Quantifying the effect of different irrigation volumes on cut-rose production
Neethling, Maria Gezina Catharina
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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.