Steve Green

Natural and induced cadmium-accumulation in poplar and willow: implications for phytoremediation.

Potentially poplars and willows may be used for the in situ decontamination of soils polluted with Cd, such as pasturelands fertilised with Cd-rich superphosphate fertiliser. Poplar (Kawa and Argyle) and willow (Tangoio) clones were grown in soils containing a range (0.6–60.6 μg g−1 dry soil) of Cd concentrations. The willow clone accumulated significantly more Cd (9–167 μg g−1 dry matter) than the two poplar clones (6–75 μg g−1), which themselves were not significantly different. Poplar trees (Beaupré) sampled in situ from a contaminated site near the town of Auby, Northern France, were also found to accumulate significant quantities (up to 209 μg g−1) of Cd. The addition of chelating agents (0.5 and 2 g kg−1 EDTA, 0.5 g kg−1 DTPA and 0.5 g kg−1NTA) to poplar (Kawa) clones caused a temporary increase in uptake of Cd. However, two of the chelating agents (2 g kg−1 EDTA and 0.5 g kg−1 NTA) also resulted in a significant reduction in growth, as well as abscission of leaves. If the results obtained in these pot experiments can be realised in the field, then a single crop of willows could remove over 100 years worth of fertiliser-induced Cd contamination from pasturelands.

Root water uptake by kiwifruit vines following partial wetting of the root zone

Rates of sap flow and root-water uptake by two 7-year old kiwifruit vines (Acinidia deliciosa) were studied in an orchard with the aim of determining the ability of the vines to alter their spatial pattern of root-water uptake following differential wetting of the root zone. Time-domain reflectometry (TDR) was used to monitor changes in the soils volumetric water content, π. The heat-pulse technique was used to monitor sap flow not only in the stem but also in several large roots to see how root flow responded with local changes in soil water availability. Prior to irrigation there was a broad correspondence between the pattern of water uptake and the distribution of root-length density. However, following irrigation, we observed a preferential uptake of water from the wetter parts of the soil and a corresponding decline in water uptake from the drier parts of the soil. Observations of root uptake by TDR following irrigation also revealed the inordinate activity of near-surface roots. The vine would preferentially draw upon near-surface water if it were available. Kiwifruit vines are able to shift rapidly their pattern of uptake, in a matter of days, away from drier parts of the root zone and begin to extract water preferentially from those regions where it is more freely available. Upon full wetting of the root zone, previously inactive roots in the dry soil of the root zone were quickly able to recover their activity. Indeed their activity following rewatering was found to be greater than it had been prior to the period of soil dryness. A rapid flush of new root growth is considered to be the mechanism that leads to this enhanced activity.

Roots : The big movers of water and chemical in soil

Root water uptake and the dynamic availability of water to plants is a phenomenon that tends to be overlooked by soil scientists, despite the often dominant role of roots as a sink for water in the soil. Water taken up by roots is vital for plant growth and the productive management of soils. In

Observations of night-time water use in kiwifruit vines and apple trees

Transpiration rates of two 7-year-old kiwifruit vines (Actinidia deliciosa) and a 10-year-old apple tree (Malus sylvestris × Red delicious) were measured for 15 days in summer using the heat-pulse technique. Environmental data were collected at the same time, every half-hour. Significant transpiration rates were observed at night, in both kiwifruit and apple, whenever the saturation deficit remained elevated. Night-time water use in kiwifruit plants ranged from 1.4 to 19.2 l, for saturation deficits of between 0.44 and 3.1 g kg−1. Nocturnal transpiration of the apple tree ranged from 0.3 to 5.3 l for saturation deficits between 0.2 and 3.3 g kg−1. Mean night-time water use for kiwifruit and apple was 19% (n = 15 nights) and 6% (n = 15), respectively of the total daily transpiration. Nocturnal transpiration increased linearly with mean saturation deficit and saturation deficit increased with mean wind speed. Nocturnal transpiration was therefore greater on windy nights. The directly measured nocturnal transpiration rates in kiwifruit were compared with rates of water use calculated using the Penman-Monteith combination equation with stomatal resistance data obtained by porometry on one night. There was satisfactory agreement between the two results.

The use of sap flow measurements for scheduling irrigation in olive, apple and Asian pear trees and in grapevines

We evaluated three approaches for scheduling irrigation in wine grape vineyards and in olive, apple and Asian pear tree orchards, based on sap flow measurements and models of plant transpiration. In the first approach, we analysed how the shape of the sap-flow profile changed in response to root-zone soil water conditions and potential evaporative demand. The second approach was based on a transpiration ratio, as defined from the actual daily water use of a target plant divided by the potential daily water use of similar-sized plants under non-limiting soil water conditions (“well-irrigated” plants). Values of the actual plant water use were always determined from measured sap flow. Two independent methods were assessed for the calculation of potential plant water use; either sap flow was measured in well-irrigated plants or we used a leaf-area based model of plant transpiration. On some occasions water stress was found to modify the shape of the sap velocity profile. However, most of the time the velocity profile was found to be an insensitive indicator for triggering irrigation. The transpiration ratio method, using measured sap flow in well-irrigated plants, was more useful for irrigation scheduling, at least for the two species (i.e. olive and grape) that were investigated here. Nonetheless, realization of such an approach in a commercial orchard may not be practical due to problems associated with irrigation management e.g. excessive vegetative growth may occur on the reference plants over time. Besides, irrigating the orchard to maintain non-limiting soil water conditions is not always the best option for water and nutrient management. The alternative transpiration ratio method based on a leaf-area based model of plant water use, yielded the best results. Modelled transpiration rates always provided reliable information not only for well-irrigated plants, but also for deficit-irrigated plants. This result lends support to the use of the method for irrigation scheduling of vineyard and orchard trees. However, the use of models does require detailed microclimate data as well as a user-friendly technique to quantify plant leaf area. From a practical viewpoint the method should encompass the spatial variability of the soil and plants within the orchard. Accurate quantification of these factors is a cornerstone of precision horticulture and such information would help to minimise risks associated with insufficient as well as excessive irrigation applications.

The response of sap flow in apple roots to localised irrigation

Abstract The oft-touted reason for the efficiency of drip irrigation is that roots can preferentially take up water from localised zones of water availability. Here we provide definitive evidence of this phenomenon. The heat-pulse technique was used to monitor rates of sap flow in the stem and in two large surface roots of a 14 year old apple tree (Malus domestica Borkh. cv. Braeburn). The aim was to determine the ability of an apple tree to modify its pattern of root water uptake in response to local changes in soil water content. We monitored the water status of the soil close to the instrumented roots by using time domain reflectometry (TDR) to measure the soils volumetric water content, θ, and by using ceramic-tipped tensiometers to measure the soils matric pressure head, h. A variation in soil water content surrounding the two roots was achieved by supplying a single localised irrigation to just one root, while the other root remained unwatered. Sap flow in the wetted root increased straight away by 50% following this drip irrigation which wetted the soil over a zone of approximately 0.6 m in diameter and 0.25 m in depth. Sap flow in the wetted root remained elevated for a period of about 10 days, that is until most of the irrigation water had been consumed. A comparative study of localised and uniform irrigation was then made. Following irrigation over the full root zone no further change in sap flow in the previously wetted root was observed when referenced to the corresponding sap flow measured in the stem of the apple tree. However sap flow in the previously dry root responded to subsequent irrigations by increasing its flow rate by almost 50%. These results show that apple roots have the capacity to transfer water from local wet areas at much higher rates than normally occurs when the entire root zone is supplied with water. They are also able to shift rapidly their pattern of uptake and begin to extract water preferentially from those regions where it is more freely available. Such an ability supports the use of drip irrigation for the efficient use of scarce water resources. We conclude that the soil-to-root pathway represents a major resistance to water uptake by apple, even at the relatively high soil water pressure heads developed during parts of this experiment, during which the tree was not even under any stress.

Sap flow as an indicator of transpiration and the water status of young apricot trees

The relationship between water loss via transpiration and stem sap flow in young apricot trees was studied under different environmental conditions and different levels of soil water status. The experiment was carried out in a greenhouse over a 2-week period (November 2–14, 1997) using three-year-old apricot trees (Prunus armeniaca cv. Búlida) growing in pots. Diurnal courses of leaf water potential, leaf conductance and leaf turgor potential also were recorded throughout the experiment. Data from four days of different enviromental conditions and soil water availability have been selected for analysis. On each of the selected days the leaf water potential and the mean transpiration rates were well correlated. The slope of the linear regression of this correlation, taken to indicate the total hydraulic resistance of the tree, confirmed an increasing hydraulic resistance under drought conditions. When the trees were not drought stressed the diurnal courses of sap flow and transpiration were very similar. However, when the trees were droughted, measured of sap flow slightly underestimated actual transpiration. Our heat-pulse measurements suggest the amount of readily available water stored in the stem and leaf tissues of young apricot trees is sufficient to sustain the peak transpiration rates for about 1 hour.

Measurement of the increased PAR and net all-wave radiation absorption by an apple tree caused by applying a reflective ground covering

Abstract Measurements were made of the effect of a reflective ground covering on the absorption of radiant energy by an apple tree ( Malus domestica Borkh . ‘Splendour’) in an orchard. Total absorbed PAR and net all-wave radiation were measured using the ‘Whirligig’ radiometer described, by McNaughton et al. (1992) (Agric. For. Meteorol., 62: 87–107) during a 4-week period in late summer. The Whirligig rotated about a tree which was 4 m tall and had a total leaf area of 35.5 m 2 . During the course of the experiment the ground beneath the tree was left uncovered, or was partially-covered or fully-covered with a reflective, aluminized foil. On a sunny day, when total incident PAR on a horizontal plane was 48 mol m −2 , the canopy absorbed about 500 mol of PAR radiation and about 140 MJ of net all-wave radiation. A linear relationship was established between daily totals of incident PAR radiation and the amount of absorbed radiation when the ground was uncovered. The influence of a reflective ground cover on the trees absorption of radiant energy was subsequently estimated by comparing measured values of absorbed radiation against those values predicted from this linear relationship. Results show that a full ground cover increased the total absorption of PAR radiation by almost 40%, whilst covering half of the ground with foil increased absorption by about 24%. A similar increase in the absorption of net all-wave radiation was recorded. Meteorological and plant measurements were also made to assess the effect of such changes on transpiration and photosynthesis. Calculations, based on measured stomatal conductance and the photosynthetic light response of sunlit and shaded leaves, showed that a complete reflective covering could increase tree photosynthesis by about 32% and transpiration by about 26%. The extra PAR reflected by the foil also caused a significant increase in PAR radiation coming into the lower parts of the canopy. Results from the literature suggest that this should lead to improved fruit quality of the fruit lower in the canopy.

Cadmium accumulation by willow clones used for soil conservation, stock fodder, and phytoremediation

Elevated levels of cadmium are often found in the soil of New Zealand pasturelands due to the long-term use of Cd-contaminated fertilisers. The accumulation of Cd in willow biomass used as stock fodder could therefore adversely affect agricultural productivity and human health. Alternatively, willows may be used for phytoremediation of Cd-contaminated soil at polluted sites. An investigation was carried out to determine the variation in Cd as well as Zn, Mn, and Fe accumulation in 15 willow clones that had been bred for soil conservation purposes. These clones were grown under controlled conditions in 20-L pots of soil containing Cd, Zn, Mn, and Fe at concentrations of 0.3, 64, 597, and 56000 mg/kg, respectively. Daily water use was measured over the final 2 weeks of the experiment and biomass accumulation was determined at the end of the experiment. We found that shrub willows had significantly higher leaf and stem Cd, Mn, and Zn concentrations than tree willows. Average leaf Cd concentrations varied widely between clones from 1.5 to 10 mg/kg. Clones with a high Cd accumulation capacity may be selected to improve the efficacy of Cd-phytoremediation, whereas clones that accumulated lower Cd concentrations may be used for stock fodder. Metal concentrations were not significantly correlated with plant water-use, or biomass production.

Drainage networks in soils. A concept to describe bypass-flow pathways

By using linear scaling factors of water characteristic functions we have been able to reconstruct a probable long-term drainage network through a sandy soil under coniferous forest in the north of Germany. The topology of the drainage network closely resembles one of mountainous streams. The fractional area of the entire profile occupied by the network was found to decrease exponentially with depth. For solutes preferentially travelling through such a network, the transport volume will therefore decrease exponentially with depth, and so the effective velocity should increase correspondingly. Assuming one-dimensional (1D), piston-flow through this effective transport volume of the network, we have been able to predict how much cumulative infiltration is, on average, needed for inert solutes to reach any given depth. Comparing our predictions with the results of a tracer experiment, we were able to estimate well the arrival time of the peak concentration of the tracer. We consider that this network analysis can be improved further by using more dynamic transport properties to define the pathways of the network.