Temperature-mediated developmental delay may limit yield of cotton in relay intercrops with wheat

Abstract

In the Yellow River valley of China, more then 1.4 million ha of cotton are grown as relay intercrops with wheat. Cotton is sown in April when winter wheat is already in the reproductive phase; thus, a wheat crop with a fully developed canopy will compete for resources with cotton plants in the seedling stage. Yields of cotton are lower in relay intercropping systems than in a monocrop, but the aggregate yield of the cotton–wheat system is greater than of monocultures of the component crops. We study the hypothesis that the lower yield of intercropped cotton is a consequence of delayed development and fruit formation of the cotton as a result of a lowered temperature experienced by seedlings in the intercrop, compared to monoculture, due to shading by wheat. Field experiments were conducted in 3 subsequent years in Anyang, Henan, China. Wheat and cotton were grown as monocrops and as strip intercrops. Four intercrop layouts were investigated, differing in number of wheat and cotton rows in a strip: 3:1, 3:2, 4:2 and 6:2. Developmental stage of the cotton was recorded at regular intervals during the growing cycle while air and soil temperatures were measured with thermocouples at several soil depths and cross-row positions in the canopy. Temperatures at and near the soil surface were substantially (on average 3 °C) lower in intercrops than in monoculture, especially on sunny days, thus lowering the rate of temperature accumulation of cotton seedlings in intercrops, compared to those in monocultures. Cotton in intercrops showed a pronounced delay in early development, e.g. attainment of the squaring stage, compared to monocrops. The period from planting to first square, expressed in thermal time (TT), lasted 531 °C d in cotton monoculture and 638–670 °C d in intercrops. There were no significant differences in developmental delay between different intercropping patterns. The formation of fruits in intercrops lagged behind by 9–15 d, compared to monoculture, while the number of fruit nodes per plant, averaged over 3 years, was reduced from 30.3 in monocrops to 19.9 in intercrops. The later formation of fruits thus results in a reduction in fruit number and also in a reduction in average age of the fruits, limiting their growth and the sink capacity of the plant as a whole. A plastic film cover increased temperatures in a 3:2 intercrop at the soil surface by 1.9 °C and at 5 cm soil depth by 2.7 °C, thus restoring the thermal conditions to levels common in monoculture. A cover with straw, however, decreased the temperature at the soil surface by 2.9 °C and at 5 cm depth by 1.3 °C. We conclude that the thermal climate in wheat–cotton intercrops is suboptimal for the cotton seedlings. The resulting delay in development of cotton culminates in a lower reproductive capacity and sink capacity. In combination with a reduced source strength, due to later and reduced canopy development in intercropped cotton, these effects result in a lint yield that is substantially lower than in monoculture cotton. This constraint can be ameliorated by measures that improve light capture and heat loading by the cotton, e.g. planting a semi-dwarf wheat or cultivation of cotton on ridges, or application of a plastic film mulch in the cotton seed bed. Early maturing cotton cultivars are at an advantage in intercropping systems.