Interest in belowground plant growth is increasing, especially in relation to arguments that shallow-rooted cultivars are efficient at exploiting soil phosphorus while deep-rooted ones will access water at depth. However, methods for assessing roots in large numbers of plants are diverse and direct comparisons of methods are rare. Three methods for measuring root growth traits were evaluated for utility in discriminating rice cultivars: soil-filled rhizotrons, hydroponics and soil-filled pots whose bottom was sealed with a non-woven fabric (a potential method for assessing root penetration ability). A set of 38 rice genotypes including the OryzaSNP set of 20 cultivars, additional parents of mapping populations and products of marker-assisted selection for root QTLs were assessed. A novel method of image analysis for assessing rooting angles from rhizotron photographs was employed. The non-woven fabric was the easiest yet least discriminatory method, while the rhizotron was highly discriminatory and allowed the most traits to be measured but required more than three times the labour of the other methods. The hydroponics was both easy and discriminatory, allowed temporal measurements, but is most likely to suffer from artefacts. Image analysis of rhizotrons compared favourably to manual methods for discriminating between cultivars. Previous observations that cultivars from the indica subpopulation have shallower rooting angles than aus or japonica cultivars were confirmed in the rhizotrons, and indica and temperate japonicas had lower maximum root lengths in rhizotrons and hydroponics. It is concluded that rhizotrons are the preferred method for root screening, particularly since root angles can be assessed.

The rapid progress in rice genotyping must be matched by advances in phenotyping. A better understanding of genetic variation in rice for drought response, root traits, and practical methods for studying them are needed. In this study, the OryzaSNP set (20 diverse genotypes that have been genotyped for SNP markers) was phenotyped in a range of field and container studies to study the diversity of rice root growth and response to drought. Of the root traits measured across more than 20 root experiments, root dry weight showed the most stable genotypic performance across studies. The environment (E) component had the strongest effect on yield and root traits. We identified genomic regions correlated with root dry weight, percent deep roots, maximum root depth, and grain yield based on a correlation analysis with the phenotypes and aus, indica, or japonica introgression regions using the SNP data. Two genomic regions were identified as hot spots in which root traits and grain yield were co-located; on chromosome 1 (39.7–40.7 Mb) and on chromosome 8 (20.3–21.9 Mb). Across experiments, the soil type/ growth medium showed more correlations with plant growth than the container dimensions. Although the correlations among studies and genetic co-location of root traits from a range of study systems points to their potential utility to represent responses in field studies, the best correlations were observed when the two setups had some similar properties. Due to the co-location of the identified genomic regions (from introgression block analysis) with QTL for a number of previously reported root and drought traits, these regions are good candidates for detailed characterization to contribute to understanding rice improvement for response to drought. This study also highlights the utility of characterizing a small set of 20 genotypes for root growth, drought response, and related genomic regions.

Root research requires high throughput phenotyping methods that provide meaningful information on root depth if the full potential of the genomic revolution is to be translated into strategies that maximise the capture of water deep in soils by crops. A very simple, low cost method of assessing root depth of seedlings using a layer of herbicide (TRIK or diuron) buried 25 or 30 cm deep in soil‐filled boxes of varying size is described that is suitable for screening hundreds or thousands of rice accessions in controlled environment conditions. Variation in cultivar sensitivity to the herbicide when injected into pots was detected but considered small in relation to the variation detected when the herbicide was buried. Using 32 rice cultivars previously characterised for root traits in rhizotron and hydroponic systems, 80% of variation in herbicide score at 35 days was explained by cultivar and herbicide score correlated strongly with rooting depth traits. Using 139 genotypes of the Bala × Azucena mapping population, heritability for herbicide symptoms reached 55% and quantitative trait loci were detected which match those previously reported in this population. In repeated experiments using different soils, the method did not always perform to its maximum potential (in terms of speed of symptom development or discrimination between cultivars). This was not due to degradation or reduced bio‐availability of the herbicide in the soil but is believed to be due to the soil water content and water release characteristics as it relates to plant water use. Therefore, when using this technique, thorough preliminary experiments to determine the best water application regime for the particular combination of soil and environmental conditions are required. The method should be applicable to seedling stage screening of rice and other crops.

A field experiment was carried out in the fields of the Field Crops Department - Faculty of Agricultural Engineering Sciences. The study included five inbred lines (ZM43W (ZE), ZM60, ZM49W3E, ZM19, CDCN5), given numbers 1, 2, 3, 4 and 5) to study the hybrid vigor and both general and special combing ability (GCA, SCA) of the half diallel mating method, for the spring and fall seasons (2016). The genetic analysis shows that all crosses gave a positive hybrid vigor for grain yield per unit area at the two population densities. the highest value is 116.20% for cross (3´5 )at low density, and 89.22% for cross( 1´4 )at high density. The hybrid vigor for all crosses is positive at two densities for dry matter yield, crop growth rate and ears weight. The highest value is 81.31%, 96.30% and 131.45% at high density for these traits for the cross (1´2), respectively. Also, this cross gave the highest value at high density for grain yield per plant (170.61%) and (85.43%) for no. of grain plant-1. The general combining ability in two densities for all studied traits. The highest positive value was (48.949) for parent 3 at low density. All values of s2sca are more than values of s2gca, and all values of s2D are more than all values of s2A. For this, all h2n.s. were low. It ranges from 1.88% for the crop growth rate to 18.82% for no. of rows ear-1 at low density and between -0.38 for the crop growth rate to 41.42 for 300-grain weight at high density. Because the values of s2D are higher than values of s2A, the values of the ratio of s2gca/s2gsca were less than one, while the value of were mor than one. This indicates that all these traits are influenced by dominance genes, and the importance of the non-additive gene action and its large contribution to the inheritance of these traits. Keywords: maize, combining ability, heritability, genetic parameters.