We have investigated the way in which the radiation absorbed by leaves affects the rate of elongation of maize (Zea mays L.) roots. In five repeated growth chamber experiments, plants previously ...grown at a photon irradiance of 23 mol m–2 d–1 received either 7 or 34 mol m–2 d–1 from day 10 to day 20 after germination. The elongation rate of primary roots steadily decreased for 4 d after reduction in irradiance and then stabilized at 60% of that in plants at high irradiance. The elongating zone was slightly shorter after 2 d at low irradiance, and was further reduced after 8 d. The concentrations of sucrose and glucose in the elongating zone were greatly decreased after 2 d at low irradiance and the gradient of both sugars was suppressed. The longer period at low irradiance affected neither sugar content nor gradient. In the same way, cell production rate was reduced after 2 d at low irradiance and was not appreciably decreased afterwards. The root zone with cell division was shorter in plants at low irradiance, but cell division rate remained nearly constant temporally and spatially, and was unaffected by the irradiance treatment. Our results suggest that primary events after a reduction in irradiance were a change in cell flux and sugar content in the elongating zone. Change in elongation rate was slower and probably the result of a time‐related developmental effect, which may be related to the change in cell production.
We analyzed the effect of short-term water deficits at different periods of sunflower (Helianthus annuus L.)leaf development on the spatial and temporal patterns of tissue expansion and epidermal ...cell division. Six water-deficit periods were imposed with similar and constant values of soil water content, predawn leaf water potential and ABA in the xylem sap, and with negligible reduction of the rate of photosynthesis. Water deficit did not affect the duration of expansion and division. Regardless of their timing, deficits reduced relative expansion rate by 36% and relative cell division rate by 39% (cells blocked at the G0-G1 phase) in all positions within the leaf. However, reductions in final leaf area and cell number in a given zone of the leaf largely differed with the timing of deficit, with a maximum effect for earliest deficits. Individual cell area was only affected during the periods when division slowed down. These behaviors could be simulated in all leaf zones and for all timings by assuming that water deficit affects relative cell division rate and relative expansion rate independently, and that leaf development in each zone follows a stable three-phase pattern in which duration of each phase is stable if expressed in thermal time (C. Granier and F. Tardieu 1998b Plant Cell Environ 21: 695-703).
In crop species, the impact of temperature on plant development is classically modelled using thermal time. We examined whether this method could be used in a non-crop species, Arabidopsis thaliana, ...to analyse the response to temperature of leaf initiation rate and of the development of two leaves of the rosette. The results confirmed the large plant-to-plant variability in the studied isogenic line of the Columbia ecotype: 100-fold differences in leaf area among plants sown on the same date were commonly observed at a given date. These differences disappeared in mature leaves, suggesting that they were due to a variability in plant developmental stage. The whole population could therefore be represented by any group of synchronous plants labelled at the two-leaf stage and followed during their development. Leaf initiation rate, duration of leaf expansion and maximal relative leaf expansion rate varied considerably among experiments performed at different temperatures (from 6 to 26 degrees C) but they were linearly related to temperature in the range 6-26 degrees C, with a common x-intercept of 3 degrees C. Expressing time in thermal time with a threshold temperature of 3 degrees C unified the time courses of leaf initiation and of individual leaf development for plants grown at different temperatures and experimental conditions. The two leaves studied (leaf 2 and leaf 6) had a two-phase development, with an exponential phase followed by a phase with decreasing relative elongation rate. Both phases had constant durations for a given leaf position if expressed in thermal time. Changes in temperature caused changes in both the rate of development and in the expansion rate which mutually compensated such that they had no consequence on leaf area at a given thermal time. The resulting model of leaf development was applied to ten experiments carried out in a glasshouse or in a growth chamber, with plants grown in soil or hydroponically. Because it predicts accurately the stage of development and the relative expansion rate of any leaf of the rosette, this model facilitates precise planning of sampling procedures and the comparison of treatments in growth analyses.
This paper reviews methods for analyzing plant performance and its genetic variability under a range of environmental conditions. Biomass accumulation is linked every day to available light in the ...photosynthetically active radiation (PAR) domain, multiplied by the proportion of light intercepted by plants and by the radiation use efficiency. Total biomass is cumulated over the duration of the considered phase (e.g., plant cycle or vegetative phase). These durations are essentially constant for a given genotype provided that time is corrected for temperature (thermal time). Several ways of expressing thermal time are reviewed. Two alternative equations are presented, based either on the effect of transpiration, or on yield components. Their comparative interests and drawbacks are discussed. The genetic variability of each term of considered equations affects yield under water deficit, via mechanisms at different scales of plant organization and time. The effect of any physiological mechanism on yield of stressed plants acts via one of these terms, although the link is not always straightforward. Finally, I propose practical ways to compare the productivity of genotypes in field environments, and a "minimum dataset" of environmental data and traits that should be recorded for that.
Grain abortion allows the production of at least a few viable seeds under water deficit but causes major yield loss. It is maximum for water deficits occurring during flowering in maize (Zea mays). ...We have tested the hypothesis that abortion is linked to the differential development of ovary cohorts along the ear and to the timing of silk emergence. Ovary volume and silk growth were followed over 25 to 30 d under four levels of water deficit and in four hybrids in two experiments. A position-time model allowed characterizing the development of ovary cohorts and their silk emergence. Silk growth rate decreased in water deficit and stopped 2 to 3 d after first silk emergence, simultaneously for all ovary cohorts, versus 7 to 8 d in well-watered plants. Abortion rate in different treatments and positions on the ear was not associated with ovary growth rate. It was accounted for by the superposition of (1) the sequential emergence of silks originating from ovaries of different cohorts along the ear with (2) one event occurring on a single day, the simultaneous silk growth arrest. Abortion occurred in the youngest ovaries whose silks did not emerge 2 d before silk arrest. This mechanism accounted for more than 90% of drought-related abortion in our experiments. It resembles the control of abortion in a large range of species and inflorescence architectures. This finding has large consequences for breeding drought-tolerant maize and for modeling grain yields in water deficit.
The improvement of crop yield has been possible through the indirect manipulation of quantitative trait loci (QTLs) that control heritable variability of the traits and physiological mechanisms that ...determine biomass production and its partitioning. This article surveys how QTL-based approaches contribute to a better understanding of the genetic basis of crop performance under environmentally constrained conditions and critically analyzes how this knowledge can assist breeders accelerate the release of cultivars better able to cope with abiotic constraints. Crop performance is the end result of the action of thousands of genes and their interactions with environmental conditions and cultural practices. During the past century, conventional breeding has been very successful in constantly raising the yield potential of crops (Campos et al., 2004Go; Borlaug and Dowswell, 2005Go; Duvick, 2005Go). This was mainly achieved with little or no knowledge of the factors governing the genetic variability exploited by breeders for crop improvement (Blum, 1988Go; Borlaug, 2007Go). However, this approach may now be insufficient, because the pressure to provide improvements at a rapid pace will mount if global climate change increases the frequency and severity of abiotic constraints. Heat stress, drought, water-logging, and salinity will probably become more prevalent in certain areas, while there will be an increased demand for agricultural products and reduced availability of agricultural land and natural resources such as water and fertilizers. Consequently, the genetic dissection of the quantitative traits controlling the adaptive response of crops to abiotic stress is a prerequisite to allow cost-effective applications of genomics-based approaches to breeding programs aimed at improving the sustainability and stability of yield under adverse conditions.
Stomatal control of species with contrasting stomatal behaviours have been investigated under natural fluctuations of evaporative demand and soil water status. Sunflower and barley (anisohydric ...behaviour) have a daytime leaf water potential (ψ1) which markedly decreases with evaporative demand during the day and is lower in droughted than in watered plants. In contrast, maize and poplar (isohydric behaviour) maintain a nearly constant ψ1 during the day at a value which does not depend on soil water status until plants are close to death. Plants were also subjected to a range of soil water potentials under contrasting air vapour pressure deficits (VPD, from 0.5 to 3 kPa) in the field, in the greenhouse or in a growth chamber. Finally, plants or detached leaves were fed with varying concentrations of artificial ABA. Stomatal conductance of well-watered plants had no response to VPD when plants were grown in natural soils, suggesting that the opposite result observed in many laboratory experiments might be linked to the low unsaturated hydraulic conductivity of usual potting substrates. The response of stomatal conductance of all studied species to the concentration of ABA in pressurized xylem sap (ABAxyl) was the same whether ABA had an endogenous origin (droughted plants) or was artificially fed. However stomatal response of maize and poplar to ABAxyl markedly changed with varying evaporative demand or ψ1, whereas this was not the case in sunflower or barley. This suggests that isohydric behaviour is linked to an interaction between hydraulic and chemical information, while anisohydric behaviour is linked to an absence of interaction. In all cases, ABAxyl was related to soil water status with common relationships for different experimental conditions, but with markedly different responses among species. Diurnal variations of ABAxyl with evaporative demand were small in all studied species. Results are synthesized in a model which accounts for observed behaviours of gs, ψ1 and ABAxyl in fluctuating conditions and for several species. The validity of this model, in particular the physiological meaning of ABAxyl, is discussed.
Stomatal aperture, transpiration, leaf growth, hydraulic conductance, and concentration of abscisic acid in the xylem sap (ABA
xyl) vary rapidly with time of day. They follow deterministic relations ...with environmental conditions and interact in such a way that a change in any one of them affects all the others. Hence, approaches based on measurements of one variable at a given time or on paired correlations are prone to a confusion of effects, in particular for studying their genetic variability. A dynamic model allows the simulation of environmental effects on the variables, and of multiple feedbacks between them at varying time resolutions. This paper reviews the control of water movement through the plant, stomatal aperture and growth, and translates them into equations in a model. It includes recent progress in understanding the intrinsic and environmental controls of tissue hydraulic conductance as a function of transpiration rate, circadian rhythms, and ABA
xyl. Measured leaf water potential is considered as the water potential of a capacitance representing mature tissues, which reacts more slowly to environmental cues than xylem water potential and expansive growth. Combined with equations for water and ABA fluxes, it results in a dynamic model able to simulate variables with genotype-specific parameters. It allows adaptive roles for hydraulic processes to be proposed, in particular the circadian oscillation of root hydraulic conductance. The script of the model, in the R language, is included together with appropriate documentation and examples.
► Water deficit reduces carbon accumulation, cell number and tissue expansion. ► Tissue expansion is loosely co-ordinated with cell division and carbon accumulation. ► The co-ordination between ...carbon accumulation, cell number and tissue expansion results from feedbacks between parallel processes and common mechanisms for several organs. ► Absence of a central co-ordination would have profound implications for plant modelling and plant breeding in dry environment.
Water deficit affects plant growth via reduced carbon accumulation, cell number and tissue expansion. We review the ways in which these processes are co-ordinated. Tissue expansion and its sensitivity to water deficit may be the most crucial process, involving tight co-ordination between the mechanisms which govern cell wall mechanical properties and plant hydraulics. The analyses of sensitivities, time constants and genetic correlations suggest that tissue expansion is loosely co-ordinated with cell division and carbon accumulation which may have limited direct effects on growth under water deficit. We therefore argue for essentially uncoupled mechanisms with feedbacks between them, rather than for a co-ordinated re-programming of all processes. Consequences on plant modelling and plant breeding in dry environment are discussed.
PDF
