Enhancing Acclimatization of Micropropagated Pistachio Through Optimization of Light Spectrum and Vapor Pressure Deficit

The light spectrum and vapor pressure deficit (VPD) are key environmental factors that significantly influence the morphophysiological development and survival of micropropagated woody plants during acclimatization. However, few studies have focused on their interactive effects under ex vitro conditions. This study examined the combined effects of four light spectra (white, blue, red, and red–blue) and two VPD levels (low: 0.2 kPa; high: 1.0 kPa) on growth, photosynthesis pigments, biochemical indices, and leaf temperature of Pistacia spp. ‘UCB1’ plantlets over a 30-day acclimatization period. The results demonstrated that red–blue light under low VPD significantly enhanced plantlet performance across multiple parameters, resulting in the highest leaflet number (79.25 pieces), stem diameter (2.13 mm), leaf dry weight (0.048 g), leaf fresh weight (0.15 g), root length (1.48 cm), and leaf area (103.3 cm2). Furthermore, this treatment markedly increased anthocyanin, total soluble carbohydrate content, and photosynthetic pigments (chlorophyll a, chlorophyll b, and carotenoids). Principal component and correlation analyses identified that red–blue light under low VPD was strongly associated with traits linked to growth and photosynthetic ability, whereas blue and white light under high VPD showed the weakest responses. Entropy-weighted TOPSIS ranked red–blue light under low VPD as the most effective treatment for balanced morpho-physiological functions during acclimatization. These findings highlight the importance of optimizing spectral quality and VPD to enhance autotrophic transition and ex vitro establishment in pistachio plantlets. These findings are important for improving ex vitro survival and large-scale propagation efficiency of micropropagated pistachio plantlets.