Canopy Leaf Area Development and Age-Class Dynamics in Cotton

Leaf senescence contributes to reduced canopy photosynthesis when new leaf initiation cannot compensate for increased leaf aging after canopy closure. However, since leaf aging is seldom addressed in whole plant investigations, it is difficult to evaluate how age-class composition influences crop productivity. Field studies were conducted to examine leaf area development in cotton (Gossypium hirsutum L.) and to incorporate this information into a simulation model for assessing canopy age-class dynamics. Simulated results indicated that leaf area produced during rapid canopy expansion (60 DAP [days after planting], LAI [leaf area index] = 2.3) was largely composed of young leaves, with >35% of the LAI being < 14 d old. However, 30 d later (90 DAP, LAI = 4.5) leaf area production was no longer sufficient to offset leaf aging and the percentage LAI < 14 d old decreased to <10%, while leaves >29 d old increased to 75%. This shift in ageclass composition toward progressively older leaves altered the relative contribution of C from differing layers within the canopy. The middle portion of the canopy was the most important contributor of C early in crop growth, supplemented by roughly equal contributions from the lower and upper canopies. However, after 90 DAP the supply of assimilate from the upper canopy rapidly increased to >45% and that of the lower canopy decreased dramatically. Towards the end of the season (120 DAP), the middle and upper portions of the canopy were about equally divided in their importance to late-season C production. These results highlight the dynamic nature of age-related processes within the cotton canopy and the importance of leaf area development in sustaining whole-plant C production. Publicatin no. 3769, Environmental Sciences Division, Oak Ridge Natl. Lab. Oak Ridge Natl. Lab is managed by Martin Marietta Energy Systems, under contract DE-AC05-840R21400 with the U.S. Dept. of Energy.