Online-monitoring of tree water stress in a hedgerow olive orchard using the leaf patch clamp pressure probe

The need for sophisticated irrigation strategies in fruit tree orchards has led to an increasing interest in reliable and robust sensor technology that allows automatic and continuous recording of the water stress of trees under field conditions. In this work we have evaluated the potential of the leaf patch clamp pressure (LPCP) probe for monitoring water stress in a 4-year-old ‘Arbequina’ hedgerow olive orchard with 1667 trees ha⁻¹. The leaf patch output pressure (P_p) measured by the LPCP probe is inversely correlated with the leaf turgor pressure (>50 kPa). Measurements of P_p were made over the entire irrigation season of 2010 (April to November) on control trees, irrigated up to 100% of the crop water needs (ET_c), and on trees under two regulated deficit irrigation (RDI) strategies. The 60RDI trees received 59.2% of ET_c and the 30RDI trees received 29.4% of ET_c. In the case of the RDI trees the irrigation amounts were particularly low during July and August, when the trees are less sensitive to water stress. At severe water stress levels (values of stem water potential dropped below ca. −1.70 MPa; turgor pressure < 50 kPa) half-inversed or completely inversed diurnal P_p curves were observed. Reason for these phenomena is the accumulation of air in the leaves. These phenomena were reversible. Normal diurnal P_p profiles were recorded within a few days after rewatering, the number depending on the level of water stress previously reached. This indicates re-establishment of turgescence of the leaf cells. Crucial information about severe water stress was derived from the inversed diurnal P_p curves. In addition P_p values measured on representative trees of all treatments were compared with balancing pressure (P_b) values recorded with a pressure chamber on leaves taken from the same trees or neighbored trees exposed to the same irrigation strategies. Concomitant diurnal P_b measurements were performed in June and September, i.e. before and after the period of great water stress subjected to RDI trees. Results showed close relationships between P_p and P_b, suggesting that the pressure chamber measures relative turgor pressure changes as the LPCP probe. Therefore the probe seems to be an advantageous alternative to the pressure chamber for monitoring tree water status in hedgerow olive tree orchards.     

叶片膨压探针诊断植物体水分状况原理及应用进展

叶片膨压探针(LPCP)技术是通过测定在恒定磁压力下叶片输出压力的衰减情况,能够实现原位连续无损监测植物叶片膨压变化,对植物水分状况的指示具有较高的准确性和敏感性.利用LPCP技术进行相关植物气孔生理、水分状况监测,并在此基础上指导灌溉,是近年来相关领域的研究热点.在梳理现有植物水分状况诊断方法的基础上,概括了 LPC...     

Soil water balance trial involving capacitance and neutron probe measurements

The objective of this study was to compare soil water measurements made using capacitance and neutron probes by means of a water balance experiment in a drainage lysimeter. The experiment was conducted in a 5-year-old drip-irrigated peach orchard (Prunus persica L. Batsch, cv. Flordastar, on GF-677 peach rootstock) planted in a clay loam textured soil located in southern Spain. Four drainage lysimeters (5 m × 5 m × 1.5 m), each containing one tree, were constructed and equipped with one lateral line containing eight drippers per tree, with a discharge rate of 2 L h⁻¹. Three access tubes for the neutron probe (NP), symmetrically facing three PVC access tubes containing the multi-depth capacitance probes (MDCP) were located perpendicularly to the drip line (0.2, 0.6 and 1 m). The results demonstrated that both the capacitance and neutron probes gave similar soil water content values under steady state hydraulic gradient conditions (0.2 m from the emitter) although some discrepancies were found in heterogeneous soil water distribution conditions (1 m from the emitter), which might be attributed to the smaller soil volume explored by the MDCP compared with the NP. Explanations for the discrepancies between both devised are presented. When water inputs and outputs were fairly constant, the volumetric soil water content could be considered to represent field saturation (θ_sat = 0.36 m³ m⁻³). When drainage was zero, there were 2 days when the soil water content was constant and could be considered as field capacity (θ_fc = 0.31 m³ m⁻³). The findings suggest that: (i) capacitance probes can be used for continuous real-time soil water content monitoring unlike the manual measurements obtained with the neutron probe; (ii) the location of the sensors is critical when used for drip irrigation scheduling and our recommendations for practical agricultural purposes would be to place MDCP sensors in the place representing the highest root density, leading the sensors to become biological sensors rather than mere soil moisture sensors; and (iii) on average, the water balance values determined by lysimeter match those calculated using the data from both probes. However, due to the smaller soil volume explored by MDCP, more of these sensors must be used to characterize the soil water status in water balance studies.     

Irrigation requirements and transpiration coupling to the atmosphere of a citrus orchard in Southern Brazil

Crop evapotranspiration (ETc) was measured as evaporative heat flux from an irrigated acid lime orchard (Citrus latifolia Tanaka) using the aerodynamic method. Crop transpiration (T) was determined by a stem heat balance method. The irrigation requirements were determined by comparing the orchard evapotranspiration (ETc) and T with the reference evapotranspiration (ETo) derived from the Penman-Monteith equation, and the irrigation requirements were expressed as ETc/ETo (Kc) and T/ETo (Kcb) ratios. The influence of inter-row vegetation on the ETc was analyzed because the measurements were taken during the summer and winter, which are periods with different regional soil water content. In this study, the average Kc values obtained were 0.65 and 0.24 for the summer and winter, respectively. The strong coupling of citrus trees to the atmosphere and the sensitivity of citrus plants to large vapor pressure deficits and air/leaf temperatures caused variations in the Kcb in relation to the ETo ranges. During the summer, the Kcb value ranged from 0.34 when the ETo exceeded 5 mm d⁻¹ to 0.46 when the ETo was less than 3 mm d⁻¹.     

Contrasting methods for estimating evapotranspiration in soybean

Crop scientists are often interested in canopy rather than leaf water estimates. Comparing canopy fluxes for multiple treatments using micrometeorological approaches presents limitations because of the large fetch required. The goal of this study was to compare leaf-scale to field-scale data by summing soil water evaporation (E) and leaf transpiration (T) versus ET using tower eddy covariance (EC) and scaling leaf transpiration to the canopy level using a two-step scaling approach in soybean [Glycine max (L.) Merr.]. Soybean transpiration represented 89-96% of E + T when combining the soil water evaporation with leaf transpiration on the five measurement days during reproductive growth. Comparing E + T versus ET from the EC system, the E + T method overestimated ET from 0.68 to 1.58 mm. In terms of percent difference, the best agreement between the two methods was 15% on DOY 235 and the worst agreement occurred on DOY 234 (41%). A two-step scaling method predicted average ET within 0.01 mm of the EC ET between 10:00 and 14:15 on an hourly time-step on DOY 227 under uniform sky conditions and average ET within 0.03 mm of the EC ET on DOY 235 under intermittent sky conditions between 10:00 and 15:15. Pooling the scaled-leaf data and comparing them with the measured EC ET data exhibited a strong linear relationship (r = 0.835) after accounting for bias (6%). Findings from this study indicate satisfactory results comparing absolute differences are likely not obtainable by summing leaf transpiration with soil water evaporation to calculate canopy water fluxes. However, scaling leaf transpiration provided a robust measure of canopy transpiration during reproductive growth in soybean under these conditions and merits additional study under different climatic and crop conditions.     

Effects of applied water and sprinkler irrigation uniformity on alfalfa growth and hay yield

This study was conducted to investigate the effects of applied water and sprinkler irrigation uniformity on alfalfa (Medicago sativa L.) growth and hay yield in a semi-arid region. Field experiments were carried out in 2006 in Varamin, Iran, on three plots of 25 m × 30 m. Each plot was subdivided into 25 subplots of 5 m × 6 m. Different irrigation depths and sprinkler water uniformities were obtained by various scenarios of sprinkler nozzle pressure. In each plot, applied water was measured at 250 points (125 points above and 125 points below canopy) and the soil water content of 40 cm deep below soil surface was monitored at 25 points, each in the center of a subplot, throughout the irrigation season. The results showed that sprinkler water and soil water content uniformity varied between 66-78 and 88-91%, respectively. The findings revealed that soil water content uniformity was around 20% higher than sprinkler water uniformity. The irrigation uniformity below the canopy was estimated to be 2.5% greater than above the canopy, and canopy-intercepted water could account for 11-15% of the total seasonal applied water. Evaluation showed that alfalfa leaf area index relies more heavily on farm water application uniformity than hay yield and crop height. The experimental results illustrated that water distribution in sprinkler irrigation systems has a direct effect on alfalfa growth, hay yield and water productivity such that the applied water reduction and the increased sprinkler water uniformity led to an increased alfalfa water productivity of 2.41 kg m⁻³.     

Evapotranspiration components determined by sap flow and microlysimetry techniques of a vineyard in northwest China: Dynamics and influential factors

Large areas of vineyards have been established in recent years in arid region of northwest China, despite limited water resources. Water to support these vineyards is mainly supplied by irrigation. Accurate estimation of vineyard evapotranspiration (ET) can provide a scientific basis for developing irrigation management. Transpiration and soil evaporation, as two main components of ET, were measured separately in a vineyard in this region by heat balance sap flow system and micro-lysimeters during the growing season of 2009. Diurnal and seasonal dynamics of sap flow and its environmental controls were analyzed. Daily sap flow rate (SR_l) increased linearly with solar radiation (R_s), but showed an exponential increase to its maximum curve as a function of vapor pressure deficit (VPD). Residuals of the two regressions both depended on volumetric soil water content to a depth of 1.0 m (VWC). VWC also significantly influenced SR_l. The relationship of them could be expressed by a piecewise regression with the turnover point of VWC = 0.188 cm³ cm⁻³, which was ∼60% of the field capacity. Conversely, soil evaporation (E_s) increased exponentially with VWC. Thus, we recommended keeping VWC in such vineyards slightly above ∼60% of the field capacity to maintain transpiration while reducing soil evaporation. Vineyard transpiration (T_s) was scaled from sap flow by using leaf area (A_l) as it explained 60% of the spatial variability of sap flow. Vine transpiration was 202.0 mm during the period from April 28 to October 5; while that of E_s was 181.0 mm. The sum of these two components was very close to ET estimated by the Bowen ratio energy balance method (386.9 mm), demonstrating the applicability of sap flow for measuring grape water use in this region.     

Water potential and turgor pressure as a selection basis for wind-grown winter wheat

Two varieties of winter wheat (Triticum aestivum L. em. Thell. ‘Ponca’ and ‘KanKing’), one considered drought sensitive (‘Ponca’) and one considered drought resistant (‘KanKing’), were grown from seed in pots of soil placed in wind or still air for 36 days to determine the effect of wind on water potentials, turgor pressure, and stomatal resistance of leaves. During the experiment, plants were well watered and soil was moist. In wind and still air, water potentials and turgor pressures of the drought sensitive wheat were higher, and stomatal resistances were lower, than those of the drought resistant wheat. Plants grown in wind had lower turgor pressures than plants grown in still air.The results showed that wind reduced turgor pressure and that measurements of water potential, turgor pressure, and stomatal resistance could be used to select varieties for windy areas.     

Using radiation thermography and thermometry to evaluate crop water stress in soybean and cotton

The use of digital infrared thermography and thermometry to investigate early crop water stress offers a producer improved management tools to avoid yield declines or to deal with variability in crop water status. This study used canopy temperature data to investigate whether an empirical crop water stress index could be used to monitor spatial and temporal crop water stress. Different irrigation treatment amounts (100%, 67%, 33%, and 0% of full replenishment of soil water to field capacity to a depth of 1.5 m) were applied by a center pivot system to soybean (Glycine max L.) in 2004 and 2005, and to cotton (Gossypium hirsutum L.) in 2007 and 2008. Canopy temperature data from infrared thermography were used to benchmark the relationship between an empirical crop water stress index (CWSI_e) and leaf water potential (Ψ_L) across a block of eight treatment plots (of two replications). There was a significant negative linear correlation between midday Ψ_L measurements and the CWSI_e after soil water differences due to irrigation treatments were well established and during the absence of heavy rainfall. Average seasonal CWSI_e values calculated for each plot from temperature measurements made by infrared thermometer thermocouples mounted on a center pivot lateral were inversely related to crop water use with r² values >0.89 and 0.55 for soybean and cotton, respectively. There was also a significant inverse relationship between the CWSI_e and soybean yields in 2004 (r² = 0.88) and 2005 (r² = 0.83), and cotton in 2007 (r² = 0.78). The correlations were not significant in 2008 for cotton. Contour plots of the CWSI_e may be used as maps to indicate the spatial variability of within-field crop water stress. These maps may be useful for irrigation scheduling or identifying areas within a field where water stress may impact crop water use and yield.     

Evaluating the Crop Water Stress Index and its correlation with latent heat and CO2 fluxes over winter wheat and maize in the North China plain

Plant water status is a key factor impacting crop growth and agricultural water management. Crop water stress may alter canopy temperature, the energy balance, transpiration, photosynthesis, canopy water use efficiency, and crop yield. The objective of this study was to calculate the Crop Water Stress Index (CWSI) from canopy temperature and energy balance measurements and evaluate the utility of CWSI to quantify water stress by comparing CWSI to latent heat and carbon dioxide (CO₂) flux measurements over canopies of winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.). The experiment was conducted at the Yucheng Integrated Agricultural Experimental Station of the Chinese Academy of Sciences from 2003 to 2005. Latent heat and CO₂ fluxes (by eddy covariance), canopy and air temperature, relative humidity, net radiation, wind speed, and soil heat flux were averaged at half-hour intervals. Leaf area index and crop height were measured every 7 days. CWSI was calculated from measured canopy-air temperature differences using the Jackson method. Under high net radiation conditions (greater than 500 W m⁻²), calculated values of minimum canopy-air temperature differences were similar to previously published empirically determined non-water-stressed baselines. Valid measures of CWSI were only obtained when canopy closure minimized the influence of viewed soil on infrared canopy temperature measurements (leaf area index was greater than 2.5 m² m⁻²). Wheat and maize latent heat flux and canopy CO₂ flux generally decreased linearly with increases in CWSI when net radiation levels were greater than 300 W m⁻². The responses of latent heat flux and CO₂ flux to CWSI did not demonstrate a consistent relationship in wheat that would recommend it as a reliable water stress quantification tool. The responses of latent heat flux and CO₂ flux to CWSI were more consistent in maize, suggesting that CWSI could be useful in identifying and quantifying water stress conditions when net radiation was greater than 300 W m⁻². The results suggest that CWSI calculated by the Jackson method under varying solar radiation and wind speed conditions may be used for irrigation scheduling and agricultural water management of maize in irrigated agricultural regions, such as the North China Plain.     

Combining an ecological model with remote sensing and GIS techniques to monitor soil water content of croplands with a monsoon climate

Soil water is an important factor affecting photosynthesis, transpiration, growth, and yield of crops. Accurate information on soil water content (SWC) is crucial for practical agricultural water management at various scales. In this study, remotely sensed parameters (leaf area index, land cover type, and albedo) and spatial data manipulated using the geographic information system (GIS) technique were assimilated into the boreal ecosystem productivity simulator (BEPS) model to monitor SWC dynamics of croplands in Jiangsu Province, China. The monsoon climate here is characterized by large interannual and seasonal variability of rainfall causing periods of high and low SWC. Model validation was conducted by comparing simulated SWC with measurements by a gravimetric method in the years 2005 and 2006 at nine agro-meteorological stations. The model-to-measurement R² values ranged from 0.40 to 0.82. Nash-Sutcliffe efficiency values were in the range from 0.10 to 0.80. Root mean square error (RMSE) values ranged from 0.028 to 0.056 m³ m⁻³. Simulated evapotranspiration (ET) was consistent with ET estimated from pan evaporation measurements. The BEPS model successfully tracked the dynamics and extent of the serious soil water deficit that occurred during September-November 2006. These results demonstrate the applicability of combining process-based models with remote sensing and GIS techniques in monitoring SWC of croplands and improving agricultural water management at regional scales in a monsoon climate.     

Estimating yield of sorghum using root zone water balance model and spectral characteristics of crop in a dryland Alfisol

This study investigated the relationship between sorghum grain yield for a range of soil depths, with the seasonal crop water stress index based on relative evapotranspiration deficits and spectral vegetation indices. A root zone water balance model was used to evaluate seasonal soil water fluctuations and actual evapotranspiration within a toposequence; soil depth varied between 30 and 75 cm and available water capacity ranged from 6.9 to 12.6% (v/v, %). An empirical model was used to determine root growth. Runoff was estimated from rainfall data using the curve number techniques of the Soil Conservation Services, combined with a soil water-accounting procedure. The high r² values between modeled and observed values of soil water in the root zone (r² > 0.70, significant at P < 0.001) and runoff (r² = 0.95, significant at P < 0.001) indicated good agreement between the model output and observed values. Canopy reflectance was measured during the entire crop growth period and the following spectral indices were calculated: simple ratio, normalized difference vegetation index (NDVI), green NDVI, perpendicular vegetation index, soil adjusted vegetation index (SAVI) and modified SAVI (MSAVI). All the vegetation indices, except for the perpendicular vegetation index, measured from booting to anthesis stage, were positively correlated with leaf area index (LAI) and yield. The correlation coefficient for spectral indices with dry biomass was relatively less than for LAI and yield. Modified SAVI recorded from booting to milk-grain stage gave the highest average correlation coefficient with grain yield. Additive and multiplicative forms of water-production functions, as well as water stress index calculated from water budget model, were used to predict crop yield. A multiple regression was carried out with yield, for the years 2001–2003, as the dependent variable and MSAVI, from the booting to the milk-grain stage of crop and relative yield values, calculated using both additive and multiplicative water production functions as well as water stress index, as the independent variables. The multiplicative model and MSAVI, recorded during the heading stage of crop growth, gave the highest coefficient of determination (r² = 0.682, significant at P < 0.001). The multiple regression equation was tested for yield data recorded during 2004; the deviation between observed and estimated yields varied from −6.2 to 9.4%. The water budget model, along with spectral vegetation indices, gave satisfactory estimates of sorghum grain yields and appears to be a useful tool to estimate yield as a function of soil depth and available water.     

Growth and evapotranspiration pattern of rajmash (Phaseolus vulgaris L.) under varying irrigation schedules and phosphate levels in a hot sub-humid climate

Soil water and phosphorus availabilities play crucial role in yield and water use pattern of pulse crops. A field experiment with rajmash (Phaseolus vulgaris L.) as a test crop was carried out during the winter seasons of 2003-2004 and 2004-2005 on a sandy loam soil (Aeric Haplaquept) in eastern India. In the main plots, irrigation was applied when the cumulative pan evaporation reached: (i) 33 mm (CPE₃₃), (ii) 44 mm (CPE₄₄) and (iii) 66 mm (CPE₆₆). In sub-plots, four levels of phosphate fertilizer were applied at: (i) 0 (P₀); (ii) 30 (P₃₀); 60 (P₆₀) and 90 kg P₂O₅ ha⁻¹ (P₉₀). Irrespective of crop growth stage, the highest leaf area index (LAI), biomass, grain yield and water use efficiency were obtained under the combination of CPE₃₃-P₉₀. Magnitude of these parameters, in general, decreased with the decrease in irrigation frequency and phosphate levels. With the advancement of crop growing period, cumulative actual evapotranspiration (CAET) increased linearly. Best matching between CAET and cumulative pan evaporation data was recorded under CPE₃₃-P₉₀ treatments. A linear relationship with high coefficient of determination was obtained between total biomass and CAET. Present study showed that crop response factor (k_y) of the crop was 1.91.     

Comparisons of energy balance and evapotranspiration between flooded and aerobic rice fields in the Philippines

The seasonal and annual variability of sensible heat flux (H), latent heat flux (LE), evapotranspiration (ET), crop coefficient (K_c) and crop water productivity (WP_ET) were investigated under two different rice environments, flooded and aerobic soil conditions, using the eddy covariance (EC) technique during 2008-2009 cropping periods. Since we had only one EC system for monitoring two rice environments, we had to move the system from one location to the other every week. In total, we had to gap-fill an average of 50-60% of the missing weekly data as well as those values rejected by the quality control tests in each rice field in all four cropping seasons. Although the EC method provides a direct measurement of LE, which is the energy used for ET, we needed to correct the values of H and LE to close the energy balance using the Bowen ratio closure method before we used LE to estimate ET. On average, the energy balance closure before correction was 0.72 ± 0.06 and it increased to 0.99 ± 0.01 after correction. The G in both flooded and aerobic fields was very low. Likewise, the energy involved in miscellaneous processes such as photosynthesis, respiration and heat storage in the rice canopy was not taken into consideration.Average for four cropping seasons, flooded rice fields had 19% more LE than aerobic fields whereas aerobic rice fields had 45% more H than flooded fields. This resulted in a lower Bowen ratio in flooded fields (0.14 ± 0.03) than in aerobic fields (0.24 ± 0.01). For our study sites, evapotranspiration was primarily controlled by net radiation. The aerobic rice fields had lower growing season ET rates (3.81 ± 0.21 mm d⁻¹) than the flooded rice fields (4.29 ± 0.23 mm d⁻¹), most probably due to the absence of ponded water and lower leaf area index of aerobic rice. Likewise, the crop coefficient, K_c, of aerobic rice was significantly lower than that of flooded rice. For aerobic rice, K_c values were 0.95 ± 0.01 for the vegetative stage, 1.00 ± 0.01 for the reproductive stage, 0.97 ± 0.04 for the ripening stage and 0.88 ± 0.03 for the fallow period, whereas, for flooded rice, K_c values were 1.04 ± 0.04 for the vegetative stage, 1.11 ± 0.05 for the reproductive stage, 1.04 ± 0.05 for the ripening stage and 0.93 ± 0.06 for the fallow period. The average annual ET was 1301 mm for aerobic rice and 1440 mm for flooded rice. This corresponds to about 11% lower total evapotranspiration in aerobic fields than in flooded fields. However, the crop water productivity (WP_ET) of aerobic rice (0.42 ± 0.03 g grain kg⁻¹ water) was significantly lower than that of flooded rice (1.26 ± 0.26 g grain kg⁻¹ water) because the grain yields of aerobic rice were very low since they were subjected to water stress.The results of this investigation showed significant differences in energy balance and evapotranspiration between flooded and aerobic rice ecosystems. Aerobic rice is one of the promising water-saving technologies being developed to lower the water requirements of the rice crop to address the issues of water scarcity. This information should be taken into consideration in evaluating alternative water-saving technologies for environmentally sustainable rice production systems.     

Development of crop water stress index of wheat crop for scheduling irrigation using infrared thermometry

This study was conducted to develop the relationship between canopy-air temperature difference and vapour pressure deficit for no stress condition of wheat crop (baseline equations), which was used to quantify crop water stress index (CWSI) to schedule irrigation in winter wheat crop (Triticum aestivum L.). The randomized block design (RBD) was used to design the experimental layout with five levels of irrigation treatments based on the percentage depletion of available soil water (ASW) in the root zone. The maximum allowable depletion (MAD) of the available soil water (ASW) of 10, 40 and 60 per cent, fully wetted (no stress) and no irrigation (fully stressed) were maintained in the crop experiments. The lower (non-stressed) and upper (fully stressed) baselines were determined empirically from the canopy and ambient air temperature data obtained using infrared thermometry and vapour pressure deficit (VPD) under fully watered and maximum water stress crop, respectively. The canopy-air temperature difference and VPD resulted linear relationships and the slope (m) and intercept (c) for lower baseline of pre-heading and post-heading stages of wheat crop were found m = −1.7466, c = −1.2646 and m = −1.1141, c = −2.0827, respectively. The CWSI was determined by using the developed empirical equations for three irrigation schedules of different MAD of ASW. The established CWSI values can be used for monitoring plant water status and planning irrigation scheduling for wheat crop.     

Effects of irrigation strategies and soils on field-grown potatoes: Gas exchange and xylem [ABA]

Gas exchange was measured in potatoes (cv. Folva) grown in lysimeters (4.32 m²) in coarse sand, loamy sand, and sandy loam and subjected to full (FI), deficit (DI), and partial root-zone drying (PRD) irrigation strategies. PRD and DI as water-saving irrigation treatments received 65% of FI and started after tuber bulking and lasted for six weeks until final harvest. Midday photosynthesis rate (A_n) and stomatal conductance (g_s) of fully irrigated (FI) plants were lowest in coarse sand and mean A_n of diurnal measurements in FI, PRD and DI tended to be lower in this soil as compared with the loamy sand and sandy loam. The results revealed that diurnal values of A_n and g_s in PRD and DI were consistently lower than FI without reaching significant differences in accordance with findings that xylem [ABA] in PRD was significantly higher than FI, and tended to be higher than in DI. Diurnal measurements showed that A_n reached peak values during mid-morning and midday, while g_s were highest during the morning. Intrinsic water use efficiency (A_n/g_s) correlated linearly well with the leaf to air vapor pressure deficit (VPD) and the slope of the line revealed the rate of A_n/g_s increase per each kPa increase in VPD, i.e. approximately 10 μmol mol⁻¹. Transpiration efficiency (A_n/T) of PRD was higher than DI, which shows slightly better efficient water use than DI. The slope of the linear relationship between transpiration efficiency and VPD decreased from −2.03 to −1.04 during the time course of the growing season, indicating the negative effect of leaf ageing on photosynthesis and thus on plant water use efficiency. This fact shows the possibility to save water during last growth stages through applying water-saving irrigations without much effect on transpiration efficiency.     

Does silicon and irrigation have impact on drought tolerance mechanism of sorghum?

Silicon absorption by crops in the form of silicic acid confers efficient utilization of available irrigation water by minimizing transpiration losses. In present study, silicon and irrigation effects on sorghum growth dynamics and drought tolerance mechanism were evaluated during 2007-2008. Two sorghum cultivars: PARCSS2 and Johar1 were treated with two levels of silicon (Si₀ = control and Si₂₀₀ = 200 ml l⁻¹ of potassium silicate per kg of soil) and irrigation (W₀ = control, crop lower limit and W₄₀ = 40 mm of water, crop upper limit). The results depicted that silicon absorption led to increased leaf water potential, growth, transpiration, net photosynthetic rate and decreased shoot to root ratio in sorghum cultivars compared to control treatment. It can be concluded that synergistic effect of silicon fertilization with ample irrigation may improve the crop stand under drought and biotic stresses.     

Leaf permeability as an index of water relations,CO2 uptake and yield of irrigated wheat

Summary An irrigation experiment was conducted on wheat in the northern Negev, Israel. The growing season rainfall was 198 mm; six irrigation treatments, ranging from 0 to 320 mm were applied at different stages of growth. The grain yields ranged from 1.20 to 5.84 t/ha. Stomatal aperture was evaluated by leaf permeability, as measured with a fast-reading viscous flow porometer. Other indices of soil-plant water status measured were: soil moisture with a neutron probe; leaf water potential with a pressure chamber; CO₂ uptake with a ¹⁴CO₂-pulse apparatus; and leaf water saturation deficit.For the penultimate and flag leaves, midday leaf permeability was highly correlated with the soil moisture in the upper 60-cm layer. CO₂-uptake, however, remained constantly high (ca. 0.8 mg m^–2s^–1 = 29 mg dm^–2h^–1) throughout a wide range of leaf permeability, from 10 down to 2 porometer units (p. u.); below this value, it decreased linearly with leaf permeability. Therefore, the value of 2 p. u. was tentatively regarded as a critical value for judging the critical values of the other indices studied; these were estimated to be: leaf water potential, –1.57 MPa = –15.7 bars; leaf saturation deficit, 18,8% and soilmoisture, 12.6% representing a 83% depletion of the available moisture in the Gilat soil. The grain yield was highly negatively correlated with the duration of period when the soil moisture was below these critical values. The use of the porometer method for evaluating water stress is discussed.