A key strategy in soybean drought research is increased stomatal sensitivity to high vapor pressure deficit (VPD), which contributes to the 'slow wilting' trait observed in the field. These experiments examined whether temperature of the growth environment affected the ability of plants to respond to VPD, and thus control transpiration rate (TR). Two soybean [Glycine max (L.) Merr.] and four wild soybean [Glycine soja (Sieb. and Zucc.)] genotypes were studied. The TR was measured over a range of VPD when plants were growing at 25 or 30°C, and again after an abrupt increase of 5°C. In G. max, a restriction of TR became evident as VPD increased above 2.0 kPa when temperature was near its growth optimum of 30°C. 'Slow wilting' genotype plant introduction (PI) 416937 exhibited greater TR control at high VPD compared with Hutcheson, and only PI 416937 restrained TR after the shift to 35°C. Three of the four G. soja genotypes exhibited control over TR with increasing VPD when grown at 25°C, which is near their estimated growth optimum. The TR control became engaged at lower VPD than in G. max and was retained to differing degrees after a shift to 30°C. The TR control systems in G. max and G. soja clearly were temperature-sensitive and kinetically definable, and more restrictive in the 'slow wilting' soybean genotype. For the favorable TR control traits observed in G. soja to be useful for soybean breeding in warmer climates, the regulatory linkage with lower temperatures must be uncoupled.

Hybrids account for nearly all commercially planted varieties of maize and many other crop plants because crosses between inbred lines of these species produce first-generation [F1] offspring that greatly outperform their parents. The mechanisms underlying this phenomenon, called heterosis or hybrid vigor, are not well understood despite over a century of intensive research. The leading hypotheses—which focus on quantitative genetic mechanisms (dominance, overdominance, and epistasis) and molecular mechanisms (gene dosage and transcriptional regulation)—have been able to explain some but not all of the observed patterns of heterosis. Abiotic stressors are known to impact the expression of heterosis; however, the potential role of microbes in heterosis has largely been ignored. Here, we show that heterosis of root biomass and other traits in maize is strongly dependent on the belowground microbial environment. We found that, in some cases, inbred lines perform as well by these criteria as their F1 offspring under sterile conditions but that heterosis can be restored by inoculation with a simple community of seven bacterial strains. We observed the same pattern for seedlings inoculated with autoclaved versus live soil slurries in a growth chamber and for plants grown in steamed or fumigated versus untreated soil in the field. In a different field site, however, soil steaming increased rather than decreased heterosis, indicating that the direction of the effect depends on community composition, environment, or both. Together, our results demonstrate an ecological phenomenon whereby soil microbes differentially impact the early growth of inbred and hybrid maize.

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Water availability for turfgrass systems is often limited and is likely to become more so in the future. Here, we conducted experiments that examined the ability of tall fescue (Festuca arundinacea Schreb.) to control transpiration with increasing vapour pressure deficit (VPD) and determined whether control was influenced by temperature. The first study was under steady-state conditions at two temperatures (21 and 27 C) and two VPDs (1.2 and 1.8 kPa). At the lower temperature, water use was similar at both VPDs, indicating a restriction of transpiration at high VPD. At 27 C, transpiration control at high VPD was weakened and root growth also declined; both responses increase susceptibility to water-deficit stress. Another series of experiments was used to examine the physiological stability of the transpiration control. Temperature and VPD were adjusted in a stepwise manner and transpiration measured across a range of VPD in the days following environmental shifts. Results indicated that VPD control acclimated to the growth environment, with adjustment to drier conditions becoming evident after~1 week. Control was again more effective at cool than at hot temperatures. Collectively, the results indicate that transpiration control by this cool season grass is most effective in the temperature range where it is best adapted.

Hybrids account for nearly all commercially planted varieties of maize and many other crop plants, because crosses between inbred lines of these species produce F1 offspring that greatly outperform their parents. The mechanisms underlying this phenomenon, called heterosis or hybrid vigor, are not well understood despite over a century of intensive research (Birchler et al. 2003). The leading hypotheses-which focus on quantitative genetic mechanisms (dominance, overdominance, and epistasis) and molecular mechanisms (gene dosage and transcriptional regulation)-have been able to explain some but not all of the observed patterns of heterosis (Stuber et al. 1992;Birchler 2015). However, possible ecological drivers of heterosis have largely been ignored. Here we show that heterosis of root biomass and germination in maize is strongly dependent on the belowground microbial environment. We found that, in some cases, inbred lines perform as well by these criteria as their F1 offspring under sterile conditions, but that heterosis can be restored by inoculation with a simple community of seven bacterial strains. We observed the same pattern for seedlings inoculated with autoclaved vs. live soil slurries in a growth chamber, and for plants grown in fumigated vs. untreated soil in the field. Together, our results demonstrate a novel, ecological mechanism for heterosis whereby soil microbes generally impair the germination and early growth of inbred but not hybrid maize.

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The noxious weed Benghal dayflower has become a severely troublesome agricultural weed in Georgia in the southeastern Unite States, and there are indications that it is moving northward. Benghal dayflower is glyphosate tolerant and possesses a high degree of reproductive elasticity, making it a formidable threat in many crop systems. The purpose of these experiments was to develop the first temperature response profiles for Benghal dayflower, and use them to evaluate whether temperature might limit its northward invasion into North Carolina and adjacent states on the U.S. east coast. Experiments focused on vegetative and early reproductive growth, stages considered crucial for establishment and competitiveness. Exposure to a range of aerial temperatures revealed that Benghal dayflower growth and production of aerial and subterranean reproductive structures were maximized at 30 C, with sharp declines occurring at cooler temperatures. When exposed to differing root temperatures in hydroponics, with a constant aerial temperature, Benghal dayflower growth did not show the same cool temperature sensitivity, but reproductive performance declined when temperatures decreased below about 29 C. The root temperature responses of several other weed species known to thrive in the climate of this geographic area also were determined. Growth of sicklepod, hemp sesbania, and jimsonweed was more sensitive than Benghal dayflower to cool temperatures, whereas the growth response of velvetleaf was similar. Based on the comparison of the Benghal dayflower temperature responses in controlled environments to (1) seasonal air and soil temperatures in the field, and (2) the temperature responses of agronomic weeds known to thrive in the region, it is concluded that cool temperatures will not restrain the northward spread of Benghal dayflower into North Carolina.