Following exposure to salinity, the root/shoot ratio is increased (an important adaptive response) due to the rapid inhibition of shoot growth (which limits plant productivity) while root growth is maintained. Both processes may be regulated by changes in plant hormone concentrations. Tomato plants (Solanum lycopersicum L. cv Moneymaker) were cultivated hydroponically for 3 weeks under high salinity (100 mM NaCl) and five major plant hormones (abscisic acid, ABA; the cytokinins zeatin, Z, and zeatin-riboside, ZR; the auxin indole-3-acetic acid, IAA; and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, ACC) were determined weekly in roots, xylem sap, and leaves. Salinity reduced shoot biomass by 50–60% and photosynthetic area by 20–25% both by decreasing leaf expansion and delaying leaf appearance, while root growth was less affected, thus increasing the root/shoot ratio. ABA and ACC concentrations strongly increased in roots, xylem sap, and leaves after 1 d (ABA) and 15 d (ACC) of salinization. By contrast, cytokinins and IAA were differentially affected in roots and shoots. Salinity dramatically decreased the Z+ZR content of the plant, and induced the conversion of ZR into Z, especially in the roots, which accounted for the relative increase of cytokinins in the roots compared to the leaf. IAA concentration was also strongly decreased in the leaves while it accumulated in the roots. Decreased cytokinin content and its transport from the root to the shoot were probably induced by the basipetal transport of auxin from the shoot to the root. The auxin/cytokinin ratio in the leaves and roots may explain both the salinity-induced decrease in shoot vigour (leaf growth and leaf number) and the shift in biomass allocation to the roots, in agreement with changes in the activity of the sink-related enzyme cell wall invertase.

Salinity limits crop productivity, in part by decreasing shoot concentrations of the growth-promoting and senescence-delaying hormones cytokinins. Since constitutive cytokinin overproduction may have pleiotropic effects on plant development, two approaches assessed whether specific root-localized transgenic IPT (a key enzyme for cytokinin biosynthesis) gene expression could substantially improve tomato plant growth and yield under salinity: transient root IPT induction (HSP70::IPT) and grafting wild-type (WT) shoots onto a constitutive IPT-expressing rootstock (WT/35S::IPT). Transient root IPT induction increased root, xylem sap, and leaf bioactive cytokinin concentrations 2- to 3-fold without shoot IPT gene expression. Although IPT induction reduced root biomass (by 15%) in control (non-salinized) plants, in salinized plants (100 mM NaCl for 22 d), increased cytokinin concentrations delayed stomatal closure and leaf senescence and almost doubled shoot growth (compared with WT plants), with concomitant increases in the essential nutrient K+ (20%) and decreases in the toxic ion Na+ (by 30%) and abscisic acid (by 20–40%) concentrations in transpiring mature leaves. Similarly, WT/35S::IPT plants (scion/rootstock) grown with 75 mM NaCl for 90 d had higher fruit trans-zeatin concentrations (1.5- to 2-fold) and yielded 30% more than WT/non-transformed plants. Enhancing root cytokinin synthesis modified both shoot hormonal and ionic status, thus ameliorating salinity-induced decreases in growth and yield.

Tomato crop productivity under salinity can be improved by grafting cultivars onto salt-tolerant wild relatives, thus mediating the supply of root-derived ionic and hormonal factors that regulate leaf area and senescence.A tomato cultivar was grafted onto rootstocks from a population of recombinant inbred lines (RILs) derived from a Solanum lycopersicum ¥ Solanum cheesmaniae cross and cultivated under moderate salinity (75 mM NaCl). Concentrations of Na + , K + and several phytohormones [abscisic acid (ABA); the cytokinins (CKs) zeatin, Z; zeatin riboside, ZR; and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC)] were analysed in leaf xylem sap in graft combinations of contrasting vigour. Scion leaf area correlated with photosystem II (PSII) efficiency (Fv/Fm) and determined fruit productivity. Xylem K + (but not Na + ), K + /Na + , the active CK Z, the ratio with its storage form Z/ZR and especially the ratio between CKs and ACC (Z/ACC and Z + ZR/ACC) were positively loaded into the first principal component (PC) determining both leaf growth and PSII efficiency. In contrast, the ratio ACC/ABA was negatively correlated with leaf biomass. Although the underlying physiological mechanisms by which rootstocks mediate leaf area or chlorophyll fluorescence (and thus influence tomato salt tolerance) seem complex, a putative potassium-CK interaction involved in regulating both processes merits further attention.

The origin and transport of the IAA responsible for rooting was studied in carnation (Dianthus caryophyllus L.) cuttings obtained from secondary shoots of the mother plants. The presence of mature leaves in the cuttings was essential for rooting. Removal of the apex and/or the youngest leaves did not reduce the rooting percentage as long as mature leaves remained attached. Removal of mature leaves inhibited rooting for a 24-day period during which the basal leaves grew and reached maturity. After this period rooting progressed as in intact cuttings. Auxin (NAA + IBA) applied to the stem base of defoliated cuttings was about 60% as effective as mature leaves in stimulating rooting. Application of NPA to the basal internode resulted in full inhibition of rooting. The view, deduced from these results, that auxin from mature leaves is the main factor controlling the rooting process was reinforced by the fact that mature leaves contained IAA and exported labelled IAA to the stem. The distribution of radioactivity after application of (5-3H)-IAA to mature leaves showed that auxin movement in the stem was basipetal and sensitive to NPA inhibition. The features of this transport were studied by applying 3H-IAA to the apical cut surface of stem sections excised from cuttings. The intensity of the transport was lower in the oldest node than in the basal internode, probably due to the presence of vascular traces of leaves. Irrespective of the localization of the sections and the carnation cultivar used, basipetal IAA transport was severely reduced when the temperature was lowered from 25 to 4 degrees C. The polar nature of the IAA transport in the sections was confirmed by the inhibition produced by NPA. Local application of IAA to different tissues of the sections revealed that polar auxin transport was associated with the vascular cylinder, the transport in the pith and cortex being low and apolar. The present results strongly support the conclusion that IAA originating from the leaves and transported in the stem through the polar auxin transport pathway was decisive in controlling adventitious rooting.

The rooting of stem cuttings is a common vegetative propagation practice in many ornamental species. A detailed analysis of the morphological changes occurring in the basal region of cultivated carnation cuttings during the early stages of adventitious rooting was carried out and the physiological modifications induced by exogenous auxin application were studied. To this end, the endogenous concentrations of five major classes of plant hormones [auxin, cytokinin (CK), abscisic acid, salicylic acid (SA) and jasmonic acid] and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid were analyzed at the base of stem cuttings and at different stages of adventitious root formation. We found that the stimulus triggering the initiation of adventitious root formation occurred during the first hours after their excision from the donor plant, due to the breakdown of the vascular continuum that induces auxin accumulation near the wounding. Although this stimulus was independent of exogenously applied auxin, it was observed that the auxin treatment accelerated cell division in the cambium and increased the sucrolytic activities at the base of the stem, both of which contributed to the establishment of the new root primordia at the stem base. Further, several genes involved in auxin transport were upregulated in the stem base either with or without auxin application, while endogenous CK and SA concentrations were specially affected by exogenous auxin application. Taken together our results indicate significant crosstalk between auxin levels, stress hormone homeostasis and sugar availability in the base of the stem cuttings in carnation during the initial steps of adventitious rooting.