Leaf senescence-like characteristics contribute to cotton's premature photosynthetic decline

Leaf and canopy photosynthesis of cotton (Gossypium hirsutum L.) declines a s the crop approaches cutout, just as the assimilate needs for reproductive growth are peaking. Our objective with this study was to determine whether this decline is due to remobilization of leaf components to support the re productive growth or due to some cue from the changing environmental condit ions during the growing season. Field studies were conducted in 1995-1996 a t Stoneville, Mississippi, using six cotton genotypes and two planting date s (early and late), which produced two distinctly different cotton populati ons reaching cutout at different times. Among the six genotypes were a phot operiod sensitive line (non-flowering) and its counter part which had photo period insensitive genes backcrossed four times to the photoperiod sensitiv e line (flowering). This pair was used to assess the degree that the photos ynthetic decline could be attributed to reproductive sink development. Leaf CO2-exchange rate (CER) and chlorophyll (Chl) fluorescence measurements we re taken in mid-August, a period corresponding to cutout for the early plan ted plots, and those leaves were collected. Leaf Chl level, soluble protein level, various soluble carbohydrate levels and Rubisco activities were ass ayed on those leaves. Averaged across years, leaf CER and soluble protein l evels were reduced approximately 14% and 18%, respectively, for the early p lanted compared to the late planted cotton. Neither leaf Chl levels or Chl fluorescence Fv/Fm values for Photosystem II yield were altered by the plan ting date. In 1996, leaves from the non-flowering line had 12% greater Chl and 20% greater soluble protein levels than the flowering line. However, in 1996, the CER of the early planted non-flowering line was reduced 10% comp ared to the late planted. Although remobilization of leaf N to reproductive growth appears to be the principle component causing the cutout photosynth etic decline, the data also indicate that environmental factors can play a small role in causing the decline.