Water use, both at the level of a single leaf and the whole plant, was studied for 1- to 4-year-old almond trees (Prunus dulcis) under arid conditions in the Negev Desert (Israel). By planting the trees into lysimeters of different volumes (7, 14 and 21 m3), the amount of water available to the plants was experimentally controlled. Each year, at the beginning of the growing season, the lysimeters, which had been filled with local homogenized loess, were watered to field capacity. The trees received different relative amounts of water in relation to their leaf area on the one hand and lysimeter volume on the other, which caused different rates of soil drying throughout the season. The following hypotheses were tested. (1) The amount of CO2 assimilated per transpiration and (2) biomass production per unit of water used increases with (a) decreasing amount of soil water applied and (b) increasing leaf area, which should enhance growth in spring during periods of low evaporative demand. At the leaf level, the ratio of daily CO2 assimilation (A) to daily transpiration (E) was independent of lysimeter size, leaf area and pre-dawn water potential, but decreased with increasing leaf-to-air vapour pressure difference (D
l). Consequently, during the course of the season, A/E decreased from spring to summer in accordance with rising D
l. However, when measured at a constant D
l, the seasonal course in A/E disappeared. At the whole plant level, the ratio of total lifetime biomass production (B) to the amount of water transpired (W) increased with leaf area (i.e. demand for water), the increase being stronger with increasing water supply. We conclude that almond trees did not adapt physiologically to a limited water supply, but maximized their carbon gain for a given amount of water available by phenological processes such as high growth rate during periods of low evaporative demand of the atmosphere.

Soil drought increases leaf and whole-plant water use of Prunus dulcis grown in the Negev Desert

This study reports on how a cold- and frost-tolerant clone of Eucalyptus nitens × nitens responds to drought stress The aim was to identify physiological traits that contribute to drought acclimation Macropropagated saplings were grown in a climate-controlled greenhouse in pots filled with coarse river sand supplied with a slow-release fertiliser One group of plants was kept regularly watered (control), and another group was subjected to four cycles of water stress (drought) whereby water was withheld for periods lasting 6, 10, 10 and 14 d, with 4 d of regular watering (recovery) inbetween A drought cycle was terminated once saplings showed signs of wilting Daily responses in stomatal conductance (g s) were similar between control and drought treatments, except on the day of termination of the cycle, when g s was significantly depressed in droughted plants During the fourth and most severe drought cycle, there was a physiological adaptation to water stress because g s was similar between control and

Physiological responses of Eucalyptus nitens × nitens under experimentally imposed water stress

This study introduces a water-driven crop model aiming to quantitatively link olive yield to climate and soil moisture dynamics using an ecohydrological approach. A mathematical model describing soil moisture, evapotranspiration and assimilation dynamics of olive orchards is developed here. The model is able to explicitly reproduce two different hydroclimatic phases in Mediterranean areas: the well-watered conditions in which evapotranspiration and assimilation assume their maximum values and the real conditions where the limitations induced by soil moisture availability are taken into account. Annual olive yield is obtained by integrating the carbon assimilation during the growing season, including the effects of vegetation water stress on biomass allocation. This numerical model has been tested on an olive orchard located in Sicily (Italy) obtaining a satisfactory reproduction of historical olive yield data. This model is useful for simulating the influence of soil moisture dynamic on biomass growth, fruit productivity, also in a context of climatic change. Copyright © 2011 John Wiley & Sons, Ltd.

Olive yield as a function of soil moisture dynamics

Tamarix chinensis Lour. (Tamarisk) is regarded as a potential shrub that improves the marginal lands, and is conversely considered as an invasive species in many areas. This study conducted controlled experiments to investigate the biomass accumulation, allocation and photosynthetic responses o f T. chinensis seedlings to different salt and water conditions. The results showed that T. chinensis seedlings died when soil salt content attained 16 to 20 g kg -1 ; total biomass, root biomass, shoot biomass and leaf biomass decreased by 63.3, 56.3, 70.6 and 62.4% respectively, from 0 to 16 g kg -1 (added NaCl content), and decrease by 37.9, 25.3, 48.4 and 37.9%, respectively, from 60 to 20% field capacity (FC, soil moisture). Water deficit led to earlier peaks of net photosynthetic rate (P N ) during the day. Relative rate of electron transport (ETR) decreased, but optimal quantum yield of photosystem II (F v /F m ) showed no significant difference (P 0.05) on biomass allocation of T. chinensis seedlings, while water deficit significantly favoured root biomass accumulation. T. chinensis seedlings were able to maintain high leaf gas exchange rates under 20% FC, but water use efficiency (WUE) decreased and sufficient water supply may increase the ability of T. chinensis to survive in higher soil salinity conditions. Keywords: Salt cedar, biomass allocation, gas-exchange, fluorescence, photosystem II.

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Effects of salt and water stress on plant biomass and photosynthetic characteristics of Tamarisk (Tamarix chinensis Lour.) seedlings

Drought has been projected to increase as
global warming progresses, putting forest health in jeopardy. Water
availability already is a major factor limiting plant growth,
especially in forests and orchards on shallow, sandy soils or in
regions with low precipitation. Furthermore, salinisation of
agricultural soils is an increasing problem, often caused by the
increased water demand for irrigation in drier climates. Research
on drought and salt resistance strategies of plants has classically
focussed on the dynamics of stem and leaf water. However, it is in
the soil where water uptake occurs and plants compete for water and
nutrients. Although an increasing number of studies are addressing
belowground traits, a better understanding of the adaptability of
tree root systems to water limitation and belowground competition
is needed to be able to predict the effects of climate change on
mature forest stands and woody crops. This study on five tree
species in temperate mixed forests and Mediterranean fruit tree
orchards aims at answering important questions on the influence of
reduced soil moisture and competitive interactions on structure and
function of tree root systems. In particular, the aims of this
study were to test whether (i) root hydraulic conductivity
decreases in response to water shortage as does shoot hydraulic
conductivity, (ii) fine roots act as ‘hydraulic fuses’ of the
soil-plant-atmosphere continuum, (iii) the below-ground competitive
ability is symmetric and directly linked to root system size, and
(iv) belowground competitive ability differs between tree species
and is not modified by resource availability. This study showed
that: i) Quercus petraea (MATT.) LIEBL. generally responded to
moderate or severe drought with an increase in root axial
conductivity, while Fagus sylvatica L. mostly did not. The same
pattern was found in salt-stressed Olea europaea L. roots, the
conductivity of which increased in response to increasing salinity.
Drought- and/or salt-adapted tree species such as Quercus petraea
and Olea europea seem to be capable of partly compensating for
water shortage-induced root biomass losses by increasing root axial
conductivity, a mechanism rarely observed in aboveground organs.
ii) Fine roots can act as ‘hydraulic fuses’ in the
soil-plant-atmosphere continuum. In addition to most previous
studies, which suggest root shedding as the underlying mechanism,
this study indicates cavitation to be another, potentially
reversible, ‘hydraulic fuse’ mechanism. This conclusion is based on
the very high vulnerability of small diameter roots of both Quercus
and Fagus to cavitation and the further increase of this
vulnerability in drought-stressed Quercus roots. iii) This study
revealed two lines of evidence for the existence of asymmetry in
below-ground competition: fine root biomass of Quercus petraea was
over-proportionally reduced in species-rich allospecific stand
patches as compared to monospecific ones, and root growth rates and
morphology depended on the competitor present but not on initial
root system size differences. iv) Tree species may be ranked
according to their belowground competitive ability. According to
data on root biomass and root growth in experimentally altered
neighbour-hoods, Fagus sylvatica seems to be a superior competitor
belowground. However, a marked asymmetry in the outcome of root
competition only developed when soil moisture in summer was
sufficiently high. Resource limitation, i.e. drought, seems to
affect the competitive ability of Fagus sylvatica and Quercus
petraea roots in a similar manner with the consequence that
species-specific differences in competitive ability are less
conspicuous in more stressful environments. Therefore, the overall
importance of direct biotic interactions belowground seems to be
reduced by limited soil water availability.

Enno Brinckmann

Papers

Biomass partitioning in response to soil drought: A pot experiment with Prunus dulcis trees during four years

Effects of atmospheric and soil drought on leaf water status and stomatal response