to adapt to environmental changes and to address main issues of food quality and environment protection is a fundamental factor in achieving sustainability. Low yield capacity of contemporary sustainable farming systems, however, is a major obstacle to future growth of sustainable agriculture. In addition, increasing pressure is placed for higher food supply due to the projected population increase. To overcome these barriers and stimulate the wide adoption of sustainable agriculture, ample supply of cultivars that satisfy the requirements for sustainability without compromising productivity is essential. Otherwise, the viability of sustainable agriculture is unsound. Moreover, plant breeding has to be a non-stop process supporting agriculture because of the ongoing climate changes. The studies of the effects of competition on crop yield and selection efficiency unravelled important findings for plant breeders. Firstly, the uppermost cultivar type is the mono-genotypic and particularly the highest evolutionary grade of ‘pure line’. Secondly, single plant selection is effective only when it is realized in the absence of competition for growth resources. Honeycomb methodology, by considering as a major principle the application of selection in the absence of competition, counteracts the disturbing effects of competition on selection effectiveness. Furthermore, the honeycomb experimental designs cope with the confounding implications of soil heterogeneity. These two findings help breeders to consider the individual plant as an evaluating and selection unit. As a consequence, the development of pure line cultivars that fully meet the needs of a sustainable agriculture is possible. Most importantly, honeycomb breeding exploits effectively not only favourable but marginal environments as well through the development of density-neutral cultivars. Marginal environments are exploited optimally when lower plant populations are used. It is of essence to realize that without the ability of exploiting successfully marginal environments which represent the majority of the production environments globally, sustainability in agriculture becomes problematic.

WET SEWAGE SLUDGE APPLICATION EFFECT ON SOIL PROPERTIES AND ELEMENT CONTENT OF LEAF AND ROOT VEGETABLES

ABSTRACT Pot experiments were conducted in a heated greenhouse to study the effects of increasing doses of sewage sludge application on vegetables grown for leaves (lettuce, endive, spinach) and roots (radish, carrots, beets), and on some soil properties. Results showed that sewage sludge application increased or decreased soil pH in cases of lower or higher values respectively, increased soil organic matter, and increased soil concentrations in all mineral elements studied, except for manganese (Mn) and cobalt (Co). Cadmium (Cd) was not detected in soil-sewage sludge mixtures. After sewage sludge application, edible plant parts of leaf and root vegetables had increased copper (Cu) and zinc (Zn) concentrations and decreased Mn concentrations, whereas iron (Fe) concentrations remained almost unchanged in the leaf vegetables and were lower in the root vegetables. Cobalt, lead (Pb), and Cd were not detected in plant tissues. Increasing doses of sewage sludge resulted in higher dry matter yield of vegetable edible parts, with those grown for roots responding stronger.

The effect of improved potential yield per plant on crop yield potential and optimum plant density in maize hybrids

Honeycomb selection in the F 2 generation of maize (Zea mays L.) hybrid PR 3183, based on line performance per se in the absence of competition, led to recycled hybrids with improved potential yield per plant (mean yield per plant in the absence of competition). In the present study six S 6 x S 6 recycled hybrids and two commercial single-cross hybrids (PR 3183 and B73 x Mo17) were tested at three plant densities (25000, 41667 and 83333 plants/ha), in two locations (Technological Education Institute farm of Florina, Greece and University farm of Thessaloniki, Greece), for 2 years (1998. 1999). The study was undertaken to assess indirectly the potential yield per plant (p), the crop yield potential (Y max ), and the optimum plant density (D opt ) of the hybrids. Estimate of p and Y max were obtained through linear regression analysis of yield per plant (Y p ) on plant density (D), expressed by the equation Y p = p-qD, with Y max being equal to (1/4)p 2 q -1 . Optimum plant density was assessed through linear regression analysis of natural logarithm of yield per plant on plant density, expressed by the equation ln(Y p ) = α-bD, with D opt being equal to l/b. The recycled hybrids had higher estimated potential yield per plant (p). than the two check hybrids, with p values being positively correlated with yield per plant of hybrids obtained experimentally in the absence of competition. Results indicated that the higher potential yield per plant decreases the optimum plant density, and renders the hybrids less density-dependent.

Wheat Landraces Are Better Qualified as Potential Gene Pools at Ultraspaced rather than Densely Grown Conditions

The negative relationship between the yield potential of a genotype and its competitive ability may constitute an obstacle to recognize outstanding genotypes within heterogeneous populations. This issue was investigated by growing six heterogeneous wheat landraces along with a pure-line commercial cultivar under both dense and widely spaced conditions. The performance of two landraces showed a perfect match to the above relationship. Although they lagged behind the cultivar by 64 and 38% at the dense stand, the reverse was true with spaced plants where they succeeded in out-yielding the cultivar by 58 and 73%, respectively. It was concluded that dense stand might undervalue a landrace as potential gene pool in order to apply single-plant selection targeting pure-line cultivars, attributable to inability of plants representing high yielding genotypes to exhibit their capacity due to competitive disadvantage. On the other side, the yield expression of individuals is optimized when density is low enough to preclude interplant competition. Therefore, the latter condition appears ideal to identify the most promising landrace for breeding and subsequently recognize the individuals representing the most outstanding genotypes.

SAMPLING THE SPATIAL HETEROGENEITY OF THE HONEYCOMB MODEL IN MAIZE AND WHEAT BREEDING TRIAS: ANALYSIS OF SECONDARY DATA COMPARED TO POPULAR CLASSICAL DESIGNS

The systematic rather than random entry arrangement of honeycomb designs (HDs) has been deployed to sample the spatial heterogeneity. This hypothesis was studied in fairly homogeneous populations, assuming that their phenotypic variability stemmed absolutely from spatial rather than genetic heterogeneity. It was based on single plant performance in two separate trials of a maize hybrid and a wheat cultivar reflecting different level of spatial heterogeneity. In general, the HDs counteracted spatial heterogeneity well, particularly when the number of evaluated entries was limited. There was a suggestion that they do well even in a high number of entries with many replicates per entry. Distribution and layout of spatial heterogeneity across the experimental area did not affect the precision of the HDs. Standardized configuration, which ensures implementation of essential principles met in other experimental models such as blocking, replication and nearest-neighbour (NN) adjustment on the same baseline, renders the honeycomb experimental pattern advantageous over the classical experimental designs like the randomized complete block (RCB), the NN method and the lattice model.

Crop adaptation to density to optimise grain yield: breeding implications

Competition between crop plants, due to resource limitation, is at the root of a considerable yield limitation, a major problem that future agriculture is faced with. Due to inter-plant variation, intra-crop competition causes plant-to-plant interference and unbalanced use of input which decreases possible profit. Acquired intra-crop competition is a priori present in farming due to spatial heterogeneity. Genetically imposed intra-crop competition is due to the plant-to-plant genetic differences, i.e. the intra-species genetic competition in multi-genotypic varieties grown alone, and both the intra- and inter-species genetic competition in intercropping multi-genotypic varieties. In general, high densities accelerate the acquired plant-to-plant variation and intensify the intra-crop competition. Considering environmental diversity, an additional yield gap element is density-reliance. Density-reliant varieties are inefficient in resource use at the single-plant level and present poor results at low densities, accompanied by variation in optimum density particularly in rain-fed agroecosystems. The remedy relies on breeding of varieties that comprise the ‘weak competitor’ ideotype(s) of improved plant yield efficiency in order to mitigate the intra-crop variation and optimise the resource use across variable conditions. To focus on both, selection at nil-competition (widely spaced plants to preclude interference for inputs) is necessary. Selection among spaced plants is further supported by the negative relationship between genotype competitive and yielding ability. The derived density-independent varieties would be capable of taking advantage of the abundance of resources in favourable seasons at low densities that are suitable for dry seasons, approaching the attainable yield across locations and seasons.

Relationships of grain carbon isotope discrimination (Δ) and ash content with yield and quality in dry bean

Dry bean ( Phaseolus vulgaris L.) is a traditional crop of north-western Macedonia, Greece, where two landraces ( plaki Prespas and Chrisoupoli ) in particular are grown. The aim of this study was to test whether the grain carbon isotope discrimination (Δ) and ash content (ASH) are related to grain yield (GY), protein content (PC) and mean grain weight (MGW) in dry bean. As a part of a honeycomb selection programme, 21 genotypes (19 lines derived via intra-landrace honeycomb selection for single plant yield at low density plus the two original populations) were grown at two densities, 1·2 and 4·8 plants/m 2 under non-limiting water conditions in a glasshouse and in the field. Genotypes differed significantly for Δ, ASH and PC under the low density. In the dense stand, genotypes differed significantly only for ASH and PC. The environment (glasshouse or field conditions) affected all the traits significantly. Neither Δ nor ASH showed strong or consistent relationships with GY and thus, they cannot serve as reliable, indirect selection criteria for GY. Strong, negative relationships between Δ and PC were found especially in the dense stand, confirming analogous results in C 3 cereals. Inconsistent genotypic ranking for grain physiological traits under the different densities and environments was indicative of large genotype×environment interaction. Genotypes performed consistently for GY and MGW under both densities, showing the strong stability of these traits.

Genetic Mechanisms Enhancing Plant Biodiversity

Biodiversity is essential for an evolving ecosystem and as a resource for further development of natural products by breeding. At present agriculture is under pressure by the demand for increased crop production and the public anticipation for sustainable cultivation practices. Undeniably, the prerequisite for adaptation of an organism to changing environmental conditions is genetic variability. The answer to the concern that the persistent accumulation of desirable alleles in a few cultivars could erode genetic variability and ultimately impede further improvement, comes from the fields of genetic and epigenetic studies that have revealed a range of mechanisms which result in remarkable variability even in narrow gene pools.

Beneficial Exploitation of Additive Genetic Effects to Improve Yield Potential per Plant in Maize

The effect of inbreeding on yield potential per plant and the load of deleterious genes in maize ( Zea mays L.) were studied. Twelve recycled hybrids obtained though inbreeding and selection on the basis line performance per se , along with their original hybrid PR 3183 and the check hybrid B73×Mo17, were tested in a R28 honeycomb trial under very low density (0.74 plants/m 2 ). Inbreeding depression was related with reduced yield potential per plant and increased CV of individual plant yield (estimated in F 1 and F 3 ), as well as with increased heterosis. The recycled hybrids were found to have improved yield potential per plant in both F 1 and F 3 , lower CV values in F 3 , and lower inbreeding depression. Results were indicative of the effective exploitation of the additive genetic effects, which reduced the load of deleterious genes, and produced less heterozygous hybrids that were characterized by improved yield potential per plant.