Reactive oxygen species (ROS) in the apoplast of cells in the growing zone of grass leaves are required for elongation growth. This work evaluates whether salinity-induced reductions in leaf elongation are related to altered ROS production. Studies were performed in actively growing segments (SEZ) obtained from leaf three of 14-d-old maize (Zea mays L.) seedlings gradually salinized to 150 mM NaCl. Salinity reduced elongation rates and the length of the leaf growth zone. When SEZ obtained from the elongation zone of salinized plants (SEZs) were incubated in 100 mM NaCl, the concentration where growth inhibition was approximately 50%, O2*- production, measured as NBT formazan staining, was lower in these than in similar segments obtained from control plants. The NaCl effect was salt-specific, and not osmotic, as incubation in 200 mM sorbitol did not reduce formazan staining intensity. SEZs elongation rates were higher in 200 mM sorbitol than in 100 mM NaCl, but the difference could be cancelled by scavenging or inhibiting O2*- production with 10 mM MgCl2 or 200 microM diphenylene iodonium, respectively. The actual ROS believed to stimulate growth is *OH, a product of O2*- metabolism in the apoplast. SEZ(s) elongation in 100 mM NaCl was stimulated by a *OH-generating medium. Fusicoccin, an ATPase stimulant, and acetate buffer pH 4, could also enhance elongation in these segments, although both failed to increase ROS activity. These results show that decreased ROS production contributes to the salinity-associated reduction in grass leaf elongation, acting through a mechanism not associated with pH changes.

The lack of association between apoplastic O(2)(*-) levels and root growth inhibition under hyper-osmotic stress leads us to hypothesize that under those conditions the role of apoplastic O(2)(*-) may be to participate in signalling processes, that convey information on the nature of the substrate that the growing root is exploring.

The effects of water stress on the infection and systemic movement of tomato spotted wilt virus (TSWV) in vegetative tomato plants are examined. Two groups of plants: water stressed (water potential −1.31 MPa) and well‐watered ones (water potential −0.65 MPa), were mechanically inoculated on the third expanded leaf from the top. Each group was then divided into a well‐watered and a water stressed one. TSWV inoculation was effective both on water‐stressed and well‐watered plants. In all groups, infection was first detected in roots, then in shoot apices and subsequently in the rest of the leaves. Systemic virus infection was somewhat slower in the water stressed plants. Post inoculation stress treatments had a significant effect in reducing infection as well as in attenuating TSWV symptoms. Results from the present work suggest that water stress may be effective in controlling virus concentration and systemic infection symptoms in tomato plants and could thus be used as a tool to study the regulation of development of TSWV infection in this system.

Sunflower (Helianthus annuus L.) has been rated as moderately salt-resistant, and variability for salt resistance has been detected within this crop. However, variability in salt-resistance mechanisms has not been assessed. Osmotic tolerance, the relation of salt resistance with whole-plant Na + and K + distribution and tissue Na + tolerance were investigated in several sunflower inbred lines. Plants were grown under controlled conditions, in pots with sand and perlite irrigated with salinized (NaCl, -0.65 MPa) nutrient solution. Osmotic tolerance was assessed from the initial effects of the salt treatment on plant elongation in eleven sunflower lines. Longterm salinity responses were evaluated in four of those lines, by assessing whole-plant growth. A principal components analysis (PCA) was run on relative-to-control growth data, and this information was used to establish a relative resistance ranking, which indicated lines HAR2 > HAR1 > HA64 > HAR5. Osmotic tolerance was observed in HA64 and HAR2. The lines showed different degrees of Na + accumulation, it was very low in some of them, but relative salt resistance was not associated to this trait. Tissue Na + tolerance was deduced by comparing the percentage of dead leaves as a function of leaf blade Na + accumulation, and it was higher in HAR1 than in the rest. These results indicate that variability for salt-resistance mechanisms exists in sunflower. Osmotic tolerance and tissue Na + tolerance were detected in different lines, highlighting that such variability may be exploited for increasing salt resistance in this crop.

Branched nodal roots comprise the largest portion of the root system mass in Chloris gayana Kunth and the effects of high salinity on nodal root appearance and elongation rates were analysed in cv. Boma in greenhouse experiments. Roots from salt‐treated plants (0·2 mol l–1 NaCl) were smaller than controls, and accumulated higher concentrations of soluble sugars and reserve lipids. The number of nodal roots was reduced by the saline treatment. Leaf, tiller and nodal root appearance were delayed by salinity but the correlation among these processes was maintained, indicating that the developmental pattern was not altered by this level of salinity, only its rate. Initial nodal root growth rates varied as a function of plant size and were decreased by salinity only after 2 weeks of treatment. When shoots of non‐salinized plants were cut, a drastic reduction in nodal root appearance was observed, suggesting emerging leaves were stronger sinks than roots for available reserves. However, when the shoots of salt‐treated plants were cut, the already depressed root appearance rate was not further reduced. This suggests that, under salinity, alterations in root ability to metabolize reserves could have been more significant than reserve availability for controlling elongation.