Viability Loss and Oxidation during Desiccation of Recalcitrant Pachira macrocarpa Seeds

This study investigated the correlation between the moisture content and viability of Pachira macrocarpa seeds, and observed the changes in seed oxidation and oxidation resistance during the desiccation process. The average moisture content of a fresh mature P. macrocarpa seed was approximately 1.93 gH_2O·g^<-1> DW. When the seeds were placed into desiccating environments at 25ºC with 60% relative humidity, seed viability and moisture content exhibited highly curve correlation. The R^2 reached 0.98, and seed viability rapidly declined as the moisture content decreased. This test was observed causing seed death occurred at a critical moisture content of 1.4 gH_2O·g^<-1> DW, where the seed germination percentage was 0%. This indicates that P. macrocarpa seeds are not only recalcitrant but also sensitive to desiccation. Furthermore, the seed oxidation resistance showed that after 0–5 days of desiccation, the superoxide anion radical (O_2 -) and hydrogen peroxide (H_2O_2) levels increased as the seed moisture content decreased. The activity of the antioxidant enzyme superoxidase dismutase (SOD) and ascorbate peroxidase (APX) also decreased with the seed moisture content. When the seeds were desiccated to a moisture content of 1.22 gH_2O·g^<-1> DW, the SOD activity was reduced to half that of a fresh seed. Although the ascorbate–glutathione cycle continued to function, the seed’s malondialdehyde (MDA) content increased 1.5 times, which expedited seed oxidation. However, the reduction of P. macrocarpa seed viability during desiccation occurred before significant oxidation, while seed death may have been caused by physical damage not metabolic injury