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in the leaf area (Gómez Del Campo et al ., 2003), xylem vessel size, and/or conductivity (Lovisolo
& Schubert, 1998).
Stomata enable a control of water regime in plants because they balance and stabilise values
of water potential existing between their leaves and the atmosphere. Stomatal closure is one
of the first responses to soil drying, and a parallel decline in photosynthesis and stomatal
conductance under progressive water stress has already been reported (Medrano et al ., 1997).
Within the framework of stomatal activities there are relationships among metabolism of
abscisic acid (ABA), hydraulic signals, regulation of activities of aquaporins and electric signals
that are manifested when measuring the water potential of leaves (Lovisolo et al., 2010). This
means that the reaction of stomata is mediated by ABA, which is produced within the
framework of a response to the stress induced by drought in roots; this newly synthesised ABA
is then transported into other parts of the plant (Loveys et al., 1984).
Plants respond to the lack of water by a quick closing of stomatal opening so that a further loss
of water via transpiration is prevented. This mechanism represents a very efficient protection
of plants against drought-induced stress.
A lack of water in soil and a leaf water deficit result also in a gradual reduction of photosyn‐
thesis and changes in assimilation of carbon and nitrogen (Chavaria & Pessoa Dos Santos,
2012, Zlatev &Cebola Lidon, 2012). Drought-induced decrease in photosynthesis is primarily
due to a stomatal closure, which lowers CO 2 availability in the mesophyll, not due to a direct
effect on the capacity of the photosynthetic apparatus (Escalona et al ., 1999). Osmotic stress is
a common feature of many abiotic stress factors, that affect grapevines (Gramer, 2010). Some
biochemical characteristics, e.g. the stability of chlorophyll, can be used for selection of
cultivars resistant to drought conditions (Sinbha & Patil, 1986, Pavloušek, 2011b).
Water-use efficiency (Wue) can be considered for the most important indicator of water
management of plants (and also in grapevine). The Wue can be defined as a balance existing
between the biomass gain (expressed in kilograms of produced biomass or in mols of assimi‐
lated CO 2 ) and losses of water (expressed as cubic meters of consumed water or mols of
transpired water). From the agronomic point of view the Wue can be defined as the volume
of yield produced per unit of consumed water (Tomás, et al., 2012). Quality of grapes is very
markedly dependent on the amount of water consumed by plants and for that reason an
improvement in efficiency of water use represents the major requirement concerning crop
sustainability and quality of grapes (Medrano et al., 2012). New aspects of Wue and actual data
concerning this indicator were dealt with and studied in many recent studies (Flexas et al.,
2010, Schultz & Stoll, 2010, Lovisolo et al., 2010, Tomás et al., 2012, Medrano et al., 2012). The
Wue is a key parameter that enables to evaluate the efficiency of water use within the agrarian
sector. It is dependent on the total amount of water consumed by plants in the course of the
growing season. This sum represents the amount of water used by plants plus water losses
caused by transpiration (Flexas et al., 2010).
For that reason it can be expected that there is a relationship between Wue on the one hand
and genetic foundations (i.e. genomes) of cultivars or rootstocks on the other. Basing on the
knowledge of Wue of individual species, rootstocks or cultivars it could be therefore possible
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