Characters in light-response curves of canopy photosynthetic use efficiency of light and N in responses to plant density in field-grown cotton

Plant density is a common practice for optimizing canopy photosynthetic capacity and improving cotton (Gossypium hirsutum L.) yield. The objective of this study was to evaluate the effects of plant density (14.5, 19.5 and 29 plants m−2 were adjusted by wide, medium and narrow row spacing, respectively) on canopy apparent photosynthetic use efficiency of light (CAPLUE) and N (CAPNUE), dry matter accumulation and yield formation of cotton plants (cv. SCRC24). Cotton in narrow row spacing responded to light interception and distribution within its canopy by significantly increasing the proportion of leaf area index (LAI) in the upper canopy and by increasing leaf N content per unit land area (Nland). Cotton in wide row spacing compared with narrow row spacing had a significantly greater proportion of LAI in the mid and lower canopy and a significantly lower Nland. The changes in canopy structure and leaf photosynthetic characteristics in response to row spacing affected the relationship between CAP and light within the canopies. Under low light conditions, CAPLUE and CAPNUE were greater in narrow row spacing than in wide row spacing. Under high light conditions, maximum CAPLUE and CAPNUE were greater in wide row spacing than in narrow row spacing. These light response curves support the hypothesis that CAPLUE and CAPNUE were maximized according to the light interception and distribution within its canopy, which is by adjusting canopy structure and leaf photosynthetic characteristics. Furthermore, there was no difference in leaf + boll dry weight among the three row spacing treatments. The leaf + boll dry weight was maintained in narrow row spacing by increasing leaf dry weight at the expense of boll dry weight. The cotton responded to wide row spacing both by increasing the single boll weight and the proportion of LAI in the mid- and lower canopy and by increasing the proportion of bolls in the upper canopy. Cotton in wide row spacing had better canopy leaf and boll distribution, therefore higher CAP, because a greater proportion of leaf area was allocated to the mid canopy and the canopy structure was optimal. This indicated that yield was influenced not only by maximum CAP, CAPLUE, and CAPNUE, but also by the spatial distribution of vegetative and reproductive organs.