INTERMITTENT SHADE EFFECT ON GAS-EXCHANGE OF COTTON LEAVES IN THE HUMID SOUTHEASTERN USA

Convective cumulus clouds intermittently shade growing plants on most days during the summer months in the southeastern USA. Previous resear ch indicates a significant delay in the recovery of stomatal conductan ce (g(s)) of cotton (Gossypium hirsutum L,) leaves following a period of shade, Our objective was to determine the effect of shade on leaf n et CO2 exchange rate (CER) and g(s) of three cotton cultivars, We moni tored CER and g(s) of greenhouse- and field-grown cotton before, durin g, and after shading plants for up to 9 min at photosynthetically acti ve radiation (PAR) levels of <300 pmol quanta m(-2) s(-1). The three c ultivars had the same gas exchange response to shade, With the imposit ion of a 6-min shade on greenhouse-grown plants at 7 weeks after plant ing (WAP), g(s) of uppermost fully expanded 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% earl y in the cutout period (cessation of vegetative growth) in 1992 and 42 % late in the cutout period in 1994. Under both greenhouse and held gr owing conditions, the low PAR levels with shade reduced CER to near 0 mu mol CO2 m(-2) s(-1). Recovery of CER to preshade levels after the s hade was removed coincided with the rate of recovery of g(s). In the g reenhouse-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 WA P) after the shade was removed, Field-grown leaves needed only 4 min t o recover to preshade levels of CER and gr following 9 min of shade, T he results suggest that, following a brief shade period, field-grown c otton leaves reacclimate within 4 min, while leaves on greenhouse-grow n plants may take longer.