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.