An Integrated View of Whole-Tree Hydraulic Architecture. Does Stomatal or Hydraulic Conductance Determine Whole Tree Transpiration? - Fig 8

<p>(A) Diameter, (B) density and (C) total lumen area of xylem vessels of taproot and basal stem in cross sections of <i>Poncirus trifoliata</i> (PT) and Cleopatra mandarin (CM) seedlings. Histological data correspond to the mean of six independent plants (n = 6) of each rootstock. The value for each plant is the mean of three visual fields of three sections from three samples per root and stem. Different letters indicate statistically significant differences (P <0.05) (LSD test).</p

Salinity and water deficit

Drought and salinity are among the environmental factors that constrain citrus productivity most dramatically. The citrus crop is predominantly cultivated in arid and semiarid regions under irrigation conditions, giving rise to problems frequently associated with salt stress rather than water deficit. Salt ions contribute significantly to osmotic adjustment under salt stress in citrus, so that leaf ion toxicity becomes the main problem to be solved. Citrus has some capacity to exclude sodium, but chloride, a plant nutrient with specific symplastic transport mechanisms, is less efficiently excluded. Therefore, resistance to salt stress is mainly associated with the rootstock ability to exclude chloride. Physiological and molecular approaches, recently supported on omics technologies, have identified stress resistance mechanisms in different citrus genotypes, including induction of earlier and stronger stress responses; stress avoidance mechanisms consisting of efficient ion exclusion from shoot organs and tight control of transpiration; rapid inhibition of photosynthesis and primary metabolism; acquisition of cell tolerance including efficient osmotic adjustment, adequate control of oxidative damage, enhancement of secondary metabolism, and biosynthesis of protective molecules. In representative stress-resistant rootstocks commonly used today, these responses lead to rapid growth inhibition. The identification of new rootstock genotypes that effectively combine stress avoidance and tolerance mechanisms to optimize both plant production under adverse environmental conditions and efficient recovery after stress represents an important achievement of current and future breeding programs