Maize leaf elongation: continuous measurements and close dependence on plant water status

Non-destructive monitoring method for leaf area of Brassica napus based on image processing and deep learning

Introduction

Leaves are important organs for photosynthesis in plants, and the restriction of leaf growth is among the earliest visible effects under abiotic stress such as nutrient deficiency. Rapidly and accurately monitoring plant leaf area is of great importance in understanding plant growth status in modern agricultural production.

Method

In this paper, an image processing-based non-destructive monitoring device that includes an image acquisition device and image process deep learning net for acquiring Brassica napus (rapeseed) leaf area is proposed. A total of 1,080 rapeseed leaf image areas from five nutrient amendment treatments were continuously collected using the automatic leaf acquisition device and the commonly used area measurement methods (manual and stretching methods).

Results

The average error rate of the manual method is 12.12%, the average error rate of the stretching method is 5.63%, and the average error rate of the splint method is 0.65%. The accuracy of the automatic leaf acquisition device was improved by 11.47% and 4.98% compared with the manual and stretching methods, respectively, and had the advantages of speed and automation. Experiments on the effects of the manual method, stretching method, and splinting method on the growth of rapeseed are conducted, and the growth rate of rapeseed leaves under the stretching method treatment is considerably greater than that of the normal treatment rapeseed.

Discussion

The growth rate of leaves under the splinting method treatment was less than that of the normal rapeseed treatment. The mean intersection over union (mIoU) of the UNet-Attention model reached 90%, and the splint method had higher prediction accuracy with little influence on rapeseed.

Flexible Wearables for Plants

The excellent stretchability and biocompatibility of flexible sensors have inspired an emerging field of plant wearables, which enable intimate contact with the plants to continuously monitor the growth status and localized microclimate in real-time. Plant flexible wearables provide a promising platform for the development of plant phenotype and the construction of intelligent agriculture via monitoring and regulating the critical physiological parameters and microclimate of plants. Here, the emerging applications of plant flexible wearables together with their pros and cons from four aspects, including physiological indicators, surrounding environment, crop quality, and active control of growth, are highlighted. Self-powered energy supply systems and signal transmission mechanisms are also elucidated. Furthermore, the future opportunities and challenges of plant wearables are discussed in detail.

Plant growth: the What, the How, and the Why

Growth is a widely used term in plant science and ecology, but it can have different meanings depending on the context and the spatiotemporal scale of analysis. At the meristem level, growth is associated with the production of cells and initiation of new organs. At the organ or plant scale and over short time periods, growth is often used synonymously with tissue expansion, while over longer time periods the increase in biomass is a common metric. At even larger temporal and spatial scales, growth is mostly described as net primary production. Here, we first address the question 'what is growth?'. We propose a general framework to distinguish between the different facets of growth, and the corresponding physiological processes, environmental drivers and mathematical formalisms. Based on these different definitions, we then review how plant growth can be measured and analysed at different organisational, spatial and temporal scales. We conclude by discussing why gaining a better understanding of the different facets of plant growth is essential to disentangle genetic and environmental effects on the phenotype, and to uncover the causalities around source or sink limitations of plant growth.

Atmospheric CO2 and VPD alter the diel oscillation of leaf elongation in perennial ryegrass: compensation of hydraulic limitation by stored-growth

We explored the effects of atmospheric CO₂ concentration (C_a ) and vapor pressure deficit (VPD) on putative mechanisms controlling leaf elongation in perennial ryegrass. Plants were grown in stands at a C_a of 200, 400 or 800 μmol mol^-1 combined with high (1.17 kPa) or low (0.59 kPa) VPD during the 16 h-day in well-watered conditions with reduced nitrogen supply. We measured day : night-variation of leaf elongation rate (LER_day  : LER_night ), final leaf length and width, epidermal cell number and length, stomatal conductance, transpiration, leaf water potential and water-soluble carbohydrates and osmotic potential in the leaf growth-and-differentiation zone (LGDZ). Daily mean LER or morphometric parameters did not differ between treatments, but LER_night strongly exceeded LER_day , particularly at low C_a and high VPD. Across treatments LER_day was negatively related to transpiration (R²  = 0.75) and leaf water potential (R²  = 0.81), while LER_night was independent of leaf water potential or turgor. Enhancement of LER_night over LER_day was proportional to the turgor-change between day and night (R²  = 0.93). LGDZ sugar concentration was high throughout diel cycles, providing no evidence of source limitation in any treatment. Our data indicate a mechanism of diel cycling between daytime hydraulic and night-time stored-growth controls of LER, buffering C_a and daytime VPD effects on leaf elongation.

Multifunctional Stretchable Sensors for Continuous Monitoring of Long-Term Leaf Physiology and Microclimate

Communication with plants to understand their growth mechanisms and interaction with the surrounding environment may improve production yield in agriculture and facilitate prevention of plant diseases and negative influence of environmental stress. Typical sensing technologies in plant biology and precision agriculture largely rely on techniques with low spatial and temporal resolutions, and fail to continuously and precisely determine localized variation in leaf physiology and microenvironments. Here, techniques to develop a multifunctional stretchable leaf-mounted sensor have been developed to offer optimized adaptability to plant growth and monitor leaf physiological and environmental conditions in continuous and highly sensitive manners. The multifunctional leaf sensor contains multiple heterogeneous sensing elements made of metal, carbon nanotube matrix, and silicon, leading to temperature, hydration, light illuminance, and strain sensing capabilities on a leaf. Evaluation under a controlled environment indicates excellent precision and accuracy of the sensor compared to conventional devices. Furthermore, indoor and outdoor experiments have demonstrated the multifunctional monitoring ability of the sensor in real situations. The multifunctional stretchable sensor holds the promise to advance monitoring techniques in plant biology and precision agriculture, resulting in improved capability to record slow and subtle physiological changes in plants and plant/environment interaction.

Water relations determine short time leaf growth patterns in the mangrove Avicennia marina (Forssk.) Vierh

High-resolution leaf growth is rarely studied despite its importance as a metric for plant performance and resource use efficiency. This is in part due to methodological challenges. Here, we present a method for in situ leaf growth measurements in a natural environment. We measured instantaneous leaf growth on a mature Avicennia marina subsp. australasica tree over several weeks. We measured leaf expansion by taking time-lapse images and analysing them using marker tracking software. A custom-made instrument was designed to enable long-term field studies. We detected a distinct diel growth pattern with leaf area shrinkage in the morning and leaf expansion in the afternoon and at night. On average, the observed daily shrinkage was 37% of the net growth. Most of the net growth occurred at night. Diel leaf area shrinkage and recovery continued after growth cessation. The amount of daily growth was negatively correlated with shrinkage, and instantaneous leaf growth and shrinkage were correlated with changes in leaf turgor. We conclude that, at least in this tree species, instantaneous leaf growth patterns are very strongly linked to, and most likely driven by, leaf water relations, suggesting decoupling of short-term growth patterns from carbon assimilation.

The Physiological Basis of Drought Tolerance in Crop Plants: A Scenario-Dependent Probabilistic Approach

Drought tolerance involves mechanisms operating at different spatial and temporal scales, from rapid stomatal closure to maintenance of crop yield. We review how short-term mechanisms are controlled for stabilizing shoot water potential and how long-term processes have been constrained by evolution or breeding to fit into acclimation strategies for specific drought scenarios. These short- or long-term feedback processes participate in trade-offs between carbon accumulation and the risk of deleterious soil water depletion. Corresponding traits and alleles may therefore have positive or negative effects on crop yield depending on drought scenarios. We propose an approach that analyzes the genetic architecture of traits in phenotyping platforms and of yield in tens of field experiments. A combination of modeling and genomic prediction is then used to estimate the comparative interests of combinations of alleles depending on drought scenarios. Hence, drought tolerance is understood probabilistically by estimating the benefit and risk of each combination of alleles.

A hydraulic model is compatible with rapid changes in leaf elongation under fluctuating evaporative demand and soil water status

Plants are constantly facing rapid changes in evaporative demand and soil water content, which affect their water status and growth. In apparent contradiction to a hydraulic hypothesis, leaf elongation rate (LER) declined in the morning and recovered upon soil rehydration considerably quicker than transpiration rate and leaf water potential (typical half-times of 30 min versus 1-2 h). The morning decline of LER began at very low light and transpiration and closely followed the stomatal opening of leaves receiving direct light, which represent a small fraction of leaf area. A simulation model in maize (Zea mays) suggests that these findings are still compatible with a hydraulic hypothesis. The small water flux linked to stomatal aperture would be sufficient to decrease water potentials of the xylem and growing tissues, thereby causing a rapid decline of simulated LER, while the simulated water potential of mature tissues declines more slowly due to a high hydraulic capacitance. The model also captured growth patterns in the evening or upon soil rehydration. Changes in plant hydraulic conductance partly counteracted those of transpiration. Root hydraulic conductivity increased continuously in the morning, consistent with the transcript abundance of Zea maize Plasma Membrane Intrinsic Protein aquaporins. Transgenic lines underproducing abscisic acid, with lower hydraulic conductivity and higher stomatal conductance, had a LER declining more rapidly than wild-type plants. Whole-genome transcriptome and phosphoproteome analyses suggested that the hydraulic processes proposed here might be associated with other rapidly occurring mechanisms. Overall, the mechanisms and model presented here may be an essential component of drought tolerance in naturally fluctuating evaporative demand and soil moisture.

Genetic and physiological controls of growth under water deficit

The sensitivity of expansive growth to water deficit has a large genetic variability, which is higher than that of photosynthesis. It is observed in several species, with some genotypes stopping growth in a relatively wet soil, whereas others continue growing until the lower limit of soil-available water. The responses of growth to soil water deficit and evaporative demand share an appreciable part of their genetic control through the colocation of quantitative trait loci as do the responses of the growth of different organs to water deficit. This result may be caused by common mechanisms of action discussed in this paper (particularly, plant hydraulic properties). We propose that expansive growth, putatively linked to hydraulic processes, determines the sink strength under water deficit, whereas photosynthesis determines source strength. These findings have large consequences for plant modeling under water deficit and for the design of breeding programs.

Physiological and growth responses to water deficit in the bioenergy crop Miscanthus x giganteus

High yielding perennial biomass crops of the species Miscanthus are widely recognized as one of the most promising lignocellulosic feedstocks for the production of bioenergy and bioproducts. Miscanthus is a C4 grass and thus has relatively high water use efficiency. Cultivated Miscanthus comprises primarily of a single clone, Miscanthus x giganteus, a sterile hybrid between M. sacchariflorus and M. sinensis. M. x giganteus is high yielding and expresses desirable combinations of many traits present in the two parental species types; however, it responds poorly to low water availability. To identify the physiological basis of the response to water stress in M. x giganteus and to identify potential targets for breeding improvements we characterized the physiological responses to water-deficit stress in a pot experiment. The experiment has provided valuable insights into the temporal aspects of drought-induced responses of M. x giganteus. Withholding water resulted in marked changes in plant physiology with growth-associated traits among the first affected, the most rapid response being a decline in the rate of stem elongation. A reduction in photosynthetic performance was among the second set of changes observed; indicated by a decrease in stomatal conductance followed by decreases in chlorophyll fluorescence and chlorophyll content. Measures reflecting the plant water status were among the last affected by the drought treatment. Metabolite analysis indicated that proline was a drought stress marker in M. x giganteus, metabolites in the proline synthesis pathway were more abundant when stomatal conductance decreased and dry weight accumulation ceased. The outcomes of this study in terms of drought-induced physiological changes, accompanied by a proof-of-concept metabolomics investigation, provide a platform for identifying targets for improved drought-tolerance of the Miscanthus bioenergy crop.

Coming of leaf age: control of growth by hydraulics and metabolics during leaf ontogeny

Leaf growth is the central process facilitating energy capture and plant performance. This is also one of the most sensitive processes to a wide range of abiotic stresses. Because hydraulics and metabolics are two major determinants of expansive growth (volumetric increase) and structural growth (dry matter increase), we review the interaction nodes between water and carbon. We detail the crosstalks between water and carbon transports, including the dual role of stomata and aquaporins in regulating water and carbon fluxes, the coupling between phloem and xylem, the interactions between leaf water relations and photosynthetic capacity, the links between Lockhart's hydromechanical model and carbon metabolism, and the central regulatory role of abscisic acid. Then, we argue that during leaf ontogeny, these interactions change dramatically because of uncoupled modifications between several anatomical and physiological features of the leaf. We conclude that the control of leaf growth switches from a metabolic to a hydromechanical limitation during the course of leaf ontogeny. Finally, we illustrate how taking leaf ontogeny into account provides insights into the mechanisms underlying leaf growth responses to abiotic stresses that affect water and carbon relations, such as elevated CO2, low light, high temperature and drought.

Developmentally equivalent tissue sampling based on growth kinematic profiling of Arabidopsis inflorescence stems

• Directional growth in Arabidopsis thaliana during bolting of the inflorescence stem makes this an attractive system for study of the underlying processes of tissue elongation and cell wall extension. Analysis of local molecular events accompanying Arabidopsis inflorescence stem elongation is hampered by difficulties in isolating developmentally matched tissue samples from different plants. • Here, we present a novel sampling approach in which specific developmental stages along the developing stem are defined nonintrusively in terms of their relative elemental growth rate by use of time-lapse imagery and subsequent derivation of growth kinematic profiles for individual plants. • Growth kinematic profiling reveals that key developmental transitions such as the point of maximum elongation rate and the point of cessation of elongation occur over broad and overlapping ranges across individuals within a population of the Columbia (Col-0) ecotype. The position of these transitions is only weakly correlated with overall plant height, which undermines the common assumption that physically similar plants have closely matched growth profiles. • This kinematic profiling approach provides high-resolution growth phenotyping of the developing stem and thereby enables the harvest, pooling and analysis of developmentally matched tissue samples from multiple Arabidopsis plants.

Environmental effects on spatial and temporal patterns of leaf and root growth

Leaves and roots live in dramatically different habitats, but are parts of the same organism. Automated image processing of time-lapse records of these organs has led to understanding of spatial and temporal patterns of growth on time scales from minutes to weeks. Growth zones in roots and leaves show distinct patterns during a diel cycle (24 h period). In dicot leaves under nonstressful conditions these patterns are characterized by endogenous rhythms, sometimes superimposed upon morphogenesis driven by environmental variation. In roots and monocot leaves the growth patterns depend more strongly on environmental fluctuations. Because the impact of spatial variations and temporal fluctuations of above- and belowground environmental parameters must be processed by the plant body as an entire system whose individual modules interact on different levels, growth reactions of individual modules are often highly nonlinear. A mechanistic understanding of plant resource use efficiency and performance in a dynamically fluctuating environment therefore requires an accurate analysis of leaf and root growth patterns in conjunction with knowledge of major intraplant communication systems and metabolic pathways.

Calcium pectate chemistry causes growth to be stored in Chara corallina: a test of the pectate cycle

Calcium pectate chemistry was reported to control the growth rate of cells of Chara corallina, and required turgor pressure (P) to do so. Accordingly, this chemistry should account for other aspects of growth, particularly the ability of plants to compensate for brief exposure to low P, that is, to 'store' growth. Live Chara cells or isolated walls were attached to a pressure probe, and P was varied. Low P caused growth to be inhibited in live cells, but when P returned to normal (0.5 MPa), a flush of growth completely compensated for that lost at low P for as long as 23-53 min. This growth storage was absent in isolated walls, mature cells and live cells exposed to cold, indicating that the cytoplasm delivered a metabolically derived growth factor needing P for its action. Because the cytoplasm delivered pectate needing P for its action, pectate was supplied to isolated walls at low P as though the cytoplasm had done so. Growth was stored while otherwise none occurred. It was concluded that a P-dependent cycle of calcium pectate chemistry not only controlled growth rate and new wall deposition, but also accounted for stored growth.

Dynamics of root growth stimulation in Nicotiana tabacum in increasing light intensity

Light intensity is crucial for plant growth. In this study, the hypothesis was tested whether a sudden increase in light intensity leads to an immediate increase of root growth. Seedlings of Nicotiana tabacum grown in agar-filled Petri dishes were subjected to light intensities of 60 and 300 micromol m(-2) s(-1), respectively. Seedling biomass, sucrose, glucose and fructose concentration as well as primary root growth increased significantly with light intensity. The dynamics of the increase in root growth were analysed here in more detail. In transition experiments from low to high light intensities, root growth increased by a factor of four within 4 d, reaching the steady-state level measured in plants that were cultivated in high-light conditions. The distribution of relative elemental growth rates along the root growth zone retained a constant shape throughout this transition. During the first three hours after light increase, strong growth fluctuations were repeatedly observed with the velocity of the root tip cycling in a sinusoidal pattern between 120 and 180 microm h(-1). These dynamic patterns are discussed in the context of hydraulic and photosynthetic acclimation to the altered conditions. Experiments with externally applied sucrose and with transgenic plants having reduced capacities for sucrose synthesis indicated clearly that increasing light intensity rapidly enhanced root growth by elevating sucrose export from shoot to root.

Functional dynamics of plant growth and photosynthesis--from steady-state to dynamics--from homogeneity to heterogeneity

Plants are much more dynamic than we usually expect them to be. This dynamic behaviour is of paramount importance for their performance under natural conditions, when resources are distributed heterogeneously in space and time. However, plants are not only the cue ball of their physical and chemical environment. Endogenous rhythms and networks controlling photosynthesis and growth buffer plant processes from external fluctuations. This review highlights recent evidence of the importance of dynamic temporal and spatial organization of photosynthesis and of growth in leaves and roots. These central processes for plant performance differ strongly in their dependence on environmental impact and endogenous properties, respectively. Growth involves a wealth of processes ranging from the supply of resources from external and internal sources to the growth processes themselves. In contrast, photosynthesis can only take place when light and CO2 are present and thus clearly requires 'input from the environment'. Nevertheless, growth and photosynthesis are connected to each other via mechanisms that are still not fully understood. Recent advances in imaging technology have provided new insights into the dynamics of plant-environment interactions. Such processes do not only play a crucial role in understanding stress response of plants under extreme environmental conditions. Dynamics of plants under modest growth conditions rise from endogenous mechanisms as well as exogenous impact too. It is thus an important task for future research to identify how dynamic external conditions interact with plant-internal signalling networks to optimize plant behaviour in real time and to understand how plants have adapted to characteristic spatial and temporal properties of the resources from their environment, on which they depend on.

The short-term growth response to salt of the developing barley leaf

Recent results concerning the short-term growth response to salinity of the developing barley leaf are reviewed. Plants were grown hydroponically and the growth response of leaf 3 was studied between 10 min and 5 d following addition of 100 mM NaCl to the root medium. The aim of the experiments was to relate changes in variables that are likely to affect cell elongation to changes in leaf growth. Changes in hormone content (ABA, cytokinins), water and solute relationships (osmolality, turgor, water potential, solute concentrations), gene expression (water channel), cuticle deposition, membrane potential, and transpiration were followed, while leaf elongation velocity was monitored. Leaf elongation decreased close to zero within seconds following addition of NaCl. Between 20 and 30 min after exposure to salt, elongation velocity recovered rather abruptly, to about 46% of the pre-stress level, and remained at the reduced rate for the following 5 d, when it reached about 70% of the level in non-stressed plants. Biophysical and physiological analyses led to three major conclusions. (i) The immediate reduction and sudden recovery in elongation velocity is due to changes in the water potential gradient between leaf xylem and peripheral elongating cells. Changes in transpiration, ABA and cytokinin content, water channel expression, and plasma membrane potential are involved in this response. (ii) Significant solute accumulation, which aids growth recovery, is detectable from 1 h onwards; growing and non-growing leaf regions and mesophyll and epidermis differ in their solute response. (iii) Cuticular wax density is not affected by short-term exposure to salt; transpirational changes are due to stomatal control.

Dynamics of leaf and root growth: endogenous control versus environmental impact

AIMS

Production of biomass and yield in natural and agronomic conditions depend on the endogenous growth capacity of plants and on the environmental conditions constraining it. Sink growth drives the competition for carbon, nutrients and water within the plant, and determines the structure of leaves and roots that supply resources to the plant later on. For their outstanding importance, analyses of internal growth mechanisms and of environmental impact on plant growth are long-standing topics in plant sciences.

SCOPE

Recent technological developments have made it feasible to study the dynamics of plant growth in temporal and spatial scales that are relevant to link macroscopic growth with molecular control. These developments provided first insights into the truly dynamic interaction between environment and endogenous control of plant growth.

CONCLUSIONS

Evidence is presented in this paper that the relative importance of endogenous control versus the impact of the dynamics of the environment depends on the frequency pattern of the environmental conditions to which the tissue is exposed. It can further be speculated that this is not only relevant within individual plants (hence leaves versus roots), but also crucial for the adaptation of plant species to the various dynamics of their environments. The following are discussed: mechanisms linking growth and concentrations of primary metabolites, and differences and homologies between spatial and temporal patterns of root and leaf growth with metabolite patterns.

A new algorithm for computational image analysis of deformable motion at high spatial and temporal resolution applied to root growth. Roughly uniform elongation in the meristem and also, after an abrupt acceleration, in the elongation zone

A requirement for understanding morphogenesis is being able to quantify expansion at the cellular scale. Here, we present new software (RootflowRT) for measuring the expansion profile of a growing root at high spatial and temporal resolution. The software implements an image processing algorithm using a novel combination of optical flow methods for deformable motion. The algorithm operates on a stack of nine images with a given time interval between each (usually 10 s) and quantifies velocity confidently at most pixels of the image. The root does not need to be marked. The software calculates components of motion parallel and perpendicular to the local tangent of the root's midline. A variation of the software has been developed that reports the overall root growth rate versus time. Using this software, we find that the growth zone of the root can be divided into two distinct regions, an apical region where the rate of motion, i.e. velocity, rises gradually with position and a subapical region where velocity rises steeply with position. In both zones, velocity increases almost linearly with position, and the transition between zones is abrupt. We observed this pattern for roots of Arabidopsis, tomato (Lycopersicon lycopersicum), lettuce (Lactuca sativa), alyssum (Aurinia saxatilis), and timothy (Phleum pratense). These velocity profiles imply that relative elongation rate is regulated in a step-wise fashion, being low but roughly uniform within the meristem and then becoming high, but again roughly uniform, within the zone of elongation. The executable code for RootflowRT is available from the corresponding author on request.

Separating growth from elastic deformation during cell enlargement

Plants change size by deforming reversibly (elastically) whenever turgor pressure changes, and by growing. The elastic deformation is independent of growth because it occurs in nongrowing cells. Its occurrence with growth has prevented growth from being observed alone. We investigated whether the two processes could be separated in internode cells of Chara corallina Klien ex Willd., em R.D.W. by injecting or removing cell solution with a pressure probe to change turgor while the cell length was continuously measured. Cell size changed immediately when turgor changed, and growth rates appeared to be altered. Low temperature eliminated growth but did not alter the elastic effects. This allowed elastic deformation measured at low temperature to be subtracted from elongation at warm temperature in the same cell. After the subtraction, growth alone could be observed for the first time. Alterations in turgor caused growth to change rapidly to a new, steady rate with no evidence of rapid adjustments in wall properties. This turgor response, together with the marked sensitivity of growth to temperature, suggested that the growth rate was not controlled by inert polymer extension but rather by biochemical reactions that include a turgor-sensitive step.

Effect of water stress on transport of [2-14C]abscisic acid in intact plants ofPhaseolus coccineus L

Eight- to 10-day-old plants ofPhaseolus coccineus, which grow on vermiculite with a water content of 12-17% of the water-holding capacity, stop growing completely, whereas water potential and relative water content are almost unaffected. [2-¹⁴C]Abscisic acid, which is applied to the midrib of a primary leaf, is transported especially to the roots and the apical bud, but not to the second primary leaf and the cotyledons. Water-stressed plants, however, export only negligible amounts of ABA from the donor leaf to the plant. Thus an accumulation of ABA occurs in the donor leaf. Consequently water stress can increase ABA concentration in leaves not only by stimulating ABA synthesis but also by inhibiting ABA transport. Recovery of growth and ABA transport after reirrigation is very weak. Water stress has no effect on ABA metabolism in bean plants.

Water transport in plants: Mechanism of apparent changes in resistance during absorption

Leaf water potentials were measured at various rates of water absorption in whole plants and detached leaves of well-watered Helianthus annuus L. The experiments were conducted in the steady state, where changes in leaf hydration did not affect the measurements but both the transpiration and growth components of absorption could be observed. Calculations of the total plant resistance to water transport showed that the resistance at low fluxes was about 30 times the resistance at high fluxes. Most of the change took place in the leaves, since similar changes could be demonstrated in detached leaves. The roots accounted for little of the change, since they varied in resistance by a factor of only 2.5 as flow varied.To ascertain whether the protoplasts of the leaves varied in resistance by an amount which could account for the change in resistance to water transport, measurements of rates of water movement in and out of the protoplasm were made when gradients in water potential between the protoplasts and the water source were varied. These showed that water movement did not occur at rates which could account for high rates of transpiration even when large differences in potential drove flow. The high temperature sensitivity of efflux confirmed that the leaf protoplasts limited flow in these experiments. When the edge of the leaf was excised and flow occurred primarily through the vascular system of the leaf, the resistance was much lower than in the protoplasts. It is therefore concluded that the leaf protoplasts represent a high resistance to water transport and that a considerable portion of the water involved in transpiration must bypass them.Calculations based on a model of water transport showed that the protoplast resistance was almost 30 times larger than the resistance of the path leading from the soil to the leaf protoplasts. The decrease in resistance of the leaves with increasing rates of absorption was therefore attributed to a decrease in water movement in and out of leaf cells, which involved a high resistance, and an increase in movement around the leaf protoplasts, which involved a low resistance. Since the experiments were conducted at the steady state, the high resistances were apparent at low rates of flow where only growth occurred, whereas the low resistances could be observed at high rates of flow because growth did not occur and flow consisted solely of transpiration. Because of the high resistance of the protoplast pathway, leaf water potentials were governed more by protoplast water movement than by transpiration over a considerable range of rates of water absorption. This may explain some of the differences in earlier work on leaf water potentials and water transport.

Studies on the action of abscisic acid on IAA-induced rapid growth of Avena coleoptile segments

A linear displacement transducer has been used to monitor the growth of a column of Avena coleoptile segments in flowing solution. IAA at 10(-5)M in phosphate buffer of pH7 promotes growth after a latent period of 10.9 min, the initial maximum growth rate occurring after 25 min. Simultaneous treatment with 10(-5) M ABA does not affect either the latent period or the initial maximum growth rate in response to the IAA treatment, but subsequently gives rise to an inhibition of growth detectable after 30 min. In contrast, pretreatment with ABA for 100 min increases the duration of the latent period and reduces the initial maximum growth rate. Removal of the ABA rapidly relieves the inhibition of IAA-induced growth but a growth rate comparable to that of material treated only with IAA is never attained. Studies using 2-[(14)C]ABA and 1-[(14)C]IAA suggest that the latent period before ABA inhibition of growth is detectable is not due to a lag in ABA uptake, and that ABA is not acting by reducing IAA uptake.

Oscillations in stomatal conductance and plant functioning associated with stomatal conductance: Observations and a model

Measurements of transpiration, leaf water content, and flux of water in a cotton plant exhibiting sustained oscillations, in stomatal conductance are presented, and a model of the mechanism causing this behaviour is developed. The dynamic elements, of the model are capacitors-representing the change of water content with water potential in mesophyll, subsidiary and guard cells-interconnected by resistances representing flow paths in the plant. Increase of water potential in guard cells causes an increase in stomatal conductance. Increase of water potential in the subsidiary cells has the opposite effect and provides the positive feed-back which can cause stomatal conductance to oscillate. The oscillations are shown to have many of the characteristics of free-running oscillations in real plants. The behaviour of the model has been examined, using an analogue computer, with constraints and perturbations representing some of those which could be applied to real plants in physiological experiments. Aspects of behaviour which have been simulated are (a) opening and closing of stomata under the influence of changes in illumination, (b) transient responses due to step changes in potential transpiration, root permeability and potential of water surrounding the roots, (c) the influence of these factors on the occurrence and shape of spontaneous oscillations, and (d) modulation of sustained oscillations due to a circadian rhythm in the permeability of roots.