•Leaf-scale VPD is a better ecosystem stressor compared to atmospheric VPD.•Ecosystem conductance response is suboptimal, i.e. typically between Medlyn’s (VPD−0.5) and Leuning’s (VPD−1) models.•Diel ...variations of canopy conductance are independent of soil moisture.•The only dependence on soil moisture is on the daily minimum ecosystem conductance.
Ecosystem conductance, which describes ecosystem regulation of water and carbon exchange and links plant functions with the environment, is a critical component in ecosystem and earth system models. However, the behaviors of ecosystem conductance at the ecosystem level and its responses to environmental conditions are still largely unclear. In this study, half-hourly data of 77 eddy-covariance sites from the FLUXNET2015 dataset were used to compare four ecosystem conductance models at the ecosystem level and determine the most consistent vapor pressure deficit (VPD) dependence across plant functional types for varying soil moisture stress levels at the subdaily time scale. We used leaf-level VPD (VPDl), a better indicator of atmospheric dryness at the leaf level, for canopy-level analysis instead of measured atmospheric VPD. Detection of the best-fitted exponent of VPDl indicates that ecosystem conductance responds to VPD between optimality-theory (i.e., VPD−0.5 dependence) and Leuning’s (i.e., VPD−1 dependence) models. Accounting for different soil moisture stress levels only affected minimum ecosystem conductance and did not affect the exponent and factor of VPDl, indicating limited diurnal soil moisture-VPDl interactions. These results indicate limited interaction between xylem and stomata at subdaily time scales and that soil moisture effects can be simplified as a regulation of minimum (soil plus canopy) conductance.
•Identified pearl millet genotypes showing contrasting transpiration response to high vapor pressure deficit (VPD).•Cloned seven PgAQP genes using homology based gene identification.•PgAQP proteins ...were found to be evolutionarily closer to maize than rice.•PgAQPs were induced in VPD-insensitive genotypes under low and high VPD conditions.•PgPIP2;1, PgPIP1;2, PgTIP2;2 and PgPIP2;6 genes followed a diurnal rhythm of expression.
Pearl millet is a crop of the semi-arid tropics having high degree of genetic diversity and variable tolerance to drought stress. To investigate drought tolerance mechanism that possibly accounts for differences in drought tolerance, four recombinant inbred lines from a high resolution cross (HRC) were selected for variability in their transpiration rate (Tr) response to vapour pressure deficit (VPD) conditions. The differential Tr response of the genotypes to increased VPD conditions was used to classify the genotypes as sensitive or insensitive to high VPD. Aquaporin (AQP) genes PgPIP1;1, PgPIP1;2, PgPIP2;1, PgPIP2;3, PgPIP2;6, PgTIP1;1 and PgTIP2;2 were cloned. Phylogenetic analysis revealed that the cloned PgAQPs were evolutionarily closer to maize AQPs than to rice. PgAQP genes, including PgPIP1;1 and PgPIP2;6 in root tissue showed a significant expression pattern with higher expression in VPD-insensitive genotypes than VPD-sensitive genotypes under low VPD conditions (1.2kPa) i.e when there is no high evaporative demand from the atmosphere. PgAQP genes (PgPIP2;1 in leaf and root tissues; PgPIP1;2 and PgTIP2;2 in leaf and PgPIP2;6 in root) followed a diurnal rhythm in leaves and roots that have either higher or lower expression levels at different time intervals. Under high VPD conditions (4.21kPa), PgPIP2;3 showed higher transcript abundance in VPD-insensitive genotypes, and PgPIP2;1 in VPD-sensitive genotypes, while rest of the PgAQPs showed differential expression. Our current hypothesis is that these differences in the expression of AQP genes under different VPDs suggests a role of the AQPs in tuning the water transport pathways with variation between genotypes.
•The midday high VPD can inhibit greenhouse tomato production in the winter season.•VPD control increases net photosynthetic rate by modifying leaf stomatal traits.•VPD control significantly ...increases tomato yield and biomass.
In greenhouses during the winter season, a high vapor pressure deficit (VPD), particularly due to air exchange during midday period, can limit plant biomass and yield. Thus, the VPD control in greenhouses is of immense importance for cultivating plants. In the present study, the effects of VPD controlled by a fogging system on greenhouse environment and tomato plant growth were studied during a winter season. The VPD was effectively reduced by the fogging system from 1.4 to 0.8kPa on average in the midday during the entire winter season. Maintaining a lower VPD in the midday increased tomato leaf stomatal index and stomatal conductance during the major part of the day, which led to increase in net photosynthetic rate. Furthermore, maintaining a lower VPD increased mean tomato biomass and yield by 17.3% and 12.3%, respectively. Thus, it is concluded that the VPD control via the fogging system promotes plant growth and productivity by improving photosynthesis during the winter season.
Terrestrial primary productivity and carbon cycle impacts of droughts are commonly quantified using vapour pressure deficit (VPD) data and remotely sensed greenness, without accounting for soil ...moisture. However, soil moisture limitation is known to strongly affect plant physiology.
Here, we investigate light use efficiency, the ratio of gross primary productivity (GPP) to absorbed light. We derive its fractional reduction due to soil moisture (fLUE), separated from VPD and greenness changes, using artificial neural networks trained on eddy covariance data, multiple soil moisture datasets and remotely sensed greenness.
This reveals substantial impacts of soil moisture alone that reduce GPP by up to 40% at sites located in sub-humid, semi-arid or arid regions. For sites in relatively moist climates, we find, paradoxically, a muted fLUE response to drying soil, but reduced fLUE under wet conditions.
fLUE identifies substantial drought impacts that are not captured when relying solely on VPD and greenness changes and, when seasonally recurring, are missed by traditional, anomaly-based drought indices. Counter to common assumptions, fLUE reductions are largest in drought-deciduous vegetation, including grasslands. Our results highlight the necessity to account for soil moisture limitation in terrestrial primary productivity data products, especially for drought-related assessments.
Pronounced warming occurring on the Tibetan Plateau is expected to stimulate alpine grassland growth but could also increase atmospheric aridity that limits photosynthesis. But there lacks a ...systematic assessment of the impact of atmospheric aridity on alpine grassland productivity. Here we combine satellite observations, flux‐tower‐based productivity, and model simulations to quantify the effect of atmospheric aridity on grassland productivity and its temporal change between 1982 and 2011. We found a negative impact of atmospheric vapor pressure deficit on grassland productivity. This negative effect becomes increasingly intensified in terms of the impact severity and extent, suggesting an increasingly important role of atmospheric aridity on productivity. We further demonstrated that this negative effect is mitigated but cannot be overcompensated by the positive effect of rising CO2. Given that vapor pressure deficit is projected to further increase by ~10–38% in the future, Tibetan alpine grasslands will face an increasing stress of atmospheric drought.
Plain Language Summary
Significant climatic warming occurred in most parts of the globe and could lead to increased atmospheric drought stress world widely. However, the trends of atmospheric drought condition gained inadequate attentions and their effects on vegetation growth are not clear. Here we combined observational and model evidences to point out a significant increasing trend of atmospheric aridity (vapor pressure deficit VPD) across Tibetan alpine grasslands during the past decades. Moreover, an increasingly detrimental role of VPD was found for grassland productivity, with respect to both extent and intensity of the impact. Under the future gas emission scenarios, VPD will further increase and ecosystems might face amplified drought stress from atmosphere, in addition to commonly recognized soil aridity.
Key Points
A significant increasing trend of VPD was found across Tibetan alpine grasslands over the past three decades
The elevated VPD played an increasingly detrimental role on grassland growth, with respect to impact extent and intensity
The negative effect of VPD would become amplified without the effect of rising CO2
Evapotranspiration (ET) is an important component of the global hydrological cycle. However, to what extent transpiration ratios (T/ET) are controlled by vegetation and the mechanisms of global‐scale ...T/ET variations are not clear. We synthesized all the published papers that measured at least two of the three components (E, T, and ET) and leaf area index (LAI) simultaneously. Nonlinear relationships between T/ET and LAI were identified for both the overall data set and agricultural or natural data subsets. Large variations in T/ET occurred across all LAI ranges with wider variability at lower LAI. For a given LAI, higher T/ET was observed during later vegetation growing stage within a season. We developed a function relating T/ET to the growing stage relative to the timing of peak LAI. LAI and growing stage collectively explained 43% of the variations in the global T/ET data set, providing a new way to interpret and model global T/ET variability.
Key Points
We reported the practical upper limit of vegetation control on T/ETFor a given leaf area, higher T/ET ratios occurred in the latter growing stage within a seasonGrowing stage and LAI explain 43% of the global T/ET variability
•A systematic review of studies on robot-centric assembly line balancing is provided.•RALBP and ALBP-HRC studies are categorized based on their optimization objectives.•The research trends in RALBP ...and ALBP-HRC are discussed using descriptive analysis.•Research gaps are identified, and directions for future studies are suggested.•The most commonly used solution methods and addressed line layouts are identified.
Assembly-line balancing is a significant issue in production systems. Employing industrial robots as the main production resource was a milestone in developing assembly lines, and emerging Industry 4.0 led industries to build collaborative assembly lines by combining robots and human operator skills. Recently, the majority of research on assembly line balancing has contributed to addressing aspects of utilizing robots in assembly lines and how they can increase line performance. Various models and methods are developed, considering different objectives and performance indicators. Despite the increasing number of studies in this area, a thorough literature review is lacking in identifying gaps, shedding light on research directions, and facilitating future development. This study systematically reviews assembly-line balancing studies targeted at assembly lines with industrial and collaborative robots. Studies are classified based on their objectives and reviewed for their solution method, line layout, and other essential specifications. A descriptive analysis is provided to assist researchers and practitioners in linking different properties of assembly lines to the objectives and applied methodologies. The results show that most studies developed models and solution methods that focused on simultaneously optimizing more than one objective. The review reveals that minimizing the cycle time is the most popular objective, and meta-heuristic algorithms are the dominant solution approaches. It is also observed that balancing assembly lines with collaborative robots has received more attention in the last five years with the emergence of Industry 4.0. The review also highlights gaps in the related literature and provides promising insights for future research.
Atmospheric pollution could significantly alter tree growth independently and synergistically with meteorological conditions. North China offers a natural experiment for studying how plant growth ...responds to air pollution under different meteorological conditions, where rapid economic growth has led to severe air pollution and climate changes increase drought stress. Using a single aspen clone (Populus euramericana Neva.) as a ‘phytometer’, we conducted three experiments to monitor aspen leaf photosynthesis and stem growth during in situ exposure to atmospheric pollutants along the urban-rural gradient around Beijing. We used stepwise model selection to select the best multiple linear model, and we used binned regression to estimate the effects of air pollutants, atmospheric moisture stress and their interactions on aspen leaf photosynthesis and growth. Our results indicated that ozone (O3) and vapor pressure deficit (VPD) inhibited leaf photosynthesis and stem growth. The interactive effect of O3 and VPD resulted in a synergistic response: as the concentration of O3 increased, the negative impact of VPD on leaf photosynthesis and stem growth became more severe. We also found that nitrogen (N) deposition had a positive effect on stem growth, which may have been caused by an increase in canopy N uptake, although this hypothesis needs to be confirmed by further studies. The positive impact of aerosol loading may be due to diffuse radiation fertilization effects. Given the decline in aerosols and N deposition amidst increases in O3 concentration and drought risk, the negative effects of atmospheric pollution on tree growth may be aggravated in North China. In addition, the interaction between O3 and VPD may lead to a further reduction in ecosystem productivity.
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•Nitrogen deposition can stimulate aspen growth by increasing canopy nitrogen uptake.•Aerosols can promote aspen growth through diffuse radiation fertilization effects.•Among all pollutants, ozone is the most important factor to inhibit aspen growth.•The effect of VPD on aspen growth is negative and aggravated by ozone pollution.
The response of leaf photosynthesis and stem growth to multiple atmospheric pollutants.
Producing more food per unit of water has never been as important as it is at present, and the demand for water by economic sectors other than agriculture will necessarily put a great deal of ...pressure on a dwindling resource, leading to a call for increases in the productivity of water in agriculture. This topic has been given high priority in the research agenda for the last 30 years, but with the exception of a few specific cases, such as water-use-efficient wheat in Australia, breeding crops for water-use efficiency has yet to be accomplished. Here, we review the efforts to harness transpiration efficiency (TE); that is, the genetic component of water-use efficiency. As TE is difficult to measure, especially in the field, evaluations of TE have relied mostly on surrogate traits, although this has most likely resulted in over-dependence on the surrogates. A new lysimetric method for assessing TE gravimetrically throughout the entire cropping cycle has revealed high genetic variation in different cereals and legumes. Across species, water regimes, and a wide range of genotypes, this method has clearly established an absence of relationships between TE and total water use, which dismisses previous claims that high TE may lead to a lower production potential. More excitingly, a tight link has been found between these large differences in TE in several crops and attributes of plants that make them restrict water losses under high vapour-pressure deficits. This trait provides new insight into the genetics of TE, especially from the perspective of plant hydraulics, probably with close involvement of aquaporins, and opens new possibilities for achieving genetic gains via breeding focused on this trait. Last but not least, small amounts of water used in specific periods of the crop cycle, such as during grain filling, may be critical. We assessed the efficiency of water use at these critical stages.
•VPD is crucial for microclimate indoor cultivation.•Managing VPD is important for the growth of spinach microgreens in McPF.•Dry weight rose by 71 % eight days after transplanting.•The height of ...plants increased 57 % in 8 days post-transplant.•VPD control enhaces yield by 18.9 % in McPF and 167 % in UMPF.
Harnessing the power of Vapor Pressure Deficit (VPD) control, this novel research illuminates a pathway to enhanced plant growth and yield within Micro-Plant Factory (McPF) and Urban Mini-Plant Factory (UMPF) environments. The research focused on the growth parameters of spinach microgreens, with the experimental data originating from each system's upper, central, and lower shelves. Results indicated significantly enhanced plant height and dry weight on the central shelves, attributed to optimal VPD control. Growth analysis also revealed higher relative growth rates (RGR) under tightly managed VPD conditions on the central shelves, even amidst varying environmental parameters. Most notably, effective VPD control translated these growth metrics into increased yields. For instance, a 71 % and 66 % increase in final plant dry weight over the lower and upper shelves was noted. These advances were mirrored in the plant height, which also saw 57 % and 103 % growth. Overall, the findings underscore the critical role of VPD control in plant growth and yield enhancement within structured environments like McPF and UMPF, offering significant insights for future research in manipulating VPD for optimal plant growth and productivity.
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