Intermittent Shade Effect on Gas Exchange of Cotton Leaves in the Humid Southeastern USA
- Philip J. Bauer ,
- E. John Sadler and
- James R. Frederick
Convective cumulus clouds intermittently shade growing plants on most days during the summer month in the southeastern USA. Previous research indicates a significant delay in the recovery of stomatal conductance (gs) of cotton (Gossypium hirsutum L .) leaves following a period of shade. Our objective was to determine the effect of shade on leaf net CO2 ex hange 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 μmol quanta m−2 s−1. The three cultivars had the same gas exchange response to shade. With the imposition of a 6-min shade on greenhouse-grown plants at 7 weeks after planting (WAP) gs of uppermost fully expandd leaves fell 43%. Two weeks later, a 6-min shade reduced gs of uppermost fully expanded leaves by 97%. Under field conditions, a 9-min shade reduced gs by 35% early in the cutout period (cessation of vegetative growth) in 1992 and 42% late in the cutout period in 1994. Under both greenhouse and field growing conditions, the low PAR levels with shade reduced CER to near 0 μmol CO2 m−2 s−1. Recovery of CER to preshade levels after the shade was removed coincided with the rate of recovery of gs. In the greenhouse-grown plants, recovery of CER to preshade levels following 6 min of shade did not occur until 7 min( at 7 WAP) and 10 min (at 9 WAP) after the shade was removed. Field-grown leaves needed only 4 min to recover to preshade levels of CER and g2 following 9 min of shade. The results suggest that, following a brief shade period, field-grown cotton leaves reacclimate within 4 min, while leaves on greenhouse-grown plants may take longer
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