Agriculture Reference
In-Depth Information
Breeding for other less obvious traits, such as abscisic acid (ABA)
sensitivity and high rate of osmotic adjustment have been implicated for
yield improvement in water-stressed crops. Several studies have shown
that improved water use effi ciency can be achieved through ABA hor-
mone control [43-45]. ABA is a hormone involved in a plant's response to
stress conditions, including drought. As water availability in the root zone
declines, ABA is synthesized by root cells and translocated to the shoot,
leading to changes in solute concentration of guard cell cytoplasm via
regulating ion channel openings in the plasma membrane, fi nally resulting
in decreased stomatal conductance. All of which translates to guard cell
defl ation and stomata closure, therefore leading to water conservation. The
genetic control of ABA physiology involves its biosynthesis, storage, dis-
tribution (transport), cell perception (receptors), and signal transduction
pathways (secondary messengers and gene activation). The best alleles of
these genetic elements can be combined to generate genotypes that opti-
mally cope with stresses. Transpiration can be reduced due to ABA over
expression in several plant species, including important and valued crops
such as tomato, cowpea, and common bean [43,46-48]. A decrease in tran-
spiration can improve growth, water status, and turgor of crops produced
in water-limited systems, allowing for potential yield gains [43,49]. To
improve water use effi ciency, genotypes that have increased sensitivity to
or production of ABA along with improved osmotic adjustment have been
suggested as a selection trait for yield increase under drought stress. Os-
motic adjustment is a cellular adaptation that enhances dehydration toler-
ance and supports yield under water stress. Rapid osmotic adjustment has
been correlated to sustained growth and yields in water-limited systems
[42]. This occurs because osmotic adjustment helps to maintain high leaf
water content and turgor, aiding the crop in the continuation of moderate
transpiration and photosynthesis under reduced leaf water potential, al-
lowing for cell stability and avoiding yield losses. Thus, by selectively
breeding for traits, whether physiological or molecular, that are advanta-
geous during drought conditions it is possible to increase yields harvested
from rain-fed or low-water input systems.
Molecular analyses using crop and model plant species are elucidating
how plants respond to drought stress and recovery by using genome-wide
expression regulation approach [50]. These studies may lead to identifi cation of
