Errors in field measurement of leaf diffusive conductance associated with leaf temperature

The ability of leaf diffusive conductance (C_L) measurements to discriminate between different treatments in field studies has generally been disappointing. Since values of C_L are sensitive to leaf temperature and temperature gradients between the leaf and air, inaccurate temperature measurements will produce substantial errors. This work compared two methods of leaf temperature measurement from which values of C_L were calculated and subsequently compared.Experiments using well-watered field- and glasshouse-grown cotton (Gossypium hirsutum L.) where diffusion porometer readings and leaf temperatures were measured indicated that the porometer thermistor reading differed markedly from leaf temperature as assessed with an infrared thermometer. Under high air vapor pressure deficit (AVPD) conditions (5.2 kPa), leaf temperature (T_L) to ambient air temperature (T_A) differences were as large as ?8°C. Porometer thermistor temperature was strongly influenced by T_A and to a lesser extent by T_L. Values of C_L calculated from porometer measurements of T_L can be up to 96% less than true C_L values. This is the result of both incorrect T_L measurement and a failure to account for the effects of the leaf to porometer-cup temperature differential. Until these two sources of error are addressed, values of C_L obtained from diffusion porometer measurements in the field must be regarded as only qualitative.     

Physiological Effects of Exposure to Arsenic,Mercury, Antimony and Selenium in the Aquatic Moss Fontinalis antipyretica Hedw.

This study assessed the effect of ambient air pollution on leaf characteristics of white willow, northern red oak, and Scots pine. Willow, oak, and pine saplings were planted at sixteen locations in Belgium, where nitrogen dioxide (NO₂), ozone (O₃), sulfur dioxide (SO₂), and particulate matter (PM₁₀) concentrations were continuously measured. The trees were exposed to ambient air during 6 months (April–September 2010), and, thereafter, specific leaf area (SLA), stomatal resistance (R _s), leaf fluctuating asymmetry (FA), drop contact angle (CA), relative chlorophyll content, and chlorophyll fluorescence (F _v/F _m) were measured. Leaf characteristics of willow, oak, and pine were differently related to the ambient air pollution, indicating a species-dependent response. Willow and pine had a higher SLA at measuring stations with higher NO₂ and lower O₃ concentrations. Willow had a higher R _s and pine had a higher F _v/F _m at measuring stations with a higher NO₂ and lower O₃ concentrations, while oak had a higher F _v/F _m and a lower FA at measuring stations with a higher NO₂ and lower O₃ concentrations. FA and R _s of willow, oak, and pine, SLA of oak, and CA of willow were rather an indicator for local adaptation to the micro-environment than an indicator for the ambient air pollution.     

Towards an improved and more flexible representation of water stress in coupled photosynthesis-stomatal conductance models

Coupled photosynthesis-stomatal conductance (A-g_s) models are commonly used in ecosystem models to represent the exchange rate of CO₂ and H₂O between vegetation and the atmosphere. The ways these models account for water stress differ greatly among modelling schemes. This study provides insight into the impact of contrasting model configurations of water stress on the simulated leaf-level values of net photosynthesis (A), stomatal conductance (g_s), the functional relationship among them and their ratio, the intrinsic water use efficiency (A/g_s), as soil dries. A simple, yet versatile, normalized soil moisture dependent function was used to account for the effects of water stress on g_s, on mesophyll conductance (g_m) and on the biochemical capacity. Model output was compared to leaf-level values obtained from the literature. The sensitivity analyses emphasized the necessity to combine both stomatal and non-stomatal limitations of A in coupled A-g_s models to accurately capture the observed functional relationships A vs. g_s and A/g_svs. g_s in response to drought. Accounting for water stress in coupled A-g_s models by imposing either stomatal or biochemical limitations of A, as commonly practiced in most ecosystem models, failed to reproduce the observed functional relationship between key leaf gas exchange attributes. A quantitative limitation analysis revealed that the general pattern of C₃ photosynthetic response to water stress may be well represented in coupled A-g_s models by imposing the highest limitation strength to g_m, then to g_s and finally to the biochemical capacity.     

Monitoring stomatal conductance of Jatropha curcas seedlings under different levels of water shortage with infrared thermography

Model simulations and experimental measurements were used to investigate the applicability of infrared thermography for the estimation of stomatal conductance and drought stress under sub-optimal meteorological conditions. The study focused on the stomatal conductance index I_g, calculated from the leaf temperature and the temperature of a dry and wet reference leaf. The simulations revealed that I_g is influenced by leaf dimension, wind speed and air temperature and not or hardly by leaf angle, albedo, relative humidity or insolation. In addition, I_g was found to be very sensitive to differences in wind speed, air temperature, insolation, leaf dimension and leaf angle between the measured and the reference leaves. In the experimental part, we evaluated if infrared thermography can be used to improve the knowledge on the water use of Jatropha curcas L., a tropical biofuel crop. Thermal images from Jatropha seedlings grown under three different drought treatments were made on a day with very variable insolation and a day with very low insolation. Smaller newly formed leaves and the active control of the leaf angle proved efficient ways of Jatropha to protect leaves under drought stress from overheating. I_g, assessed in four different ways, and four simplified drought stress indices were derived and related to the measured stomatal conductance (g_s) of the seedlings. The strongest correlation with g_s and the highest discriminative power between the different water treatments were achieved when I_g was calculated by taking the average leaf temperature per plant and the temperature of the dry and wet reference leaves of this plant, rather than the average temperature of several reference leaves. Using the difference between the dry reference and the measured leaf (T_dry − T_l) as a simplified index gave similar results, although correlations were weaker. Other simplified thermal indices were not well correlated with leaf stomatal conductance or with water treatment. Recommendations were formulated for the measurement of I_g and (T_dry − T_l).     

Comparison of the eddy covariance and automated closed chamber methods for evaluating nocturnal CO2 exchange in a Japanese cypress forest

Underestimation of nocturnal CO₂ respiration using the eddy covariance method under calm conditions remains an unsolved problem at many flux observation sites in forests. To evaluate nocturnal CO₂ exchange in a Japanese cypress forest, we observed CO₂ flux above the canopy (F_c), changes in CO₂ storage in the canopy (S_t) and soil, and trunk and foliar respiration for 2 years (2003–2004). We scaled these chamber data to the soil, trunk, and foliar respiration per unit of ground area (F_s, F_t, F_f, respectively) and used the relationships of F_s, F_t, and F_f with air or soil temperature for comparison with canopy-scale CO₂ exchange measurements (=F_c + S_t). The annual average F_s, F_t, and F_f were 714 g C m⁻² year⁻¹, 170 g C m⁻² year⁻¹, and 575 g C m⁻² year⁻¹, respectively. At small friction velocity (u_*), nocturnal F_c + S_t was smaller than F_s + F_t + F_f estimated using the chamber method, whereas the two values were almost the same at large u_*. We replaced F_c + S_t measured during calm nocturnal periods with a value simulated using a temperature response function derived during well-mixed nocturnal periods. With this correction, the estimated net ecosystem exchange (NEE) from F_c + S_t data ranged from −713 g C m⁻² year⁻¹ to −412 g C m⁻² year⁻¹ in 2003 and from −883 g C m⁻² year⁻¹ to −603 g C m⁻² year⁻¹ in 2004, depending on the u_* threshold. When we replaced all nocturnal F_c + S_t data with F_s + F_t + F_f estimated using the chamber method, NEE was −506 g C m⁻² year⁻¹ and −682 g C m⁻² year⁻¹ for 2003 and 2004, respectively.     

The sensitivity of modeled SO2 fluxes and profiles to stomatal and boundary layer resistances

A higher-order closure model for canopy/surface exchange is presented and applied to an oak-hickory forest to model SO₂ deposition for typical summertime conditions. The model is then used as a tool to investigate the sensitivity of modeled fluxes (the deposition velocity) and concentration profiles to the parameters used to compute the leaf boundary layer and stomatal resistances. Both the deposition velocity and concentration profiles show little sensitivity to variations in the leaf boundary-layer resistance (r _b) since it generally comprises only a small fraction of the total resistance to diffusion from the air to the sub-stomatal cavity. The deposition velocity (V _d ) is more sensitive to variations in the minimum stomatal resistance (r _s ,min) and light response coefficient (β) than r _b .It was found that 50% variations in β give a maximum difference of 0.2 cm s^?1 in V _d while 30% variations in r _s , min produce a maximum difference of near 0.3 cm s^?1.     

Water use efficiency in a soybean field: influence of plant water stress

Measurements were made in 1980 over a fully-developed soybean (Glycine max (L.) Merrill) canopy at Mead, Nebraska to determine how crop water status influences photosynthesis, evapotranspiration and water use efficiency. Water use efficiency was calculated in terms of the CO₂—water flux ratio (CWFR). Micrometeorological techniques were used to measure the exchange rates of CO₂ and water vapor above the crop canopy. Crop water status was evaluated by reference to volumetric soil moisture (θ_v), stomatal resistance (r_s), and leaf water potential (ψ) measurements.Stomatal resistance (r_s) was independent of ψ when the latter was greater than ?1.1 MPa. r_s increased sharply as ψ dropped below this threshold. Canopy CO₂ exchange (F_c) decreased logarithmically with increasing r_s under strong irradiance. Although F_c was found to be strongly correlated with r_s, the influence of low values of ψ and of high air temperature cannot be discounted since these factors affect the enzymatic reactions associated with photosynthesis. Stomatal closure also reduced evapotranspiration and influenced the partitioning of net radiation.Under strong irradiance the CO₂ water flux ratio (CWFR) decreased with increasing stomatal resistance. This observation is at variance with predictions of certain early ‘resistance’ models, but substantiates predictions of some recent models in which leaf energy balance considerations are incorporated.     

Empirical and optimal stomatal controls on leaf and ecosystem level CO2 and H2O exchange rates

Linkage between the leaf-level stomatal conductance (g_s) response to environmental stimuli and canopy-level mass exchange processes remains an important research problem to be confronted. How various formulations of g_s influence canopy-scale mean scalar concentration and flux profiles of CO₂ and H₂O within the canopy and how to derive ‘effective’ properties of a ‘big-leaf’ that represents the eco-system mass exchange rates starting from leaf-level parameters were explored. Four widely used formulations for leaf-level g_s were combined with a leaf-level photosynthetic demand function, a layer-resolving light attenuation model, and a turbulent closure scheme for scalar fluxes within the canopy air space. The four g_s models were the widely used semi-empirical Ball-Berry approach, and its modification, and two solutions to the stomatal optimization theory for autonomous leaves. One of the two solutions to the optimization theory is based on a linearized CO₂-demand function while the other does not invoke such simplification. The four stomatal control models were then parameterized against the same shoot-scale gas exchange data collected in a Scots pine forest located at the SMEAR II-station in Hyytiälä, Southern Finland. The predicted CO₂ (F_c) and H₂O fluxes (F_e) and mean concentration profiles were compared against multi-level eddy-covariance measurements and mean scalar concentration data within and above the canopy. It was shown that F_c comparisons agreed to within 10% and F_e comparisons to within 25%. The optimality approach derived from a linearized photosynthetic demand function predicted the largest CO₂ uptake and transpiration rates when compared to eddy-covariance measurements and the other three models. Moreover, within each g_s model, the CO₂ fluxes were insensitive to g_s model parameter variability whereas the transpiration rate estimates were notably more affected. Vertical integration of the layer-averaged results as derived from each g_s model was carried out. The sensitivities of the up-scaled bulk canopy conductances were compared against the eddy-covariance derived canopy conductance counterpart. It was shown that canopy level g_s appear more sensitive to vapor-pressure deficit than shoot-level g_s.     

Coupled model of stomatal conductance–photosynthesis–transpiration for paddy rice under water-saving irrigation

To improve the performance of a coupled model based on a Leuning–Ball stomatal conductance (g_s) model for rice under water deficit conditions, leaf temperature rising (ΔT) was incorporated into the Leuning–Ball model and a revised coupled model for simulation of stomatal conductance g_s–net photosynthesis rate (P_n)–transpiration rate (T_r) was developed based on data collected from a rice paddy with nonflooded controlled irrigation in 2003 and 2004. Both a Leuning–Ball and revised Leuning–Ball and coupled model based on both were evaluated with internal conductance (g_ic) determined by different equations. The performance of the Leuning–Ball model was improved under water deficit condition by incorporating ΔT, and the revised Leuning–Ball model performed better than the Leuning–Ball model in the coupled model of stomatal conductance–photosynthesis–transpiration for rice under water deficit conditions. Meanwhile, accuracy in g_ic calculation is essential for simulation of P_n, but not for simulation of T_r. Thus, leaf temperature rising ΔT is suitable as a leaf water status indicator in a simulation of rice leaf gas exchange response to water deficit conditions.     

大豆幼苗光合特性对锰营养的响应

采用溶液培养方法,设Mn2+浓度为0、0.05、0.50、5、30、50.mg/L,探讨了2种大豆品种(浙春2号、东北大豆854-11)的幼苗光合特性对不同锰浓度的响应。结果表明,低锰浓度提高了大豆叶片的初始荧光(Fo)、最大荧光(Fm)、PSⅡ原初光能转化效率(Fv/Fm)、潜在光化学活性(Fv/Fo)和光化学猝灭系数(qP),高锰降低了Fo、Fm、Fv/Fm、Fv/Fo、qP。随着锰营养的增加,非光化学猝灭系数(qN)增大。适量的锰浓度显著提高了大豆的净光合速率(Pn)、蒸腾速率(Tr)、气孔导度(Gs),降低了气孔阻力(Rs)和细胞间CO2浓度(Ci),随着锰浓度的逐渐增大,降低了Pn、Tr、Gs,提高了Rs、Ci。0.50.mg/L下的锰浓度有最大的Fo,5.mg/L下的锰浓度有最大的Fm、qP、Fv/Fm、Fv/Fo,表明0.505～mg/L的锰浓度有利于大豆的光合作用。在50.mg/L的锰浓度下,两个大豆品种有最大的qN、Rs、Ci和最小的Tr、Pn,此时两个品种大豆耗散了过剩的激发能,降低了大豆叶片的光合速率,对大豆已产生了一定的伤害。两个品种大豆光合特性对锰的响应存在着基因型差异,浙春2号较东北大豆耐锰胁迫。     

Fire interacts with season to influence soil respiration in tropical savannas

Soil respiration (R_s) is the second-largest source of CO₂ to the atmosphere in terrestrial systems. In tropical savannas seasonal moisture availability and frequent fires drive ecosystem dynamics and may have a considerable impact on soil carbon (C) cycling, including R_s. In order to test the effect of fire on soil C cycling we measured R_s in annually burnt and unburnt plots in wet and dry seasons at a long-term fire experiment established in savanna woodlands of northern Australia. There was a significant interaction between season and fire, with highest rates of daily R_s (722 mmol CO₂ m⁻² d⁻¹) observed in the wet season on unburnt, leaf litter patches. The three fold higher R_s rate on unburnt plots in the wet season was due to greater root-derived respiration (R_root: 356 mmol CO₂ m⁻² d⁻¹), while smaller changes to soil-derived respiration (R_soil: 51 mmol CO₂ m⁻² d⁻¹) were simply the result of C moving through decomposition rather than combustion pathways. Relationships between instantaneous R_s and soil temperature showed hysteresis with variable direction, suggesting that season and fire treatment also influence the soil depth at which CO₂ is produced. We suggest that (1) changes to fire regimes, through active management or climate change, in tropical savannas could have an impact on R_s, and (2) the direct effect of fire on soil C cycling is limited to the removal of aboveground litter inputs.     

On the relationship between incoming solar radiation and daily maximum and minimum temperature

A relationship between atmospheric transmittance and the daily range of air temperature is developed. The relationship is T_t = A[1—exp(—BΔT^c)] where T_t is the daily total atmospheric transmittance, ΔT is the daily range of air temperature, and A, B, and C are empirical coefficients, determined for a particular location from measured solar radiation data. Tests on three data sets indicate that 70–90% of the variation in daily solar radiation can be accounted for by this simple model.     

Impact of high cadmium and nickel soil concentration on selected physiological parameters of Arundo donax L.

The purpose of this study was to investigate the effects of high cadmium and nickel soil concentrations on selected physiological parameters of Arundo donax L. A 2-year pot experiment was held in the field and the pots were irrigated with aqueous solutions of Cd and Ni in concentrations of 5, 50 and 100 ppm, against the control (tap water). At the end of the cultivation periods the pots soil was divided into three equal zones and total and NH₄OAc extractable Cd and Ni concentrations were determined. The top zone exhibited the highest metal content. Cadmium and nickel total concentrations at the end of the experiment were up to 973.8 mg kg⁻¹ and 2543.3 mg kg⁻¹ respectively, while NH₄OAc extractable Cd was up to 291.7 mg kg⁻¹ and Ni up to 510.3 mg kg⁻¹. Stomatal conductance ranged between 0.3 and 0.8 mol CO₂ m⁻² s⁻¹, intercellular CO₂ concentration ranged between 212.9 and 243.0 ppm CO₂, stomatal resistance between 0.6 and 1.3 s cm⁻¹, chlorophyll content (SPAD values) between 46.3 and 57.0 and chlorophyll fluorescence (F_v/F_m) ranged between 0.8 and 0.9. All studied physiological parameters did not show statistically significant differences among control and heavy metal treated plants for both years; therefore, high soil cadmium and nickel concentration did not inhibit stomatal opening and did not affect the function of the photosynthetic machine of A. donax plants.     

Estimating stomatal diffusion resistance for Douglas-fir,lodgepole pine and white oak under light saturated conditions

Stomatal diffusion (leaf) resistance (r_s) of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), lodgepole pine (Pinus contorta Dougl. ex Loud.) and white oak (Quercus alba L.) in a variety of natural environments is described during conditions of light saturation as a function of the product of the predawn (base) xylem pressure potential of twigs (ψ_xb) or the soil water potential (ψ_s), and the saturation deficit of the atmosphere (δ_e). This general functional form was derived from an analysis based on an Ohm's law analog for water flow in the soil—plant—atmosphere system. The specific form used in each case was determined using regression analysis, where dependent variables considered were xylem pressure potential of the twigs or leaves (ψ_x) and air temperature (T) as well as δe and ψ_xb (or ψ_s). For three independent sets of Douglas-fir data, it is shown qualitatively that a single function describes the combined data as well as the functions derived for each of three data sets individually. The results imply that it may be possible to describe stomatal response of Douglas-fir, and perhaps other species, over a range of environmental conditions using the same model.     

Sapflow characteristics and climatic responses in three forest species in the semiarid Loess Plateau region of China

In the semiarid Loess Plateau region of China, ecosystems are frequently affected by water shortages. Late spring and early summer are periods when forest communities tend to suffer from soil drought. To clarify the water-use strategies of the main species in this area, the xylem sap flow of trees from three species in the field was monitored for three successive periods in summer using Granier-type thermal dissipation probes. Vapor pressure deficit (VPD), solar radiation (R_s) and soil moisture had varying influences on sap flux density (F_d) in the species. Normalized F_d values could be fitted to VPD using an exponential saturation function, but the fit was better with a derived variable of transpiration (VT), an integrated index calculated from VPD and R_s. From differences in model coefficients, the species were roughly divided into two types with contrasting drought sensitivity. The exotic Robinia pseudoacacia was defined as drought-sensitive type. It showed higher sapflow increases in response to rainfall, suggesting a high water demand and high influence of soil water conditions on transpiration. This species showed relatively late stomatal response to increasing VPD. The wide-peak pattern of diurnal sapflow course also suggests relatively low stomatal regulation in this species. The drought-insensitive type consisted of the naturally dominant Quercus liaotungensis and an indigenous concomitant species, Armeniaca sibirica, in the forest. The sap flow of these species was not very sensitive to changes in soil water conditions. The results suggest that typical indigenous species can manage the water consumption conservatively under both drought and wet conditions. Variations in water use strategies within indigenous species are also detected.     

Long-term management impacts on soil C,N and physical fertility: Part II: Bad Lauchstadt static and extreme FYM experiments

Manure is a source of plant nutrients and can make a valuable contribution to soil organic matter (SOM). Two experimental sites were studied on a Halpic Phaeozem soil near Bad Lauchstadt in Germany. The first experiment, called the static experiment, commenced in 1902. The impact of fresh farmyard manure (FYM) (0, 20 and 30 t ha⁻¹ 2 year⁻¹) combined with P, K and N fertiliser application on total organic C (C_T), labile C (C_L), non-labile C (C_NL), total N (N_T), mean weight diameter (MWD) and unsaturated hydraulic conductivity (K_unsat) was investigated. The second experiment commenced in 1984 and investigated the effect of extreme rates of fresh FYM applications (0, 50, 100 and 200 t ha⁻¹ year⁻¹) and cropping, or a continuous tilled fallow on the same soil properties. At both sites a nearby grassland site served as a reference. On the static experiment, FYM application increased all C fractions, particularly C_L, where application of 30 t ha⁻¹ 2 year⁻¹ increased C_L by 70% compared with no FYM application. Fertiliser additions to the static experiment had a positive influence on C fractions while N_T increased from both FYM and fertiliser application. MWD increased as a result of FYM application, but did not reach that of the grassland site. Both fertiliser and FYM application increased K_unsat (10 mm tension) on the static experiment. In the second experiment application of 200 t ha⁻¹ year⁻¹ of FYM increased concentrations of C_L by 173% and of C_NL by 80%, compared with no FYM application to make them equivalent to, or greater than the grassland site. A continuously tilled fallow resulted in significant decreases in all C fractions, N_T and MWD compared with the cropped site, while K_unsat (10 mm tension) was increased on the 0 and 50 t ha⁻¹ year⁻¹ treatments as a result of a recent tillage. There was no difference in K_unsat between the cropped and the continuous tilled fallow at FYM applications of 100 and 200 t ha⁻¹ year⁻¹. There were similar significant positive correlations of all C fractions and N_T with MWD on both experimental sites but the relationships were much stronger on the extreme FYM experiment. Weaker relationships of C fractions and N_T with K_unsat (10 mm tension) occurred for the static experimental site but these were not significant for the extreme FYM experimental site. The strongest relationship between C fractions and K_unsat was with C_L. This research has shown that applications of FYM can increase SOM and improve soil physical fertility. However, the potential risk of very high rates of FYM on the environment need to be taken into consideration, especially since the application of organic materials to soils is likely to increase in the future.     

GAS EXCHANGE AND GROWTH OF TRITICALE SEEDLINGS UNDER DIFFERENT NITROGEN SUPPLY AND WATER STRESS

The effect of nitrogen (30 and 120 mg N per cuvette) on photosynthetic rate of four cultivars of triticale (‘Bolero’, ‘Grado’, ‘Largo’, and ‘Lasko’) grown 14 days in phytotron was strongly modified by water content (75, 45 and 35% of full water capacity). For plants grown under 35% of full water capacity, it was higher when they were grown under 30 than under 120 mg N/cuvette (9.88 and 8.76 μmol CO₂ m^?2 s^?1, respectively) but for plants grown under 45 and 75% of full water capacity there were not significant differences. Transpiration, stomatal conductance, photosynthetic water use efficiency, and internal water use efficiency were not influenced by nitrogen doses independently of water content. Photosynthetic rate, transpiration, stomatal conductance, photosynthetic water use efficiency, and dry matter of studied cultivars of triticale grown under 45 and 35% of full water capacity and both nitrogen doses were lower than for plants grown under 75% of full water capacity. With lowering of water content stomatal conductance was decreasing similarly as photosynthetic rate e.g. for plants grown under 35% of full water capacity as compared with those grown under 75% of full water capacity average stomatal conductance decreased from 0.209 to 0.138 mol H₂O m^?2 s^?1 and photosynthetic rate from 13.69 to 9.32 μmol CO₂ m^?2 s^?1 and as a result there were not significant differences in internal water use efficiency for all studied combinations (67.09 μmol CO₂ mol^?1 H₂O) which shows that stomatal factors were mainly responsible for changes of photosynthetic rate. With lowering of water content from 75 to 35% of full water capacity the decrease of photosynthetic rate and stomatal conductance was much higher than the decreases of transpiration (from 3.57 to 3.02 mmol H₂O m^?2 s^?1) what shows not direct dependence of transpiration on stomatal conductance (water use efficiency decreased from 3.87 to 3.10 μmol CO₂ mmol^?1 H₂O). The effect of nitrogen on dry matter production was strongly modified by water availability e.g. for plants grown under 35% of full water capacity, dry matter was similarly independent of nitrogen dose but for plants grown under 45 and 75% of full water capacity dry matter was significantly higher than when they were grown under 120 (79.05 and 86.75 mg, respectively) or with 30 mg N/cuvette (74.03 and 80.30 mg, respectively).     

Plant water relations in lychee: diurnal variations in leaf conductance and leaf water potential

Diurnal variations in leaf conductance and leaf water potential were investigated in 6-year-old lychee trees (Litchi chinensis sonn. cv. Bengal) during the dry season in subtropical Queensland, Australia. Leaf conductance and leaf water potential fell from 0.56 mol m⁻² s⁻¹ and -0.1 MPa in the morning to 0.08 mol m⁻² s⁻¹ and -1.0 MPa, respectively, in the middle of the day and then recovered slowly during the afternoon. Most of the variation in leaf conductance and leaf water potential could be attributed to a negative response to leaf-air water vapour concentration gradient (r ≈ 0.90, P < 0.01).The marked sensitivity of lychee stomata to relative humidity suggests that CO₂ assimilation could be reduced dramatically at high vapour concentration gradient and is one of the possible explanations for the poor fruit retention of lychee in dry environments. It is possible that the productivity of lychee at low humidities could be improved by techniques which decrease leaf-air vapour concentration gradient, such as overhead intermittent misting.     

Evapotranspiraton estimation based on scaling up from leaf stomatal conductance to canopy conductance

Evapotranspiraton (ET) estimation based on scaling up from leaf stomatal conductance (g_s) to canopy conductance (g_c) is important in improving effective use and evaluation of agricultural water resources. Taking a summer maize field in north China as an example, after the response of g_s to main environmental factors was analyzed based on the measured value, the Jarvis model for g_s was established and calibrated. Then the weighted integration model (WI model) was established on the basis of weighted model (W model) after considering the difference of intercept diffuse radiation by shaded leaves in different canopy heights and nonlinear relationship between g_s and the photosynthetically active radiation (PAR) to improve g_c estimation for shaded leaves using integration equation. Meanwhile the estimation accuracy of W and WI models for g_c was compared, and then field ET was estimated using the Penman-Monteith equation. Results indicate that the variation of g_s was similar to that of PAR and the Jarvis model could better express the response of g_s to PAR, vapour pressure deficit and air temperature. Compared to the W model, WI model could effectively improve the estimation accuracy of g_c, with the relative error of 4.4%. Penman-Monteith equation overestimated λET by 9.4% using the estimated g_c by the W model, but underestimated λET by 2.3% using the estimated g_c by the WI model. Therefore, Penman-Monteith equation can estimate maize field ET using the estimated g_c by WI model in the region.     

Response of Fertigation Under Buried Straw Layer on Growth,Yield, and Water-fertilizer Productivity of Chinese Cabbage Under Greenhouse Conditions

In the present study, the effect of buried straw (two levels; M_S; buried straw layer and M_N; no straw layer), nitrogen fertilizer (two levels: F_H, 120 kg ha^?1 and F_L, 80 kg ha^?1), and deficit drip irrigation with irrigation frequency of 2 days (two irrigation regimes: I₅₀; 50% and I₃₀; 30% of evapotranspiration) was investigated on a greenhouse-grown leafy cabbage for two consecutive seasons. The results indicated that in both the seasons, T₅ (M_NF_HI₅₀) gave higher yield when compared to all other treatments but it also utilized more water and fertilizer. On the other hand, with a 5–10% decrease in yield comparing to T₅ (M_NF_HI₅₀), T₂ (M_BSF_LI₅₀) and T₄ (M_SF_LI₃₀) saved 33% of fertilizer. T₄ (M_BSF_LI₃₀) also gave the highest water and fertilizer use efficiencies when compared to all other treatments. However, it was clearly noted that T₄ (M_SF_LI₃₀) treatment could save water and nitrogen without a significant decrease in fresh yield of Chinese cabbage. Hence, T₄ (M_sF_LI₃₀) is the recommended strategy to manage water and nitrogen fertilizer for getting optimal leafy cabbage plant growth and yield.