S. B. Kikuta

Graphical evaluation and partitioning of turgor responses to drought in leaves of durum wheat

The relationship between relative water content (R) and turgor potential (Ψp) may be derived from pressure-volume (PV) curves and analyzed in various ways. Fifty PV curves were measured with the pressure chamber on leaves of durum wheat (Triticum durum L.). The plots of Ψp versus R were highly variable and could not be adequately described by a single mathematical function. The area below the curve was therefore determined by means of an area meter. This procedure gave the integral of turgor from full saturation to the turgor-loss point. Responses to drought treatment could thus be quantified and partitioned into effects of osmotic adjustment and elastic adjustment. These two adjustment responses, which are probably of different metabolic origin, together improve turgor maintenance in durum wheat considerably.

Do water‐limiting conditions predispose Norway spruce to bark beetle attack?

Drought is considered to enhance susceptibility of Norway spruce (Picea abies) to infestations by the Eurasian spruce bark beetle (Ips typographus, Coleoptera: Curculionidae), although empirical evidence is scarce. We studied the impact of experimentally induced drought on tree water status and constitutive resin flow, and how physiological stress affects host acceptance and resistance. We established rain-out shelters to induce both severe (two full-cover plots) and moderate (two semi-cover plots) drought stress. In total, 18 sample trees, which were divided equally between the above treatment plots and two control plots, were investigated. Infestation was controlled experimentally using a novel ‘attack box’ method. Treatments influenced the ratios of successful and defended attacks, but predisposition of trees to infestation appeared to be mainly driven by variations in stress status of the individual trees over time. With increasingly negative twig water potentials and decreasing resin exudation, the defence capability of the spruce trees decreased. We provide empirical evidence that water-limiting conditions impair Norway spruce resistance to bark beetle attack. Yet, at the same time our data point to reduced host acceptance byI. typographus with more extreme drought stress, indicated by strongly negative pre-dawn twig water potentials.

Osmotic potential of Norway spruce [Picea abies (L.) Karst.] secondary phloem in relation to anatomy

Abstract. Variations in water status of secondary phloem of Picea abies (L.) Karst., caused by (1) radial and (2) vertical differences within the tree, (3) seasonal influences, and (4) tree class, and their relation to bark anatomy were investigated. The water status parameters measured were the osmotic potential at full saturation [Ψo (sat)], the in situ osmotic potential [Ψo (in situ)], the in situ water content (Cin situ), and the in situ relative water content (Rin situ). Ψo (sat) reached most negative values in the conducting part of the secondary phloem, whereas Ψo (in situ) was similar in conducting (PC) and non-conducting secondary phloem (PN). The remarkable discontinuity in the radial course of Cin situ and Ψo (sat) at the transition from PC to PN can be attributed to the degeneration of sieve cells and Strasburger cells. In PC, the vertical decrease of Ψo (sat) towards the crown was compensated by an increase in Rin situ, so that Ψo (in situ) did not change along the stem. With stem height, Ψo (sat) decreased and PC width increased. The determining factor for vertical gradients in Ψo (sat) was the distance to the sources; similar gradients were also measured in PN. Seasonal differences in Ψo (sat) could only be detected in PC, where they corresponded exactly to changes of Ψo (sat) in needles. Suppressed trees showed less negative Ψo (sat) values in PC, smaller annual secondary phloem increments and smaller radial lumen diameter of living sieve cells than predominant or dominant trees.

Ecophysiology of Long-Distance Water Transport in Trees

We give a short overview over some basics of tree water relations and the likely impact of future climate changes on the functionality of the soil-plant-atmosphere continuum. We start with a short account of the biophysics of water transport and explain some methods for determining relevant parameters in the field and in the laboratory. Important results are described next: the variable values of total water potential in crowns, the use of pressure-volume curves established in the lab for a detailed analysis of leaf water relations in the field, parameters indicating water stress, and the stability of water columns in the xylem. A look at the water relations of small plants shows that the numerical values of key parameters in seedlings or herbs are not much different from those in tall trees; this enigma may have to do with evolutionary selection for a cautious use of soil water reserves. There are still a number of weak spots in our understanding of water transport in trees. We have only a very general knowledge of root distribution over the soil layers and of fine root turnover. Also, cavitation in the different size classes of roots has not been investigated sufficiently. The most important unknown however is probably the distribution of resistances over the whole length of the xylem, which depends on xylem structure at different points in the plant. Finally we speculate about possible effects of future climate changes. There are a number of relevant climate factors which can influence plant water relations in both positive and negative ways; they are described in some detail. Changes of these factors will not be uniform, not even on small scales, let alone over continents or the whole globe. It depends on their combinations whether tree water stress will increase or decrease at a given site. It seems however safe to conclude that the existence of trees on our planet is far from being jeopardized: trees have existed uninterruptedly since the late Devonian, including long periods with climates far more extreme than the ones predicted for the next 100 years. This positive outlook does however not pertain to the fate of single tree species, which may well succumb to increased competition by better adapted ones.

Xylem embolization by the pressure collar in Salix viminalis L.: sites for embolization and ultrasound acoustic emission

ABSTRACT Pressurized air (3.5 MPa) produced massive embolization and loss of hydraulic conductivity in detached willow twigs (Salix viminalis L.) enclosed in a pressure collar. A state of tension in the xylem water column was not a necessary precondition for this embolization. Ultrasound acoustic emissions on the distal side and close to the collar were recorded only when air-saturated water was leaving the collar, whether moved by transpiration or by an overpressure applied to a reservoir at the base of the twig. Water released from embolized xylem elements increased the water potential of the leaves. The results show that air entry into xylem elements inside the pressure collar is the main mechanism responsible for the loss of hydraulic conductivity with the pressure collar technique, and that ultrasound signals originating within the collar are not sensed on the outside.