Snap bean (Phaseolus vulgaris L.) is a horticultural crop that is enhanced during Fall in Rio Grande do Sul State, which can be favored by growing inside a plastic greenhouse (protected cropping). The estimation of the time interval between the appearance of two successive nodes on a dicot stem, known as plastochron, is important in modeling studies and for crop management. The objective of this study was to evaluate the plastochron and the yield of snap bean grown inside plastic greenhouse (protected environment) and in the open field. An experiment was carried out in Santa Maria, RS, Brazil, during the Fall 2009 in two environments: protected environment (plastic house covered with 100 mu m of thickness low density polyethylene) and in the open field. Solar radiation, air temperature, thermal time, the main stem node appearance (plastochron) and pod yield were determined. Solar radiation was, 26% lower in the protected environment, and minimum, mean and maximum air temperatures were greater in the protected environment. The plastochron was higher (lower node appearance rate) in the protected environment whereas pod yield was similar in the two environments. The reduction in solar irradiation, caused by the plastic cover, was pointed out as a major cause for the lower node appearance rate (greater plastochron) of the plants in the protected environment whereas reproductive growth in the protected environment was similar to the one in the open field most likely because of the increase in the diffuse fraction of solar radiation that partially compensated the plastic shading, and because of the less plants exposure to possible mechanic stresses as wind and hydraulic during the hottest hours of the day when water demand is the highest.
