Drought adaptation of field grown wheat in relation to soil physical conditions

In order to investigate the effects of soil texture on possible non-hydraulic signals under field conditions, spring wheat plants (Triticum aestivum L. cv. Cadensa) grown in sand and loam soils and with a well developed root system were exposed to slow soil drying in the late vegetative stage of growth. Soil water potential and content were measured daily at different depths and plant responses were measured in flag leaves. When the average soil water potential in the top soil layers (0–25 cm depth in sand and 0–45 cm depth in loam) dropped to –60 or –70 kPa and the lower soil layers were still at field capacity, morning xylem [ABA] (0.03–0.04 vs. 0.06–0.08 mmol m-3) and midday leaf ABA concentration increased (250–300 vs. 400–450 ng/g DW) and leaf conductance decreased relatively to well-watered (control) plants (0.75–0.88 vs. 0.64–0.70 mol m-2 s-1). These responses took place before any decrease in leaf water potential occurred as compared with control plants, indicating that they were triggered by root-borne signals due to reduced root water status in the top soil layers. At this stage the soil water content was as low as 6% by volume, the fraction of roots in ‘wet’ soil was 0.12 and relative available soil water was 45% in sand and still high 20%, 0.48 and 70%, respectively, in loam of the whole soil profile indicating that roots were responding to soil water availability and not soil water content at a certain evaporative demand. In addition, similar responses occurred at high and low evaporative demands (3.4–5.2 vs. 0.6–4.0 mm/day of potential evapotranspiration). This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization of photosynthesis of flag leaves in a wheat hybrid and its parents grown under field conditions

Two wheat cultivars, one with high yield and the other with a high tolerance against oxidative stress, were compared with a hybrid of these two cultivars by investigating their photosynthetic characteristics of flag leaves. From the beginning of flowering to the 17th day, CO(2) assimilation rate (P(max)) was maintained and there were no appreciable differences between the hybrid and its parents. P(max) showed no decrease at noon compared to that in the morning. From the 20th to the 30th day of flowering, P(max) decreased significantly, and this decrease was significantly less in the hybrid than in its parents. The actual photosystem II (PSII) efficiency (Phi(PSII)) and the maximal efficiency of PSII photochemistry (F(v)/F(m)) showed a significant decrease only on the 30th day after anthesis; this decline was much less marked in the hybrid relative to its parents, both in the morning and at noon. A decrease occurred in Phi(PSII) and F(v)/F(m) at noon when compared to that in the morning, but this decrease was less marked in the hybrid than in its parents. Rubisco activity decreased significantly from the 13th day of flowering and was higher in the morning than at noon both in the hybrid and its parents. However, the hybrid always showed a higher value of Rubisco activity. The activities of posphoenolpyruvate carboxylase and pyruvate phosphate dikinase showed similar changes to those in Rubisco activity, particularly from the 20th to 30th day. The results of this study suggest that the higher photosynthetic capacity of the flag leaf in the hybrid can help to accumulate more dry material, and may be the physiological basis for higher yield over its parents.     

Photoinhibition of photosynthesis in willow leaves under field conditions

Chlorophyll fluorescence of leaves of a willow (Salix sp.) stand grown in the field in northern Sweden was measured on several occasions during the growing season of 1987. For leaves that received mostly full daylight, the F _V/F _P ratio declined roughtly 15% in the afternoon on cloudless days in July (F _P is the fluorescence at the peak of the induction curve obtained at the prevailing air temperature after 45 min of dark adaptation, and F _V is variable fluoresence, F _V=F _P-F _O, where F _O is minimal fluorescence). There was no decrease in the F _V/F _P ratio on cloudy days, while the effect was intermediate on changeable days. In view of this light dependence, together with the fact that the decline in the F _V/F _P ratio was paralleled with an equal decline in the corresponding fluorescence ratio F _V/F _M at 77K, and a similar decline in the maximum quantum yield of O₂ evolution, it is suggested that the decline in the F _V/F _P ratio represents a damage in photosyntem II attributable to photoinhibition. Recovery of the F _V/F _P ratio in dim light following a decline on a cloudless day took 7–16 h to go to completion; the F _V/F _P ratio was fully restored the following morning. When all active leaves of a peripheral shoot were compared, the F _V/F _P ratio in the afternoon of a day of bright light varied greatly from leaf to leaf, though the majority of leaves showed a decline. This variation was matched by a pronounced variation in intercepted photon flux density. When leaves developed in the shade were exposed to full sunlight by trimming of the stand an increased sensitivity to photoinhibition was observed as compared to peripheral leaves. The present study indicates that peripheral willow shoots experienced in the order of 10–20% photoinhibition during an appreciable part of their life. This occurred even though the environmental conditions were within the optimal range of photosynthesis and growth.Abbreviations and symbols F _O minimum fluorescence - F _P fluorescence at the peak of the induction curve obtained at normal ambient temperatures - F _V variable fluorescence - F _M maximum fluorescence obtained at 77K - PPFD photosynthetic photon flux density     

Phytoremediation of pyrene in a Cecil soil under field conditions

We evaluated the effects of annual ryegrass (Lolium multiflorum Lam.) and phosphorus (P) availability on the dissipation of pyrene added at a concentration of approximately 600 mg kg-1 dry soil in the top 7.5 cm of a Cecil loamy sand (fine, kaolinitic, thermic Typic Kanhapludults) in a 10-month experiment under field conditions in Clemson, South Carolina. Plastic canopies were installed to prevent flooding of plots and raindrop dispersion of pyrene. Treatment factors were pyrene, vegetation, and available P levels. Each of the eight treatments had four replicates. The soil was adjusted to low and high P concentrations (an average of 41 and 66 kg extractable P ha-1, respectively). After a 175-d lag period for all treatments, the rate of pyrene removal followed first-order kinetics. The first-order rate constant was significantly higher in nonvegetated (0.098 d-1) than vegetated treatments (0.034 d-1). These data suggest that the presence of easily biodegradable organic matter from plant roots slowed the removal rate of pyrene. The levels of available P did not affect the rate of pyrene dissipation. Pyrene decreased below the detection limit of 6.25 mg kg-1 dry soil in all treatments after 301 d.     

Lipoxygenase from the leaves of wheat grown under different water supply conditions

Lipoxygenase (LOG) in protein fractions isolated from the leaves of substituted wheat lines was investigated. Three molecular forms of the enzyme were detected. A water deficiency caused the induction of a membrane-bound form (mLOG) and resulted in a decrease in the activity of "soluble" enzymes (s1LOG) and (s2LOG) in most genotypes. A correlation analysis demonstrated the dependence between the level of enzymatic activity and indices of resistance to drought. A genetic control of the s 1 LOG and s2LOG activity at an optimal water supply level was associated with chromosomes 1A, 1D, 3A, 5A, 5B, and 5D, while under the conditions of the modeled soil drought, it was associated with chromosomes 1B and 1D.     

Lipoxygenase from the leaves of wheat grown under different water supply conditions

Lipoxygenase (LOG) in protein fractions isolated from the leaves of substituted wheat lines was investigated. Three molecular forms of the enzyme were detected. A water deficiency caused the induction of a membrane-bound form (mLOG) and resulted in a decrease in the activity of “soluble” enzymes (s1LOG) and (s2LOG) in most genotypes. A correlation analysis demonstrated the dependence between the level of enzymatic activity and indices of resistance to drought. A genetic control of the s1LOG and s2LOG activity at an optimal water supply level was associated with chromosomes 1A, 1D, 3A, 5A, 5B, and 5D, while under the conditions of the modeled soil drought, it was associated with chromosomes 1B and 1D.     

Silicification of the adaxial epidermis of leaves of a panicoid grass in relation to leaf position and section and environmental conditions

Many studies relate silica content in plants with internal or external factors; however, few works analyse the effect of these factors on the silicification of different cell types. In this study, we examined the effect of leaf section and leaf position, and environmental conditions on the percentages of silicified epidermal cells of a native Pampean panicoid grass, Bothriochloa laguroides D. C. Pilger. Two different environmental situations were selected for the collection of plants: a natural wetland and a quartzite quarry, located in the southeast Buenos Aires province, Argentina. Clarification and staining methodologies were applied so as to study the distribution of silicified cells in different sections of leaves of the plants collected. Two and three-factor anovas were applied to the data. Between 13% and 19% of total cells of the adaxial epidermis of leaf blades were silicified. Typical silica short cells were the largest contributor to total silicified cells (53-98%), while the second largest contributor was bulliform cells (0-30%). Percentages of total silicified cells were higher in superior than in inferior leaves, while values from leaf sections varied. When collection sites were compared, plants growing in Los Padres pond, where the silica content in soils is higher, had the higher percentage of silicified cells. Among all types of cell, bulliform cells showed differences in the proportion of silicified cells between leaf position and section and collection site. These results show that silica availability in soils is an important factor that conditions silica accumulation and overlaps with the transpiration effect.     

Effects of short-term flooding on growth,yield and mineral composition of wheat on sodic soil under field conditions

In sodic soils of the Indo-Gangetic alluvial plains of Northern India, flooding for short periods often occurs during the growing season of wheat, leading to low yields. A field study was therefore conducted to evaluate the effects of short-term flooding on growth, yield and mineral composition of wheat (Triticum aestivum Linn. emend. Fiori and Paol) in a sodic soil (pH 8.9, exchangeable sodium percentage 25). Flooding wheat for 2,4 and 6 days at the time of first irrigation (25-day old plants), significantly reduced tillering, plant height, delayed head emergence and resulted in 17.6, 29.0 and 46.7% reduction in grain yield, respectively, Flooding decreased oxygen diffusion rate (ODR) values, restricted root grwoth and reduced ion uptake, especially of N, P, K, Ca, Mg and Zn and led to higher absorption of Na, Fe and Mn. Under the conditions of this experiment, the reduced growth and yield of wheat resulting from short-term flooding was not due to Mn, Fe and Na toxicity but may be due to reduced uptake of nutrients resulting from O₂ deficiency in the soil.     

Radiocarbon--a low-impact tool to study nutrient transport by soil fungi under field conditions

Here, we present a new in-situ method to study the uptake of amino acids by soil fungi. We injected 14C-labeled glycine into a marshland soil and measured the rate and the 14C signature of CO2 respired from sporocarps of Pholiota terrestris over 53.5 h and 2 m. We also determined the incorporation of glycine-C into sporocarp tissue. The 14C signature of the CO2 and tissue was quantified by accelerator mass spectrometry. After the label application, the rate of CO2 flux and its 14C signature from chambers with sporocarps were significantly higher than from chambers without sporocarps, and then declined with time. Postlabel, the 14C signature of the sporocarp tissue increased by 35 per thousand. We show that this approach can be used to study below-ground food webs on an hourly time-scale while minimizing the perturbation of competitive relationships among soil microorganisms and between plants and soil microorganisms. Additionally we show that care must be taken to avoid confounding effects of sporocarp senescence on rates and radiocarbon signatures of respired CO2.     

Dehydration tolerance of spring wheat and its relation to plant growth and productivity under soil drought conditions

Studies performed on spring wheat (Triticum aestivum L. var.lutescens), cv. Kaspar and Kolibri revealed differences between cultivars in leaf dehydration tolerance. The differences were found to be responsible for the changes in plant growth and productivity observed under drought conditions. Thus, determination of leaf dehydration tolerance may allow the prediction of the plant response to the water stress in terms of plant growth and grain yield. Differentiated drought responses of both cultivars are described. This work was supported by Project No. 9.1.1. coordinated by the Institute of Plant Breeding and Acclimatization.     

Reexamination of silicon effects on rice growth and production under field conditions using a low silicon mutant

Silicon (Si) is a beneficial element for healthy growth and high and sustainable production of rice, but the mode of action of the beneficial effects has not been well understood. We carried out field trials for four years at two different locations to re-examine the effects of Si on the growth and production of rice using a low silicon rice (lsi1) mutant. The mutant accumulated much lower Si at each growth stage compared with the wild-type rice (Oryza sativa L. cv Oochikara), but there was no difference in the accumulation of other nutrients including N, P, and K. Measurements at different growth stages showed that low Si in the mutant hardly affected the tiller number, chlorophyll content (SPAD value), and root growth. The plant height and shoot dry weight of the wild-type rice were slightly higher than those of the mutant at a later growth stage, but the difference was not significant between the two lines. However, grain yield was reduced by 79–98%, depending on year, due to a low Si accumulation in the mutant, which showed the largest effect of Si on rice production among all studies reported so far. Among the yield components, the percentage of filled spikelets was mostly affected, being only 13.9% of the wild-type rice in the mutant. The grain color of the mutant became brown because of excessive transpiration and infection of pathogens. These results indicate that Si increases rice yield mainly by enhancing the fertility of spikelets.     

Inverse determination of heterotrophic soil respiration response to temperature and water content under field conditions

Heterotrophic soil respiration is an important flux within the global carbon cycle. Exact knowledge of the response functions for soil temperature and soil water content is crucial for a reliable prediction of soil carbon turnover. The classical statistical approach for the in situ determination of the temperature response (Q₁₀ or activation energy) of field soil respiration has been criticised for neglecting confounding factors, such as spatial and temporal changes in soil water content and soil organic matter. The aim of this paper is to evaluate an alternative method to estimate the temperature and soil water content response of heterotrophic soil respiration. The new method relies on inverse parameter estimation using a 1-dimensional CO₂ transport and carbon turnover model. Inversion results showed that different formulations of the temperature response function resulted in estimated response factors that hardly deviated over the entire range of soil water content and for temperature below 25°C. For higher temperatures, the temperature response was highly uncertain due to the infrequent occurrence of soil temperatures above 25°C. The temperature sensitivity obtained using inverse modelling was within the range of temperature sensitivities estimated from statistical processing of the data. It was concluded that inverse parameter estimation is a promising tool for the determination of the temperature and soil water content response of soil respiration. Future synthetic model studies should investigate to what extent the inverse modelling approach can disentangle confounding factors that typically affect statistical estimates of the sensitivity of soil respiration to temperature and soil water content.     

Phosphatidylethanolamine in wheat and barley leaves under water stress

Phosphatidylethanolamine could not be detected in the leaves of less drought-tolerant varieties of wheat (S-308) and barley (BG-25) when the plants wer     

Relative growth rates of leaves from soybean grown under drought-stressed and irrigated field conditions

Abstract. The relative growth rates and leaf area were graphed against leaf area, normalized with respect to final leaf area, to assess the applicability of the Lockhart cell wall expansion equation to soybean, Glycine max (L.) Merr., leaf development under field conditions. For leaves that had completed more than 20% of their growth, relative growth rates decreased linearly with an increase in the normalized leaf area, indicating that these leaves were undergoing strictly expansive growth. Drought stress significantly decreased the relative growth rate of these larger leaves. Small leaves which had completed less than 20% of their growth, were found to have highly variable relative growth rates. The large variability in relative growth rates indicated that the Lockhart cell wall expansion equation was inadequate to evaluate the growth of these young leaves. Drought stress had virtually no influence on the relative growth rates observed in the small leaves.     

Pulse-labelling a cover crop with 13C to follow its decomposition in soil under field conditions

A preliminary study was conducted using the stable isotope ¹³C to pulse label the cover crop phacelia (Phacelia tanacetifolia) to examine its decomposition in soil, under field conditions. Plants were grown, in pots, in the greenhouse and after four weeks of growth were labelled with ¹³CO₂ six times, at 1–2 week intervals. A single chamber was placed over the pots, and ¹³CO₂ was generated, inside the chamber, by injecting lactic acid into sodium carbonate (99 atom % ¹³C). For calculating the quantity of Na₂CO₃ required, a target enrichment of 5 atom% ¹³C within the shoots of plants, assuming no respiration losses, was used. When harvested, at flowering, the mean enrichment of the shoot material was 3.0466 atom% ¹³C, or 1.9654 atom% excess ¹³C. To assess uniformity of labelling within plants, the shoot of a single plant was divided into leaves and stem from three sections of equal length. Ninety-three percent of this plant's dry matter had a ¹³C enrichment within 20 % of the weighted mean. At a field site with sandy soil, ¹³C labelled shoot and root material were combined and mixed with soil (0–15 cm). The soil was sampled 16 and 179 days later to determine the recovery of the added excess ¹³C in soil total C. The recoveries in soil (0–30 cm) were, respectively, 78 and 40 % at 16 and 179 days; there was appreciable variation associated with the recovery data from day 16, much less so at day 179. Methodological procedures for (i) enhancing the uniformity of labelling with ¹³C within plants, and (ii) minimising variability in the recovery of ¹³C from soil are suggested. ei]R Merckx     

Growth and yield responses of cowpea and maize to inoculation withGlomus mosseae in sterilized soil under field conditions

Summary The effects of inoculation of cowpea and maize withGlomus mosseae and sowing into field soil fumigated with formalin are reported. Root infection was significantly, though temporarily, decreased and grain yield increased, by sterilization. Inoculation increased root infection, shoot yields, P contents and grain yield. IITA Journal Series Paper No. 205.     

Decomposition of humic acids by incubation in a soil water-extract under various conditions of oxygen availability

Summary Decomposition of humic acids suspended in a soil water-extract under various conditions of oxygen availability (aerobic, anaerobic, alternating aerobic/anaerobic) led to the formation of fulvic acids, lower molecular weight compounds, and humin, a more complex mixture of insoluble compounds. The transformation of humic acids and formation of new humic compounds were higher under aerobic conditions than under either anaerobic or alternating aerobic/anaerobic ones.Although subjected to decomposition under greatly differing conditions of oxygen availability, the residual humic acids showed essentially the same types of chemical alterations, an increase of functional groups containing oxygen and a decrease of hydrolysable ternary and quarternary fractions.