IN-VITRO HARDENING OF RED RASPBERRY BY CO2 ENRICHMENT AND REDUCED MEDIUM SUCROSE CONCENTRATION

Micropropagated 'Festival' red raspberry (Rubus idaeus L.) shoots were rooted in specially constructed plexiglass chambers in ambient (340 /- 20 ppm) or enriched (1500 +/- 50 ppm) CO2 conditions on a medium co ntaining 0, 10, 20, or 30 g sucrose/liter. Plantlet growth and leaf (C O2)-C-14 fixation rates were evaluated before and 4 weeks after ex vit ro transplantation. In vitro CO2 enrichment promoted in vitro hardenin g; it increased root count and length, plantlet fresh weight, and phot osynthetic capacity but did not affect other variables such as plantle t height, dry weight, or leaf count and area. No residual effects of i n vitro CO2 enrichment were observed on 4-week-old transplants. Sucros e in the medium promoted plantlet growth but depressed photosynthesis and reduced in vitro hardening. Photoautotrophic plantlets were obtain ed on sucrose-free rooting medium under ambient and enriched CO2 condi tions and they performed better ex vitro than mixotrophic plantlets gr own with sucrose. Root hairs were more abundant and longer on root tip s of photoautotrophic plantlets than on mixotrophic plantlets. The max imum CO2 uptake rate of plantlet leaves was 52% that of greenhouse con trol plant leaves. This did not change in the persistent leaves up to 4 weeks after ex vitro transplantation. The photosynthetic ability of persistent and new leaves of 4-week-old ex vitro transplants related n either to in vitro CO2 nor medium sucrose concentration. Consecutive n ew leaves of transplants took up more CO2 than persistent leaves. The third new leaf of transplants had photosynthetic rates up to 90% that of greenhouse control plant leaves. These results indicate that in vit ro CO2 enrichment was beneficial to in vitro hardening and that sucros e may be reduced substantially or eliminated from red raspberry rootin g medium when CO2 enrichment is used.