Effects of continuous cropping on growth and photosynthesis and nutrient absorption as well as yield and quality traits of melon grown in greenhouse

Abstract：【Objective】Planting the same crop for many years creates continuous cropping problem. The effects of continuous cropping on melon (Cucumis melo L.) under protected cultivation need to be studied. The growth and dry mass allocation in melon plants at leaf developing stage; photosynthesis, nutrient uptake and distribution at different growing stages of plants, an yield and quality traits of melon fruits were determined to investigate the effects of continuous cropping on melon.【Methods】'Henan sweet'variety was used as material and planted in pots filled with soils from greenhouses in which melons had been planted for 0 (control) , 1, 3, 5, 7 and 10 years. The height of plant, diameter of stalk, number and area of the true leaves, dry mass of shoot and root were determined 15 days after planting. Indexes of photosynthesis were determined at different phases of melon development. Also, total N, P and K contents in different organs were determined. The flesh thickness, diameter, lengthen, and fresh weight were determined when fruits were ripe. The contents of vitamin C, total acid, soluble solids, soluble proteins, free amino acids and total sugars in ripe fruit were measured.【Results】The plant height, the number and largest area of the true leaf, the volume and length of the root at 15 d after planting all presented a downward trend with the years of continuous cropping. Dry mass of the shoot and root also showed a downward trend, but the dry mass ratio of shoot to root was the opposite, increasing with theyears of continuous cropping. The result indicated that the root of melon plant might be much more susceptible to continuous cropping. The photosynthesis of melon plants at flowering stage and fruit set stage was stronger than at vine elongation stage. It was weaker at fruit set stage than at flowering stage.The result suggested that photosynthetic assimilate accumulated chiefly during the period of flowering.Net photosynthetic rate, total chlorophylls, transpiration rate and stomatal conductance at flowering stage, all declined with the years of continuous cropping, but the ratio of chlorophyll a to chlorophyll b apparently increased. It indicated that the ability of melon plants to catch light decreased with continuous cropping. Net photosynthetic rate, the total chlorophyll, transpiration rate and stomatal conductance also showed a decline trend at both vine elongation stage and fruit set stage. Intercellular CO2 concentration presented a rising trend at the vine elongation stage with years of continuous cropping years, but it decreased at flowering stage and fruit set stage. At vine elongation stage, total N content in the leaf, vine and root decreased at with the years of continuous cropping. Compared to the vine elongation stage, N was more accumulated during flowering and fruiting. At flowering and fruiting stages, total N content showed a raising trend with the years of continuous cropping. Total N content was highest in root and lowest in vine at different stages. Total P content in the leaf, vine and root was relatively stable at different stages. Total P content was lowest in the root at flowering stage and fruit set stage, but it was highest in root at vine elongation stage. Total K content in the leaf, vine and root generally decreased with the years of continuous cropping. And total K content in the root was greater influenced by continuous cropping at vine elongation stage and flowering stage than in the other organs. It was lowest in the root at the flowering and fruit set stages, but highest at vine elongation stage. The results also showed that the total acid content in fruit increased significantly with the prolonged years of continuous cropping, and when melon was planted in soils under 10 years of continuous cropping, total acid increased by 18.3% compared with the control. The soluble solid and the total sugar contents in fruits decreased under the prolonged cropping years.【Conclusion】Melons grown in greenhouse begin to show continuous cropping problems when they have been continuously planted for 3-5 years. In general, long-term continuous cropping is unfavorable for the growth of melon plant, especially for the development of the roots. Long-term continuous cropping also reduces the net photosynthesis of melon plants. It reduces the absorption and accumulation of K in different organs, decreases the yield and quality of fruit, and worsens the taste of fruit. Therefore, it is recommended that melons should not be continuously planted in a greenhouse for more than 5 years.