Abstract:
Many investigators have suggested that current photosynthate
supply can limit adventitious root formation on leafy cuttings.
Accordingly, studies were undertaken to determine the effects of
altering photosynthesis and carbohydrate levels on adventitious root
formation by leafy cuttings of a number of different species.
Pea cuttings rooted at high CO₂ (1800 μl/liter) or 87% relative
humidity (high RH) had increased root dry weight and root length
compared to those rooted at normal CO₂ (350 μl/liter) or low RH
(55%). The difference in root dry weight was due to increased
photosynthesis and hence increased carbohydrate supply under high CO₂
or RH. The increased root length on the cuttings rooted at high CO₂
or RH, however, was probably at least in part due to increased water
potential (Ψw). Cuttings of 8 ornamental species were also rooted at
elevated CO₂ (1200 μl/liter). Root system size was increased in 5 of
the species and was unaffected in the other 3 indicating that the
effects of CO₂ enrichment during rooting are species dependent. In
some species CO₂ enrichment during rooting decreased the time to
obtain heavy, well-rooted cuttings probably by increasing
carbohydrate levels and raising Ψw.
When net photosynthesis by pea cuttings was reduced by several
different treatments the number of roots per cutting decreased, but
this decrease was overcome by the application of indole-3-butyric
acid (IBA) or partially overcome by the application of exogenous
sugars. These results suggest that when daily net CO₂ fixation per
cutting was reduced both endogenous auxin and carbohydrates
simultaneously limited the number of roots formed per cutting. The
application of IBA to cuttings rooted during light saturated
photosynthesis also enhanced rooting suggesting that endogenous auxin
limited rooting regardless of the supply of current photosynthate.
Experiments were conducted to alter carbohydrate levels in
Rhododendron cuttings by storing cuttings in the dark or by rooting
them under different shade levels. Although these treatments had
some influence on carbohydrate levels, the effects were not large
enough to significantly influence rooting or were opposed by other
effects which masked the influence of carbohydrates on rooting.
