This review considers stomatal conductance as an indicator of genotypic differences in the growth response to water stress. The benefits of using stomatal conductance are compared with photosynthetic ...rate and other indicators of genetic variation in water stress tolerance, along with the use of modern phenomics technologies. Various treatments for screening for genetic diversity in response to water deficit in controlled environments are considered. There is no perfect medium: there are pitfalls in using soil in pots, and in using hydroponics with ionic and non-ionic osmotica. Use of mixed salts or NaCl is recommended over non-ionic osmotica. Developments in infrared thermography provide new and feasible screening methods for detecting genetic variation in the stomatal response to water deficit in controlled environments and in the field.
Ber (Ziziphus mauritiana Lamk.) is a major fruit tree crop of the north-west Indian arid zone. In a study of the physiological basis of drought tolerance in this species, two glasshouse experiments ...were conducted in which trees were droughted during single stresscycles. In the first experiment, during a 13 d drying cycle, pre-dawn leaf water (ψleaf) and osmotic (ψπ) potentials in droughted trees declined from −0.5 and −1.4 MPa to −1.7 and −2.2 MPa, respectively, for a decrease in relative water content (θ) of 14%. During drought stress, changes in sugar metabolism were associated with significant increases in concentrations of hexose sugars (3.8-fold), cyclitol (scylloinositol; 1.5-fold), and proline (35-fold; expressed per unit dry weight), suggesting that altered solute partitioning may be an important factor in drought tolerance of Ziziphus. On rewatering, pre-dawn ψleaf and θ recovered fully, but ψπ remained depressed by 0.4 MPa relative to control values, indicating that solute concentration per unit water content had changed during the drought cycle. Evidence for osmotic adjustment was provided from a second study during which a gradual drought was imposed. Pressure-volume analysis revealed a 0.7 MPa reduction in osmotic potential at full turgor, with ψleaf at turgor loss depressed by ∼ 1 MPa in drought-stressed leaves. Coupled with osmotic adjustment, during gradual drought, was a 65% increase in bulk tissue elastic modulus (wall rigidity) which resulted in turgor loss at the same θ in both stressed and unstressed leaves. The possible ecological significance of maintenance of turgor potential and cell volume at low water potentials for drought tolerance in Ziziphus is discussed.
This paper outlines the various sensor technologies available for monitoring grapevine performance remotely. These sensors range from optical reflectance sensors (including multispectral and ...hyperspectral instruments that give a wide range of possible vegetation indices), through thermal imagers and their use for derivation of stress indices related to evaporation rate, to fluorescence sensors that provide information on photosynthesis and metabolite biochemistry. Mention is also made of Lidar and ultrasound ranging sensors and microwave sensors. The relative advantages of the different ways in which these sensors can be deployed, from satellite through aircraft and unmanned aerial vehicles to mobile and fixed in‐field platforms, are discussed. Applications to crop management and precision farming, irrigation scheduling and phenotyping are outlined.
Changes in canopy structure parameters (leaflet orientation, leaflet inclination and leaf area index) were measured in crops of beans (Phaseolus vulgaris L.) in the field as the canopy developed ...between July and October. These changes were compared with the corresponding changes in seasonal light transmission. The beans showed clear heliotropic behaviour, with preferential orientation of leaflets towards the sun's beam, especially on sunny days. Nevertheless a significant proportion of the leaves pointed in other directions, with as much as 20% oriented towards the north. The highest proportion of leaf inclinations was in the range 30-40 degrees on cloudy days and between 40 degrees and 50 degrees on sunny days. Two methods were compared for assessing changes in light transmission: (a) the use of a Sunfleck Ceptometer and (b) the use of continuous records obtained with sensors installed in the canopy. Over the growth period studied, the total of the leaf plus stem area indices (L(S)) increased from 0.26 to 5.2 with the transmission coefficient (tau) for photosynthetic photon flux density (PPFD), obtained using the Ceptometer, correspondingly decreasing from 0.72 to 0.05, and the canopy extinction coefficient decreasing from 1.4 to 0.62. The continuous records of light transmission gave generally similar estimates of tau. Some contrasting leaf angle distribution functions were compared for estimation of L(S) from the light measurements. The best leaf angle function to predict L(S) from the observed light transmission was a conical function corrected by the degree of heliotropism.
High resolution multispectral images obtained from different view angles at the field level provide a potentially valuable new approach to the remote sensing of canopy structure as an indicator of ...water or nutrient stress to the vegetation. Extraction of frequency distributions of radiance from such images gives more information than does the simple bi-directional reflectance factor which averages the responses for all the surfaces in a large-area pixel. We describe here a preliminary analysis of this novel use of the Dundee low-cost Variable Interference Filter Imaging Spectrometer (VIFIS) for this type of study.
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