Wall Extensibility and the Growth of Salt Stressed Leaves
An understanding of the physical restraints underlying the inhibition by environmental stresses of leaf expansion and the consequent reductions in photosynthetic area and crop yield may facilitate the selection or engineering of more resistant varieties. Physical restraints to leaf expansion may involve reductions in turgor and/or the mechanical extensibility of growing cell walls. Several recent reports suggest that reductions in wall extensibility rather than turgor may underlie growth inhibition induced by salinity and water stress. We have developed a rapid method for assaying the plastic, elastic and total extensibilities of growing leaves in vivo. Extensibility was assayed by applying and removing a 2 g weight to and from the leaf tip and assaying the resultant changes in position with a sensitive displacement transducer (LVDT). The technique was used to investigate the degree to which inhibition of primary leaf elongation in salinised maize seedlings is associated with reductions in mechanical extensibility. Segmental analysis of expanding epidermal tissues revealed that the capacity for cell elongation was inhibited throughout the basal growing zone of the leaf, but that this could be reversed within hours of transferring plants to non-saline nutrient solutions. Leaf elongation rates in plants salinised for three days were 9 µm min-1 as compared with 16 µm min-1 in control plants, a reduction of 44%. The elastic extensibility of the leaves was only slightly reduced by salinity, but plastic extensibility was reduced by 32%. This reduction occurred in the growing tissues since the plastic extensibility of mature fully expanded leaf tissues was comparatively low and unaffected by salinisation. Moreover, the reduction was not caused by hydraulic or turgor differences between the growing leaf tissues since similar reductions were measured after freeze-thaw treatment of the growing tissues to dissipate turgor and hydraulic restraints. Similar results were also obtained after short-term (20 minute) exposure to NaCl or PEG. We conclude that sustained reductions in the extensibility of expanding cell walls make an important contribution to the observed inhibition of leaf growth by salinity stress.
KeywordsMaize Seedling Physical Restraint Primary Leaf Mechanical Extensibility Barley Leave
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