Studies on citrus root-stocks. III

Abstract

1. Growth response of citrus seedlings to various soil temperatures in summer was observed with 8 varieties planted in soil contained in glass pots (1, 800 ml). The glass pots were kept in the glass room in the thermostatic boxes heated electrically (Fig. 1) for 36 or 45 days during the period from April to August, 1954. The soil temperatures in the pots were maintained at 20_??_22°C, 30_??_32°C and 40_??_42°C. Citrus Natsudaidai, C. intermedia and C. grandis var. Hogen, heat resistant varieties, showed their maximum growth rates at 30_??_32°C, while C. junos, Poncirus trifoliata, C. Tachibana and C. Tamurana at 20_??_22°C, and C. leiocarpa at 2_??_32°C (Table 1, Fig. 2, 3). But the heat resistant varieties showed a better growth even at 20_??_22°C than the rest. At 40_??_42°C, the former varieties grew only slightly, but the latter scarecely. These results seem to have shown that C. Natsudaidai, C. intermedia and C. grandis var. Hogen are most suitable as root-stock followed by C. junos in the summer soil in Kochi, where the soil temperature reaches 24_??_31°C at 0_??_30cm in depth in summer. 2. The roots of citrus seedlings of 10 varieties were soaked in the water baths at 5 and 10°C from February 7 to March 6, 1955. The results observed are shown in Fig. 6. Growth response of citrus seedlings to various soil temperatures in winter was observed with 12 varieties planted in soil contained in glass pots (500 ml). These glass pots were kept in field in the thermostatic boxes from December 23, 1956 to April 30, 1957. The soil temperature of the high heated plot was maintained at 18_??_23°C, those of the low heated and nonheated plots at 9_??_15°C and 3_??_13°C, respectively. The results observed are shown in Fig. 4, 5, 6 and 7. 3. The seedling roots in the heated plots commenced an earlier elongation, 45_??_56 days earlier at 18_??_23°C and 26_??_38 days earlier at 9_??_15°C than those in the non-heated plot (Fig. 4 and 5). When the soil temperature was below 23°C in winter, roots of Poncirus trifoliata, C. junos and C. leiocarpa, the most cold resistant varieties, showed the most rapid elongation, followed by those of C. Hassaku, C. Aurantium, C. Funadoko, C. Tamurana and C. Tachibana, and the root elongation of C. sinensis var. Fukuhara, var. brasiliensis and C. intermedia, less cold resistant varieties, was delayed. And the increment of root elongation in the cold resistant varieties decreased as the soil temperature was increased. 4. Generally speaking, the roots in the heated plots showed a better daily elongation over a long period than those in the non-heated plot (Fig. 4). As in Fig. 6 and 7, the cold resistant varieties showed a better growth at 9_??_15°C; especially, Poncirus trifoliata, C. junos and C. leiocarpa grew most vigorously. On the other hand, the growth of the less cold resistant varieties was retarded. And Poncirus trifoliata, C. leiocarpa and C. junos increased in fresh weight a little less than twice as much as at 3_??_13°C; C. Aurantium, C. Funadoko, C. Tamurana and C. sulcata twice to three times; C. Natsudaidai, C. intermedia and C. Tachibana a little over three times; C. sinensis increased more greatly. In consequence, the most suitable rootstocks for the winter soil in Kochi, where the soil temperature in winter remains at 8.5°C or above at 50cm or more in depth, will be the varieties with deep root system, that is, C. junos and C. Natsudaidai, and the most cold resistant varieties, namely, Poncirus trifoliata, C.junos and C. leiocarpa, followed by C. Funadoko and C. Aurantium.