Influence of exogenous IAA and GA on seed germination, vigor and their endogenous levels in Cunninghamia lanceolata

To improve seed germination of Chinese fir (Cunninghamia lanceolata (Lamb) Hook), IAA and GA₃ treatments were applied. The most significant promotion by IAA and GA₃ occurred at the concentration of 10⁻⁴ M and 10⁻⁵ M, respectively. 10⁻⁴ M IAA treatment increased germination rate, germination index, seedling length, seedling weight by 41%, 47%, 30%, 28% compared to the control, and 12%, 23%, 15%, 11% compared to target moist chilling. Seed vigor and endogenous hormone levels were analyzed to elucidate the physiological mechanisms. Electrical conductivity test showed that electrolyte leakage rate of the control seeds was 10.4 µS.cm⁻².g⁻¹ while it was decreased to 5.6 µS.cm⁻².g⁻¹ after 10⁻⁴ M IAA soaking. Furthermore, oxygen sensing test showed that 10⁻⁴ M IAA treatment increased oxygen metabolism rate (OMR) by 17% and it reduced increased metabolism time (IMT) by 31%, critical oxygen pressure (COP) by 62%, relative germination time (RGT) by 36%, suggesting that IAA promoted seed respiration. In addition, gas chromatography/mass spectrometry analysis showed that IAA soaking reduced endogenous IAA levels by 32% while it increased endogenous GA₁ contents by 347%, suggesting that feedback inhibition of IAA biosynthesis and promotion of GA₁ biosynthesis might be involved in seed vigor and germination improvement.