AUDIO DETECTION OF CAVITATION

According to my notebooks we first detected acoustic signals from plants on 6th February 1963. The sounds were produced on a record-player amplification system and filled our converted-museum laboratory in Aberdeen university. The elation produced was partially on account of the champagne-cork sounds which flooded the laboratory, but also from the fact that earlier work in previous months, based on the removal of embolisation, had predicted they might be found. Sadly, validation of such predictions is rare and not often so easily realised in science! The main avenue of this research was the uptake of water by seriously waterstressed leaves.

INTRODUCTION

In previous experimental work the variation of evapotranspiration (ET), stomatal conductance (gs) and predawn leaf water potential (Ψ) were studied in an irrigated tomato crop in Portugal; the midday stomatal conductance decreased when a Ψ of -0.4 MPa was attained (Katerji, Itier & Ferreira, 1988). Above this limit there was considerable scatter and no clear relationship between these variables existed. In spite of that, ET decreased as soon as one day after irrigation (Itier, Ferreira & Katerji, 1988) and this was associated with a decrease in gs (Ferreira, 1987). What is the reason for stomatal closure so soon after irrigation? Two hypotheses were considered: 1) local advection reduces the air humidity conditions close to the leaves inducing stomatal closure and 2) the roots send a signal to the shoot (as described by Davies & Mansfield, 1988) and induce stomatal closure.

MATERIALS AND METHODS

Measurements were made in the field in Coruche, Portugal (lat 38°57′, long 8°32′, alt 30m). Tomato plants (Lycopersicon esculentum Mill.) were planted in a loamy soil in rows for furrow irrigation. Four plots (A, B, C and D) were used, each with a 50 m fetch in the center (Fig. 1).

INTRODUCTION

A number of papers have been published concerning the theoretical estimates of the efficiency of symplast as the water pathway in plants. The authors assume that the symplast is the most important pathway for the intercellular transport of water and other substances (Arisz, 1956; Tyree, 1970; Newman, 1976). This assumption permits us to account for the dependence of tissue water regime on inhibitors and activators of metabolism better than other explanations. Hitherto, there have been no direct measurements of symplastic water transport due to the absence of any method that differentiates symplast transfer against a background of apoplast and transmembrane transport. The development of nuclear magnetic resonance (NMR) technique allowed us for the first time to distinguish the symplast component from the total intercellular transfer (Anisimov et ai, 1983).

The principle of measuring water diffusion just through the symplast is based on the relaxation suppression of the apoplast water NMR signal by doping the intercellular space with paramagnetic ions of high relaxation efficiency, such as manganese and gadolinium.

INTRODUCTION

Numerous structural features combine to determine the overall drought resistance of a plant species, including the architecture of the water conducting system (Baas, 1982; Carlquist, 1977; Ewers, 1985; Lo Gullo, Salleo & Rosso, 1986; Salleo & Lo Gullo, 1986, 1989a, 1990; Zimmermann, 1978, 1983).

INTRODUCTION

The laurel forest of the Macaronesian archipelagos (Canary Islands, Madeira, Azores) is restricted to moist and humid mountain slopes where humidity brought by the trade winds condenses to form clouds (Ceballos & Ortuno, 1976). The tree species of these forests are obviously not competitive at low humidity. This inability to withstand arid conditions may arise from (i) an insufficient xylem capacity for water transport to the crowns, (ii) an ineffective stomatal regulation of transpiration, (iii) a low drought tolerance of the leaves. These functional attributes of the laurel forest tree species were investigated comparatively and compared with pertinent data of Mediterranean species.

MATERIALS AND METHODS

The species are named in the legend of Fig.l. Xylem anatomy and drought tolerance of the leaves were studied with plant material collected in the Anaga Mountains of Tenerife, Canary Islands (28.5° N, 15.9° W). Transpirational water loss was assessed with leaves from greenhousegrown plants. No differences in structure and function could be recognized between field-collected and cultivated plants. The theoretical water supply capacity of the twigs was calculated according to the Hagen- Poiseuille formula (Huber, 1956) using data from microscopic determinations of vessel numbers and diameters.

INTRODUCTION

For technical reasons, it is very difficult to measure negative pressures in the xylem of transpiring plants directly. An artificial osmotic cell has been constructed using reverse osmosis membranes to study phenomena of negative pressure (Steudle & Heydt, 1988; Zhu, Steudle & Beck, 1989). The cell has been also used as an analytical device (osmotic sensor; Steudle & Stumpf, 1989) to measure the concentration of certain solutes in solutions. The sensivity and selectivity of the osmometer could be increased by coupling the osmotic process to a chemical reaction.

MATERIAL AND METHODS

Reverse osmosis membranes were used to separate the cell interior from the medium. ‘Cell turgor’ was measured with the aid of a pressure transducer and was recorded continuously. The system was calibrated in the range of positive and negative pressures by applying gas pressures to both sides of the transducer membrane. During the experiments, the calibration and the proper function of the transducer could be checked (Steudle & Heydt, 1988). The thickness of the osmotic cell was 0.1 to 0.2 mm and the diameter 4 mm. The cell was filled with a non-permeating solute to build up a positive turgor in the presence of hypotonic solutions. In order to create negative pressures (tensions), the external medium was replaced by hypertonic solutions of non-permeating solutes.

RESULTS AND DISCUSSION

When permeating solutes were added to the medium, biphasic changes of turgor were observed. From these pressure-time curves, the hydraulic conductivity (Lp), the permeability (Ps), and the reflection (σs) coefficients of the membrane were evaluated.

INTRODUCTION

The water budget of trees is linked to the water potential gradient between soil and atmosphere. Transpiration of a tree crown depends on stomatal conductivity, radiation, air temperature, water vapour saturation deficit of the air and wind speed. Variations are influenced by the soil water potential, the conducting tissues, the root system and the amount of leaves. The base of the trunk is the optimal place to measure transpiration of a whole tree assuming that storage is negligible.

The intention of this research work was to complement a programme of gas exchange measurements by heat pulse velocity measurements. The spatial and temporal variation of heat pulse velocity was investigated in a single tree. The correlation between heat pulse velocity and weather conditions during the vegetation period was analysed.

MATERIALS AND METHODS

The determinations were carried out on a single beech tree growing in a beech stand (Luzulo-Fagetum) which has been intensively investigated during the last 20 years (Bl research area of the IBP, International Biological Programme, “Solling-Projekt”, Germany; Ellenberg, Mayer & Schauermann, 1986).

INTRODUCTION

Although there has been increasing awareness in the past decade of the potential significance of xylem embolisms for plant water relations, there is still an incomplete understanding of the importance of cavitations and consequent xylem embolism as a major factor in plant adaptation to dry environments. In this chapter we attempt to provide a theoretical basis for consideration of the significance of xylem embolisms and the different ways by which different plant species survive and grow in drought conditions. It might be expected that any loss of functional xylem as a result of xylem embolism would be disadvantageous. It is possible, however, that adaptation to dry conditions may involve either mechanisms that minimise the occurrence of cavitations, or mechanisms that minimise their detrimental consequences. In general it seems likely that more ‘conservative’ behaviours that minimise cavitations (for example by reducing water loss through stomatal closure) will also reduce the chances of high productivity. Circumstances where some controlled cavitation may be advantageous in terms of productivity are identified and appropriate stomatal control functions are outlined.

INTRODUCTION

The basic discoveries of nuclear induction made in 1946 laid down the fundamentals of the nuclear magnetic resonance (NMR technique, which has had an explosive development, with application in all branches of physics, chemistry and biology. The sophisticated 13C and 31P NMR spectroscopy has been successfully applied to studies of metabolism and transport processes in plants (Loughman & Ratcliffe, 1984; Roberts, 1987).

Since our interest here is in water transport in plants we shall consider those NMR techniques which can measure resonance and relaxation of hydrogen nuclei, i.e. protons, in water molecules, and will also show that the proton NMR signal of water in plants is dependent on the state of water in the tissue and on the motion of the fluid within the NMR probe.

INTRODUCTION

The ratio of actual (Tr) to potential (PTr) transpiration is less than unity because the leaf epidermis offers a resistance to water vapour flow. In a “Marsh” grapefruit {Citrus paradisiMacf.) orchard, the ratio was found to be relatively small (0.3) when soil water availability was not limited but it dropped to 0.2 when the soil dried out to about -80 kPa in the main root zone (Cohen, 1991). In the computation of potential transpiration, using a meteorological model, the resistance of the canopy to vapour diffusion is considered zero and this explains the above ratio of measured to potential transpiration. Numerous field and laboratory measurements of stomata have shown the relationship between transpiration and stomatal conductance (Hall & Schulze, 1982). On the other hand, several micrometeorological models have been used successfully to predict evapotranspiration for non-water-stressed vegetation without taking into account the stomatal characteristics (Kanemasu, Stone & Powers, 1976; Ritchie & Jordan, 1972).

INTRODUCTION

Pressure-volume (PV) analysis (Tyree & Hammel, 1972) enables the determination of several plant water relations parameters. The measured values are “bulk tissue averages”, which characterize the plant tissues better when they are uniform {i.e. only leaves). In plant species with small leaves, for example conifers, PV analysis can only be performed on shoots which consist of leaves, wood and bark. The resultant water relation parameters then represent a complex tissue; this can be disadvantageous in studying the physiology of leaves or other plant parts.

Through the combined use of PV analysis of spruce shoots and capillary microcryoscopy of sap pressed from needles, determinations of the most important water relations parameters, needle apoplastic water content and bulk osmotic pressure in the needle symplast, were possible.

MATERIALS AND METHODS

Pressure-volume analysis

In late summer 1987, water potential isotherms from different sized shoots of a 25-year-old, 16 meter tall Norway Spruce {Picea abies [L.] Karst.) were generated (Gross & Koch, 1991a). The sample shoots were kept in a temperature regulated pressure chamber during the entire experiment (Gross & Pham-Nguyen, 1987) and were dehydrated through stepwise increases in pressure. From the water potential isotherms, the bulk osmotic pressure at full turgor (π0), and the bulk osmotic pressure when turgor initially reaches zero (πp), the amount of symplastic (Wo) and apoplastic (Wa) water was determined.

INTRODUCTION

Forests are naturally regenerated either via sexual reproduction or by vegetative propagation. In the first case, the production of seeds, their germination and the establishment of seedlings can be encouraged by several silvicultural techniques.

On the other hand, regeneration of many species can occur by growth and development of the meristematic centres located at the stump or root levels. The sprouts and suckers thereby produced give rise to a forest stand called coppice; sometimes, within coppice stands some trees are left uncut for a maximum of three to four consequent harvests, to provide some regeneration from seeds, in order to replace the declining stumps. These trees left for sexual reproduction are called standards. Coppice and coppice-with-standards are old silvicultural systems, widely used for firewood and timber production in Southern Europe. They have received renewed attention in recent years because they represent a valid option for new silvicultural goals such as fast growing biomass plantations (Ferm & Kauppi, 1990) and tropical agroforestry systems (Stewart, 1980).

INTRODUCTION

It has long been known that the water content of the woody tissues of trees undergo seasonal fluctuations, but the fact was overlooked in most discussion of the theory of water transport and little attention was given to the underlying processes. Some of the most convincing early data illustrating the phenomenon were provided by Gibbs, working on Canadian forest trees in the 1930s (Gibbs, 1958). He showed that the wood of young trees of Betula populiferaunderwent an annual cycle of water content, varying from 100 per cent of dry weight in the early spring to as little as 60 per cent in the late summer (Fig. 1). Fluctuations were most extreme, and earlier in the season, in the upper part of the stem; and the rise in the spring was immediately after the thawing of the soil.

INTRODUCTION

Xylem embolism occurs as a result of water stress and freezing of xylem sap and it has the immediate consequence of reducing the hydraulic conductivity of the xylem. Longer-term consequences could include reduced growth and dieback.