Effect of varying red soil moisture on amino acid and polyamine content in citrus (Citrus unshiu Marc.) leaves

In this study, 2-year pot seedlings of satsuma mandarin (Citrus unshiu Marc.cv. Miyagawa Wase) were selected and grown in Quaternary red clay soils to explore how varying soil water content impacts the ecophysiological factors of the citrus plant. Soil water content was monitored in real time by FDR and strictly controlled in five treatments (SWC30, SWC45, SWC60, SWC75 and SWC90) with 30%, 45%, 60%, 75% and 90% soil field capacity, respectively. The study shows that nitrogen accumulation in citrus leaves decreases with scanty or excess SWC. Total free amino acid (y) decreases with increasing SWC(x):y= -0.028 2x+ 12.049; R²= 0.852 4^*; n = 50. There is a negative relation between proline(y) and SWC (x) when SWC is below 75%:y = -0.015 2x+4.224; R² = 0.860 5^*; n = 50. Putrescine (Put) reaches the minimum when SWC is 75%. The change in spermidine (Spd) content follows a parabolic curve with increasing SWC, attaining the maximum at 45% SWC. Maximum of spermine (Spm) occurs at 60% SWC. There is a significant relation between Spd(y) and Put(x): y = 0.240 4x² - 51.337x +2 976.4; R² = 0.858 6^*; n= 50. When SWC drops below 75%, a significant linear relation develops between (Spd+Spm)/Put ratio (y) and SWC(x): y = 0.011 2x+0.173; R² = 0.851 8^*; n = 50. Spd and Spm are the main physiological adjustment factors of polyamine (PAs), which is produced under water stress condition. The degree of the effect of water stress on citrus is determined mainly by (Spd＋Spm)/Put ratio. This ratio is a crucial index for assessing the effects of soil water stress on citrus plants.