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Intermittent Shade Effect on Gas Exchange of Cotton Leaves in the Humid Southeastern USA

USDA-ARS Coastal Plains Soil, Water, and Plant REs. Ctr., 2611 W. Lucas St., Florence, SC 29501-1241

James R. Frederick

Dep. of Agronomy, Clemson Univ., Pee Dee Res. and Educ. Ctr., Florence, SC 29501

^* Corresponding author (bauer{at}florence.ars.usda.gov.

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 (g_s) 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 g_s of three cotton cultivars. We monitored CER and g_s 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) g_s of uppermost fully expandd leaves fell 43%. Two weeks later, a 6-min shade reduced g_s of uppermost fully expanded leaves by 97%. Under field conditions, a 9-min shade reduced g_s 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 g_s. 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

Received for publication September 22, 1995.