| Summary: | Rose plants in general, and especially rose plants of the Hybrid Tea cultivar, Grand Gala, are known
for their demanding nature with respect to fertiliser, irradiance levels, night and day temperatures,
carbon dioxide concentrations as well as irrigation water. The objective of the current experiment was
to quantify the effect of different irrigation volumes on the production of cut-roses grown in a
commercial greenhouse under South African conditions. Data was collected from late summer up to
early winter (1 February - 16 June 1999) and consisted of, (1) yield data, (2) stem-quality data and (3)
data of the photosynthesis parameters and environmental variables.
The rose plants were subjected to three different volumes of irrigation water (treatments 2X, X and
1/2X). Treatment X sewed as the control treatment since the plants of this treatment received the same
volume of water (late summer: 1.2l plant-1 day-1; early winter: 1.0l plant-1 day-1) as the rest of the
plants in the greenhouse. The plants of treatment 2X received twice the volume of water than plants
of treatment X, while the plants of treatment 1/2X received only half.
The number of stems yielded by each of the replicates of the different irrigation treatments, over the
20 week experimental period, was determined. The data collected for the stems of the replicas of each
irrigation treatment was pooled into three major length classes viz. short (40 - 84.9 cm), medium (85 -
109.9 cm) and long (1 10 - 165 cm) and the number of stems per length class determined after it was
divided into minor length classes ranging from 40 cm to 165 cm, in 5 cm intervals. The 2X, X and
1/2X treatments yielded 759, 699 and 654 short stems, 2927, 2776 and 2868 medium length stems,
1372, 1409 and 1737 long stems and a total number of stems of 5058, 4884 and 5258 per 556 plants
respectively. The number of short stems harvested did not differ significantly. The number of
medium stems yielded by the 2X treatment was significantly higher than that yielded by the other two
treatments. The 1/2X treatment yielded a significantly higher number of long stems as well as the total
number of stems than that yielded by treatment X and 2X. The Productivity Index (PI) was
determined for the stems in the different major length classes of each treatment as well as for the total
number of stems yielded by each treatment. PI was determined by dividing the fresh weight by the
average length of the specific stems and then multiplying the answer with the total number of stems
harvested per length class or per treatment. The PIS of the total number of stems harvested per
treatment were 2388,2285 and 2401 (g cm-1 total number of stems) for the 2X, X and 1/2X treatments
respectively. As the PI is a function of the total number of stems, it concurs with the yield data, only
emphasising the fact that the 1/2X treatment yielded a significantly higher number of stems than the 2X
and X treatments.
The quality of the stems in the "long" length class was found to be of the highest, while the stems in
the "short" length class were of the lowest quality and that of the stems in the "medium" length class
to be of intermediate quality. The quality criteria measured were the leaf area, fresh weight, flower
bud length and the stem diameter. It was also observed that the quality of the leaf area, fresh weight
and flower bud length increased as the winter period was closing in. The stem diameter of the short
stems also increased as the experiment continued, while the stem diameter of the medium and long
stems decreased slightly. When the stems within a certain length class were compared, no significant
difference was found between the different irrigation treatments with respect to the four mentioned
quality criteria; however, the stems in the "long" length class had the highest Quality Index (QI), while
the short stems had the lowest. The QI was determined for the stems of the three irrigation treatments
in each length class by dividing the average fresh weight by the average stem length of the stems of
that specific length class. The general trend was a slight inclination in the QI over the 20-week period.
No significant difference was found when the QI of the stems in a specific length class was compared,
irrespective of the irrigation treatment.
Measurements of the photosynthesis parameters as well as the environmental variables were taken at
07h00, 12h00 and 15h30 daily. The photosynthesis parameters included the rate of photosynthesis
(A), transpiration rate (E), stomatal conductance (G) and the internal C02 concentration (C,), while the
environmental variables included the irradiance level (I), ambient temperature (T,) and external C02
concentration (C,). The Water Use Efficiency (WUE) was determined by calculating the ratio
between A and E. No significant differences were found between the respective photosynthesis
parameters of the three irrigation treatments measured at 07h00, 12h00 and 15h30 respectively.
It can be concluded that the treatment receiving the lowest volume of water (treatment 1/2X) had the
highest productivity, since it had the highest PI-value for the total number of stems harvested.
Although no significant differences were found between the photosynthesis parameters of the different
irrigation treatments, the average leaf surface area of the stems yielded by the 1/2X treatment as well as
the Leaf Area Index (LAI) of the plants was the highest. This effectively means that the plants of the
1/2X treatment had the highest photosynthetic capacity, which explains the high productivity of the
plants of this treatment. === Thesis (M.Sc.)--North-West University, Potchefstroom Campus, 2004.
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