| Summary: | The role of light and temperature on flowering of South African Watsonia species
were evaluated to assess the potential for this genus as a commercial flower crop.
Species were selected that represent different climatic regions of South Africa, with
the aim of understanding how ecologically distinct species perform under cultivation.
The four selected species were W. borbonica and W. tabularis (winter-rainfall area),
W. angusta (shared rainfall) and W. pillansii (summer-rainfall area).
In order to establish the optimum temperature required for flowering, plants were
exposed for 12 weeks to three temperature regimes (12/7 °C, 21/15 °C and 29/21 °C)
after attaining their first and/or second leaves. A temperature shift of 12/7 °C was
used to assess if the plants had a vemalisation requirement. Controls were
maintained under 25 % shade under natural conditions, with an average temperature
of 24/7 °C. An elevated temperature of 29/21 °C was detrimental to plant growth.
Moderate temperatures of 21/15 °C significantly (P<0.001) increased the height and
the number of leaves produced per plant relative to the 12/7 °C treatment. These
temperatures significantly (P<0.001) increased the total number of flowers produced
per plant compared to low temperatures. However, flowering percentage and quality
of flowers were reduced.
A low temperature regime of 12/7 °C was efficient in satisfying vernalisation
requirements and inducing flowering in four selected species. However, the total
number of leaves produced per plant was signifcantly reduced. The summer-rainfall
species, W pillansii, displayed a qualitative response to vernalisation, as no flowering
was observed in non-vernalised plants. Two winter-rainfall species, W borbonica and
W. tabularis, demonstrated a quantitative response to vernalisation. These species
flowered at non-vernalising temperatures. W angusta behaved like the winter-rainfall
species in terms of flowering. Overall, a vernalisaton treatment marginally reduced
days to flower while flowering percentage was increased compared to other
temperature regimes. However, there was no increase in the total number of flowers
produced per plant. Low temperatures were not only effective for flower induction, but
also for releasing corm dormancy, thus synchronising growth. Storing corms at either
4 or 10 QC resulted in 100 % sprouting within 4-6 weeks.
The role of daylength in flowering of Watsonia plants was established by subjecting
plants to long days (LO) of 16 h light and 8 h dark and to short days (SO) of 8 h light
and 16 h dark. The number of leaves and flowering were significantly (P<0.01)
promoted under the LO regime. However, there was strong temperature and
daylength interaction in terms of flowering potential, as at low temperatures flowering
was induced irrespective of daylength. In W. pillansii, flowering was obtained under
both regimes (LO and SO) applied at the second leaf stage. Flowering in W.
borbonica and W. tabularis was only observed under the LO regime at the second
leaf stage. In both species, flowering was also obtained in SD-treated plants,
provided treatment occurred after the formation of the third leaf. However, the total
number and quality of flowers were reduced.
To examine the effect of light intensity on flowering, plants at different developmental
stages (first and/or second or beyond the third leaf stage) were exposed to
photosynthetically active radiation (PAR) of 150 jJmol m-2s-1 or 39.5 jJmol m-2s-1 for 7
weeks. Exposure to low light intensity at either developmental stage compromised
leaf quality. No flowering was observed following low light intensity treatment during
the first to third leaf stages, even though plants were exposed to low temperature and
LO regimes, both of which promoted flowering. Observation of the shoot apical
meristem revealed that the second leaf stage was critical as the anatomical transition
to flowering occurred at this level. When beyond the third leaf stage, low light intensity
did not prevent flowering. However, the number of flowers produced per plant was
reduced compared to plants maintained at 150 jJmol m-2s-1. Thus, light intensity
played a role in both plant morphogenesis and flowering. LDs were effective in
promoting vegetative growth whereas high light intensity and low temperature
regimes played pivotal roles in flower induction. This makes them useful horticulture
tools to produce desirable Watsonia plants for commercialisation. === Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2006.
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