The short-term growth response to salt of the developing barley leaf.

Recent results concerning the short-term growth response to salinity of the developing barley leaf are reviewed. Plants were grown hydroponically and the growth response of leaf 3 was studied between 10 min and 5 d following addition of 100 mM NaCl to the root medium. The aim of the experiments was to relate changes in variables that are likely to affect cell elongation to changes in leaf growth. Changes in hormone content (ABA, cytokinins), water and solute relationships (osmolality, turgor, water potential, solute concentrations), gene expression (water channel), cuticle deposition, membrane potential, and transpiration were followed, while leaf elongation velocity was monitored. Leaf elongation decreased close to zero within seconds following addition of NaCl. Between 20 and 30 min after exposure to salt, elongation velocity recovered rather abruptly, to about 46% of the pre-stress level, and remained at the reduced rate for the following 5 d, when it reached about 70% of the level in non-stressed plants. Biophysical and physiological analyses led to three major conclusions. (i) The immediate reduction and sudden recovery in elongation velocity is due to changes in the water potential gradient between leaf xylem and peripheral elongating cells. Changes in transpiration, ABA and cytokinin content, water channel expression, and plasma membrane potential are involved in this response. (ii) Significant solute accumulation, which aids growth recovery, is detectable from 1 h onwards; growing and non-growing leaf regions and mesophyll and epidermis differ in their solute response. (iii) Cuticular wax density is not affected by short-term exposure to salt; transpirational changes are due to stomatal control.

Cytokinin flows from Hordeum vulgare to the hemiparasite Rhinanthus minor and the influence of infection on host and parasite cytokinins relations.

Using the facultative root hemiparasite Rhinanthus minor L. and Hordeum vulgare L. as a host, the flows, depositions and metabolism of zeatin-type cytokinins [zeatin (Z), zeatin riboside (ZR), zeatin nucleotide (ZN)] within the host, the parasite and between host and parasite have been studied during the period 41-54 d after planting (i.e. ~30-43 d after successful attachment of the parasite to the host). Parasitism decreased the synthesis of Z in the root (by 57%) and decreased xylem flows (by 56%) and metabolism (by 71%) in leaf laminae. However, phloem flows of Z were increased by 3-fold in the host barley. The deposition of Z in the roots of Rhinanthus and the flows in xylem and phloem were increased by 20, 12, 29-fold, respectively, after successfully attaching to the host barley. However, net biosynthesis of Z in Rhinanthus roots decreased by 35% after attachment. This indicates that a large portion (70%) of xylem flow of Z in attached Rhinanthus was extracted from the host. In singly growing Rhinanthus plants, the balance of Z deposition in the shoot was negative (i.e. Z was metabolised and exported back to root in the phloem). Xylem flows and deposition of ZR and ZN showed comparable quantitative changes after attachment. A significant deposition of Z, ZR and ZN was detected in the haustoria of the Rhinanthus / barley association. The possible physiological functions of the large quantities of Z and ZR and ZN derived from the host barley, for the improved leaf development and the stomatal reactions of the parasitising Rhinanthus are discussed.

Rapid and tissue-specific changes in ABA and in growth rate in response to salinity in barley leaves.

The addition of 100 mM NaCl to the root medium of barley plants caused the rapid cessation of elongation of the growing leaf three, followed by a sudden resumption of growth during the following hour. The idea that resumption of growth is preceded and mediated by rapid and tissue-specific changes in ABA concentration and by changes in transpiration was tested. Leaf elongation velocity was recorded continuously using linear variable displacement transducers (LVDT), ABA was determined by immunoassay, and transpiration and stomatal conductivity were measured gravimetrically and by porometry, respectively. Within 10 min following addition of salt, ABA increased 6-fold in the distal portion of the leaf elongation zone; in the proximal portion, ABA accumulated with a delay. In the portion of the growing blade that had emerged ABA increased 3-fold and remained elevated during the following 20 min. This preceded a decrease in transpiration and stomatal conductivity, which, in turn, coincided with growth resumption. Twenty hours following the addition of salt, the ABA concentrations had returned to the level before stress. Leaf elongation velocity was still reduced. It is concluded that NaCl causes a rapid increase in ABA in the transpiring portion of the growing leaf. This leads to a decrease in transpiration. As a result, xylem water potential is expected to rise. The moment that the water potential gradient between the xylem and the peripheral cells in the growth zone favours water uptake again into the latter, leaf elongation resumes. The results suggest that ABA causes different responses in different leaf regions, all aimed at promoting the resumption of leaf growth.