Hamlyn G. Jones

MODELLING DIURNAL TRENDS OF LEAF WATER POTENTIAL IN TRANSPIRING WHEAT

SUMMARY (1) Measurements were made of the diurnal changes in the leaf water potential (*L) of flag leaves of winter-sown and spring-sown wheat in three different years, and on crops grown with different water stresses. Parallel measurements were also made of leaf conductance to water vapour and of the environmental variables necessary to calculate transpiration rate from the flag leaf. (2) A range of models were fitted to the diurnal trends in evaporation, using cubic spline approximations, in an attempt to interpret the changes in water potential and to develop a simple model for predicting leaf water potential. (3) An Ohms Law analogue, with a constant resistance to flow through the soil-plant pathway, explained more than 50% of the variation in leaf water potential. This model, however, was always inferior to models which incorporated a variable resistance (decreasing asymptotically with increasing flux) and/or a capacitance to simulate the hysteresis in the relationship between water potential and evaporation rate. The more sophisticated models usually accounted for more than 90% of the variation in hourly means of leaf water potential. (4) The validity of this type of model, which considers only flow through the flag leaf, and the interpretation of the estimated parameters is discussed.

Infra-red Thermography for High Throughput Field Phenotyping in Solanum tuberosum

The rapid development of genomic technology has made high throughput genotyping widely accessible but the associated high throughput phenotyping is now the major limiting factor in genetic analysis of traits. This paper evaluates the use of thermal imaging for the high throughput field phenotyping of Solanum tuberosum for differences in stomatal behaviour. A large multi-replicated trial of a potato mapping population was used to investigate the consistency in genotypic rankings across different trials and across measurements made at different times of day and on different days. The results confirmed a high degree of consistency between the genotypic rankings based on relative canopy temperature on different occasions. Genotype discrimination was enhanced both through normalising data by expressing genotype temperatures as differences from image means and through the enhanced replication obtained by using overlapping images. A Monte Carlo simulation approach was used to confirm the magnitude of genotypic differences that it is possible to discriminate. The results showed a clear negative association between canopy temperature and final tuber yield for this population, when grown under ample moisture supply. We have therefore established infrared thermography as an easy, rapid and non-destructive screening method for evaluating large population trials for genetic analysis. We also envisage this approach as having great potential for evaluating plant response to stress under field conditions.

Coping with drought: stress and adaptive responses in potato and perspectives for improvement

Potato (Solanum tuberosum L.) is often considered as a drought sensitive crop and its sustainable production is threatened due to frequent drought episodes. There has been much research aiming to understand the physiological, biochemical, and genetic basis of drought tolerance in potato as a basis for improving production under drought conditions. The complex phenotypic response of potato plants to drought is conditioned by the interactive effects of the plants genotypic potential, developmental stage, and environment. Effective crop improvement for drought tolerance will require the pyramiding of many disparate characters, with different combinations being appropriate for different growing environments. An understanding of the interaction between below ground water uptake by the roots and above ground water loss from the shoot system is essential. The development of high throughput precision phenotyping platforms is providing an exciting new tool for precision screening, which, with the incorporation of innovative screening strategies, can aid the selection and pyramiding of drought-related genes appropriate for specific environments. Outcomes from genomics, proteomics, metabolomics, and bioengineering advances will undoubtedly compliment conventional breeding strategies and presents an alternative route toward development of drought tolerant potatoes. This review presents an overview of past research activity, highlighting recent advances with examples from other crops and suggesting future research directions.

Can root electrical capacitance be used to predict root mass in soil

BACKGROUNDnElectrical capacitance, measured between an electrode inserted at the base of a plant and an electrode in the rooting substrate, is often linearly correlated with root mass. Electrical capacitance has often been used as an assay for root mass, and is conventionally interpreted using an electrical model in which roots behave as cylindrical capacitors wired in parallel. Recent experiments in hydroponics show that this interpretation is incorrect and a new model has been proposed. Here, the new model is tested in solid substrates.nnnMETHODSnThe capacitances of compost and soil were determined as a function of water content, and the capacitances of cereal plants growing in sand or potting compost in the glasshouse, or in the field, were measured under contrasting irrigation regimes.nnnKEY RESULTSnCapacitances of compost and soil increased with increasing water content. At water contents approaching field capacity, compost and soil had capacitances at least an order of magnitude greater than those of plant tissues. For plants growing in solid substrates, wetting the substrate locally around the stem base was both necessary and sufficient to record maximum capacitance, which was correlated with stem cross-sectional area: capacitance of excised stem tissue equalled that of the plant in wet soil. Capacitance measured between two electrodes could be modelled as an electrical circuit in which component capacitors (plant tissue or rooting substrate) are wired in series.nnnCONCLUSIONSnThe results were consistent with the new physical interpretation of plant capacitance. Substrate capacitance and plant capacitance combine according to standard physical laws. For plants growing in wet substrate, the capacitance measured is largely determined by the tissue between the surface of the substrate and the electrode attached to the plant. Whilst the measured capacitance can, in some circumstances, be correlated with root mass, it is not a direct assay of root mass.

A new physical interpretation of plant root capacitance

Capacitance has been used as a non-destructive measure of root system size for 30 years. The equipment required is cheap and simple to apply in both field and laboratory. Good linear correlations have been reported between capacitance and root mass. A model by F. N. Dalton, predicting a linear relationship between these two variables, has become accepted widely. This model was tested for barley (Hordeum vulgare) grown hydroponically using treatments that included: raising roots out of solution, cutting roots at positions below the solution surface, and varying the distance between plant electrode and the solution surface. Although good linear correlations were found between capacitance and mass for whole root systems, when roots were raised out of solution capacitances were not linearly related to submerged root mass. Excision of roots in the solution had negligible effect on the measured capacitance. These latter observations conflict with Dalton’s model. Capacitance correlated linearly with the sum of root cross-sectional areas at the solution surface and inversely with distance between plant electrode and solution surface. A new model for capacitance is proposed that is consistent with these observations.

Field phenotyping of potato to assess root and shoot characteristics associated with drought tolerance

AimsPotatoes are a globally important source of food whose production requires large inputs of fertiliser and water. Recent research has highlighted the importance of the root system in acquiring resources. Here measurements, previously generated by field phenotyping, tested the effect of root size on maintenance of yield under drought (drought tolerance).MethodsTwelve potato genotypes, including genotypes with extremes of root size, were grown to maturity in the field under a rain shelter and either irrigated or subjected to drought. Soil moisture, canopy growth, carbon isotope discrimination and final yields were measured. Destructively harvested field phenotype data were used as explanatory variables in a general linear model (GLM) to investigate yield under conditions of drought or irrigation.ResultsDrought severely affected the small rooted genotype Pentland Dell but not the large rooted genotype Cara. More plantlets, longer and more numerous stolons and stolon roots were associated with drought tolerance. Previously measured carbon isotope discrimination did not correlate with the effect of drought.ConclusionsThese data suggest that in-field phenotyping can be used to identify useful characteristics when known genotypes are subjected to an environmental stress. Stolon root traits were associated with drought tolerance in potato and could be used to select genotypes with resilience to drought.

The use of indirect or proxy markers in plant physiology

There are many cases in crop science and physiology where direct measurement of a particular environmental or physiological variable or plant response is difficult; it is therefore often convenient to use indirect proxy indicators to increase the ease and speed of their estimation. As an example, remote sensing of leaf or canopy spectral or thermal properties is increasingly being used for the indirect estimation of processes such as photosynthesis, stomatal conductance, canopy nitrogen content and even plant water status (Jones & Vaughan 2010). In some cases, the variable used as a proxy is closely related to the process of interest: for example, chlorophyll fluorescence can be used as a direct measure of photosynthetic electron transport, which is itself closely associated with photosynthetic carbon fixation. In other cases, such as thermal sensing of canopy temperature which has been widely proposed as a measure of crop water ‘stress’ (e.g. Idso et al. 1981), the proxy may be rather more remote. In this case, leaf temperature is used as an indirect estimator of stomatal conductance, which in turn is assumed to indicate plant water status because stomata tend to close as water status declines. Although such an indirect proxy can be useful, especially in isohydric plants where a relatively constant leaf water status is maintained by stomatal closure as soil water availability decreases, it is necessary to remember that the association of the proxy with the variable of interest may not be robust under all conditions. In the case of canopy temperature, although it is affected by stomatal conductance, other factors such as wind speed, radiation and humidity also affect leaf temperature, so it is necessary to simultaneously measure or correct these factors if temperature is to be used as an effective proxy for stomatal conductance (Leinonen et al. 2006). Major limitations to the use of proxy measures include the fact that the value of the inverse prediction involved is limited both by errors in measurement of the proxy and by scatter in the relationship between the proxy and the original measure. In any application of proxy measures it is critical to assess the accuracy of the inverse prediction.

Chilling requirement of ribes cultivars

It is usually thought that adequate winter chill is required for the full flowering of many temperate woody species. This paper investigates the sensitivity of blackcurrant bud burst and flowering to natural weather fluctuations in a temperate maritime climate, and compares a range of chill models that have been proposed for assessing the accumulation of winter chill. Bud break for four contrasting cultivars are compared in an exceptionally cold and in a mild winter in Eastern Scotland. The results confirm the importance of chilling at temperatures lower than 0°C and demonstrate that no single chilling function applies equally to all blackcurrant cultivars. There is a pressing need for further model development to take into account the relationship between chilling temperatures and warming temperatures occurring both during and after the chill accumulation period.

Assessing Drought Responses Using Thermal Infrared Imaging.

Canopy temperature, a surrogate for stomatal conductance, is shown to be a good indicator of plant water status and a potential tool for phenotyping and irrigation scheduling. Measurement of stomatal conductance and leaf temperature has traditionally been done by using porometers or gas exchange analyzers and fine-wire thermocouples attached to the leaves, which are labor intensive and point measurements. The advent of remote or proximal thermal sensing technologies has provided the potential for scaling up to leaves, plants, and canopies. Thermal cameras with a temperature resolution of <0.1 K now allow one to study the temperature variation within and between plants. This chapter discusses some applications of infrared thermography for assessing drought and other abiotic and biotic stress and outlines some of the main factors that need to be considered when applying this to the study of leaf or canopy temperature whether in controlled environments or in the field.

A method for automatic segmentation and splitting of hyperspectral images of raspberry plants collected in field conditions

Hyperspectral imaging is a technology that can be used to monitor plant responses to stress. Hyperspectral images have a full spectrum for each pixel in the image, 400–2500xa0nm in this case, giving detailed information about the spectral reflectance of the plant. Although this technology has been used in laboratory-based controlled lighting conditions for early detection of plant disease, the transfer of such technology to imaging plants in field conditions presents a number of challenges. These include problems caused by varying light levels and difficulties of separating the target plant from its background. Here we present an automated method that has been developed to segment raspberry plants from the background using a selected spectral ratio combined with edge detection. Graph theory was used to minimise a cost function to detect the continuous boundary between uninteresting plants and the area of interest. The method includes automatic detection of a known reflectance tile which was kept constantly within the field of view for all image scans. A method to split images containing rows of multiple raspberry plants into individual plants was also developed. Validation was carried out by comparison of plant height and density measurements with manually scored values. A reasonable correlation was found between these manual scores and measurements taken from the images (r2xa0=xa00.75 for plant height). These preliminary steps are an essential requirement before detailed spectral analysis of the plants can be achieved.