(2001)  Page(s) 123-136.  
 
Leaf orientation...was considered by Ding and Liu (1991) as an important character for distinguishing P. japonica (spreading) from P. obovata (ascending. According to our field observations, however, this character is unstable even within a population....
Leaf indumentum. This character was used by Japanese botanists for recognizing infraspecific taxa both in Paeonia obovata (var. typica and var. glabra) and P. japonica (var. japonica and var. pilosa). We examined hundreds of specimens, both in the field and herbaria, and found that leaf indumentum on the lower surface was an extremely variable character, from entirely glabrous to densely hirsute or pubescent..... the recognition of Paeonia obovata var. glabra and P. japonica var. pilosa based on leaf indumentum is unreasonable. In addition, we examined hundreds of herbarium specimens from China, Japan and the Korea Peninsula and projected their indumentum states onto the distribution map of the complex (Fig. 3). It can be seen that leaf indumentum is correlated to some extent with the geographical distribution and ploidy. The plants of this complex in W. Hubei, NW. Sichuan. N. Chongqing, Shaanxi, Shanxi, and W. Henan are all tetraploid and nearly always densely hirsute or pubescent except those in Shanxi, which were variable from mostly hairy (e.g. Hong et al. H98009) to mostly glabrous or sparsely hairy (e.g. Hong et al. H98010)....
Sepals. They exhibit the least variation, are nearly always 3, very occasionally 2 or 4 in number and mostly rounded at the top....
The most important character used for distinguishing P. japonica from P. obovata is the colour of petals, rose-purple in P. obovata and white in P. japonica. Our extensive observations revealed that the colour pattern in this complex is not so simple, but actually varies from purely white ...., white with pinkish shade at the base or periphery....in W. Henan, rose ...., pink-red ...., red .... purple-red ....to purple-red with two white stripes on the back...
The orientation of petals was said by Ding and Liu (1991) to be a valuable character for distinguishing white-flowered P. japonica (ascending) from red-flowered P. obovata (spreading). However, the two orientations could be found in each species....
Petals are usually 5 or 6, but flowers with 4 ...or 7 petals...occasionally occur. Even within a population....the number of petals varied from 4 to 6....
The number of stamens was used by Ding and Liu (1991) as an important character in distinguishing P. japonica (100-280) from P. obovata (14-75). Our extensive observations did not confirm this......the number of stamens varied from (58) 70 to 230 in the white-flowered form (P.japonica), and from 21 to 110 in the red-flowered form (P. obovata). Although there is some difference in number of stamens between the two colour forms, the two ranges overlap much. Therefore, as number of stamens varies continuously in the complex, this is not a diagnostic character for distinguishing the two colour forms as separate species....
The number of carpels was found to vary from 1 to 5, usually 2 or 3 in both the white-flowered and red-flowered forms....
According to Ding and Liu (1991), the orientation of mature follicles can be used for distinguishing P. obovata (strongly recurved) and P. japonica (slightly recurved). However, follicles recurve gradually during ripening and the two states can be found in both P. obovata and P. japonica....
....diploids and tetraploids occur in both the white-flowered (P. japonica) and the red-flowered form (P. obovata). Therefore, the chromosome data do not support the recognition of P. japonica as an independent taxon. However, ploidy level is shown to be correlated with the geographical distribution. Nearly all the populations sampled are tetraploids in the western part of distribution area of the complex: Shanxi, Shaanxi. NW. Sichuan. N. Chongqing, W. Ik-nan and W. Hubei... But there are two exceptions: In Lushi County, W. Henan, there is a mixed population....in which both red-flowered and white-flowered forms were found to grow together, and both 2n= 10 and 2n = 20 were counted....
....it is justified to recognize only one species in the complex, P. obovata, with two subspecies: the typical subspecies for the populations in the northern, eastern and southern parts of the distribution range, which are diploid (very occasionally tetraploid in the Far East of Russia and in the Changbai Mountains, Jilin, China) and mostly glabrous to moderately pubescent or hirsute, and the subspecies willmottiae for the populations in the western part of the distribution range, which are tetraploid (diploid and tetraploid only in one mixed population in Lushi County, W. Henan Province), and mostly densely (less frequently moderately) hirsute or pubescent.

(2001)  Page(s) 123-136.  
 
....it is justified to recognize only one species in the complex, P. obovata, with two subspecies: the typical subspecies for the populations in the northern, eastern and southern parts of the distribution range, which are diploid (very occasionally tetraploid in the Far East of Russia and in the Changbai Mountains, Jilin, China) and mostly glabrous to moderately pubescent or hirsute, and the subspecies willmottiae for the populations in the western part of the distribution range, which are tetraploid (diploid and tetraploid only in one mixed population in Lushi County, W. Henan Province), and mostly densely (less frequently moderately) hirsute or pubescent.