Response of Vegetable Seed Germination to Solar Radiation Penetrating through Soil.

Abstract

Response of vegetable seeds to irradiation and emergence of plants seeded at various depths were investigated to clarify the effects of solar radiation through soil on vegetable seed germination. Seeds of eight vegetable species were germinated in Petri dishes under 11-h irradiation per day. Seed germination was delayed in tomato (Licopersicon esculentum Mill.) but accelerated in perilla (Perilla ocymoides L.) and Japanese hornwort (Cryptotaenia japonica Hassk.) with increase in irradiation at the intensities higher than 0.4 Wm-2. Seeds of Japanese radish (Raphanus sativus L.), watermelon (Citrullus lanatus Matsum.), and Chinese cabbage (Brassica campestris L.) showed delayed germination at more than 4-6 Wm-2. No effect of irradiation on lettuce (Lactuca sativa L.) and carrot (Daucus carota L.) seed germination was seen. For tomato, Japanese radish and Japanese hornwort, the effects of irradiation time on germination were also investigated. Tomato seed germination was delayed and Japanese hornwort seed germination was accelerated with increase in irradiation time beyond 2 h per day. The emergence of tomato and Japanese hornwort covered with Shimokuriyagawa loam soil (Kuriyagawa soil) and vermiculite at depths of less than 5 mm, 5-10 mm and 10-15 mm was observed. Plants emerged more rapidly from 5-10 mm depths than from less than 5 mm depth in tomato. The plants seeded at 10-15 mm depths emerged as rapidly as those at 5-10 mm depths using vermiculite but later than those at other depths using Kuriyagawa soil, probably due to high bulk density. The early emergence of Japanese hornwort was fastest from less than 5 mm depth. The plants seeded at 5-10 mm depths did not emerge much slower than those at less than 5 mm depth. A seeding depth of 5-10 mm was suitable for the rapid emergence of those vegetables covered with both the soil and vermiculite.