Physiological response, phytohormone signaling, biomass production and water use efficiency of the CAM plant Ananas comosus under different water and nitrogen regimes

The objective of this study was to investigate and unravel the mechanisms for the impact of soil water regimes and N application rates on growth, physiological responses, phytohormone signaling, water use efficiency (WUE) and nitrogen recovery efficiency (NRE) of crassulacean acid metabolism (CAM) plant pineapple. The experimental treatments included three soil water regimes (90%, 70% and 50% of soil water holding capacity, representing well watered, mildly and moderately water stressed conditions) and N application rates (109, 218 and 473 mg kg-1 soil). Results showed that the well watered and mildly water stressed treatments increased the shoot dry biomass by 70.7–110.9% and the plant water use by 25.7–30.4%, consequently, the plant WUE was significantly improved compared to the moderately water stressed treatment. The specific leaf N content was significantly and positively correlated with plant carbon (C) accumulation that was increased by 66.9–89.9%, implying that the enhanced specific leaf N content in the well watered and mildly water stressed treatments could have facilitated the carbon fixation, thus increased the shoot biomass accumulation. Moreover, the well watered and mildly water stress treatments significantly increased leaf δ18O, indicating the significantly higher transpiration in line with the markedly increased plant water use due to both the large leaf water concentration and the characteristic diel pattern of stomatal conductance associated with CAM. The enhanced leaf δ13C and plant WUE in the well watered and mildly water stress treatments were ascribed to the enhanced specific leaf N content and the improved leaf relative water content. The moderately water stressed treatment decreased leaf and root water potential while significantly intensified root endogenous ABA due to water deficit. The [ZR], [IAA] and [GA3] in the leaves and roots interacted complicatedly with water and N rates. The well watered and mildly water stressed treatments enhanced ability of the roots to absorb water and nutrients from the soil, resulting in the significantly higher N and 15N accumulation. Conclusively, in the production of Ananas comosus maintaining high soil water supply is critical to achieve improved growth, water and fertilizer-N use efficiencies.