Surfaces and interfaces o er new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides 1-4 . Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO 3 (001) surface 5-7 to reveal a remarkably complex evolution of electron-phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO 3 /SrTiO 3 interface 8-13 , our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron-phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO 3 -based 2DELs.Carrier concentration is a key parameter defining the ground state of correlated electron systems. At the LaAlO 3 /SrTiO 3 interface, the 2DEL density can be tailored by field-effect gating. As the system is depleted of carriers, its ground state evolves from a high-mobility 2DEL 4 into a two-dimensional superconductor 8-10 with pseudogap behaviour 11 and possible pairing above T c (ref. 12). An analogous 2DEL can be induced by doping the (001) surface of SrTiO 3 . As for the interface, the surface 2DEL is confined by a band-bending potential in SrTiO 3 and consists of an orbitally polarized ladder of quantum confined Ti t 2g electrons that are highly mobile in the surface plane [5][6][7]14 . Thus far, the surface 2DEL has been studied only at carrier densities around 2 × 10 14 cm −2 , approximately a factor of five higher than typically observed at the LaAlO 3 /SrTiO 3 interface [5][6][7] . In the following, we present ARPES data extending to lower carrier densities that are directly comparable to the LaAlO 3 /SrTiO 3 interface. We achieve this by preparing SrTiO 3 (001) wafers in situ, which results in well-ordered clean surfaces that can be studied by ARPES over extended timescales, as they are less susceptible to the ultraviolet-induced formation of charged oxygen vacancies reported for cleaved SrTiO 3 5,7,15,16 . Details of the sample preparation are given in Methods. Figure 1a shows an energy-momentum intensity map for a 2DEL with a carrier density of n 2D ≈ 2.9 × 10 13 cm −2 estimated from the Luttinger volume of the first light subband and the two equivalent heavy subbands (see Supplementary Section 2). The most striking features of this data are replica bands at higher binding energy following the dispersion of the primary quasiparticle (QP) bands. The replica bands are all separated by approximately 100 meV and progressively lose intensity, but can be visualized up to the third replica in the curvature plot shown in Fi...

H igh temperature stress is an important yield limiting factor in both spring and winter wheat (Triticum aestivum L.). At the present rates of greenhouse gas emissions and population growth, it is expected that mean surface air temperatures will increase in the range of 1.8 to 5.8°C by the end of this century (Intergovernmental Panel on Climate Change, 2007). It is predicted that future climates will not only be associated with an increase in mean temperatures (Easterling et al., 1997) but also with an increase in the frequency of episodes of high temperatures . In addition, climate models foresee that there will be a relatively greater increase in nighttime temperatures as compared to daytime temperatures. Over the past century global daily minimum temperatures increased more than twice compared to increases in daily maximum temperatures (Easterling et al., 1997). Recent studies have shown that historical yields of rice (Oryza sativa L.; Peng et al., 2004) and wheat (Lobell et al., 2005) were strongly correlated with minimum (nighttime) temperatures, rather than daytime maximum temperatures. Decreasing rice yields in the Philippines were related to increasing nighttime temperatures (Peng et al., 2004), and increasing wheat yields in Mexico were related to decreasing nighttime temperatures (Lobell et al., 2005). ABSTRACTClimate models predict greater increases in nighttime temperature in the future. The impacts of high nighttime temperature on wheat (Triticum aestivum L.) are not well understood. Objectives of this research were to quantify the impact of high nighttime temperatures during reproductive development on phenology, physiological, vegetative, and yield traits of wheat. Two spring wheat cultivars (Pavon-76 and Seri-82) were grown at optimum temperatures (day/night, 24/14°C; 16/8 h light/dark photoperiod) from sowing to booting. Thereafter, plants were exposed to four different nighttime temperatures (14, 17, 20, 23°C) until maturity. The daytime temperature was 24°C across all treatments. There were signifi cant infl uences of high nighttime temperatures on physiological, growth, and yield traits, but no cultivar or cultivar by temperature interactions were observed. High nighttime temperatures (>14°C) decreased photosynthesis after 14 d of stress. Grain yields linearly decreased with increasing nighttime temperatures, leading to lower harvest indices at 20 and 23°C. High nighttime temperature (≥20°C) decreased spikelet fertility, grains per spike, and grain size. Compared to the control (14°C), grain fi lling duration was decreased by 3 and 7 d at night temperatures of 20 and 23°C, respectively. High nighttime temperature increased the expression of chloroplast protein synthesis elongation factor in both cultivars suggesting possible involvement of this protein in plant response to stress.

No abstract

High temperature and drought stress are among the two most important environmental factors influencing crop growth, development and yield processes. These two stresses commonly occur in combination. Objectives of this research were to investigate the independent and combined effects of high temperature and drought stress during grain filling on physiological, vegetative and yield traits and expression of a chloroplast protein synthesis elongation factor (EF‐Tu) of wheat (Triticum aestivum L.). Two spring wheat cultivars (Pavon‐76 and Seri‐82) were grown at control temperatures (CT; day/night, 24/14 °C; 16/8 h photo/dark period) from sowing to heading. Thereafter, one half of the plants were exposed to high temperature stress (HT; 31/18 °C in Exp. 1 and 34/22 °C in Exp. 2), drought stress (withholding water), or a combination of both HT and drought stress. There were significant influences of HT and/or drought stress on physiological, growth and yield traits. There was no cultivar or cultivar by temperature or cultivar by drought interaction effects on most traits. The decreases in leaf photosynthesis were greater at HT compared with drought alone throughout the stress period, and the combination of HT and drought had the lowest leaf photosynthetic rates. Overall, HT or drought had similar effects (about 48–56 % decrease) on spikelet fertility, grain numbers and grain yield. High temperature decreased grain numbers (by 56 % averaged across both experiments) and individual grain weight (by 25 %), while, respective decreases due to drought were 48 % and 35 %. This suggests that the grain numbers were more sensitive to HT and grain weights to drought for the range of temperatures tested in this research. The interaction between HT and drought stress was significant for total dry weights, harvest index and spikelet fertility, particularly when HT stress was severe (34/22 °C). The combined effects of HT and drought were greater than additive effects of HT or drought alone for leaf chlorophyll content, grain numbers and harvest index. High temperature stress and the combination of HT and drought stress but not drought stress alone resulted in the overexpression of EF‐Tu in both spring wheat cultivars.