Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 25, 1996
Publication Date: N/A
Interpretive Summary: Isolated clouds that cast a brief shadow commonly occur over cotton fields in the southeastern USA. Information is needed on how these brief periods of shade affect plants. We studied how a brief shade period affects cotton leaf photosynthesis. Following a nine-minute shade period, leaves of field-grown cotton recovered to preshade levels of photosynthesis within four minutes after removing the shade. Plants grown in the greenhouse took up to ten minutes to recover to preshade levels of photosynthesis after a six minute shade. The cotton varieties we tested had the same leaf photosynthesis response to brief shade. These results indicate scientists can resume measuring photosynthesis shortly after a cloud passes. Also, scientists who develop computer simulation models of cotton growth may use these results to improve their models.
Technical Abstract: Convective cumulus clouds intermittently shade growing plants on most days during the summer months in the southeastern USA. Previous research indicated a significant delay in the recovery of stomatal conductance (gs) of cotton (Gossypium hirsutum L.) leaves following a shade event. Our objective was to determine the impact of shade on leaf net CO2 exchange rate (CER) and gs of three cotton cultivars. We monitored CER and gs of greenhouse- and field-grown cotton before, during, and after shading plants for up to 9 min at photosynthetically active radiation (PAR) levels of <300 umol quanta m-2/s. The three cultivars had the same gas exchange response to shade. A 6-min shade reduced gs of uppermost fully expanded leaves of greenhouse-grown plants by 43% and 97% at 7 wk and 9 wk after planting, respectively. Under field conditions, a 9-min shade reduced gs by 35% in early-August 1992 and 42% in late-August 1994. Under both greenhouse and field growing conditions, the low PAR levels with shade reduced CER to nea 0 umol CO2 m-2/s. The recovery of CER following shade to preshade levels was dependent on reopening of stomata. In the greenhouse-grown plants, up to 10 min was needed for recovery of CER to preshade levels. Field-grown leaves needed only 4 min to recover to preshade levels of CER and gs. The results suggest that following a brief shade, field-grown cotton leaves reacclimate within 4 min. Leaves on greenhouse-grown plants may take longer.