Changes in plant–soil hydraulic pressure gradients of soybean in response to soil drying

Plant gas‐exchange response to drying soil in many instances tracks a common pattern when expressed as a function of fraction of transpirable soil water (FTSW). There is little decrease in gas exchange until FTSW decreases to a value in the range of about 0.3–0.45, then with further drying gas exchange declines approximately linearly. This unique pattern is hypothesised to reflect mainly changes in the water potential gradient between bulk soil and plant. The primary objective was to directly document the basis of this response by measuring the hydrostatic pressure gradient required in the soil to maintain leaf xylem at zero potential with decreasing FTSW. Pots in which soybean (Glycine max) plants were grown were placed in a pressure chamber and the pressure adjusted to maintain zero water potential in a leaf petiolule. These results showed a small, relatively constant hydrostatic pressure had to be applied to the soil to maintain zero leaf xylem water potential until FTSW decreased to approximately 0.3–0.45. Thereafter, the required hydrostatic pressure gradient increased as FTSW continued to decrease. Hydraulic conductance was calculated to be relatively stable early in the drying cycle, and then decrease as the soil dried to comparatively high FTSW of 0.5–0.7.     

Genotypic variation within sorghum for transpiration response to drying soil

Aims

Sorghum is commonly grown under dryland conditions, where yields are limited by soil water deficits. Yield increase may be possible by selecting genotypes that express traits that are desirable for water-limited conditions. Water conservation is one possible trait for increasing yield since this trait could increase water availability during critical stages of crop development. Water conservation could be achieved by slower transpiration rate with soil drying initiated at a high fraction of transpirable soil water (FTSW) so that the use of soil water is extended over a longer period of time. This water conservation strategy may allow the crop to have water available during the critical phase of grain filling. Therefore, the objective of this study was to compare genotypes of sorghum for possible differences in the threshold for the decline in transpiration.

Methods

Sixteen sorghum genotypes were selected for this study based on differences in their sensitivity to elevated vapor pressure deficit (VPD). These genotypes were subjected to dry-down experiments in pots in a greenhouse to determine the threshold FTSW for the decrease in transpiration rate as the soil dried.

Results

Differences in the FTSW threshold for transpiration decline were observed to range among genotypes from 0.32 to 0.48. The genotypes differed between low and high FTSW thresholds in the same manner as they did for a breakpoint in the VPD or lack of breakpoint, respectively. Those genotypes with high FTSW thresholds exhibited the hypothesized desired trait. However, they did not exhibit the water conserving trait of limited transpiration rate at high VPD. On the other hand, those genotypes with a low FTSW threshold were those selected for limited transpiration rate at high VPD. These genotypes also differed based on their transpiration rate under well-watered conditions with the genotypes with a low FTSW threshold exhibiting a low transpiration rate.

Conclusions

These results demonstrated that among the sorghum genotypes there are several alternative traits for enhancing soil water conservation for growth under dry land conditions.     

Sequence of drought response of maize seedlings in drying soil

Leaf elongation in monocotyledonous plants is sensitive to drought. To better understand the sequence of events in plants subjected to soil drying, leaf elongation and transpiration of maize seedlings ( Zea mays L.) of 4 cultivars were monitored continuously and the diurnal courses of the root and leaf water relations were determined. Results from this study indicate the following sequence of drought response: Leaf elongation decreased before changes in the leaf water relations of non‐growing zones of leaf blades were detected and before transpiration decreased. Reductions in leaf elongation preceded changes in the root water potential (ψ_w). Root ψ_w was not a very sensitive indicator of soil dryness, whereas the root osmotic potential (ψ_s) and root turgor (ψ_p) were more sensitive indicators. The earliest events observed in drying soil were a significant increase in the largest root diameter class (1 720 to 1 960 gm) and a decrease in leaf elongation ( P = 0.08) 2 days after withholding water. Significant increases in root length were observed 2 days later. Soil drying increased the number of fine roots with diameters of <240 µm. Slight increases in soil strength did not affect leaf elongation in the drying soil.     

Stomatal control in tomato with ABA-deficient roots: response of grafted plants to soil drying

The hypothesis that ABA produced by roots in drying soil is responsible for stomatal closure was tested with grafted plants constructed from the ABA-deficient tomato mutants, sitiens and flacca and their near-isogenic wild-type parent. Three types of experiments were conducted. In the first type, reciprocal grafts were made between the wild type and sitiens or flacca. Stomatal conductance accorded with the genotype of the shoot, not the root. Stomates closed in all of the grafted plants in response to soil drying, regardless of the root genotype, i.e. regardless of the ability of the roots to produce ABA. In the second type of experiment, wild-type shoots were grafted onto a split-root system consisting of one wild-type root grafted to one mutant (flacca or sitiens) root. Water was withheld from one root system, while the other was watered well so that the shoots did not experience any decline in water potential or loss of turgor. Stomates closed to a similar extent when water was withheld from the mutant roots or the wild-type roots. In the third type of experiment, grafted plants with wild-type shoots and either wild-type or sitiens roots were established in pots that could be placed inside a pressure chamber, and the pressure increased as the soil dried so that the shoots remained fully turgid throughout. Stomates closed as the soil dried, regardless of whether the roots were wild type or sitiens. These experiments demonstrate that stomatal closure in response to soil drying can occur in the absence of leaf water deficit, and does not require ABA production by roots. A chemical signal from roots leading to a change in apoplastic ABA levels in leaves may be responsible for the stomatal closure.     

The effect of vapor pressure deficit on maize transpiration response to a drying soil

A decline in plant transpiration has been widely observed to occur within a fairly stable range of threshold values of fraction transpirable soil water (FTSW), usually 0.3–0.4. However, the stability of this function has not been compared at various levels of atmospheric vapor pressure deficit (VPD). Soil hydraulic conductivity is likely to be involved in determining the threshold where water supply is limiting. Thus, it was hypothesized that at a high VPD resulting in increased transpiration rates, the FTSW threshold for the decline of transpiration rates as a result of drying soil would be increased. This study was undertaken in controlled environment chambers with two maize (Zea mays L.) hybrids (Pioneer Brand Hybrids `3165' and `3737') so as to subject plants to four VPD levels (1.1, 2.0, 2.9 and 3.6 kPa) during a soil drying experiment. In contrast to the original hypothesis, there was little ( 0.05 FTSW) change in the threshold FTSW in response to increased VPD for either hybrid. In fact, over the narrow 0.31–0.38 FTSW range observed, the two hybrids showed opposite trends in FTSW threshold as VPD increased. These results supported the view that the FTSW threshold for the decline in transpiration with drying soil is stable, showing little sensitivity to changes in VPD.     

Abscisic acid and ethylene interact in wheat grains in response to soil drying during grain filling

Grain filling is an intensive transportation process regulated by soil drying and plant hormones. This study investigated how the interaction between abscisic acid (ABA) and ethylene is involved in mediating the effects of soil drying on grain filling in wheat (Triticum aestivum). Two wheat cultivars, cv. Yangmai 6 and cv. Yangmai 11, were field-grown, and three irrigation treatments, well-watered, moderately soil-dried (MD) and severely soil-dried (SD), were imposed from 9 d post anthesis until maturity. A higher ABA concentration and lower concentrations of ethylene and 1-aminocylopropane-1-carboxylic acid (ACC) were found in superior grains (within a spike, those grains that were filled earlier and reached a greater size) than in inferior grains (within a spike, those grains that were filled later and were smaller), and were associated with a higher filling rate in the superior grains. An increase in ABA concentration and reductions in ethylene and ACC concentrations in grains under MD conditions increased the grain-filling rate, whereas much higher ethylene, ACC and ABA concentrations under SD conditions reduced the grain-filling rate. Application of chemical regulators gave similar results. The results did not differ between the two cultivars. The grain-filling rate in wheat is mediated by the balance between ABA and ethylene in the grains, and an increase in the ratio of ABA to ethylene increases the grain-filling rate.     

土壤干旱胁迫下非水力根信号调控夏玉米气体交换对大气环境的响应

对不同程度土壤干旱胁迫下夏玉米非水力根信号的产生以及气体交换过程对大气环境的响应进行了试验研究。充足底墒播种后采用3个土壤水分处理等级(0~200cm土壤相对湿度为>80%、60%~70%、40%~50%,代号为W T1、W T2和W T3)。生育期内遮去自然降水。试验结果表明,在拔节期轻度和中度土壤干旱胁迫的情况下,玉米根系合成大量ABA传输到地上部分,参与控制气孔开度和气体交换过程对大气环境变化的响应并调节水分消耗。在日变化过程中,当光强和水汽压亏缺较高时,由于蒸腾速率较高,非水力根信号物质向冠层的传输速率也较高,ABA在叶片中的累积影响了气孔开张对光强响应的敏感度,气孔开度受到抑制,并且随着ABA累积和浓度的增加,气孔抑制作用越强;在水汽压亏缺较低的情况下,非水力根信号物质向冠层的传输速率较低,ABA的代谢过程以及再分配过程能够保证这种信号物质保持在低水平,从而保证一定程度的气孔开度和光合、蒸腾速率。这种策略能够使夏玉米在轻、中等干旱条件下保证最大的光合作用,同时在可能的胁迫情况下降低蒸腾作用以提高水分利用效率。     

Transient response of amplified metallothionein genes in CHO cells to induction by alpha interferon.

Alpha interferon treatment of CHO cells elicits the rapid synthesis of many gene products, including metallothionein (MT), a protein which avidly binds heavy metals such as zinc, cadmium, and copper. Since MTs appear to have a pleiotropic role in the cell, ranging from metal detoxification to free-radical scavenging, interferon treatment may trigger a generalized defense mechanism. Activation by interferon, however, was transient, with MT mRNA being maximally detectable by a cytodot procedure within the first hour. Subsequent addition of interferon was ineffective until 7 h after the initial treatment. The action of zinc, a potent inducer of MT, however, remained independent of alpha interferon induction. The transient nature of induction by interferon was examined for altered rate of MT mRNA turnover.     

Photosynthesis,root respiration,and grain yield of spring wheat in response to surface soil drying

The aims of this research were to test the influence of surface soil drying on photosynthesis, root respiration and grain yield of spring wheat (Triticum aestivum), and to evaluate the relationship between root respiration and grain yield. Wheat plants were grown in PVC tubes 120 cm in length and 10 cm in diameter. Three water regimes were employed: (a) all soil layers were irrigated close to field water capacity (CK); (b) upper soil layers (0–40 cm from top) drying (UD); (c) lower soil layer (80–120 cm from top) wet (LW). The results showed that although upper drying treatment maintained the highest root biomass, root respiration and photosynthesis rates at anthesis, the root respiration of the former was significantly (P < 0.05) lower than the latter at the jointing stage. There were no differences in water use efficiency or harvest index between plants from the upper drying and well-watered treatment. However, the grain weight for plants in the upper drying treatment was significantly (P< 0.05) higher than that of in well-watered control. The results suggest that reduced root respiration rate and the amount of photosynthates utilized by root respiration in early season growth may also have contributed to improve crop production under soil drying. Reduced root activity and root respiration rate, in the early growth stage, not only increased the photosynthate use efficiency (root respiration rate: photosynthesis ratio), but also grain yield. Rooting into a deeper wet soil profile before grain filling was crucial for spring wheat to achieve a successful seedling establishment and high grain yield.     

Water relations and root activities of Buchloe dactyloides and Zoysia japonica in response to localized soil drying

Effects of localized soil drought stress on water relations, root growth, and nutrient uptake were examined in drought tolerant ‘Prairie’ buffalograss [Buchloe dactyloides (Nutt.) Engelm.] and sensitive ‘Meyer’ zoysiagrass (Zoysia japonica Steud.). Grasses were grown in small rhizotrons in a greenhouse and subjected to three soil moisture regimes: (1) watering the entire 80-cm soil profile (well-watered control); (2) drying 0–40 cm soil and watering the lower 40 cm (partially dried); (3) and drying the entire soil profile (fully dried). Drying the 0–40 cm soil for 28 days had no effect on leaf water potential (Ψ _leaf ) in Prairie buffalograss compared to the well-watered control but reduced that in Meyer zoysiagrass. Root elongation rate was greater for Prairie buffalograss than Meyer zoysiagrass under well-watered or fully dried conditions. Rooting depth increased with surface soil drying; with Prairie buffalograss having a larger proportion of roots in the lower 40 cm than Meyer zoysiagrass. The higher rates of water uptake in the deeper soil profile in the partially dried compared to the well-watered treatment and by Prairie buffalograss compared to Meyer zoysiagrass could be due to differences in root distribution. Root ¹⁵N uptake for Prairie buffalograss was higher in 0–20 cm drying soil in the partially dried treatment than in the fully dried treatment. Diurnal fluctuations in soil water content in the upper 20 cm of soil when the lower 40 cm were well-watered indicated water efflux from the deeper roots to the drying surface soil. This could help sustain root growth, maintain nutrient uptake in the upper drying soil layer, and prolong turfgrass growth under localized drying conditions, especially for the deep-rooted Prairie buffalograss. This revised version was published online in June 2006 with corrections to the Cover Date.     

Transpiration response of Arabidopsis,maize, and soybean to drying of artificial and mineral soil

Water-deficit stress is a major constraint on plant productivity and consequently, is a major focus of much research. Stress is often imposed on plants in these experiments by withholding water from the artificial potting media on which the plants are grown. No attention has been given, however, to the possibility of differences in the dynamics of stress imposition between that resulting from dehydration of the artificial rooting media and that of drying of mineral soil. The objective of this research was to compare transpiration rates during drying of a mineral soil and of an artificial potting mixture for three test species: Arabidopsis thaliana, maize (Zea mays), and soybean (Glycine max). These results showed major differences in transpiration response between the two soil media. Drying of mineral soil confirmed previous observations that no decrease in transpiration rates occurred until 0.27–0.34 of the extractable water remained in the soil. Thereafter, there was essentially a linear decrease in transpiration with further soil drying. In contrast, transpiration rates of plants grown on the potting mixture began to decrease when about 0.6–0.7 of the extractable water still remained in the soil. Consequently, plants grown on the potting mixture as compared to the mineral soil were exposed to stress very early in the drying cycle and the stress was much more prolonged over a wide range of soil moistures. Caution is warranted in extrapolating to natural, mineral soils the results obtained from plants subjected to water-deficits using artificial potting mixtures.     

The use of comparative quantitative proteomics analysis in rice grain-filling in determining response to moderate soil drying stress

Moderate soil drying (MSD) stress at the grain filling stage can improve grain filling efficiently and thus increase grain yield. To elucidate the molecular response of grain filling to MSD stress, a labeling LC-based quantitative proteomics approach using tandem mass tags was applied to determine the changes in leaf and grain protein abundance level at 15 days after flowering. A total of 2109 leaf proteins and 3220 grain proteins were detected, and 251 leaf proteins and 220 grain proteins were differentially expressed under MSD stress. Based on MapMan ontology, differentially expressed proteins in leaf and grain were categorized within 22 and 18 functional categories, respectively. The patterns observed were interesting in that in some categories such as photosynthesis-related protein in leaf and cell division related proteins in grain showed higher expression abundant under MSD stress, which facilities increasing the source supply and sink size. In other categories, such as carbohydrate metabolism and mitochondrial electron transport, surprisingly showed a completely different expression pattern between leaf and grain under MSD stress, which led to faster and better remobilization of carbon from leaf to grain. Additionally, the complicated functional network including the small GTP-binding proteins, calmodulin, and 14-3-3 proteins play an important role in regulation carbon remobilization mediated by the stressful signals from soil after rice plants were treated with MSD at grain-filling stage. The findings provide theoretical evidence for better quality control and scientific improvement of rice in practice.     

Cellular basis for the differential response of mouse kallikrein genes to hormonal induction.

The expression of many mouse kallikrein genes in the salivary gland is sexually dimorphic and inducible in females by administration of testosterone or thyroxine. Induction is slow (3-7 days) and is accompanied by the non-uniform differentiation of the cell type expressing these genes from striated duct (SD) cells (female) to granular convoluted tubule (GCT) cells (male). One kallikrein gene, mGK-6, is expressed at an apparently constant total level in male and female and is not induced by either hormone. In situ hybridization histochemistry shows that all kallikrein genes analyzed exhibit uniform cellular distribution of expression in the SD cells of the female. The hormonally mediated differentiation of some, but not all, of these cells has different effects on kallikrein gene expression--mGK-6 is repressed while other kallikreins are induced--leading to non-uniform distribution of expression.     

Photosynthesis,respiration, and carbon allocation of two cool-season perennial grasses in response to surface soil drying

The study was conducted to investigate carbon metabolic responses to surface soil drying for cool-season grasses. Kentucky bluegrass (Poa pratensis L.) and tall fescue (Festuca arundinaceae Schreb.) were grown in a greenhouse in split tubes consisting of two sections. Plants were subjected to three soil moisture regimes: (1) well-watered control; (2) drying of upper 20-cm soil (upper drying); and (3) drying of whole 40-cm soil profile (full drying). Upper drying for 30 d had no dramatic effects on leaf water potential (Ψ_leaf) and canopy photosynthetic rate (P_n) in either grass species compared to the well-watered control, but it reduced canopy respiration rate (R_canopy) and root respiration rate in the top 20 cm of soil (R_top). For both species in the lower 20 cm of wet soil, root respiration rates (R_bottom) were similar to the control levels, and carbon allocation to roots increased with the upper soil drying, particularly for tall fescue. The proportion of roots decreased in the 0-20 cm drying soil, but increased in the lower 20 cm wet soil for both grass species; the increase was greater for tall fescue. The Ψ_leaf, P_n, R_canopy, R_top, R_bottom, and carbon allocation to roots in both soil layers were all significantly higher for upper dried plants than for fully dried plants of both grass species. The reductions in R_canopy and R_top in surface drying soil and increases in root respiration and carbon allocation to roots in lower wet soil could help these grasses cope with surface-soil drought stress. This revised version was published online in June 2006 with corrections to the Cover Date.