A comparative study of three leaf wetness sensors

The laboratory and field performance of two electrical resistance (ER) sensors of leaf surface wetness were compared with that of a beta-ray gauge (BRG). The BRG provided the most accurate measurements of wetness duration, which were in agreement with visual observations. A Campbell and a cotton cloth ER sensor consistently underestimated the duration of leaf surface wetness compared to the value obtained with the BRG in a dew chamber. However, the response of the Campbell sensor improved considerably with increase in the severity of dewfall. A superior performance of one of the two ER sensors could not be decisively established on the basis of the field experiments of 1989 and 1990 on soybean and tobacco crops, respectively. For studies where accurate measurements of surface wetness are critical, it is suggested that a beta-ray gauge should be used.     

Electronic leaf wetness duration sensor: why it should be painted

The purpose of this study was to compare and evaluate the performance of electronic leaf wetness duration (LWD) sensors in measuring LWD in a cotton crop canopy when unpainted and painted sensors were used. LWD was measured with flat, printed-circuit wetness sensors, and the data were divided into two periods of 24 days: from 18 December 2001 to 10 January 2002, when the sensors were unpainted, and from 20 January to 13 February 2002, when the sensors were painted with white latex paint (two coats of paint). The data analysis included evaluating the coefficient of variation (CV%) among the six sensors for each day, and the relationship between the measured LWD (mean for the six sensors) and the number of hours with dew point depression under 2 °C, used as an indicator of dew presence. The results showed that the painting markedly reduced the CV% values. For the unpainted sensors the CV% was on average 67% against 9% for painted sensors. For the days without rainfall this reduction was greater. Comparing the sensor measurements to another estimator of LWD, in this case the number of hours with dew point depression under 2 °C, it was also observed that painting improved not only the precision of the sensors but also their sensitivity, because it increases the ability of the sensor to detect and measure the wetness promoted by small water droplets.     

植物叶片形成露水的室内模拟

叶片在空气中形成露水是干旱、半干旱地区植物水分来源之一,具有重要的生态意义.本文借助人工智能气候室和叶片温度自控系统,对影响露水形成的环境温湿度、叶片温度和叶片倾角进行调节,研究了叶倾角、环境温湿度、露点-叶温差对叶片露水累积速率和叶片露水量的影响.结果表明: 叶片露水累积速率和叶片最大露水量均随叶倾角的增加而减小,但随着环境温度、湿度和露点-叶温差的增加而增大.叶片处于水平状态时,叶片露水在达到最大露水量前呈线性快速递增,露水达到最大露水量(0.80 mm)后处于稳定值;当叶片有倾角时,叶片露水达到一定值就会发生叶片露水滑落现象,使得叶片露水量呈现锯齿形变化,且露水累积速率明显变慢.     

LEAF SURFACE WETNESS AND GAS EXCHANGE IN THE POND LILY NUPHAR POLYSEPALUM (NYMPHAEACEAE)

As part of a continuing study of the effects of leaf surface wetness on gas exchange, the occurrence of leaf surface wetting by dewfall and associated effects on photosynthesis were evaluated for floating and aerial leaves of the pond lily Nuphar polysepalum Engelm. Because of nighttime radiation exchange with a cold sky, high humidity, and the presence of adaxial stomata, we predicted that pond lily leaves would be particularly susceptible to wetting events such as dewfall. A substantial reduction in net photosynthesis (up to 20%) occurred for leaves that were experimentally misted to simulate leaf wetting by dewfall. Aerial leaves remained below dewpoint temperatures for long periods on clear nights. However, floating leaves rarely approached dewpoint temperatures at night because minimum nighttime temperatures of leaves were up to 10 C warmer than air temperature. Thus, floating leaves of N. polysepalum did not experience dew formation primarily because of strong thermal coupling to a substrate (water) that was much warmer than air temperature at night. This coupling to a warmer substrate prevented a potentially strong inhibition of photosynthetic CO₂ exchange the following morning.     

Measuring dew formation and its threshold value for net radiation loss on top leaves in a paddy rice crop by using the dewball: a new and simple instrument

In a field experiment on dew formation in tropical paddy rice, the threshold value for the nocturnal net radiative loss required for dew formation was investigated during 23 nights from February to April 1994. The onset and cessation of dew were visually observed on both the top leaves of a rice crop and on a glass sphere, the ”dewball”, installed in the field 1.0 m above the ground. The threshold value for the nocturnal net radiative loss (R _n,thresh) necessary for dew formation on the ball was deduced from the maximum zenith angle on the ball surface reached by the dew formed. R _n,thresh was found to be linearly related to the nightly minimum vapour-pressure deficit. This linear relationship as well as the Penman-Monteith equation (an energy-balance approach) were used to predict both the time of dew onset and the duration of dew on the ball. These predicted values agreed well with the observed results. Dew duration and time of dew onset on the ball were well correlated. During most nights, dew began to form on the top leaf surfaces at almost the same time as it reached the zenith angle of 60° on the ball. A linear relationship was found between the observed daily duration of dew on the top leaf surfaces of the rice crop and the estimated time when dew reached the zenith angle of 60° on the ball. This relationship gave as accurate an estimation of dew duration on the top leaf surface of the rice crop as did the Penman-Monteith combination equation. This study showed that the dewball is a potentially useful device for observing dew formation. Received: 26 April 1999 / Revised: 8 May 2000 / Accepted: 9 May 2000     

陕北黄土丘陵区露水量及影响因子

为探明中国黄土丘陵半干旱区露水量特征及其影响因子,利用叶片湿度传感器(LWS)在陕西安塞县进行1年的监测试验.结果表明: 安塞地区2015—2016水文年露水总量为29.85 mm,日最大露水量可达0.82 mm.露水凝结量主要呈现日变化和季节变化两种特征,日变化过程中露水凝结量大多发生在18:00至次日7:00;季节变化中露水在秋季凝结量最大,夏季次之,春、冬季最少.在时空分布上,露水和降雨高度互补,露水大多产生于晴朗无云夜间,但降雨前后几天更有利于露水凝结.空气相对湿度和气温-露点差是影响露水凝结的主要因素,露水量与相对湿度呈显著正相关(r=0.726,P<0.01),与气温-露点差呈显著负相关(r=-0.725,P<0.01);风速、风向在一定程度上影响露水凝结,但风速(r=0.133,P<0.01)、风向(r=0.219,P<0.01)与露水量之间的相关性较弱.     

Infection of sunflower leaves by ascospores of Sclerotinia sclerotiomm

Ascospores of Sclerotinia sclerotiorum infected fully expanded sunflower leaves in the absence of added nutrients, wounds or senescent tissue. The site of infection was confined to a specific region around the junction of the leaf blade and the petiole and was associated with sites of sucrose secretion by the host. In these areas ascospores germinated and formed extensive colonies on the leaf surface. Simple appressoria were visible at 24 h, and complex appressoria at 48 h, after inoculation. Ascospores that germinated in areas where sucrose was not secreted produced germtubes that were shorter (less than 50 μm at 48 h after inoculation) than those produced at sites where sucrose was secreted (greater than 5 mm in length). Infections also developed on severely wounded main leaf veins. Leaf wetness was required for infection, with maximal infection occurring after a 72 h dew period.     

Infection periods in relation to the natural development of hop downy mildew (Pseudoperonospora humuli)

The relationships between temperature and surface wetness and subsequent infection of hop tissues by P. humuli were examined on potted plants and detached leaves kept in temperature-controlled growth rooms. Periods of wetness which would just allow leaf infection ranged from 1 1/2 h at 30d? to 24 h at 5d?. The corresponding ranges for shoots were: light infection, 3 h at 19–23d? to 6 h at 8–10d?; severe infection, 4 h at 19–23d? to 8 h at 12–13d?. These data were used to relate the development of downy mildew in an unsprayed hop garden during 1967 and 1968 to periods with temperature/surface wetness suitable for minimum (minor infection periods) and severe infection (major infection periods). In 1967 a sudden outbreak of infected basal shoots (spikes) was related to an isolated major infection period. By contrast, early in 1968, major shoot infection periods did not arise and spikes appeared gradually in response to a succession of minor infection periods. More spikes were formed in 196 than in 1967; this was not related to the incidence of infection periods but probably reflected the relatively higher concentrations of airborne sporangia early in 1968. In both years outbreaks of leaf and lateral shoot infection could be traced to major infection periods caused by rain; sudden disease increases again originated from isolated infection periods. There was a close similarity between the incubation period for each principal disease outbreak and that expected from growth-room experiments. Major infection periods occurred more frequently at the end of June 1968, resulting in a higher final concentration of diseased tissue than in 1967. Predicted major infection periods failed to induce large disease increases when dew alone provided wetness or when no airborne sporangia could be detected.     

西双版纳热带雨林干季林冠层雾露形成的小气候特征研究

对西双版纳热带雨林干季林冠层雾露形成的小气候特征进行了观测研究。结果表明,雾露首先形成于最上林冠层,林下露水的形成迟于林上3～4h,林下雾是由上层雾变浓、下沉而来。夜间,雾形成前,气温高于叶表温;雾形成后,气温则低于叶表温,且气温及叶表温均有回升。雾露的形成不仅凝结了水汽进入森林,同时也对森林起到了一定的保温作用,这对热带雨林的生存和发展具有致关重要的作用。     

Patterns of leaf surface wetness for montane and subalpine plants

The frequency and duration of water on leaf surfaces have important consequences for plant growth and photosynthetic gas exchange. The objective of the present study was to compare the frequency and duration of leaf wetness under natural field conditions among species and to identify variation in structural features of leaves that may reduce surface wetness. During June–September 1992 in the central Rocky Mountains (USA), natural leaf wetting due to rain and dewfall was observed on 79 of 89 nights in open meadow habitats compared to only 29 of 89 nights in the understorey. Dew formation occurred at relative humidities that were often well below 100% because of radiational heat exchange with cold night skies and low wind speeds (< 0.5 m s^?1). A survey of 50 subalpine/montane species showed that structural characteristics associated with the occurrence and duration of leaf surface wetness differed among species and habitats. Both adaxial and abaxial surfaces accumulated moisture during rain and dewfall events. Leaf surfaces of open-meadow species were less wettable (P= 0.008), and had lower droplet retention (P= 0.015) and more stomata P= 0.017) than adjacent understorey species. Also, leaf trichomes reduced the area of leaf surface covered by moisture. Ecophysiological importance is suggested by the high frequency of leaf wetting events in open microsites, influences on growth and gas exchange, and correspondence between leaf surface wettability and habitat.     

Spatial variability of leaf wetness duration in different crop canopies

The spatial variability of leaf wetness duration (LWD) was evaluated in four different height-structure crop canopies: apple, coffee, maize, and grape. LWD measurements were made using painted flat plate, printed-circuit wetness sensors deployed in different positions above and inside the crops, with inclination angles ranging from 30 to 45°. For apple trees, the sensors were installed in 12 east-west positions: 4 at each of the top (3.3 m), middle (2.1 m), and bottom (1.1 m) levels. For young coffee plants (80 cm tall), four sensors were installed close to the leaves at heights of 20, 40, 60, and 80 cm. For the maize and grape crops, LWD sensors were installed in two positions, one just below the canopy top and another inside the canopy. Adjacent to each experiment, LWD was measured above nearby mowed turfgrass with the same kind of flat plate sensor, deployed at 30 cm and between 30 and 45°. We found average LWD varied by canopy position for apple and maize (P<0.05). In these cases, LWD was longer at the top, particularly when dew was the source of wetness. For grapes, cultivated in a hedgerow system and for young coffee plants, average LWD did not differ between the top and inside the canopy. The comparison by geometric mean regression analysis between crop and turfgrass LWD measurements showed that sensors at 30 cm over turfgrass provided quite accurate estimates of LWD at the top of the crops, despite large differences in crop height and structure, but poorer estimates for wetness within leaf canopies.     

Environmental factors influencing the infection of wheat by Puccinia graminis

Germination rate and total germination of Puccinia graminis uredospores were directly related to pustule age and duration between spore collections. Partial drying of the spores enhanced germination rate; keeping them for 18 h at 100% r.h. reduced both rate and total germination. Spores germinated in polystyrene dishes between 4 and 29 °C and optimally between 15 and 23 °C Light (3 cal/cm²/h) had little effect on germination on moist surfaces but inhibited germination on the leaf. In Hybrid 229/8 wheat this effect was more pronounced than in var. Little Club. The number of primary infections increased linearly with duration of surface wetness with a narrow temperature optimum at 23.5 °C. Two phases of infection could be distinguished: germination (requiring darkness and capable of taking place over a wide temperature range) and penetration (requiring light and slightly higher temperature than for germination). Stomatal closure caused by subjecting the plants to water stress led to proporational reductions in infection. The results are discussed in relation to dew formation.     

毛乌素沙地南缘沙丘生物结皮对凝结水形成和蒸发的影响

在水分极度匮乏的荒漠生态系统,凝结水是除降雨之外最重要的水分来源之一,它对荒漠生态系统结构、功能和过程的维持产生重要的影响。为探明半干旱沙区生物结皮表面的凝结水形成和蒸发特征,采用自制的微型蒸渗计(直径7 cm、高5 cm的PVC管)实验观测了不同类型地表(裸沙、浅灰色藻类结皮、黑褐色藻类结皮和苔藓结皮)对凝结水形成和蒸发的影响。结果表明:(1)观测期间共有20次凝结水形成记录,除降雨天气外,几乎每天都能观测到水分凝结现象;(2)不同类型地表凝结水总量依次为(1.998±0.075),(2.326±0.083),(2.790±0.058)和(3.416±0.068) mm,生物结皮表面的凝结水总量显著大于裸沙(P < 0.05);随生物结皮的发育,不同类型生物结皮表面的凝结水总量呈增加的趋势,凝结水总量之间差异显著(P < 0.05);观测期间不同类型地表日平均凝结水量依次为(0.100±0.003),(0.116±0.004),(0.140±0.002)和(0.171± 0.003) mm,不同类型地表日平均凝结水量之间差异极显著(P < 0.01);(3)凝结水形成过程的观测结果显示,凝结水19:00开始形成,23:00-凌晨1:00形成不明显,1:00-7:00继续形成,除浅灰色藻类结皮外,太阳升出后在黑褐色藻类结皮和苔藓结皮表面继续形成少量的凝结水;凝结水7:30开始蒸发,10:30到11:00之间结束蒸发,凝结水在裸沙和浅灰色藻类结皮中的保持时间显著大于黑褐色藻类结皮和苔藓结皮中的保持时间(P < 0.05);(4)凝结水的形成受大气温度、地表温度、空气相对湿度和大气地表温度差等气象因素的影响,但其形成过程不与某一个气象因素呈简单的线性关系。     

Vertical patterns and duration of surface wetness in an old-growth tropical montane forest, Indonesia

In tropical montane forests, the wetness of leaf surfaces is an important parameter which may influence gas exchange, growth and vitality of leaves, and forest productivity. Thirty surface wetness sensors were operated during May–August 2004 in a vertical profile inside an old-growth lower montane rain forest of Central Sulawesi, Indonesia, with the objective to analyse spatial and temporal patterns of surface wetness and to relate wetness duration to the microclimate inside the stand. The canopy was wet during 25–30% of time in this study period. In a dry period, however, surface wetness lasted for only 5% of the time, whereas the canopy was wet during 45–55% of the time in a rainy period. In the lower shade canopy, surface wetness continuously existed for periods of up to 22 h and more, although rainfall occurred only during afternoon thunderstorms of limited duration. The long duration of surface wetness has implications for forest interception models, which assume a complete drying of the canopy between subsequent rainfall events. In periods with rainfall, leaf wetness typically occurred in the afternoon, evening and first half of the night because intercepted water persisted on the leaves until about midnight. In dry periods, in contrast, surface wetness was mainly caused by dewfall in the second half of the night, and it occurred mainly in the uppermost canopy where radiative heat losses resulted in a substantial under-cooling of the leaves. Ecophysiological and hydrological importance is suggested by the long duration of surface wetting in this stand with possible implications for gas exchange, leaf growth, leaf colonization by epiphylls and the forest water balance.     

Susceptibility of clover species to fungal infection: the interaction of leaf surface traits and environment

Many foliar pathogens require free water to germinate; therefore, disease pressure should favor plants that are able to repel water. For a suite of 18 sympatric clover species (Trifolium and Medicago, Fabaceae), we evaluated leaf traits affecting leaf wetness and susceptibility to infection by the fungal pathogen Stemphylium sp., causal agent of Stemphylium leaf spot. Spore germination increased with time in free water, and the relative susceptibility of host plants to infection was proportional to the duration of water retention on leaves. Larger leaves captured more water and retained it longer. Unexpectedly, trichomes and leaf wettability did not affect water capture. For clovers planted within natural clover populations at two sites, infection was threefold greater at the wetter site. At the drier site, water retention on the leaf surface was an important predictor of infection rates across host species, but persistent fog and dew at the wetter site reduced the importance of rapid leaf drying. Our results suggest that plant adaptations that reduce water retention on leaves may also reduce disease incidence, but the selective advantage of these traits will vary among habitats.     

Leaf wetness duration measurement: comparison of cylindrical and flat plate sensors under different field conditons

In general, leaf wetness duration (LWD) is a key parameter influencing plant disease epidemiology, since it provides the free water required by pathogens to infect foliar tissue. LWD is used as an input in many disease warning systems, which help growers to decide the best time to spray their crops against diseases. Since there is no observation standard either for sensor or exposure, LWD measurement is often problematic. To assess the performance of electronic sensors, LWD measurements obtained with painted cylindrical and flat plate sensors were compared under different field conditions in Elora, Ontario, Canada, and in Piracicaba, São Paulo, Brazil. The sensors were tested in four different crop environments—mowed turfgrass, maize, soybean, and tomatoes—during the summer of 2003 and 2004 in Elora and during the winter of 2005 in Piracicaba. Flat plate sensors were deployed facing north and at 45° to horizontal, and cylindrical sensors were deployed horizontally. At the turfgrass site, both sensors were installed 30 cm above the ground, while at the crop fields, the sensors were installed at the top and inside the canopy (except for maize, with a sensor only at the top). Considering the flat plate sensor as a reference (Sentelhas et al. Operational exposure of leaf wetness sensors. Agric For Meteorol 126:59–72, 2004a), the results in the more humid climate at Elora showed that the cylindrical sensor overestimated LWD by 1.1–4.2 h, depending on the crop and canopy position. The main cause of the overestimation was the accumulation of big water drops along the bottom of the cylindrical sensors, which required much more energy and, consequently, time to evaporate. The overall difference between sensors when evaporating wetness formed during the night was around 1.6 h. Cylindrical sensors also detected wetness earlier than did flat plates—around 0.6 h. Agreement between plate and cylinder sensors was much better in the drier climate at Piracicaba. These results allow us to caution that cylindrical sensors may overestimate wetness for operational LWD measurements in humid climates and that the effect of other protocols for angling or positioning this sensor should be investigated for different crops.     

Influence of Temperature, Wetness Duration, and Leaf Type on the Quantification of Monocyclic Parameters of Bean Rust

Monocyclic parameters of bean rust (Uromyces phaseoli var. typical) were quantified in growth chambers, on rwo bean cultivars for three temperatures (17, 21, and 25 °C), two types of leaves (unifoliolate and trifoiiolate leaves), and nine leaf wetness periods (0, 4, 7, 10, 13, 16, 19, 22, and 25 hrs). The expression of disease was greatly influenced by past-inoculation temperatures. The incubation and latent periods were shortest at 21 °C for both cultivars and leaf types. For both cultivars, trifoiiolate leaves were more susceptible than unifoliolate leaves. A wetness period of at least four hours was required for disease to occur. The maximum disease efficiency for both cultivars occurred with 22 hrs of leaf wetness at 17 °C. The disease efficiencies for temperatures of 17–29 °C and leaf wetness periods of 0–25 hrs were adequately described by a response-surface model. Because of the great influence of temperature and leaf wetness on infection, bean rust is unlikely to occur at high temperatures (> 25°C) and short leaf wetness periods (< 7 hrs).     

Self-propulsion of dew drops on lotus leaves: a potential mechanism for self cleaning

This study shows that condensation on the hierarchically structured lotus leaf can facilitate self-propulsion of water droplets off the surface. Droplets on leaves inclined at high angles can be completely removed from the surface by self-propulsion with the assistance of gravity. Due to the small size of mobile droplets, light breezes may also fully remove the propelled droplets, which are typically projected beyond the boundary layer of the leaf cuticle. Moreover the self-propelled droplets/condensate were able to remove contaminants (eg silica particles) from the leaf surface. The biological significance of this process may be associated with maintaining a healthy cuticle surface when the action of rain to clean the surface via the lotus effect is not possible (due to no precipitation). Indeed, the native lotus plants in this study were located in a region with extended time periods (several months) without rain. Thus, dew formation on the leaf may provide an alternative self-cleaning mechanism during times of drought and optimise the functional efficiency of the leaf surface as well as protecting the surface from long term exposure to pathogens such as bacteria and fungi.     

Drechslera sp. (Pyrenophora chaetomioides (Speg.)), a Potential Biocontrol Agent for Bromus sterilis and Other Bromus spp.

Infection by a Drechslera sp. (perfect stage, Pyrenophora chaetomioides (Speg.)), isolated from Bromus sterilis , killed B. sterilis , B. commutatus and B. secalinus. B. diandrus and B. hordeaceus were both infected but not killed. Successful infection required a 24-h dew period. Reduction of the dew period to 8 h significantly reduced the infection of all Bromus spp. tested as determined by leaf necrosis. Inoculation with a low inoculum concentration (2 104 conidia/ml) produced little dry weight reduction, but at 2 105 conidia/ml with an 8-h dew period the dry weights of B. commutatus , B. diandrus , B. secalinus and B. sterilis were reduced by 11-25%. Extending the dew period to 24 h resulted in 77% mortality of B. sterilis and 93% mortality of B. commutatus and B. secalinus.     

Effects of Leaf Wetness Duration and Inoculum Level on Resistance of Wheat Genotypes to Pyrenophora tritici-repentis

Percent leaf necrosis and lesion length on wheat genotypes increased markedly with increasing duration of leaf wetness (up to 24h or 48 h) following inoculation with Pyrenophora tritici-repentis. A long wetting duration favoured less disease development on resistant (Fink's'), and moderately resistant (Bon/YR/3/F3570//KAL/BB) genotypes than on susceptible Glenlea. No significant difference in per cent necrosis was detected among the upper three leaf positions within a genotype. A long wetness duration had a varying effect on the resistance of wheat genotypes, depending upon the inoculum level. Increasing the inoculum level along with the leaf wetness period increased the per cent leaf necrosis on all three wheat genotypes tested. However, the ranking of the genotype for resistance did not alter even after prolonged duration of leaf wetness (up to 96 h) and/or high inoculum level (12000 conidia/ml water). Various post-inoculation wet-periods in combination with high conidia concentrations in inoculum should be used in identifying highly resistant germplasm in breeding populations at the seedling stage of the wheats.