The incidence and implications of clouds for cloud forest plant water relations

Although clouds are the most recognisable and defining feature of tropical montane cloud forests, little research has focussed on how clouds affect plant functioning. We used satellite and ground‐based observations to study cloud and leaf wetting patterns in contrasting tropical montane and pre‐montane cloud forests. We then studied the consequences of leaf wetting for the direct uptake of water accumulated on leaf surfaces into the leaves themselves. During the dry season, the montane forest experienced higher precipitation, cloud cover and leaf wetting events of longer duration than the pre‐montane forest. Leaf wetting events resulted in foliar water uptake in all species studied. The capacity for foliar water uptake differed significantly between the montane and pre‐montane forest plant communities, as well as among species within a forest. Our results indicate that foliar water uptake is common in these forest plants and improves plant water status during the dry season.     

Leaf surface wetness in sorghum and resistance to shoot fly, Atherigona soccata: role of soil and plant water potentials

In experiments with potted plants, the relationships between soil matric potential, plant water potential and production of water droplets (leaf surface wetness) on the folded central whorl leaf of seedlings of sorghum genotypes that are either resistant or susceptible to shoot fly (Atherigona soccata) damage were investigated. Differences in soil matric potentials in the pots affected the plant water status, which in turn had profound effects on the production of water droplets on the central whorl leaf of the sorghum genotype susceptible to shoot fly. There was no consistent variation in the relationship between plant water potential and soil matric potential of resistant and susceptible sorghum genotypes. However, there was very little or practically no water droplets on the central whorl leaf of the resistant genotypes, indicating that the production of water droplets is not solely the result of internal water status of the plant. It is suggested that leaf surface wetness is genetically controlled and that an understanding of the mechanism by which water is transferred to the leaf surface will enhance breeding for resistance to shoot fly.     

Resistance in sorghum to shoot fly Atherigona soccata: Evidence for the source of leaf surface wetness

Leaf surface wetness (LSW) of the central whorl leaf of sorghum seedlings has been associated with susceptibility to shoot fly. Previous physical and physiological evidence suggested that LSW originates from the plant. This was confirmed by radioactive labelling methods using tritium and carbon-14. Tritiated water applied to the soil of potted seedlings was translocated to the surface of the whorl leaf. There were significant differences in the amount of tritiated water collected from susceptible (CSH 5) and resistant (IS 18551) genotypes. Studies with carbon-14 labelling of sorghum seedlings indicated the presence of (small amounts of) solutes in the surface water that may affect larval movement and survival.     

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.     

Foliar water uptake: Processes,pathways, and integration into plant water budgets

Nearly all plant families, represented across most major biomes, absorb water directly through their leaves. This phenomenon is commonly referred to as foliar water uptake. Recent studies have suggested that foliar water uptake provides a significant water subsidy that can influence both plant water and carbon balance across multiple spatial and temporal scales. Despite this, our mechanistic understanding of when, where, how, and to what end water is absorbed through leaf surfaces remains limited. We first review the evidence for the biophysical conditions necessary for foliar water uptake to occur, focusing on the plant and atmospheric water potentials necessary to create a gradient for water flow. We then consider the different pathways for uptake, as well as the potential fates of the water once inside the leaf. Given that one fate of water from foliar uptake is to increase leaf water potentials and contribute to the demands of transpiration, we also provide a quantitative synthesis of observed rates of change in leaf water potential and total fluxes of water into the leaf. Finally, we identify critical research themes that should be addressed to effectively incorporate foliar water uptake into traditional frameworks of plant water movement.     

Harmless nectar source or deadly trap: Nepenthes pitchers are activated by rain, condensation and nectar

The leaves of Nepenthes pitcher plants are specialized pitfall traps which capture and digest arthropod prey. In many species, insects become trapped by 'aquaplaning' on the wet pitcher rim (peristome). Here we investigate the ecological implications of this capture mechanism in Nepenthes rafflesiana var. typica. We combine meteorological data and continuous field measurements of peristome wetness using electrical conductance with experimental assessments of the pitchers' capture efficiency. Our results demonstrate that pitchers can be highly effective traps with capture rates as high as 80% but completely ineffective at other times. These dramatic changes are due to the wetting condition of the peristome. Variation of peristome wetness and capture efficiency was perfectly synchronous, and caused by rain, condensation and nectar secreted from peristome nectaries. The presence of nectar on the peristome increased surface wetness mainly indirectly by its hygroscopic properties. Experiments confirmed that pitchers with removed peristome nectaries remained generally drier and captured prey less efficiently than untreated controls. This role of nectar in prey capture represents a novel function of plant nectar. We propose that the intermittent and unpredictable activation of Nepenthes pitcher traps facilitates ant recruitment and constitutes a strategy to maximize prey capture.     

内蒙古典型草原作物系数的动态模拟与确定

作物系数是计算作物需水量必不可少的参数。利用2008年野外水分试验和4个气象站近26年的土壤水分和气象等常规观测资料, 以相关分析和回归分析等统计学方法为基础, 根据水量平衡原理计算了内蒙古典型草原区的作物系数, 分析了其在生长期和不同站点间的变化规律; 建立了典型草原标准作物系数与返青后年日数和大于0 ℃积温的模拟方程, 相关指数在0.94以上。在分析湿润指数、叶面积指数和盖度与作物系数关系的基础上, 提出标准作物系数的气候修正方法和胁迫条件下作物系数的修正方法。同时, 与修正后的联合国粮农组织(FAO)推荐值比较后得出, 生长季标准作物系数的平均值为0.60, 最大值为1.02; 不同生长阶段作物系数的典型值分别为: 初始生长期0.40, 生长中期0.93, 生长后期0.80, 相应的阈值范围为0.35-0.45、0.85-1.00和0.70-0.90。通过旬蒸散量的模拟计算值与蒸渗仪实测结果的比较, 平均相对误差在20%-24%之间, 生长旺盛期大多低于10%, 从而初步证明该文提出的方法在内蒙古典型草原区有较好的适用性。     

三江源区干湿变化特征及其影响

受全球气候变化影响,过去的几十年里,位于青藏高原东部的三江源区气象、水文特征发生了显著变化。干湿状况反映了区域水分和气候特征,研究气候变暖背景下的干湿变化特征,对揭示区域环境对气候变化的响应以及水文-生态演变问题有重要价值。利用近58 a的水文气象数据,采用霍尔德里奇(Holdridge)潜在蒸散率(Potential Evapotranspiration Rate,PER)代表干燥度,用累计距平、Pettitt突变点检测及逆距离加权法研究基于PER的三江源区干湿变化特征和分布,探讨气候变化背景下各气象要素变化对干湿变化带来的可能影响。结果表明:(1)时间序列上,三江源区整体PER值表现出上升趋势(P0.05),且在1997年发生突变(P0.1),干旱化有增加趋势;(2)空间分布上,PER呈现自东南向西北递增的变化格局,大部分站点的PER增加趋势显著;(3)通过相关性分析,PER与降水及相对湿度呈显著地负相关,与气温和日照显著正相关;气温是三江源暖干化的主要因素。     

Infection of Blackcurrant Leaves by Drepanopeziza ribis in Relation to Weather Conditions and Leaf Position

Drepanopeziza ribis causes the leaf spot disease of blackcurrant ( Ribes nigrum ) and may lead to severe premature leaf-fall. Artificial inoculation studies were carried out to investigate infection of leaves by D. ribis conidia in relation to environmental conditions and leaf position (age) on cvs. Baldwin and Ben Hope in April and July 2007. All leaves on a number of selected extension shoots on potted three-year old plants were inoculated with conidia and then incubated under different conditions: 10, 17.5 and 25°C each with five wet periods (4, 8, 12, 24 and 30 h). Number of infected leaves was determined. The two cultivars differed significantly in their susceptibility to conidial infection: cv. Baldwin was much more susceptible than cv. Ben Hope. Older leaves on extension shoots were more susceptible to conidial infection than younger leaves. Increasing length of wetness duration led to increasing incidence of leaves infected, particularly when inoculated in July. However, the effects of temperature were inconclusive and generally very small in comparison with other factors. Field epidemics were monitored over three years (2005–07). Field data confirmed the main findings from controlled inoculation studies: severe disease was associated with very wet conditions and older leaves. Furthermore, they also suggested that significant disease increase only occurred from late July onwards.