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In many plant species, long-term exposure to elevated CO2 concentration results in a reduction in photosynthetic capacity, known as acclimation. This process is mainly explained by a feedback inhibition mechanism. The supply of a fraction of the nitrogen (N) in the nutrient solution as NH4+ can play an important role in the maintenance of photosynthetic activity and could mitigate the acclimation process. The aims of the present work were to study the photosynthetic response of sweet pepper (Capsicum annuum L.) to CO2 enrichment in Mediterranean greenhouse conditions, throughout the crop growth cycle and to evaluate the supply of NH4+ in the nutrient solution as a strategy to enhance the long-term response to CO2 at different levels of salinity. The experiment was conducted in two identical greenhouses: one with CO2 enrichment according to the ventilation, maintaining a high concentration when the vents were closed and a near-atmospheric level when the vents were open and one without. Sweet pepper plants were grown in both greenhouses, being irrigated with two levels of water salinity and two N sources: (i) NO3- and (ii) NO3- plus NH4+. A reduction in the response of photosynthesis to high CO2 concentration was found in the enriched plants after 135 days of CO2 supply, with respect to the reference plants. The leaf photosynthesis rate measured at high CO2 concentration showed a closer relationship with the leaf N concentration than the non-structural carbohydrate concentration. The relative yield gain of the CO2-enriched plants progressively decreased after reaching a maximum value; this was probably associated with the photosynthetic acclimation process. This decrease was delayed by the use of NH4+ in the nutrient solution at low salinity. Knowledge of the crop phase when acclimation to high CO2 concentration occurs can be the basis for deciding when to impose an early cessation of CO2 application, as a strategy to improve the economic efficiency of CO2 supply in Mediterranean conditions.

Regulated deficit irrigation (RDI) and partial root zone irrigation (PRI) were compared for 4 years at two irrigation volumes (110mm year-1 (1) and 78mm year-1 (2)) in field-grown grafted Monastrell grapevines (Vitis vitifera L.) to distinguish the effects of deficit irrigation from specific PRI effects. PRI-1 and RDI-1 vines received ~30% of the crop evapotranspiration (ETc) from budburst to fruit set, 13-15% from fruit set to veraison and 20% from veraison to harvest. RDI-2 and PRI-2 vines received around 20% of ETc from budburst to fruit set, no irrigation from fruit set to veraison, and recovery (21-24% ETc) thereafter. Compared with RDI-1, PRI-1 increased irrigation depth and total soil water (θv) availability in the root zone, and stimulated greater fine root growth and water uptake. Increased soil volume exploration supported greater canopy water use, vegetative development, biomass accumulation and internal water storage capacity. PRI-1 vines had higher stomatal conductance, lower leaf-level water use efficiency and increased leaf xylem sap concentration ([X-ABA]leaf) following reirrigation. Compared with RDI-2, PRI-2 decreased total θv availability, fine root growth and water uptake, gas exchange, leaf water status, [X-ABA]leaf, biomass accumulation and storage capacity. Xylem ABA decreased with total θv availability in PRI-2, probably from limited sap flow when θv in drying soil was low (≈20%). For this rootstock-scion combination, high irrigation volumes applied to the wet part of the roots (θv>30%) are critical for increasing root-to-shoot ABA signalling and growth, and improving performance under semiarid conditions.

To characterise the effect of bacterial inoculants (Azospirillum brasilense and Pantoea dispersa) on the response of sweet pepper (Capsicum annuum L.) to saline stress, plants were exposed to 0, 40, 80 and 120mM NaCl in solution. The effect on plant growth; leaf gas exchange; NO3-, Cl-, K+ and Na+ accumulation; and chlorophyll fluorescence and content were investigated. Total plant DW was reduced significantly by salinity but when inoculants were applied, DW was increased. Inoculated plants showed higher DW accumulation in the roots. Salinity levels up to 80mM NaCl did not affect the net assimilation rate in inoculated plants but 40mM NaCl was enough to reduce this parameter in non-inoculated plants. The leaf area ratio was not modified substantially by inoculation. The leaf Cl- concentration of inoculated plants was reduced at the highest salinity, compared with control plants, and NO3- concentration increased markedly. A higher K+:Na+ ratio was found in inoculated plants. Leaf photosynthesis and stomatal conductance were impaired significantly at moderate, but not low, salinity, the effect of inoculation being enough to maintain higher stomatal conductance under higher stress. The photochemical efficiency of PSII and the relative chlorophyll content were not affected by the inoculants. Thus, the effects of the inoculants on the response to salinity were due mainly to stomatal regulation of photosynthesis rather than effects on biochemical limitations on photosynthesis. These results indicate the benefits of these bacterial inoculants in ameliorating the deleterious effect of NaCl in a salt-sensitive crop like sweet pepper.