Gas exchange in paphiopedilum: lack of chloroplasts in guard cells correlates with low stomatal conductance.

Net photosynthesis and stomatal conductance were measured in attached leaves of Paphiopedilum insigne. At 20 degrees C and a vapor-pressure deficit of 0.5 kilopascal, both net photosynthesis and stomatal conductance were light-saturated below 0.2 millimole per square meter per second, a response typical of shade plants. The absolute values of photosynthetic rate and conductance however were remarkably low, presumably reflecting an adaptation to the low-light, limited-nutrient habitat characteristic of these orchids. The leaves also showed a vapor-pressure deficit response, with net photosynthesis and conductance varying over a 2-fold range between 0.3 and 1.6 kilopascals.These results confirm that Paphiopedilum stomata are functional. The correlation between achlorophyllous guard cells and low conductance rates, however, singles them out as an exceptional biological system, exhibiting basic differences from typical stomata in higher plants. Available evidence showing that guard-cell chloroplasts are needed to sustain high conductance rates at moderate to high irradiances indicates that the genetic changes leading to the loss of chloroplast differentiation in Paphiopedilum guard cells were not deleterious because of the low conductance rates characteristic of this genus.