Physiological Responses of Hard Red Winter Wheat to Infection by Wheat streak mosaic virus.

Wheat streak mosaic virus (WSMV) causes significant yield loss in hard red winter wheat in the U.S. Southern High Plains. Despite the prevalence of this pathogen, little is known about the physiological response of wheat to WSMV infection. A 2-year study was initiated to (i) investigate the effect of WSMV, inoculated at different development stages, on shoot and root growth, water use, water use efficiency (WUE), and photosynthesis and (ii) understand the relationships between yield and photosynthetic parameters during WSMV infection. Two greenhouse experiments were conducted with two wheat cultivars mechanically inoculated with WSMV at different developmental stages, from three-leaf to booting. WSMV inoculated early, at three- to five-leaf stage, resulted in a significant reduction in shoot biomass, root dry weight, and yield compared with wheat infected at the jointing and booting stages. However, even when inoculated as late as jointing, WSMV still reduced grain yield by at least 53%. Reduced tillers, shoot biomass, root dry weight, water use, and WUE contributed to yield loss under WSMV infection. However, infection by WSMV did not affect rooting depth and the number of seminal roots but reduced the number of nodal roots. Leaf photosynthetic parameters (chlorophyll [SPAD], net photosynthetic rate [Pn], stomatal conductance [Gs], intercellular CO2 concentration [Ci], and transpiration rate [Tr]) were reduced when infected by WSMV, and early infection reduced parameters more than late infection. Photosynthetic parameters had a linear relationship with grain yield and shoot biomass. The reduced Pn under WSMV infection was mainly in response to decreased Gs, Ci, and SPAD. The results of this study indicated that leaf chlorophyll and gas exchange parameters can be used to quantify WSMV effects on biomass and grain yield in wheat.

Response of Aegilops species to drought stress during reproductive stages of development.

Drought stress is an important abiotic factor limiting wheat yield. Thirty-one accessions of Aegilops species belonging to five species were screened to identify species/accessions tolerant to drought stress and to measure traits associated with the tolerance. Plants were grown at full irrigation, 25/19°C day/night temperature and an 18h photoperiod. At anthesis (Feekes 10.5.1), drought stress was imposed by withholding water for 16 days. Controls were continuously irrigated. Drought stress decreased chlorophyll content, grain number, individual grain weight and grain yield by 31, 25, 68 and 76% compared with the control. Aegilops geniculata Roth had greater tolerance to drought stress for yield (48% decline from control) compared with other species (>73% decline from control). The tolerance was associated with higher grain number spike-1 and heavier grains. A. geniculata, GenBank accession number TA 10437, was highly tolerant to drought stress with <20% yield decline and a drought stress susceptibility index (DSI) <0.5, whereas TA 1802, TA 2061, TA 1814, TA 1819 were identified as moderately tolerant to drought stress (20-40% yield decline and DSI<1.0). Our results suggest a presence of genetic variability among Aegilops species that can be utilised in breeding wheat for tolerance to drought stress at reproductive stages.

Effects of drought and high temperature stress on synthetic hexaploid wheat.

Drought and high temperature often occurs simultaneously, causing significant yield losses in wheat (Triticum aestivum L.). The objectives of this study were to: (i) quantify independent and combined effects of drought and high temperature stress on synthetic hexaploid wheat genotypes at anthesis and at 21 days after anthesis; and (ii) determine whether responses to stress varied among genotypes. Four synthetic hexaploid and two spring wheat genotypes were grown from emergence to anthesis (Experiment I) and emergence to 21 days after anthesis (Experiment II), with full irrigation and 21/15°C day/night temperature. Thereafter, four treatments were imposed for 16 days as (a) optimum condition: irrigation+21/15°C, (b) drought stress: withhold irrigation+21/15°C, (c) high temperature stress: irrigation+36/30°C and (d) combined stress: withhold irrigation+36/30°C. Results indicated a decrease in leaf chlorophyll, individual grain weight and grain yield in an increasing magnitude of drought