Moderate drought and less fertilizer input promote growth and polysaccharide accumulation in Polygonatum sibiricum Red.

Objectives We explored differences in photosynthetic characteristics, growth development, and polysaccharide content of Polygonatum sibiricum Red. under different fertilizers and water stress. The aim was to provide a scientific basis for water and fertilizer requirements in the cultivation and production of P. sibiricum Red.

Methods Four single-factor irrigation potted experiments were carried out. The irrigation experiment comprised five irrigation treatments (400 mm, 500 mm, 600 mm, 700 mm, and 800 mm) to simulate severe drought, moderate drought, mild drought, sufficient irrigation, and excessive irrigation. The single N, P, and K fertilizer experiments were composed of low, theoretical, experienced, and excess levels under 680 mm irrigation (i.e., average annual precipitation). The detailed quantity of nitrogen fertilizer (urea) applied were 110, 225, 375, and 525 kg/hm²; 260, 525, 750, and 975 kg/hm² for phosphorus fertilizer(diammonium phosphate), and 75, 150, 225, and 300 kg/hm² for potassium fertilizer (potassium sulphate). At the mid growth stage of P. sibiricum, the net photosynthetic rate (P_n), transpiration rate (T_r), stomatal conductance (G_s), and intercellular CO₂ concentration (C_i) were measured with a photosynthetic meter, and the fluorescence parameter values were determined using a chlorophyll fluorescence detector. Plant height, root length, leaf area, and fresh and dry weight of rhizome were recorded. A UV-Vis spectrophotometer was used to determine polysaccharide content in the rhizome.

Results The highest photosynthetic capacity and polysaccharide content of P. sibiricum was recorded under moderate drought stress. The growth and development of P. sibiricum was vigorous under sufficient irrigation; however, the photosynthetic rate and polysaccharide content decreased compared with that of moderate drought. P_n, G_s and T_r values were higher under excessive N fertilization than other N treatments. The P_n curve had an N-shape with an increasing N fertilization rate. The highest F_m was observed under low N, experienced P, and low K rate. The leaves of P. sibiricum showed an evident trend of Chlorophyll A Fluorescence Transient (OJIP) under different water and fertilizer rates. The plant height and biomass were higher under low N and low K rates than those in the others. The highest polysaccharide content was recorded under the experienced fertilization rate. There was a significant correlation between plant height and rhizome biomass (P < 0.05) and an extremely significant correlation between leaf area and polysaccharide content (P < 0.01). The elongation of plant height led to high P. sibiricum yield, and leaves with the larger area had higher polysaccharide content.

Conclusions Moderate drought and slightly low fertilizer application rate than experience, primarily N and K fertilizer, could maintain rhizomes high biomass and large leaf area, improve vigorous photosynthetic efficiency, and high polysaccharide content in the rhizome of P. sibiricum.