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An investigation into possible water savings with sub-surface irrigation (Part II) - Plant water stress, growth, yield and quality
Philip Myburgh, ARC Infruitec-Nietvoorbij, Stellenbosch
Key words: Grapes, irrigation, water potential, shoot growth, yield, raisin quality
Introduction
As a result of periodical droughts, producers in the Lower Orange River region have to contend with water restrictions. The current Water Act also stipulates that licences to use water for agriculture will only be allocated if the water is being used effectively. What is more, the Water Allocation Reform (WAR) policy, in terms of which water may be allocated to communities that did not have access to it previously, is soon to be implemented by the Department of Waterworks and Forestry (Roux, 2006). The WAR policy will put additional pressure on existing water resources, and may possibly result in existing allocations being reduced. Consequently there will be more pressure on producers in future to use the available water more effectively. Sultanina (Sultana, Thompson’s Seedless) is the most important cultivar in the Lower Orange River. If this cultivar experiences water deficits in summer as well as winter, it may seriously jeopardise grape yield and quality (Myburgh, 2003a; Myburgh, 2003b). Water deficits may also be detrimental to the vigour and yield of other cultivars such as Chenin blanc and Colombar, that are also cultivated in the region (Van Zyl & Weber, 1981; Van Zyl, 1984). It is therefore essential that any water saving practices do not impact negatively on the profitability of grapevines. The purpose of the project was to compare sub-surface irrigation methods on alluvial soils to more traditional above-ground methods with a view to possible water conservation, without reducing yield levels and grape quality.
Materials and methods
The field trial was carried out near Upington on Sultanina, which is being cultivated for the production of dried grapes. Different ways of applying water sub-surface were compared to furrows and above-ground drip (Table 1). The wetting effectiveness of drippers with regard to flow rate, as well as the depth at which it is installed, also formed part of the investigation. Each irrigation treatment was repeated four times in a randomised block design. On each trial site the 12 experiment grapevines were bordered by two grapevines at the ends of the experiment rows, as well as on both sides by two rows to prevent overlapping of treatment effects. The treatments were applied from the 2001/02 to 2003/04 seasons. More comprehensive information about the soil conditions, irrigation methods and quantities as well as other viticultural aspects have been published (Myburgh, 2007).
Plant water stress was quantified by measuring leaf water potential at various stages during the course of the trial. These measurements were taken either pre-dawn or at 12:00. On two occasions the daily course of the water stress was also measured. On 23 January 2004 leaf water potential over the course of the day was measured on a wet and dry experiment plot, respectively. Unlike the usual two-hourly intervals, the leaf water potential of three grapevines on each of the two plots was measured every 15 minutes from 09:45 to 18:00. Shoot growth was quantified by weighing the shoots at pruning. Yield was determined by weighing the total amount of grapes on each experiment plot and converting this figure to tons per hectare. Berry mass as well as sugar and acid content of the grapes were determined at harvest. Sun dried as well as leached raisins were made, whereafter the quality thereof was graded according to commercial standards at the processing plant in Upington. The water use efficiency was quantified in terms of the production water use efficiency (WUEp), which is defined as the grape mass (kg) produced per unit volume irrigation water (m3) (Myburgh, 2003c).
Results and discussion
Plant water stress: On 28 November 2001, just before the clogged porous pipe was replaced by above-ground dripper lines (Myburgh, 2007), the leaf water potential of grapevines irrigated using furrows (T1), above-ground drip (T2), sub-surface drip (T4) and porous pipe (T8), was -1.47 MPa, -1.46 MPa, -1.44 MPa and -1.78 MPa, respectively during the warmest time of the day. There was consequently considerably more water stress in the grapevines that had been exposed to water deficits as a result of the clogged porous pipe. The leaf water potential in grapevines of the measured treatments was also lower than -1.2 MPa, which is generally accepted to be the water status at which negative effects on plant physiological processes start manifesting themselves (Williams et al., 1994). At about 12:00 on 25 November 2002 there was a tendency towards less water stress only in grapevines that were irrigated using furrows (Table 1). This indicated that furrows resulted in more effective wetting than the other methods. The scope of this investigation did not allow for an acceptable explanation why water stress in grapevines of the drainage pipe treatment on the day in question was significantly higher compared to T1.
Measurement of leaf water potential over the course of the day on 17 December 2003 once again showed that there were no significant differences in water stress between grapevines of the respective treatments during berry development (Fig. 1). Water stress in grapevines irrigated with drippers at 30 cm depth (T4), was in fact significantly higher a few times in the course of the day compared to some of the other treatments. However, there is not a meaningful explanation why water stress was higher in T4 grapevines at the specific times. Leaf water potential of all the treatments was lower than -1.2 Mpa during the warmest time of the day, which indicates that there was in fact a degree of water stress that may have hampered the physiological activities during the day. Before sunrise leaf water potential of all the treatments was higher than -0.4 MPa (Fig. 1). This was also an indication that severe negative effects on plant physiological processes had not yet manifested themselves (Williams et al., 1994). Consequently there was sufficient water in the soil for the water status in the grapevines to recover during the night.
During the 2003/04 season leaf water potential before sunrise was measured shortly before the harvest on 22 January 2004. The water status in grapevines of all the treatments varied from -0.21 MPa to -0.28 MPa and there weren’t any significant differences between any treatments (data not shown). The high values indicated that the water status in the grapevines also recovered during ripening in the course of the night. The next day the intensive measurements showed that for the most part of the day, water stress in grapevines in the wetter soil was lower than in the ones in the drier soil (Fig. 2). However, leaf water potential had an oscillating tendency in the dry as well as the wet plot during the warmest part of the day. Seeing that the atmospheric conditions did not fluctuate over short periods in the course of the day (Fig. 3), the oscillations were probably caused by partial stomatic closure. The regulation of the water stress was so effective that the stress in grapevines on the drier plot was even lower in the course of the afternoon than stress on the wetter plot. It is also clear that regulating did not cause a linear increase in water stress to coincide with the decrease in air moisture over the course of the day, or with the increase in vapour pressure deficit (VPD) (Fig. 3). The aforegoing explains to a certain extent why there were generally no prominent differences in water status between treatments. It also proves in no uncertain terms that Sultanina is able to prevent serious water stress under warm, dry conditions, and is therefore able to perform well in the Lower Orange River region.
Vegetative growth: The average growth vigour was comparable to that of Sultanina which was also irrigated using furrows in a previous investigation, but lower compared to grapevines that were flood irrigated over the total surface (Myburgh, 2003c). The shoot growth was also comparable to that of Sultanina (clone H4) that received micro-sprinkler irrigation twice weekly on sandy soil away from the river (Myburgh, 2003a). Although there were signs of water stress during the day, grapevines of all the treatments were vigorous enough for the development of sufficiently strong bearing canes. Over the three seasons grapevines irrigated using the different permanent systems (T2 to T7), showed the same vigour compared to the furrow irrigation (T1) (Table 2).
Yield: In general good berry mass (ca. 1.9 g/berry) for raisin production was obtained by all the irrigation methods (Table 2). This was even higher than the 1.7 g/berry where Sultanina on sandy soil away from the river was irrigated twice a week using micro-sprinkler irrigation which wetted the total surface (Myburgh, 2003a). Slightly higher water stress that occurred during the first two seasons probably caused the average berry size of grapes that were irrigated sub-surface using drainage pipe (T7), to be smaller than berries of grapevines irrigated using furrows (T1). Where water was applied sub-surface with higher flow rate drippers (T6), the bigger berries could possibly be ascribed to more water being applied (Myburgh, 2007). The rest of the sub-surface irrigation treatments did not have a significant effect on average berry mass compared to furrow irrigation or above-ground drip.
Average yields obtained with the H5 clone over three seasons, were relatively high compared to the long term average of approximately 20 t/ha for Sultanina in this particular region. The yields were also considerably higher compared to that of Sultanina in alluvial soil irrigated over the total surface (Myburgh, 2003c). However, the latter results were obtained with less fertile "Orange River" Sultanina. The yields were comparable to Sultanina (clone H4) that received micro-sprinkler irrigation twice weekly on sandy soil away from the river (Myburgh, 2003a). None of the respective irrigation treatments had a significant effect on yield, however (Table 4). Except for the high flow rate drippers (T6), more or less the same quantity of water was given in all the treatments (Myburgh, 2007). The tendency to higher yield with the furrows was therefore possibly due to the fact that the irrigations were given in one application throughout. This resulted in more effective wetting compared to the other treatments (Myburgh, 2007). Where irrigation with higher flow rate drippers was applied sub-surface (T6), the tendency to higher yield could possibly be ascribed to more water. Seeing that most of the water and root distribution patterns were comparable (Myburgh, 2007), bigger, more effective root systems could not have caused the tendency to higher yields of T1 and T6 compared to some of the other treatments.
Despite the gradual drying out of the soil during the warmest part of the season (Myburgh, 2007), yields were considerably higher than the average for dried grape vineyards along the Lower Orange River. On the other hand the gradual drying out of the soil could have restricted vigorous shoot growth. The vineyard was to a certain extent irrigated according to a regulated deficit irrigation strategy. It seems therefore that it may be possible to prevent the vigorous shoot growth that occurs on the alluvial soils, in particular when such an irrigation strategy is followed, without sacrificing yield. If deficit irrigation is applied, however, all soil and viticultural management practices will have to be optimal.
Sugar and acid content in the must: The average sugar and acid content of the must were 21.5°B and 5.8 g/L, respectively, over the three seasons. The different sub-surface irrigation treatments did not have a significant effect on sugar and acid content in the must compared to furrow irrigation or above-ground drip during any of the three seasons (data not shown). Despite the relatively high crop load and signs of water stress during the day, none of the treatments caused problems with regard to an increase in sugar. During all three seasons the grapes of all the treatments could be harvested on the same day.
Raisin quality: During the 2002/03 season the sun dried raisins dessicated to such an extent during a heat wave in the course of a weekend that they could not be properly graded. The leached raisins were damaged by rains during the 2003/04 season to such an extent that they could not be graded either. For both sun dried and leached raisins data could therefore only be collected over two seasons. The respective sub-surface irrigation treatments did not have a significant effect on average percentage choice grade sun dried or leached raisins compared to furrows or above-ground drip during either of the two seasons (Table 3). There is no acceptable explanation why sub-surface drip at 2.3 L/hour at 15 cm and 45 cm depths resulted in less standard grade raisins compared to some of the other treatments.
Water use efficiency: The total amount of irrigation water that was applied to all the treatments on average per annum (Table 4), was considerably lower than the 15 000 m3/ha to 18 000 m3/ha quotas that are currently allocated to producers along the Lower Orange River. The average production water use efficiency of Sultanina was considerably higher (Table 4) compared to the approximately 2 kg/m3 where flood irrigation was applied over the total surface (Myburgh, 2003a). Drip irrigation, be it above-ground or sub-surface, did not improve the production water use efficiency compared to furrows (Table 4). It should be borne in mind, however, that flood irrigation applied in short beds is more effective than when applied in long beds. It could therefore be that the short furrows that were used in the trial were not entirely representative of the situation in practice and may have benefited growth and yield ahead of drip irrigation.
Conclusions
It is fascinating that the minimum leaf water potential over the hottest part of the day was below -1.2 MPa on all the days in question, but that sufficient vigour and above-average yields could be maintained while retaining quality. The only exception was where the clogged porous pipe caused serious water deficits. Similar values were also measured previously in Sultanina in alluvial soil (Myburgh, 2003c). This indicated that the water stress threshold value of -1.2 MPa could possibly have been too high for dried grape Sultanina along the Lower Orange River. A value of -1.8 MPa on the other hand showed that the grapevines experienced serious water deficits. It may therefore be more realistic to use a threshold value of -1.5 MPa as an indication that irrigation is necessary. With furrows or drip irrigation considerable water savings, while retaining above-average yield and acceptable raisin quality, are indeed possible for the cultivation of grapes on the alluvial soils of the Lower Orange River. Drip irrigation, be it above-ground or sub-surface, did not offer additional water savings compared to furrows.
Recommendations
Acknowledgements
The Agricultural Research Council for infrastructure and other resources, Winetech for partial funding as well as Mr Leon van der Walt and the Soil Science personnel at ARC Infruitec-Nietvoorbij for technical assistance.
For more information contact Philip Myburgh on myburghp@arc.agric.za.
Literature references
Myburgh, P.A., 2003a. Responses of Vitis vinifera L. cv. Sultanina to level of soil water depletion under semi-arid conditions. S. Afr. J. Enol. Vitic. 24, 16 - 24.
Summary
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