Greenhouse microclimatic variations:: Effects on water relations and gas exchange on bell pepper (Capsicum annuum)

The effects of sunny (1700 mmol m(-2) s(-1)) and cloudy days (< 1000 mu mol m(-1) s(-1)) on microclimatic conditions and on gas exchange and water relations were studied in three hybrids of C. annuum under greenhouse conditions. Microclimatic measurements included temperature and humidity at 4, 2.5 and 1 m above soil surface. In both radiation conditions, temperatures and humidity at 4 to 1.5 m were similar, but temperature was lower on cloudy days. With lower radiation, stomatic conductance was lower, reducing CO2 assimilation and transpiration rates. Leaf water potentials were significantly lower at mid-day on high radiation days. The relationship between humidity and temperature in the greenhouse was linear, while a logarithmic regression had better fit to the relationships between photosynthetic photon flow and CO2 assimilation. The relative humidity recorded at mid-day on sunny days (47-50%) suggests the use of atomizers to prevent increments in vapor pressure deficits which induce stomata to close.