Germination response of the chasmogamous and cleistogamous seeds of Viola philippica to drought

Viola philippica presents a dimorphic breeding system, or a dimorphic mating system, in which both chasmogamous (CH) and cleistogamous (CL) seeds are produced on the same plant. Here, the response of seed germination to drought (induced by moistening the germination paper with PEG-6000 solution) was studied under laboratory conditions. The following results were obtained. (1) Water absorption by both types of seeds under drought was characterized by a “fast–slow–fast” pattern. Moreover, upon treatment with PEG solution at 15%, 20%, and 25% concentrations, both types of seeds failed to reach the water absorption stage within 48 h. In the control group(without drought treatment), CH seeds started germinating after 60 h and CL seeds after 66 h of water absorption. (2) Both types of seeds could germinate following treatment with PEG solution at 0～25% concentrations. However, as drought intensified, the initial germination time and the time at which seed germination reached 50% of the maximum (TG₅₀) increased for both types of seeds. At PEG concentration of 15%, the TG₅₀ of CL seeds was longer than that of CH seeds. At PEG concentration of 25%, the percentage rate and final percent germination of CL seeds were significantly lower than those of CH seeds (P < 0.05); as such, the final percent germination of CL seeds did not reach 50%. Drought reduced radical length, mild drought promoted plumule length, and severe drought inhibited plumule growth. V. philippica is somewhat drought tolerant, although the drought response differs between its CH and CL seeds. V. philippica likely produces different seeds through a dimorphic breeding system to maximize plant fitness in heterogeneous environments.