In their findings, as root biomass increased beyond a certain threshold it negatively impacted grain yields, perhaps due to increased costs of maintaining a large root system. This point appears worthy of detailed study as it may point to which root system traits will be beneficial in a given environment and how they might impact yields if conditions become favorable. Perhaps a large root system may be beneficial when water is limited; however, will a large root system remain an advantage if water becomes sufficient? These types of questions need to be answered before efforts are made toward modifying crop root system traits. Understanding the genetics of root system traits does not have to wait until we have all the answers. Since root traits are highly plastic and regulated by a number of small-effect loci it will likely take some time to unravel these complex traits. Yaseen and Malhi reported that wheat genotypes varied significantly in their allocation of dry matter in roots and shoots. Frageria also demonstrated that root dry weight was genetically controlled as well as being influenced by the environment. Currently,grow table the neglect of selecting for root system traits is mainly due to the difficulty of measuring root system traits of field-grown plants .
As our understanding of root system genetics is improved new tools such as genetic markers associated with desired traits can be utilized by breeders for marker-assisted breeding efforts. Two systems were used in the experiments described below. One consisted of tubes fitted with a plastic sleeve filled with sand. A filter paper covering two holes punctured at the bottom of the sleeve allowed for drainage. In the standard system water is applied to the top of the tube . In a modified version an additional plastic sleeve was fitted to allow for water to be delivered from the bottom up.The second system consisted of pots lined with a plastic sleeve, filled with sand and four holes punctured in the plastic for drainage. Pots were brought above water holding capacity and allowed to drain for 24 hours before being planted. Peters Excel fertilizer was injected to the irrigation water at a 1:100 ratio. Every irrigation event in the experiments included this ratio of fertilizer.Three doubled haploid bread wheat populations were used to measure total root biomass at various stages of growth. Parents were selected for their root biomass among 16 spring wheat land races and modern cultivars tested by Waines et al. . Cv. Sonora ranked among the highest, Foisy was intermediate, and CBdeM had a low total root biomass. These parents have other contrasting phenotypes for traits such as drought tolerance, plant height, days to heading, awn type, and seminal root angle. Detailed information about genotyping, linkage mapping and general descriptions of each population can be found in the first chapter of this dissertation. Populations Sonora x CBdeM , Sonora x Foisy , and CBdeM x Foisy have 146, 141, and 128 lines respectively. In 2013 the three parental lines were evaluated for root biomass in 80cm tubes using the standard method.
These were grown for 30 days and 60 days as preliminary evaluations at two growth stages. Plants were grown in a factorial design with four replications. Doubled haploid progeny were grown in one gallon pots with four replications in a randomized complete block design for 21 and 28 days in 2014 and 2015 respectively. The 2016 experiments included 100 randomly selected progeny from each population with three replications due to constraints on greenhouse space. Plants were grown until heading . Plants were watered as needed to keep the sand at water holding capacity for the duration of the experiments. To gain a broad perspective about the relationship between shoot and root biomass, data from multiple experiments were combined. Data sets from experiments running for a similar duration were combined to fit into a similar scale. Following these criteria three sets of data were created. The first set was created from the data of the trade off experiments described below and data kindly provided by Dr. Harun Bektas . The second set is from the data collected during the allelic variation experiments and the mapping population data collected in 2016 mentioned bellow. The third set is from the 2014 and 2015 mapping population experiments described below. Raw values for individual were used to create scatter plots fitted with a Loess smoothing curve with an alpha of 0.75 with a quadratic degree using statistical analysis software, Statistix.Plants were grown to maturity in the modified tube system and the pot system. Seeds of Pavon and Yecora Rojo were imbibed for 24 hours before planting. For each cultivar two treatments plus a control were run in a factorial design with two replications in fall 2015 and winter 2016. The control was given 500mL of water daily, from the top down.
The fist treatment received water from the bottom only starting at two weeks of growth. Water from the bottom was kept at the furthest point at which the roots reached within the sand as visible through the clear plastic sleeve of two check tubes. As the roots grew into the water profile the water level was continuously dropped until the roots reached the bottom of the tube. The second treatment received water from both the top and the bottom daily. Water from the bottom was maintained at 50cm to prevent roots from growing deeper and 500mL of water was added each day from the top. The pot experiments had three replications in fall 2015 and winter 2016 setup in a factorial design having three treatments and a control. Each pot, for both cultivars,ebb flow table was kept at water holding capacity until three different phenological stages: booting, heading and anthesis at which point water was cut entirely. Treatments are termed as drought-at booting, drought-at-heading, and drought-at-anthesis. After that point, any plant showing severe water stress was given water to prevent death. This point was determined when leaves began to wilt and curl beyond mild symptoms. The control was given ample water to maintain the sand at water holding capacity throughout the experiment. During the experiments days to booting, days to heading, days to anthesis, and days to maturity were recorded. Days to booting was recorded at the point when the flag leaf emerged. Days to heading was recorded at the point when the head split the boot. Days to anthesis was recorded at the point when the first anthers became dehiscent. Days to maturity was recorded at the point when the grain was ready to harvest. At maturity the total number of tillers and total number of fertile tillers were counted. Heads were harvested and the shoots were cut at the soil surface to separate them from the roots. Shoot biomass is reported without grain yield included in the total biomass. Heads were threshed to record grain yield for each plant. For the tube experiments, root length was measured and root biomass above 30 cm was separated from the root biomass bellow 30 cm and weighed separately. The analyses of variance for traits in each experiment were based on mean values of the experimental units and considered significant where p ≤ 0.05. Genotype means were used for LSD all-pairwise comparisons where α = 0.05. In experiments involving the mapping populations, broad sense heritability and Pearson‟s correlation coefficients for shoot and root biomass were calculated on a mean basis across four replications. Genomic regions associated with root and shoot biomass were detected by the software package IciMapping using linkage maps for the mapping populations as described previously and the mean value of four seedlings of each genotype from four replicates in 2014 and 2015; in 2016 the mean value of three seedlings from three replications was used. The composite interval mapping method with a step of 1cM was used and the threshold for QTL detection was determined using 1000 permutations where α = 0.05. Markers in the linkage maps were renamed using the index number provided by Wang et al. preceded by the chromosome designation. In total 6,353 data points were included to create the scatter plots for the combined data analysis of the relationship between root and shoot biomass . The first set included 1,243 data points . The second set had 1,342 points . The third set had a total of 3,768 data points .
In the first set of data, root mass versus shoot mass and grain yield were plotted as those experiments went until maturity. All other experiments were concluded earlier having only shoot and root biomass collected. Data for the fall and winter 2016 trade off experiments were not significantly different so the data were combined. In the pot experiments Pavon 76 did not show significant differences between drought treatments and the control for days to booting, days to heading, or days to anthesis with means of 49.1, 54.3, and 59.1 days respectively. However, days to maturity showed significant differences between the treatments and the control, with means of 92.8 and 137.5 days respectively. The total number of tillers was also significantly different between the treatments and the control with means of 9.3 and 34.3, respectively. The total number of fertile tillers showed significant differences within treatments as well as between treatments and the control. Plants experiencing drought-at booting had means of 5.5 fertile tillers, those at-heading and at-anthesis 8.3 fertile tillers, and the control had means of 32.2 fertile tillers per plant. For shoot biomass, the treatments showed significantly less biomass than the control with a mean of 11.9 grams versus 36.0 grams for the control. Root biomass was significantly different between plants receiving drought-at-anthesis versus those at-booting and plants receiving drought-at-heading had a similar mean when compared to either of the other treatments. Mean root biomass for plants receiving drought at booting, heading, and anthesis were 3.5, 4.9, and 5.1 grams respectively. All drought treatments were significantly less than the control which had a mean total root biomass of 7.1 grams. Grain yield for the drought treatments were significantly less than the control which had a mean yield of 51.3 grams. Plants receiving drought-at-heading yielded the second highest with a mean of 12.0 grams and both the drought-at-booting and drought-at-anthesis treatments yielded the lowest with a mean of 6.3 grams. Similar to Pavon 76 in the pot experiments, Yecora Rojo did not show significant differences between drought treatments and the control for days to booting, days to heading, or days to anthesis with means of 29.6, 36.7, and 41.6 days, respectively. Days to maturity showed a significant difference between the treatments and the control with means of 77.9 and 106.5 days, respectively. The total number of tillers for the drought treatments was 6.6 which was significantly lower than the control with 9.5 tillers per plant. Treatments showed significant differences with the drought-at-booting and drought-at-anthesis treatments having means of 5.0 and 7.0 fertile tillers respectively. Plants receiving drought-at-heading were intermediate between the other two treatments with a mean of 6.0 tillers per plant which was not significantly different the other treatments. All treatments had lower means than the control which had 8.8 fertile tillers per plant. For shoot biomass all treatments were similar with a mean of 3.1 grams which was significantly different from the control of 4.7 grams. The control had the highest root biomass with 2.6 grams per plant and the drought-at-anthesis treatment had the next largest root biomass with 1.9 grams per plant. Both the booting and heading treatments had a mean of 1.3 grams per plant. Grain yield also showed significant difference within the treatments and between the treatments and the control. The control yielded 11.3 grams per plant, the drought-at-anthesis yielded 5.7 grams per plant, and the drought-at heading treatment yielded 4.7 grams per plant which was not significantly different from the drought-at-anthesis treatment or the drought-at-booting treatment, and the drought-at booting treatment yielded the lowest with 3.6 grams per plant. In the tube experiments Pavon did not show significant differences between the treatments and control for days to booting, days to heading, days to anthesis, or days to maturity with means of 55.13, 60.6, 64.8, and 125.2 respectively.