Methods to estimate changes in soil water for phenotyping root activity in the field.

BACKGROUND AND AIMS: There is an urgent need to develop new high throughput approaches to phenotype roots in the field. Excavating roots to make direct measurements is labour intensive. An alternative to excavation is to measure soil drying profiles and to infer root activity. METHODS: We grew 23 lines of wheat in 2013, 2014 and 2015. In each year we estimated soil water profiles with electrical resistance tomography (ERT), electromagnetic inductance (EMI), penetrometer measurements and measurements of soil water content. We determined the relationships between the measured variable and soil water content and matric potential. RESULTS: We found that ERT and penetrometer measurements were closely related to soil matric potential and produced the best discrimination between wheat lines. We found genotypic differences in depth of water uptake in soil water profiles and in the extent of surface drying. CONCLUSIONS: Penetrometer measurements can provide a reliable approach to comparing soil drying profiles by different wheat lines, and genotypic rankings are repeatable across years. EMI, which is more sensitive to soil water content than matric potential, and is less effective in drier soils than the penetrometer or ERT, nevertheless can be used to rapidly screen large populations for differences in root activity.

Measurement of the matric potential of soil water in the rhizosphere.

The availability of soil water, and the ability of plants to extract it, are important variables in plant research. The matric potential has been a useful way to describe water status in a soil-plant system. In soil it is the potential that is derived from the surface tension of water menisci between soil particles. The magnitude of matric potential depends on the soil water content, the size of the soil pores, the surface properties of the soil particles, and the surface tension of the soil water. Of all the measures of soil water, matric potential is perhaps the most useful for plant scientists. In this review, the relationship between matric potential and soil water content is explored. It is shown that for any given soil type, this relationship is not unique and therefore both soil water content and matric potential need to be measured for the soil water status to be fully described. However, in comparison with water content, approaches for measuring matric potential have received less attention until recently. In this review, a critique of current methods to measure matric potential is presented, together with their limitations as well as underexploited opportunities. The relative merits of both direct and indirect methods to measure matric potential are discussed. The different approaches needed in wet and dry soil are outlined. In the final part of the paper, the emerging technologies are discussed in so far as our current imagination allows. The review draws upon current developments in the field of civil engineering where the measurement of matric potential is also important. The approaches made by civil engineers have been more imaginative than those of plant and soil scientists.

Capture and use of solar radiation, water, and nitrogen by sugar beet (Beta vulgaris L.).

Sugar beet is spring-sown for sugar production in most sugar beet-growing countries. It is grown as a vegetative crop and it accumulates yield (sugar) from very early in its growth cycle. As long as the sugar beet plants do not flower, the sugar accumulation period is indefinite and yield continues to increase. This paper reviews the success of the sugar beet crop in capturing and using solar radiation, water and mineral nitrogen resources. The prospects for improved resource capture and therefore increased sugar yield are also considered, particularly the potential to increase solar radiation interception in the future by sowing the crop in the autumn.

Genotypic variation for drought tolerance in Beta vulgaris.

Insufficient soil moisture during summer months is now the major cause of sugar beet yield losses in the UK. However, selection for increased drought tolerance has not been a breeding priority until recently. Genetic variation for drought tolerance is an essential prerequisite for the development of more stress-tolerant varieties, but commercial sugar beet varieties seem to have similar yield responses to drought. The objective of this study was to assess the degree of genotypic variation for drought tolerance within a wide range of sugar beet germplasm and genebank accessions within Beta. Thirty sugar beet genotypes were screened under field drought conditions, and putative drought tolerant and sensitive lines (in terms of yield reduction in polythene-covered vs. irrigated plots) were identified. Significant genotype x water treatment interactions were found for dry matter yield and relative leaf expansion rate. Genotypic differences for drought susceptibility index were also significant. Differential sensitivity of seedling shoot growth to water deficit was examined by comparing 350 genebank accessions in a simple growth chamber screen. Methods of data management were devised to highlight lines for entry into subsequent field tests. The results of the field and seedling screens indicate that there is variation for tolerance to water deficits within sugar beet and related types, and that there are lines that show greater drought tolerance than selected commercial varieties. Divergent lines showing contrasting behaviour should aid in the identification of key morpho-physiological traits that confer drought tolerance.

Proline Accumulation in Maize (Zea mays L.) Primary Roots at Low Water Potentials (I. Requirement for Increased Levels of Abscisic Acid).

Previous work showed that the concentration of proline (Pro) increases greatly in the primary root tip of maize (Zea mays L.) at low water potentials ([psi]w). It was also shown that the maintenance of root elongation at low [psi]w depends on increased levels of abscisic acid (ABA). In this study we have assessed whether ABA is required for the increase in Pro concentration. Seedlings were grown in vermiculite of various [psi]w, and endogenous ABA levels were decreased using either fluridone (FLU) or the vp5 mutant to inhibit carotenoid (and ABA) synthesis. In both treatments, Pro concentrations at low [psi]w were substantially decreased throughout the apical centimeter, which encompassed the elongation zone. Pro concentrations in FLU-treated roots were restored by addition of 7 [mu]M ABA to the vermiculite, which raised the internal ABA content to the level in untreated roots at the same [psi]w. Pro and water content profiles were combined with published growth-velocity distributions to calculate the distribution of net Pro and water deposition rates using the continuity equation. At a [psi]w of -1.6 MPa, the rate of Pro deposition in the root tip was decreased by 75% in FLU-treated compared to untreated roots. FLU treatment increased root diameter and, therefore, water content per unit length, but water deposition rates decreased due to the dominant influence of reduced longitudinal expansion. Thus, the decrease in Pro concentration was attributable entirely to the decrease in Pro deposition. The results demonstrate that increased ABA is required for high rates of Pro deposition and, thereby, high Pro concentrations in the growing region of maize primary roots at low [psi]w.

Water deficit induces abscisic Acid accumulation in endosperm of maize viviparous mutants.

To determine whether abscisic acid (ABA) accumulation in endosperms of water-limited maize (Zea mays L.) plants is from synthesis in maternal plant organs or from intraendosperm synthesis, plants heterozygous for viviparous (vp) genes were self-pollinated to create endosperm genotypes capable (+/-/-; +/+/-; +/+/+) or incapable (-/-/-) of carotenoid and ABA synthesis. The mutants vp2, vp5, and vp7, each in W22 inbred background, were utilized. Both in wild-type endosperms capable of ABA synthesis and in mutants incapable of ABA synthesis, ABA concentrations at 15 days after pollination were substantially increased in response to plant water deficit. We conclude that ABA synthesis in maternal organs was the source of ABA that accumulated in endosperms in response to plant water deficit.

Influence of water deficit on maize endosperm development : enzyme activities and RNA transcripts of starch and zein synthesis, abscisic Acid, and cell division.

In maize (Zea mays L.), drought during the post-pollination stage decreases kernel growth and often leads to grain yield losses. Kernels in the apical region of the ear are more severely affected than basally positioned kernels. We hypothesized that water deficit during early endosperm development might inhibit kernel growth by decreasing endosperm cell division, and that this response might be mediated by changes in endosperm abscisic acid (ABA) levels. Greenhouse-grown maize, cultivar Pioneer 3925, was subjected to water limitation from 1 to 15 days after pollination (DAP), spanning the period of endosperm cell division and induction of storage product accumulation. Water deficit decreased the number of endosperm nuclei during the treatment period; the most substantial effect was in the apical region of ears. Correspondingly, endosperm fresh weight, starch accumulation and dry mass at maturity were decreased by water limitation. Abscisic acid concentrations in endosperm were quantified by enzyme-linked immunosorbent assay. Water deficit increased ABA concentration in apical-region endosperm by four-fold compared to controls. ABA concentrations were also increased in middle and basal regions of the ear, but to a lesser extent. Two key enzymes in the starch synthesis pathway, sucrose synthase and granule-bound ADP-glucose starch synthase, and zein, the major storage protein in maize endosperm, were studied as markers of storage product synthesis. Water deficit did not affect sucrose synthase enzyme activity or RNA transcript abundance relative to total RNA. However, ADP-glucose starch synthase activity and RNA transcript abundance decreased slightly in apical-region endosperm of water-limited plants by 15 DAP, compared with well-watered controls. In contrast to starch, there was no treatment effect on the accumulation of zein, evaluated at either the polypeptide or RNA level. We conclude that under the conditions tested, the establishment of starch and zein synthetic potential in endosperm was only slightly affected by plant water deficit during the early phase of kernel growth, and that capacity for growth and starch accumulation was affected by the extent to which cell division was inhibited. Based on correlative changes in ABA concentration and cell division we suggest that ABA may play a role in inhibiting endosperm cell division during water limitation.