Tomato growth as affected by soil extract of continuously cropped okra

Single tillage system, single variety and excessive pursuit for economic efficiency have exacerbated the barriers of continuous cropping of greenhouse tomato (Lycopersicum esculentum). Although its root secretions can cause autotoxicity, okra (Hibiscus esculentus) is a new vegetable with great economic prospect in China. Rotation is an effective way to avoid continuous cropping obstacles, however, in practice, rotation of tomato and okra inhibited growth and impeded fruiting of tomato. To clear the hypotrophy between tomato and okra, soils of okra continuously cropped for 1 year and 10 years were used to conduct an experiment with tomato seeds and seedlings. Water extracts of two soils were diluted into 1 000 mg·mL^-1, 2 000 mg·mL^-1 and 3 000 mg·mL^-1 to treat tomato seeds, and into 125 mg·mL^-1, 250 mg·mL^-1 and 500 mg·mL^-1 to cultivate tomato seedlings. Physiological and biochemical analysis and seed root and stem morphology observation of tomato were conducted to investigate the influence on the below-ground and above-ground growth of tomato to determine the effects of okra soil extracts on tomato seed germination and seedling growth. The results showed that under the same concentration of soil extract, with continuous cropping years increase, the main root of tomato deformed with more but thin lateral root. Tomato seedling also showed increased root forks and root activity significantly lowered and with abnormal and green-lost top leaves compared to the control (total nutrient solution, CK) treatment. The activity of antioxidant enzyme, and MDA and proline contents were also significantly different from CK. Under different concentrations of soil abstracts for the same continuous cropping year, main root deformity appeared gradually in germinated tomato seeds as the concentration of soil extract increased. Also lateral roots increased and most relatively thin, tomato seedling rootlets increased and root tip number and rootlets were respectively 1 146 and 3 321 for the 1-year and 2 291 and 1 947 for the 10-year continuous cropping, all significantly higher than those under the control (1 071 and 385, respectively). Root activity of tomato seedlings firstly rose and then fell. At 3 days of cultivation, root activity of tomato seedlings under soil abstracts with over 250 mg·mL^-1 concentration of 1 year and 10 years continuous cropping were lower than that of CK. Antioxidant enzyme activity, MDA and proline contents were also significantly different from CK. Above all, some allelechemicals were secreted by okra roots, including the substances vestigital in the soil, which had negative effects on subsequent tomatoes. The substances were enriched with increasing years of continuous cropping, which caused more damage to succeeding tomato crops.