Sensitivity of Gramineacous Leaf Growth to Abscisic Acid: Its Potential Importance to Leaf Growth of Plants in Drying Soil.

Recent reports have indicated that leaf growth of plants in drying soil can be regulated by chemical signals originating from the roots. A potential signal is the plant hormone abscisic acid, ABA. This thesis aimed to identity sources of variation in the sensitivity of cereal leaf growth to ABA using a detached shoot leaf elongation assay. Preliminary experiments showed that altering the nutrient composition or pH of the feeding solution had no effect on sensitivity. Assay variability resulted in considerable uncertainty over whether observed genotypic differences in sensitivity were real, or reproducible. Increasing the air temperature around the detached shoots greatly increased the sensitivity of leaf growth to ABA. Increased concentrations of ABA in the elongation zone ( [EZ-ABA] ) of detached shoots seemed to account for this increased sensitivity. When detached shoots were fed sap from droughted plants, sap ABA concentration could not explain the growth inhibitory activity. Measurement of [EZ-ABA] accounted for this "unexplained" growth inhibition. When intact plants were subjected to a slowly developing drought, growth was reduced by 35 % without any increases in [EZ-ABA]. Measurement of leaf water relations and xylem sap pH similarly failed to account for the growth inhibition. The diurnal growth pattern of droughted plants suggested an interaction of water relations with chemical signals. Use of a coleoptile growth assay showed that individual application of mild osmotic stresses or ABA did not inhibit growth. However, osmotic stress and ABA applied together significantly reduced growth. This interaction may be an important mechanism in explaining leaf growth inhibition of droughted plants. It also accounts for the relative (compared to droughted plants) insensitivity of leaf elongation to ABA in the leaf elongation assay

Daily irrigation attenuates xylem abscisic acid concentration and increases leaf water potential of Pelargonium×hortorum compared with infrequent irrigation

The physiological response of plants to different irrigation frequencies may affect plant growth and water use efficiency (WUE; defined as shoot biomass/cumulative irrigation). Glasshouse-grown, containerized Pelargonium × hortorum BullsEye plants were irrigated either daily at 100% of plant evapotranspiration (ET) (well-watered; WW), or at 50% ET applied either daily [frequent deficit irrigation (FDI)] or cumulatively every 4 days [infrequent deficit irrigation (IDI)], for 24 days. Both FDI and IDI applied the same irrigation volume. Xylem sap was collected from the leaves, and stomatal conductance (gs) and leaf water potential (Ψleaf) measured every 2 days. As soil moisture decreased, gs decreased similarly under both FDI and IDI throughout the experiment. Ψleaf was maintained under IDI and increased under FDI. Leaf xylem abscisic acid (ABA) concentrations ([X-ABA]leaf) increased as soil moisture decreased under both IDI and FDI, and was strongly correlated with decreased gs, but [X-ABA]leaf was attenuated under FDI throughout the experiment (at the same level of soil moisture as IDI plants). These physiological changes corresponded with differences in plant production. Both FDI and IDI decreased growth compared with WW plants, and by the end of the experiment, FDI plants also had a greater shoot fresh weight (18%) than IDI plants. Although both IDI and FDI had higher WUE than WW plants during the first 10 days of the experiment (when biomass did not differ between treatments), the deficit irrigation treatments had lower WUE than WW plants in the latter stages when growth was limited. Thus, ABA-induced stomatal closure may not always translate to increased WUE (at the whole plant level) if vegetative growth shows a similar sensitivity to soil drying, and growers must adapt their irrigation scheduling according to crop requirements

Stem girdling uncouples soybean stomatal conductance from leaf water potential by enhancing leaf xylem ABA concentration

To understand the impact of shoot-to-root ABA transport on water potential of, and xylem ABA concentration in, different plant tissues during soil drying, soybean (Glycine max cv. Siverka) plants were subjected to drought and girdling in a factorial experiment. Girdling was achieved by surgically excising the phloem tissue from just above the cotyledonary node. After girdling and withholding water, ABA concentrations were determined in xylem saps extracted from individual leaves, detached shoots and de-topped roots, after measuring stomatal conductance (gs), tissue water potentials, and root ABA concentrations. Soil drying decreased water potential throughout the plant and approximately doubled xylem ABA concentrations, coinciding with stomatal closure. Girdling slightly enhanced water potential, especially in droughted plants. Girdling diminished the soil-drying induced increase in xylem sap ABA concentration, and completely prevented root tissue ABA accumulation. Furthermore, girdling decreased root ABA concentration and increased leaf xylem ABA concentration of well-watered (WW) plants. Stomatal conductance declined linearly with leaf water potential only in intact plants, while gs declined as leaf xylem ABA concentration increased, independently of girdling. Thus shoot to root ABA transport not only determines (soil-drying induced) root ABA accumulation, but also limits ABA accumulation in the shoot to maintain stomatal opening of WW plants

Gravimetric phenotyping of whole plant transpiration responses to atmospheric vapour pressure deficit identifies genotypic variation in water use efficiency

There is increasing interest in rapidly identifying genotypes with improved water use efficiency, exemplified by the development of whole plant phenotyping platforms that automatically measure plant growth and water use. Transpirational responses to atmospheric vapour pressure deficit (VPD) and whole plant water use efficiency (WUE, defined as the accumulation of above ground biomass per unit of water used) were measured in 100 maize (Zea mays L.) genotypes. Using a glasshouse based phenotyping platform with naturally varying VPD (1.5 to 3.8 kPa), a 2-fold variation in WUE was identified in well-watered plants. Regression analysis of transpiration versus VPD under these conditions, and subsequent whole plant gas exchange at imposed VPDs (0.8 to 3.4 kPa) showed identical responses in specific genotypes. Genotype response of transpiration versus VPD fell into two categories: 1) a linear increase in transpiration rate with VPD with low (high WUE) or high (low WUE) transpiration rate at all VPDs, 2) a non-linear response with a pronounced change point at low VPD (high WUE) or high VPD (low WUE). In the latter group, high WUE genotypes required a significantly lower VPD before transpiration was restricted, and had a significantly lower rate of transpiration in response to VPD after this point, when compared to low WUE genotypes. Change point values were significantly positively correlated with stomatal sensitivity to VPD. A change point in stomatal response to VPD may explain why some genotypes show contradictory WUE rankings according to whether they are measured under glasshouse or field conditions. Furthermore, this novel use of a high throughput phenotyping platform successfully reproduced the gas exchange responses of individuals measured in whole plant chambers, accelerating the identification of plants with high WUE

Exploring the use of recombinant inbred lines in combination with beneficial microbial inoculants (AM fungus and PGPR) to improve drought stress tolerance in tomato

At a world scale, tomato is an important horticultural crop, but its productivity is highly reduced by drought stress. Combining the application of beneficial microbial inoculants with breeding and grafting techniques may be key to cope with reduced tomato yield under drought. This study aimed to investigate the growth responses and physiological mechanisms involved in the performance under drought stress of four tomato recombinant inbred lines (RIL) after inoculation with the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis and the plant growth promoting rhizobacteria (PGPR) Variovorax paradoxus 5C-2. Results showed a variation in the efficiency of the different tomato RILs under drought stress and a differential effect of the microbial inoculants, depending on the RIL involved. The inoculants affected plant parameters such as net photosynthetic capacity, oxidative damage to lipids, osmolyte accumulation, root hydraulic conductivity or aquaporin abundance and phosphorylation status. RIL66 was the one obtaining maximum benefit from the microbial inoculants under drought stress conditions, due likely to improved CO2-fixation capacity and root hydraulic conductivity. We propose that RIL66 could be selected as a good plant material to be used as rootstock to improve tomato growth and productivity under water limiting conditions. Since RIL66 is highly responsive to microbial inoculants, this grafting strategy should be combined with inoculation of R. irregularis and V. paradoxus in order to improve plant yield under conditions of drought stress

Whole plant chamber to examine sensitivity of cereal gas exchange to changes in evaporative demand

Background: Improving plant water use efficiency (WUE) is a major target for improving crop yield resilience to adverse climate change. Identifying genetic variation in WUE usually relies on instantaneous measurements of photosynthesis (An) and transpiration (Tr), or integrative measurements of carbon isotope discrimination, at the leaf level. However, leaf gas exchange measurements alone do not adequately represent whole plant responses, especially if evaporative demand around the plant changes. Results: Here we describe a whole plant gas exchange system that can rapidly alter evaporative demand when measuring An, Tr and intrinsec WUE (iWUE) and identify genetic variation in this response. An was not limited by VPD under steady-state conditions but some wheat cultivars restricted Tr under high evaporative demand, thereby improving iWUE. These changes may be ABA-dependent, since the barley ABA-deficient mutant (Az34) failed to restrict Tr under high evaporative demand. Despite higher Tr, Az34 showed lower An than wild-type (WT) barley because of limitations in Rubisco carboxylation activity. Tr and An of Az34 were more sensitive than WT barley to exogenous spraying with ABA, which restricted photosynthesis via substrate limitation and decreasing Rubisco activation. Conclusions: Examining whole plant gas exchange responses to altered VPD can identify genetic variation in whole plant iWUE, and facilitate an understanding of the underlying mechanism(s)

Distinctive phytohormonal and metabolic profiles of Arabidopsis thaliana and Eutrema salsugineum under similar soil drying

Main conclusions: Arabidopsis and Eutrema show similar stomatal sensitivity to drying soil. In Arabidopsis, larger metabolic adjustments than in Eutrema occurred, with considerable differences in the phytohormonal responses of the two species. Although plants respond to soil drying via a series of concurrent physiological and molecular events, drought tolerance differs greatly within the plant kingdom. While Eutrema salsugineum (formerly Thellungiella salsuginea) is regarded as more stress tolerant than its close relative Arabidopsis thaliana, their responses to soil water deficit have not previously been directly compared. To ensure a similar rate of soil drying for the two species, daily soil water depletion was controlled to 5–10% of the soil water content. While partial stomatal closure occurred earlier in Arabidopsis (Day 4) than Eutrema (from Day 6 onwards), thereafter both species showed similar stomatal sensitivity to drying soil. However, both targeted and untargeted metabolite analysis revealed greater response to drought in Arabidopsis than Eutrema. Early peaks in foliar phytohormone concentrations and different sugar profiles between species were accompanied by opposing patterns in the bioactive cytokinin profiles. Untargeted analysis showed greater metabolic adjustment in Arabidopsis with more statistically significant changes in both early and severe drought stress. The distinct metabolic responses of each species during early drought, which occurred prior to leaf water status declining, seemed independent of later stomatal closure in response to drought. The two species also showed distinct water usage, with earlier reduction in water consumption in Eutrema (Day 3) than Arabidopsis (Day 6), likely reflecting temporal differences in growth responses. We propose Arabidopsis as a promising model to evaluate the mechanisms responsible for stress-induced growth inhibition under the mild/moderate soil drying that crop plants are typically exposed to

Deep roots and soil structure

In this opinion article we examine the relationship between penetrometer resistance and soil depth in the field. Assuming that root growth is inhibited at penetrometer resistances > 2.5 MPa, we conclude that in most circumstances the increases in penetrometer resistance with depth are sufficiently great to confine most deep roots to elongating in existing structural pores. We suggest that deep rooting is more likely related to the interaction between root architecture and soil structure than it is to the ability of a root to deform strong soil. Although the ability of roots to deform strong soil is an important trait, we propose it is more closely related to root exploration of surface layers than deep rooting

How useful are volunteers for visual biodiversity surveys? An evaluation of skill level and group size during a conservation expedition

The ability of volunteers to undertake different tasks and accurately collect data is critical for the success of many conservation projects. In this study, a simulated herpetofauna visual encounter survey was used to compare the detection and distance estimation accuracy of volunteers and more experienced observers. Experience had a positive effect on individual detection accuracy. However, lower detection performance of less experienced volunteers was not found in the group data, with larger groups being more successful overall, suggesting that working in groups facilitates detection accuracy of those with less experience. This study supports the idea that by optimizing survey protocols according to the available resources (time and volunteer numbers), the sampling efficiency of monitoring programs can be improved and that non-expert volunteers can provide valuable contributions to visual encounter-based biodiversity surveys. Recommendations are made for the improvement of survey methodology involving non-expert volunteers