Abstract: 【Objective】Under-mulch-drip irrigation is a precision irrigation technology which combines drip irrigation with mulched with plastic film to form. It is of great importance to study the effect of soil moisture content on production and distribution of photosynthate and seed yield of cotton for realizing water saving and high yield with under-mulch-drip irrigation. 【Method】With the materials of different water-sensitivity Xinluzao 10 and Xinluzao 13, we designed three soil moisture content treatments, controlled the lower limited of soil water content in 0~60 cm soil layer at 85%~90%, 70%~75% and 55%~60% of the field water capacity and upper limit of soil water content at field capacity by applying drip irrigation. Leaf gas exchange and isotopic tracer techniques were used to investigate the change of photosynthesis, production and distribution of photosynthate and seed yield during flowering and boll-setting stage with different soil moisture content under field condition.【Result】Different soil moisture content had significant influence on the production and distribution of photosynthate and yield. Under 60% field water capacity, export and distribution percentage 14C-assimilates in boll increased, but photosynthetic rate and photosynthesis products accumulation of aerial part reduced, so the seed yield was lower. Under 90% field water capacity, leaf photosynthetic rate was low and distribution percentage 14C-assimilates in stem and leaf significant increased. Under 75% field water capacity, photosynthetic rate was high, photosynthesis products accumulation of aerial part were more weighty and distribution percentage 14C-assimilates in various organs were moderate, hence the seed yield obvious increased. The differences of response to drip irrigation among varieties were great. Xinluzao 10 under water deficits, photosynthetic rate was low and rapidly reduced in later growth stage, photosynthesis products accumulation of aerial part were light and seeding yield was low. Xinluzao 13 under water abundances, photosynthesis products accumulation of aerial part significant increased, but distribution percentage 14C-assimilates in boll significant reduced, so the seed yield was lower than 75% field water capacity. 【Conclusion】Water deficiency before drip in cotton field caused photosynthetic rate and photosynthesis products accumulation of aerial part significant decreased which led to seed yield obvious reduced. The main reason that seed yield decreased under water abundance before drip in cotton field was distribution percentage 14C-assimilates in bud and boll significant reduced. 70%~75% field water capacity took precedence over insignificant decreased cotton plant photosynthetic rate and photosynthesis products accumulation of aerial part under mulch drip irrigation, at the same time promoted distribution percentage 14C-assimilates in boll, the seed yield and water efficiency significant increased. According to the differences of response to soil moisture content among varieties, instituting irrigation scheduling will be propitious to exploit the potential of increasing yield and water saving under mulch drip irrigation.

Key words: Gossypium hirsutum L., Under-mulch-drip irrigation, Soil moisture content, Photosynthesis, Distribution of photosynthates, Yield