To determine whether root-supplied ABA alleviates saline stress, tomato (Solanum lycopersicum L. cv. Sugar Drop) was grafted onto two independent lines overexpressing the SlNCED1 (9-cis-epoxycarotenoid dioxygenase) gene (NCED OE) and wild type rootstocks. After 200 days of salinity irrigation (EC = 3.5 dS m-1), plants with NCED OE rootstocks had 30% higher fruit yield, but root biomass and lateral root development was reduced. Although NCED OE rootstocks upregulated ABA-signalling (AREB, ATHB12), ethylene-related (ACCs, ERFs), aquaporin (PIPs) and stress-related (TAS14, KIN, LEA) genes, downregulation of PYL ABA receptors and signalling components (WRKYs), ethylene synthesis (ACOs) and auxin responsive factors occurred. Elevated SlNCED1 expression enhanced ABA levels in reproductive tissue while ABA catabolites accumulated in leaf and xylem sap suggesting homeostatic mechanisms. NCED OE also reduced xylem cytokinin transport to the shoot and stimulated foliar 2-isopentenyl adenine (iP) accumulation and phloem transport. Moreover, increased xylem gibberellin GA3 levels in growing fruit trusses was associated with enhanced reproductive growth. Improved photosynthesis without changes in stomatal conductance was consistent with hormone-mediated alteration of leaf growth and mesophyll structure, which combined with lower assimilate requirement in the roots and systemic changes in hormone balances could explain enhanced vigour, reproductive growth and yield under saline stress.

Variation in solar ultraviolet (UV) radiation induces a wide-range of plant responses from the cellular to whole-plant scale. We demonstrate here for the first time that partial stomatal closure caused by UV exposure significantly increases leaf temperature independently of any increase in incident energy on the leaves. Significant leaf warming in response to UV radiation was consistent in tomato (Solanum lycopersicum L. ) across different experimental approaches. Exposure to UV radiation significantly decreased stomatal conductance and increased leaf temperature by up to 2°C in field experiments where solar UV was attenuated using filters. Smaller but significant increases in leaf temperature due to decreases in stomatal conductance occurred in multi-day controlled environment (CE) growth room experiments and in short-term (< 2 hours) irradiance response experiments, both using fluorescent lamps to provide UV treatments. We show that leaf warming due to partial stomatal closure is independent of any direct warming effects of UV manipulations. We discuss the implications of UV-induced warming both for crop production and understanding broader plant and ecosystem responses to UV radiation.

Coffee flowering requires a period of water deficit followed by rainfall to break flower bud dormancy and promote anthesis. Since drought followed by re-watering can increase shoot ethylene production, we investigated changes in root, leaf and flower bud ethylene production and expression of genes within the ethylene biosynthesis and signalling pathways and their relationship to coffee flowering. Drought decreased foliar and flower bud ethylene production without changing root ethylene production, even though all tissues likely accumulated the ethylene precursor ACC (1-aminocyclopropane-1-carboxylic acid), since ACS gene expression was maintained while ACO gene expression decreased. The ethylene receptor CaETR4-like was not differentially expressed in leaves under water deficit, but it was downregulated in roots. Re-watering restored shoot ethylene production, which seems important in promoting anthesis. 1-MCP, an ethylene action inhibitor, triggered coffee anthesis without re-watering the plants, which hitherto was considered essential to allow flowering. 1-MCP positively regulated ethylene biosynthesis genes (CaACS1-like and CaACO1-like), similar to re-watering, and downregulated CaETR4-like, suggesting that changes in ethylene levels and sensitivity are required to promote coffee anthesis. Thus, drought and re-watering-induced changes in ethylene levels and sensitivity allow coffee flowering, while the growth regulator 1-MCP can potentially regulate anthesis time and intensity.

Atmospheric CO2 levels influences plant growth and susceptibility to pathogens and pests but there is little information on the mechanisms involved. We therefore studied how elevated atmospheric CO2 concentrations (eCO2) effects shoot branching and the performance of the pea aphid on wild type peas and on mutants that are defective in either strigolactone (SL) synthesis or signalling. Shoot branching and dry biomass accumulation were increased under eCO2 conditions in all lines. However, eCO2 decreased shoot water content only in the wild type peas but not in the SL mutants. Growth under eCO2 decreased the levels of salicylic acid and increased jasmonic acid in the wild type peas, without any significant effect on aphid fecundity. However, aphid numbers were increased on the SL mutants under both ambient and eCO2 conditions. Of the shoot phytohormones measured in this study, only gibberellic acid (GA3) was decreased in the SL mutants, an effect that was exacerbated by eCO2. There was a negative correlation between aphid fecundity and shoot GA3 levels. Taken together, these studies provide confirm that eCO2 modifies the levels of defence hormones and provides the first evidence that SL and GA3 are important regulators of plant responses to phloem-feeding insects.