The recurrent California drought necessitates investigation of the relationship between water application, crop yields, and management practices. The majority of cultivars in many crops are genetically dwarfed which allows the application of larger amounts of water and fertilizer in return for higher yields and ease of harvesting. This project used bread wheat as a model system to investigate the water use and water-use efficiency of taIl and dwarf cultivars. Four near-isogenic lines, rhtrht (tall), RhtlRhtl (semidwarf), Rht2Rht2 (semidwarf), and Rht3Rht3 (dwarf), in 'Maringa' bread wheat background and four of their near-isogenic Fl hybrids derived from crossing the original lines were used to determine the effects of dwarfing genes on plant height, water use, grain yield, total dry matter, and wateruse efficiency in well-watered and droughted pot experiments in the glasshouse. The nearisogenic lines and their six F 1 hybrids were also grown in well-watered and droughted field conditions. The glasshouse season lasted 158 days, whereas the field season took 149 days between planting and harvesting. Carbon isotope discrimination was determined as a measure of transpiration efficiency. The near-isogenic lines used similar amounts of water in well-watered (12 kg per 158 days) and droughted (9 kg per 158 days) pot experiments. The Rht3Rht3 dwarf line actually used 3% less water than the tall line in a well-watered situation, and 5% less water than tall line in a droughted situation, but these differences were not significant in this experiment. Plant height ranged from 60 to 124 em and from 53 to 121 ern in well-watered and droughted pot experiments, and it varied from 50 to 94 cm and from 49 to 90 cm in well-watered and droughted field experiments, respectively. Total dry matter, grain yield, transpiration efficiency (total dry matter/water used), and water-use efficiency (grain yield/water used) declined with plant height in well-watered glasshouse conditions. No significant relationships were found between plant height and these traits in droughted glasshouse conditions. Carbon isotope discrimination was negatively correlated with transpiration efficiency, but significantly so only in well-watered pot experiments. Plant height was negatively associated with carbon isotope discrimination in both well-watered and droughted pot and field experiments. Grain yield and aboveground dry matter also declined with plant height in field conditions. In most cases, the dwarfing genes reduced shoot dry matter more than grain yield, therefore, harvest index of the semidwarf and dwarf lines was higher than that of the tall standard line. The dwarfism caused by Rhtl, Rht2, and Rht3 genes had, in general, depressing effects on transpiration efficiency, water-use efficiency, total dry matter, and grain yield. An optimum range for plant height was determined (90- 100 ern) using these near-isogenic lines, below which shoot dry matter, grain yield, and water-use efficiency were significantly reduced.

Bread wheat (Triticum aestivum L.) is the second most cultivated cereal crop after rice. Many present day bread wheats carry a centric rye-wheat translocation 1RS.1BL in place of chromosome 1B. The increased grain yield of translocation lines is positively associated with root biomass. To map loci controlling root characteristics, homoeologous recombinants of 1RS with 1BS were used to generate an integrated genetic map comprised of 20 phenotypic and molecular markers, with an average spacing of 2.5 cM. To identify the chromosomal region associated with rooting ability, root phenotypic data were subjected to Quade analysis to compare genotypes. The distal 15% of the rye 1RS arm may carry QTL for greater rooting ability in bread wheat.

To identify QTL for individual root traits, a phenotyping experiment was conducted involving recombinants from each marker interval of the 1RS-1BS genetic map. An empirical Bayes method was applied to estimate additive and epistatic effects for all possible marker pairs simultaneously in a single model. This method has an advantage for QTL analysis in minimizing the error variance and detecting interaction effects between loci with no main effect. Four common regions were identified to involve a total of 15 QTL effects, six additive and nine epistatic, for different root traits in 1RS wheat. Three of four regions were localized in the distal 15% of the 1RS arm.

The effect of different dosages (0 to 4) of the 1RS translocation on root morphology and anatomy in bread wheat was determined. The F1 hybrid with single dose of 1RS and 1AS arms showed heterosis for root and shoot biomass. Root biomass was positively associated with increase in dosage of 1RS. This study also provided evidence of the presence of gene(s) influencing root anatomical traits. The central metaxylem vessel diameter was negatively associated with increasing 1RS dosage. Wheat genotypes with higher number of 1RS translocation arms may have inherent morphological and anatomical advantage over normal bread wheat to survive under stress conditions.

Background and aims

Methods

Key results

Conclusions

Bread wheat is an allohexaploid crop with a large and complex genome structure. It was one of the first crops domesticated by human beings in the Near East and it had dramatic effects on human history. The amount of energy gained per hour of work from wheat was much higher than hunting and gathering. Ancient farmers continuously selected it to increase seed size, grain yield, and straw yield to feed a growing population; this scenario has not changed for thousands of years. Plant scientists, breeders, and farmers are still working to improve grain yield to support growing demand. Introduction of genetic variation with novel alleles has been a major component of plant breeding, especially after the major genetic bottlenecks of the last century.

I have aimed to retrieve some of the available genetic variation within the Triticeae tribe by screening wild wheat relatives, landraces, modern wheats, and a synthetic wheat population. All of the above materials were evaluated to genetically locate and identify root system traits.

Breeding for drought tolerance requires selection for traits that improve water uptake and use efficiency. Root system traits are a major component to improve water acquisition. Here we report significant genotypic variation for root traits within and between wheat wild relatives, bread wheat landraces, modern wheats, and the Synthetic W7984* Opata M 85 doubled haploid (SynOpDH) mapping population. Up to four-fold difference for root size within wild accessions, eight-fold difference within landraces and modern wheats and fourteen-fold difference within the progeny of the SynOpDH population was observed.

This large range in genotypic variation may provide many useful alleles for breeders, especially those who target rain-fed growth conditions in their breeding programs. Even though studying the root system is technically challenging, and time and labor intensive, an urgent need for drought tolerant crops makes it a necessity. There is a major need for research leading to a complete understanding of the genetic control of the wheat root system. Advances in genotyping technologies, marker assisted selection, and fast / accurate phenotyping may provide useful tools to select root traits such as, deep root biomass, number of seminal and nodal roots, and root angle with less labor and time, to develop drought tolerant wheat cultivated varieties.

It was reported that wheat cultivars with reduced metaxylem diameters in roots produced higher grain yields compared to those with larger metaxylem diameters under drought conditions, but not under well-watered conditions. Multiple dosages of rye 1RS translocations in wheat were reported to reduce root metaxylem diameter. The dosage number of 1RS translocations was inversely proportional to the root metaxylem diameter.

Two greenhouse experiments and a field experiment were carried out at University of California, Riverside in 2012 and 2013 under well-watered and terminal drought treatments to test whether translocation lines of wheat cv. Pavon 76 containing 1(RA1), 2 (RA2 and RD2), and 4 dosages (RAD4) of 1RS produced higher grain yields than the control with zero dosage (R0) under terminal drought. Drought in the field experiment reduced grain yield significantly. Grain yield of RA2 (23.9 g-1) was significantly greater than those of RD2 (20.6 g-1) and RAD4 (17.3 g-1) under well-watered field conditions. Also, grain yield of RAD4 was significantly less than those of RA1, R0, and RD2 under well-watered conditions. Under drought field conditions, only significant difference in grain yield was found between RA1 (14.7 g-1) and RAD4 (12.4 g-1). Harvest index was significantly greater in well watered (44.2%) than in droughted (34.6%) field conditions. On average, genotype R1 (42.3%) and RA2 (40.6%) had higher harvest index than RD2 (38.3%) and DAR4 (35.5%) in the field. Also, R0 (40.2) and RD2 (38.3) had higher harvest index than DAR4. In general, Pavon 1RS.1AL carrying double doses of 1RS showed higher grain yields under wet treatments. Pavon 1RS.1AL-1RS.1DL carrying four doses of 1RS produced largest shoot and root biomasses, but relatively had least grain yield. There was a negative correlation between the grain yield loss and metaxylem diameter in roots at mild stress levels (~36%).