Sugar transport together with environmental conditions controls time lags between xylem and stem diameter changes

In the present study the seasonal patterns of time lags between diurnal xylem and whole stem diameter variations at the top and at the base of two Scots pine trees (Pinus sylvestris L.) were compared. The diameter variations were measured during the summers of 2001 and 2002. Time lags were determined using the cross‐correlation method. The lags were found to vary in time according to the different stages of growth. At the top the xylem lagged behind the whole stem between the beginning of stem growth and the end of shoot growth in both years. In 2001 the time lags at the base showed a similar behaviour during stem growth. That kind of seasonal pattern of the time lags would result from the changes in the sink strength due to changing growth rate at different parts of the tree and the differences in the annual rhythm of growth and water availability in the soil (based on precipitation measurements) between the years 2001 and 2002 were reflected in the patterns. The time lags of shrinking and swelling periods during high and low photosynthetic activity (measured using a shoot chamber) were also compared. It was found, for example, that in 2001 in the middle of the growing season at the top of the tree the whole stem lagged on average 15 min more behind the xylem on the days of high photosynthetic activity than on the days of low or moderate. These results show for the first time that the transportation of carbohydrates and variable sink activity could be detected during the growing season in field conditions using stem and xylem diameter variation measurements. Furthermore, these results provide evidence of the pressure gradient‐driven flow also in the phloem of gymnosperms.     

Time lags for xylem and stem diameter variations in a Scots pine tree

Diameter variations in the xylem and whole stem (i.e. over bark) stem of a Scots pine (Pinus sylvestris L.) tree were measured at four heights over a 23 d period at 5 min intervals. Cross‐correlation analysis was used to calculate time lags between the measurements. Xylem diameter measurements at the different heights had time lags varying from 10 to 50 min, measurements at the lower heights lagging behind the most. This result was in good agreement with the cohesion theory of transpiration. For the whole stem diameter measurements, the treetop lagged behind all other heights and the shortest lags were midway along the stem. Changes in whole stem diameter always lagged behind those of xylem stem diameter (30–110 min), and at all heights. The considerable differences in the behaviour of xylem and whole stem diameter support the Münch hypothesis of phloem flow. Time lags calculated separately for the shrinkage (morning) and swelling (afternoon) periods indicated shorter time lags during the swelling periods. The non‐destructive methods used show promise in the simultaneous study of flow dynamics of xylem and phloem in trees.     

New understanding on phloem physiology and possible consequences for modelling long-distance carbon transport

Most current models of assimilate carbohydrate partitioning are based on growth patterns observed under a range of experimental conditions, from which a set of empirical rules are derived to simulate partitioning. As a result, they are not good at extrapolating to other conditions; this requires a mechanistic approach, which only transport-resistance (TR) models currently provide. We examine an approach to incorporating recent progress in phloem physiology into the TR approach, which leads to a 'minimalist' Munch model of a branched system with competing sinks. In vivo whole-plant measurements have demonstrated that C-flow rates are dependent not only on the properties of the sink, but also on the properties of the whole transport system, and the detailed dynamics of this behaviour is mimicked by the proposed model. This model provides a sound theoretical framework for an unambiguous definition of sink and source strengths, with sink priority being an emergent property of the model. Further developments are proposed, some of which have already had limited application, to cope with the complexity of plants; the emphasis is on a modular approach, together with the importance of choosing the appropriate scale level for both structure and function. Whole-plant experiments with in vivo measurement of the phloem dynamics will be needed to help with this choice.     

Modeling xylem and phloem water flows in trees according to cohesion theory and Münch hypothesis

Water and solute flows in the coupled system of xylem and phloem were modeled together with predictions for xylem and whole stem diameter changes. With the model we could produce water circulation between xylem and phloem as presented by the Münch hypothesis. Viscosity was modeled as an explicit function of solute concentration and this was found to vary the resistance of the phloem sap flow by many orders of magnitude in the possible physiological range of sap concentrations. Also, the sensitivity of the predicted phloem translocation to changes in the boundary conditions and parameters such as sugar loading, transpiration, and hydraulic conductivity were studied. The system was found to be quite sensitive to the sugar-loading rate, as too high sugar concentration, (approximately 7 MPa) would cause phloem translocation to be irreversibly hindered and soon totally blocked due to accumulation of sugar at the top of the phloem and the consequent rise in the viscosity of the phloem sap. Too low sugar loading rate, on the other hand, would not induce a sufficient axial water pressure gradient. The model also revealed the existence of Münch “counter flow”, i.e., xylem water flow in the absence of transpiration resulting from water circulation between the xylem and phloem. Modeled diameter changes of the stem were found to be compatible with actual stem diameter measurements from earlier studies. The diurnal diameter variation of the whole stem was approximately 0.1 mm of which the xylem constituted approximately one-third.     

煤矿复垦区土壤水动力学特性对下渗过程的影响

煤矿复垦区土壤水对植物生长、溶质运移以及土壤环境的变化起着至关重要的作用。定量揭示煤矿复垦区土壤水下渗过程是亟待诠释的关键科学问题。本研究通过测定典型矿区不同深度土壤非饱和导水率、容重、总孔隙度和粒径等水动力学参数,结合染色示踪试验,刻画矿区非饱和带土壤水运移过程。染色示踪结果显示30、60 L和90 L这3种实验下渗水量条件下,水流沿X方向侧向扩散的最大距离分别为10、30 cm和35 cm,沿Y方向侧向扩散的最大距离分别为10、25 cm和30 cm。互相关函数显示随着下渗水量增大,水流扩散作用也在加强,但过多水量并没有明显增加下渗深度和扩散距离。吸力大于300 hpa时,0.01—0.05 mm土壤粒径含量和非饱和导水率呈负相关关系;吸力和非饱和导水率采用指数函数拟合效果较好(r~20.9),对拟合参数a、b和土壤容重(x)进行回归分析:a=0.0015x~2-0.00499x+0.0004,b=0.0583x~2+0.1234x-0.072。同一吸力下土壤容重大的土样非饱和导水率较小;吸力值为300 hpa是非饱和导水率的转折点;非饱和导水率和土壤容重呈现负相关关系,和总孔隙度呈现正相关关系,且二者的相关性随吸力的增加而降低。     

The 'hydrology' of leaves: co-ordination of structure and function in temperate woody species

The hydraulic conductance of the leaf lamina (K_lamina) substantially constrains whole‐plant water transport, but little is known of its association with leaf structure and function. K_lamina was measured for sun and shade leaves of six woody temperate species growing in moist soil, and tested for correlation with the prevailing leaf irradiance, and with 22 other leaf traits. K_lamina varied from 7.40 × 10^?5 kg m^?2 s^?1 MPa^?1 for Acer saccharum shade leaves to 2.89 × 10^?4 kg m^?2 s^?1 MPa^?1 for Vitis labrusca sun leaves. Tree sun leaves had 15–67% higher K_lamina than shade leaves. K_lamina was co‐ordinated with traits associated with high water flux, including leaf irradiance, petiole hydraulic conductance, guard cell length, and stomatal pore area per lamina area. K_lamina was also co‐ordinated with lamina thickness, water storage capacitance, 1/mesophyll water transfer resistance, and, in five of the six species, with lamina perimeter/area. However, for the six species, K_lamina was independent of inter‐related leaf traits including leaf dry mass per area, density, modulus of elasticity, osmotic potential, and cuticular conductance. K_lamina was thus co‐ordinated with structural and functional traits relating to liquid‐phase water transport and to maximum rates of gas exchange, but independent of other traits relating to drought tolerance and to aspects of carbon economy.     

Real time monitoring of biofilm development under flow conditions in porous media

Biofilm growth can impact the effectiveness of industrial processes that involve porous media. To better understand and characterize how biofilms develop and affect hydraulic properties in porous media, both spatial and temporal development of biofilms under flow conditions was investigated in a translucent porous medium by using Pseudomonas fluorescens HK44, a bacterial strain genetically engineered to luminesce in the presence of an induction agent. Real-time visualization of luminescent biofilm growth patterns under constant pressure conditions was captured using a CCD camera. Images obtained over 8 days revealed that variations in bioluminescence intensity could be correlated to biofilm cell density and hydraulic conductivity. These results were used to develop a real-time imaging method to study the dynamic behavior of biofilm evolution in a porous medium, thereby providing a new tool to investigate the impact of biological fouling in porous media under flow conditions.     

The role of soil hydraulic conductivity on the spatial and temporal variation of root water uptake in drip-irrigated corn

A multiplexed TDR system and a heat-pulse system for stem sap flow measurements were used to determine the spatial and temporal pattern of root water uptake in field-grown corn. The TDR probes, 0.15 and 0.30 m in length, were buried vertically in the soil profile to a depth of 0.95 m below the soil surface and heat-pulse sensors were installed on the plant base. Nocturnal readings from TDR probes were used successfully to differentiate the two components of moisture change: root uptake and net drainage. The instantaneous rate of water extraction by the plant measured by the heat-pulse system agreed well with the integrated rate of root water uptake measured frequently (at half-hour or hourly intervals) by the TDR probes. This agreement enabled further exploration into the cause of the evolution of the spatial and temporal patterns of root water uptake during a drying cycle. The results indicated that right after irrigation in the well-watered soil profile, it is the spatial distribution of the roots that mainly determines the typical pattern of root extraction, in addition to the fact that the roots near the plant base are more effective than those farther away. The higher density and effectiveness of the roots near the plant base dry the soil rapidly so that soil hydraulic conductivity soon becomes a limiting factor for water uptake. Further analysis revealed that a decrease in root uptake occurs near the plant base under a given atmospheric demand when the relative bulk soil hydraulic conductivity decreases to 0.002K _r. This suggests that low conductivity (high resistance) in the soil near the plant base is the initial cause for downward and lateral shifting of the root uptake pattern. Note that this critical value of hydraulic conductivity is not universal since it depends on the soil type and atmospheric water demand during the period under observation. Therefore, prior to the application of moisture content or suction head as measures of water availability or to control irrigation scheduling, it is suggested that these parameters be calibrated by the soil K() or K() curves, respectively, for the expected atmospheric water demand for the specific crop and growing period.     

Root functioning in tropical high-elevation forests: Environmental vs. biological control of root water absorption

Lowered temperatures may reduce the root water uptake of tropical trees at high elevations through several mechanisms; however, field studies to test their relevance are lacking. We measured sap flux density (J) in small-diameter tree roots across a 2000-m elevation transect in a tropical mountain forest for quantifying the effects of temperature (T_a), VPD and soil moisture (θ) on root water flow and uptake at different elevations. Recently developed miniature heat balance-sap flow gauges were applied to roots of about 10 mm in diameter in mountain forest stands at 1050, 1890 and 3060 m a.s.l. in the Ecuadorian Andes and the measured flow was related to anatomical properties of the root xylem. Between 1050 and 3060 m, mean J decreased to about a third. VPD was the most influential environmental factor controlling J at 1050 and 1890 m, while T_a was the key determinant at 3060 m. Large vessels were absent in the root xylem of high-elevation trees which resulted in a 10-fold decrease of theoretical hydraulic conductivity (k_h^theor) between 1050 and 3060 m. We conclude that both physical limitations (reduced VPD, increased viscosity of water) and biological constraints (large decrease of k_h^theor) result in a significantly reduced J and root water uptake of the trees in high-elevation tropical forests.     

季节性干旱对白皮乌口树(Tarenna depauperata Hutchins)水分状况、叶片光谱特征和荧光参数的影响

干热河谷稀树灌丛常绿植物能够忍受长达半年以上的季节性干旱胁迫,但对这些常绿植物响应干旱胁迫的生理生态机制研究很少。本研究以干热河谷稀树灌丛优势常绿植物白皮乌口树(Tarenna depauperata Hutchins)为研究对象,分别在雨季和干季测定其叶片的水势、压力-体积曲线、气体交换参数、叶片光谱特征以及叶绿素荧光和P700的光能分配。结果显示:受严重季节性干旱胁迫的影响,与雨季相比,干季的凌晨叶片水势(Ψpd)下降至-4.5 MPa,水分传导的叶比导率(KL)下降了49.5%,叶绿素反射指数(NDVI)下降了40.6%,花青素反射指数(ARI)上升至0.074(约为雨季的12.3倍),并且雨季和干季的叶片水势、水分传导效率、叶绿素含量和花青素含量均差异显著(P0.05)。与雨季相比,干旱导致光系统Ⅱ(PSⅡ)最大光化学量子效率(Fv/Fm)显著下降至0.72(P0.05),即PSⅡ发生光抑制,而光系统Ⅰ(PSⅠ)的活性(Pm)未发生明显变化;干季叶片的最大非光化学淬灭(NPQ)增加了31%,而激发的最大环式电子传递速率(CEF)下降了66%。表明长期干旱胁迫使CEF的激发受到强烈抑制,即光能捕获效率的降低和NPQ的增强促进了白皮乌口树在长期干旱胁迫下的光保护。     

Stomatal control of xylem embolism

Abstract. The potential role of stomatal closure in the control of xylem embolism is investigated by means of a simple model of hydraulic flow in plants. Maintenance of a maximally efficient conducting system requires the stomata to close in an appropriate fashion as evaporative demand increases in order to prevent shoot water potentials falling below the threshold value at which cavitations occur. The model showed that the optimal stomatal behaviour required depends on soil water availability. Further analysis of the model demonstrated that there could be certain circumstances where loss of a proportion of the conducting tissue by embolisms can, perhaps surprisingly, be beneficial in terms of maximizing stomatal aperture and hence short-term productivity. The results are discussed in relation to the signals controlling stomatal aperture, and it is shown that (1) optimal control cannot be obtained using information on leaf water potential alone, and (2) information relating to soil water potential is a necessary requirement for optimal control.