WATER-LOSS AND PHOTOSYNTHESIS OF PLUM PLANTLETS IS INFLUENCED BY RELATIVE-HUMIDITY DURING ROOTING IN-VITRO

Shoots of Mr. S. 2/5, a plum rootstock selected from Prunus cerasifera seedlings, were rooted in vitro at 100%, 90%, 80% and 70% relative hu midity (r.h.). Decreases in r.h. led to increased resistance to water loss when the plantlets were subsequently exposed to a low environment al relative humidity. Statistically significant differences were obser ved after only 20 min of exposure. Differences increased progressively over time up to the end of the 90 min exposure period. In 80% and 70% r.h. treatments, the leaf waxy mesh was denser and presented thicker filaments. In addition,the reduction in r.h induced a decrease in stom atal density; stomata were also more ellipsoidal and had a smaller ope ning. Net photosynthetic activity at the end of the in vitro rooting s tage showed a slower rate in plantlets obtained from the 80% r.h. trea tment compared with 100% r.h. plantlets; however, the total CO2 uptake over an 8 h period of illumination tended to be similar in plantlets from both r.h. treatments.