Physiological determinants of high yielding ultra-narrow row cotton: Canopy development and radiation use efficiency

► We compared leaf area, light interception and radiation use efficiency in UNR cotton and 100 cm spaced rows. ► Early light interception was higher in the UNR crop. ► UNR cotton had a 17% higher canopy extinction coefficient. ► UNR cotton had 19% lower radiation use efficiency. ► We concluded that the higher plant density in the UNR crop was excessive for optimal RUE. Ultra-narrow row cotton (UNR, with rows spaced less than 40 cm apart) has long been proposed to have the potential to increase yields while reducing the time to crop maturity. Investigations have shown that biomass accumulation in high-input, high yielding UNR (25 cm spaced rows with yields greater than 1800 kg lint ha −1 ) cotton is similar to conventionally spaced rows (100 cm) despite a three-fold increase in plant density, indicating a limitation on individual plant growth. This study investigates whether the increased plant density in UNR crops (36 plants m −2 ) leads to differences in canopy development, radiation use efficiency (RUE) and light interception contributing to plant growth limitations. Three experiments over three years compared UNR treatments to conventionally spaced treatments in high-input production systems and found that early canopy development (leaf area index LAI) and consequently early interception was higher in the UNR crops in two of the three experiments. This resulted in a 17% higher seasonal canopy extinction coefficient ( k ) in UNR crops over the season. However, seasonal RUE of the UNR crop was 19% lower as increases in light interception were not accompanied by increased total dry matter. Light distribution through the canopy was poorer (higher k ) in the UNR crop and LAI continued to increase in the UNR crop after maximum light interception was reached, which combined with a lower leaf nitrogen concentration may have reduced the photosynthetic efficiency of the UNR crop. We conclude that differences in canopy light interception and the efficiency of conversion of light to biomass were the primary factors responsible for differences in the pattern of biomass accumulation between UNR and conventionally spaced cotton.