Abstract:

Cotton (Gossypium hirsutum) is an important crop in the Xinjiang Province in Northwest China, and high yields of high quality cotton can be produced under irrigation in the arid, continental climate of this region. In order to increase cotton yields in Xinjiang, we studied the effects of planting density (three treatments: 60 000 plants·hm-2, 180 000 plants·hm-2 and 300 000 plants·hm-2) on canopy photosynthetic rate, canopy structure and yield formation in two cotton varieties, `Xinluzao 6' and `Xinluzao 7'. Our results showed that canopy photosynthetic rates increased with planting density before full boll stage. Canopy photosynthetic rates of `Xinluzao 6' and `Xinluzao 7' at a planting density of 300 000 plants·hm-2 were 29.58 μmol CO2·m-2·s-1 and 26.16 μmol CO2·m-2·s-1 respectively, which was 10.4% and 6.5% higher than that of the medium density treatment of 180 000 plant·hm-2. In later growth periods after full boll stage, the canopy photosynthetic rate was maintained at a relatively high level in the medium density treatment, remained low in the low density treatment, but decreased rapidly in the high density treatment due to the decline of the leaf area index (LAI). LAI increased with planting density before full boll stage and attained maximum LAI at full boll stage. Generally, the canopy photosynthetic rate at each growth stage was significantly correlated with LAI (R=0.935*-0.998*, n=5) except for the `Xinluzao 6' in later full boll stage. The mean foliage angle of inclination, an important index for describing canopy structure, increased with planting density, whereas two other indices of canopy structure, the canopy transmission coefficient for radiation penetration and canopy transmission coefficient for diffuse penetration, decreased with planting density.Very high planting densities negatively impacted the canopy structure, resulting in a rapid decline of canopy photosynthetic rate. The accumulation of colonial photosynthetic product increased with an increase in planting density during the growth period. In the high density treatment, colonial photosynthetic product accumulation of `Xinluzao 6' and `Xinluzao 7' was 13 700 kg·hm-2 and 13 153 kg·hm-2, respectively, which was 1.5% and 2.0% higher than that of the medium density treatment, and 34.3% and 54.6% higher than that of the low density treatment. The allocation of photosynthetic product to flower buds and bolls was enhanced from full flower stage to later full boll stage in the medium and high density treatments. Total boll number per unit area increased with planting density. Weight per boll in the medium density planting was 5.61 g and 5.48 g in the two varieties, which was 2.6% and 11.5% heavier than in the high density treatment for the two varieties and 8.7% and 13.5% heavier than in the low density treatments. The canopy photosynthetic rate at full flower stage and boll opening stage were significantly correlated with lint yield. Therefore, the enhancement of the canopy photosynthetic rates during the full boll and opening boll stage could lead to increased of yields of lint.