Germplasm Evaluation For Low-phosphorus Tolerance In Maize Under Hydroponics Culture

Maize (Zea mays L.) is a versatile cereal crop and can grow in tropical, subtropical and temperate agro-climatic conditions. Phosphorus (P) is a second most important macro-element that is essential for plant growth and development. Plants have developed complex responsive and adaptive mechanisms for foraging, remobilizing and recycling of phosphorus to retain P homeostasis. Low-phosphorus (LP) in the soil is a major yield-limiting factor in maize production, particularly in low-input agriculture and developing countries. The present studies experimental breeding approaches were applied to reveal morpho-physiological mechanisms underlying natural variation for LP tolerance in maize and to find ways to explore this variation.A total of550maize germplasm, including338from two RIL populations and69temperate and143tropical maize inbreds, were evaluated for seedling traits in hydroponic under LP (2.5×10-6mol L"-1of KH2PO4) and normal phosphorus (NP)(2.5×104mol L-1of KH2PO4) conditions. Descriptive statistics and analysis of variance revealed a wide range of variability for LP tolerance related traits. Estimated broad-sense heritability (h2) for all the measured traits ranged from70%to91%, indicating that all the traits were highly inheritable. Genetic variances were low to moderate (0.05-0.31) for most seedling traits, indicating strong treatment effects and/or complex genetic architecture. Best linear unbiased predictor (BLUP) analysis found a strong positive correlation between BLUPs and means of the traits. Shoot length was significantly correlated with other root traits, indicating that direct selection based on maximum shoot length (MSL) might be sufficient for improvement of other traits. The first two principal components (PCs) explained about81.27%of the total variation among lines for the eight maize seedling traits. The relative magnitudes of eigenvectors for the first principal component was59.35%, explained mostly by total dry matter (TDM), shoot dry weight (SDW), root dry weight (RDW) shoot fresh weight (SFW), root fresh weight (RFW), maximum root length (MRL) and MSL. Genotype by trait (GXT) biplot revealed superior genotypes with combinations of favorable traits. The average genetic distance was3.53, ranging from0.25to20.01, indicating high levels of variability among the germplasm. A multi-trait selection index was calculated based on principal component analysis (PCA) using all measured traits, and30accessions with tolerance to LP stress were selected. These lines could be of potential use for breeding LP tolerance maize.Root network system (RNS) traits were measured from images of220inbred lines using GiA Root software. The inbred lines grew up to15days under hydroponic conditions in the high lux plant growth room with LP and NP treatments. Analysis of variance revealed a wide range of variability among the inbred lines, and heritability estimates ranged from0.59to0.95for all RNS traits, indicating consistency across experiments. The proportions of genetic variance ranged from0.01-0.60in the maize RNS traits. There was a strong positive, linear relationship between best linear unbiased predictors and estimated means. The Euclidean genetic distances ranged from0.61to29.33, indicating high levels of variability among the inbred lines. The first three PCs explained more than79%of total genetic variation, which were mostly contributed by network length (NWL), network surface area (NWSA), network perimeter (NWP), network area (NWA), maximum number of roots (MANR), median number of roots (MENR), network volume (NWV), network convex area (NWCA), specific root length (SRL), network depth (NWD), number of connected components (NCC) and network width (NWW). The G×T biplot revealed superior genotypes with combinations of favorable traits. Some outstanding genotypes with higher values of most RNS traits were identified. These lines could be of potential use for breeding LP tolerance maize.P deficiency in plants triggers many transcriptional, biochemical, and physiological changes that ultimately help the plants absorb P from the soil or improve the P use efficiency. Substantial genetic variation in P efficiency exists among the maize genotypes. It is expected that integration of systems biology with high-throughput, high-dimensional and precision phenotyping will contribute to the development of maize varieties tolerant to LP stress.