IN-VITRO HARDENING OF RED RASPBERRY THROUGH CO2 ENRICHMENT AND RELATIVE-HUMIDITY REDUCTION ON SUGAR-FREE MEDIUM

Micropropagated shoots of red raspberry (Rubus idaeus L. 'Comet') were rooted on modified Murashige-Skoog medium lacking sucrose, in special ly 'constructed plexiglass chambers, under ambient (340 +/- 20 ppm) or enriched (1500 +/- 50 PPM) CO2 and ambient (ca. 100 %) or reduced (90 +/- 5 %) relative humidity. Cultured plantlets were evaluated for the ir survival, rooting and relative vigor, leaf and root number, stem an d root length, total leaf area, total fresh and dry weight, gas exchan ge rate, and stomatal features, prior to transplantation to soil and a t intervals for 6 wk ex vitro. In vitro CO2 enrichment promoted plantl et growth, rooting and both the survival and early growth of transplan ts. CO2 enrichment increased stomatal aperture of plantlet leaves but did not apparently increase water stress at transplantation. Reduced i n vitro RH did not affect plantlet growth but decreased stomatal apert ures and stomatal index on leaves of cultured plantlets and promoted b oth the survival and early growth of transplants. In vitro CO2 and RH levels did not affect the photosynthetic rate of either plantlets or t ransplants. Only the stomata on leaves of plantlets from the ambient C O2 and reduced RH treatment were functional. Normal stomatal function was not observed in persistent leaves of transplants from the other tr eatments, even 2 wk after transplantation. In vitro CO2 enrichment act ed synergistically with RH reduction in improving growth of plantlets both in vitro and ex vitro. Hardened red raspberry plantlets obtained through CO2 enrichment and RH reduction survived direct transfer to am bient greenhouse conditions without the necessity for specialized ex v itro acclimatization treatment.