Quantifying water use and water productivity of high performing apple orchards of different canopy sizes

General aim: 

To determine the water use, yield and quality of selected high performing apple cultivars from planting to full-bearing in selected climatic zones and specific soils.

Specific objectives:

• To measure unstressed apple orchard water use according to seasonal growth stages from planting to full-bearing.
• To model the water balance of apple orchards according to seasonal growth stages from planting to full-bearing for future extrapolation to other apple cultivars and climatic zones.
• To determine the water productivity in full-bearing orchards in terms of crop yield in relation to quality.

The study was conducted in the KBV and EGVV production regions in the Western Cape. Both regions have a Mediterranean-type climate although their microclimates differ. KBV experiences cold winters with the long-term average minimum daily air temperatures for the coldest month (July) being 3 to 4 °C and occasional snowfalls. Summers are generally hot and dry with a mean maximum air temperature for the hottest month (February) reaching 28 to 29 °C. In contrast, EGVV experiences milder winters and summers as the weather is

moderated by proximity to the Atlantic Ocean to the south west. Mean minimum daily temperatures in winter are between 8 and 9 °C while average maximum summer temperatures are between 25 and 26 °C.

Cultivars studied were Golden Delicious, which is most widely planted in South Africa occupying approx. 24% of the area under apples, and the blushed cultivar Cripps’ Pink and its close relatives Cripps’ Red and Rosy Glow. Both the Golden Delicious and the blushed cultivars are high yielding. The blushed cultivars were selected as these are high value late season cultivars with the highest growth potential. We hypothesized that the blushed cultivars used the greatest amount of water given that they maintain a high leaf area for longer compared to the other cultivars. The specific blushed cultivar variant used depended on the availability of suitable orchards in a particular growing region. However, as the blushed cultivar variants are all close relatives, no differences in eco-physiological  responses and water use patterns were expected.

Data were collected over three growing seasons namely 2014/15, 2015/16 and 2016/17 (Table I). In 2014/15, data were collected from October to June in two full-bearing and two non-bearing orchards in KBV. The mature full-bearing orchards had a high effective canopy cover varying from 45 to 52% while the young non-bearing orchards had a low canopy cover between 14 and 26%. In the 2015/16 season, data were collected also in four orchards, comprising two full-bearing and two non-bearing orchards, but in the EGVV region. In the 2016/17 season measurements were taken in two orchards in each production region with medium canopy cover ranging from 30 to 44%. Soil types were predominantly sandy to sandy loam except for the full-bearing ‘Cripps’ Pink’ at Radyn, non-bearing ‘Golden Delicious’ at Vyeboom and the ‘Cripps’ Pink’ at Dennebos, all in EGVV. These orchards had dark red clayey loam soils with a high stone content.

All orchards were irrigated using the micro-sprinkler system. There was one micro-sprinkler per tree delivering between 30 and 35 litres of water per hour. Irrigation frequency ranged from two to three times per week with each event lasting for one to two hours early in the season. The irrigation frequency increased to daily or several times a day during the hot summer months in some orchards.

Three methods were used to quantify the orchard evapotranspiration (ET) to reduce uncertainties in the water use estimates. These included the open path eddy covariance method, which was deployed at selected window periods during the growing season due to equipment limitations. The second technique was the soil water balance approach which

was used in two orchards each season, also due to equipment limitations. Thirdly, additional ET data were derived from the remote sensing “FruitLook” product. Interpolation of the eddy covariance ET to seasonal water use was done using a dual source ET model. We adopted and propose improvements to the Shuttleworth and Wallace model applied to apple orchards with varying canopy cover.

Table I. Summary of the study sites used in the KBV and EGVV production regions from 2014-2017. High, medium and low canopy cover denotes >45%, 30-44% and <30% vegetation cover, respectively.

Additional data collected include the orchard leaf area index (LAI – m² of leaf area per m² of ground area), volumetric soil water content at various depths and wet/dry spots in some orchards, soil properties, orchard floor evaporation, tree water status, leaf stomatal conductance and gas exchange rates, yield and fruit quality.

Using the data collected on all eight productive orchards (medium and high canopy cover, both cultivars and both regions) for seasonal water use and yield, we calculated the water use efficiency, defined as kg fruit per m³ of water used. This was done based on both measured tree transpiration and modelled orchard evapotranspiration. The gross orchard value from packout and price data (thus integrating all quality parameters) were calculated and combined with seasonal water use to estimate the water productivity of all the orchards. This represents the commercial value of the harvest (in Rand) per m³ of water used.

In the 2014/15 season, the Koue Bokkeveld sites received ~  240  mm  of  rainfall between  1 October 2014 and 30 June 2015. The total short grass reference evapotranspiration (ET_o) over the same period was ~ 1 260 mm which was more than five times higher than the rainfall. The maximum air temperature for the season reached ~ 37 C on 3 March 2015 while the daily ET_o peaked at ~ 8.8 mm. Climatic conditions in EGVV during the 2015/16 season were somewhat milder than those measured the previous year in KBV. Maximum air temperature in EGVV was 39.7 C measured on 30 December 2015 with the maximum daily ET_o of 7.3 mm. The seasonal total rainfall (247 mm) was similar to that received in KBV the preceding year although the ET_o was significantly lower at 1 065 mm. Weather conditions during the 2016/17 season followed similar trends to the previous years although the rainfall was significantly lower due to the prevailing drought.

The solar radiation and water vapour pressure deficit of the air (VPD) were the main climatic factors driving water use of both the Golden Delicious and the blushed cultivars. There was a strong linear relationship (R²>0.70) between the daily solar radiation and the daily total transpiration of the unstressed trees. However, the relationship between the transpiration and VPD was non-linear with peak transpiration reached at VPDs between 2.0 and 3.0 kPa. Seasonal total transpiration of the trees was better related to canopy cover than to crop load as shown in Table II. Orchards with high yields e.g. the full-bearing ‘Cripps’ Pink’ in KBV and EGVV did not necessarily have the highest transpiration rates. Variations in tree  transpiration rates in full-bearing orchards were a result of differences in canopy size. ‘Cripps’ Pink’ orchards, for example, had relatively small and open canopies due to pruning and use of shoot growth retardants such as Regalis® (a formulation of Prohexadione-Ca). Open canopies expose the fruit to solar radiation for anthocyanin synthesis to occur and to promote the development of the red fruit colour. On the other hand, full-bearing ‘Golden Delicious’ orchards had larger canopies since these are managed to provide more shade to fruit which are susceptible to sunburn.

The maximum unstressed seasonal transpiration of mature high yielding ‘Cripps’ Pink’ and ‘Golden Delicious’ orchards was in the range 6 000 to 8 000 m³ ha^-1 depending on canopy cover. The maximum orchard ET varied from 9 000 to just over 10 000 m³ ha^-1 season^-1. In young orchards seasonal total transpiration ranged from 1 330 m³ ha^-1 in the low density plantings in EGVV to 2 710 m³ ha^-1 in the high density orchards in KBV. Seasonal ET was very high (> 5 000 m³ ha^-1) in all the young orchards. This was due to the large exposed orchard floor area which increased the soil and cover crop evaporation fluxes.

The long growing season of ‘Cripps’ Pink’ did not translate to higher seasonal water use compared to the shorter growing season of ‘Golden Delicious’. This was because the winter (May and June) transpiration contributed less than 12% of the seasonal total transpiration in these orchards. Tree transpiration contributed between 65 and 82% to the total orchard ET in full-bearing orchards – depending on canopy cover. In young orchards, orchard floor evaporation accounted for more than 60% of ET, which was clearly excessive.

Leaf level measurements of stomatal conductance and gas exchange (photosynthesis and transpiration rates), together with measurements of stem water potential, corroborated the results obtained using sap flow techniques and measurements of soil water content. In some cases, periods of water stress were identified, but in general the data confirmed that the orchards were not water stressed. All orchards (except for some periods in EGVV where afternoon cloud develops) showed the characteristic decline in stomatal conductance from morning to afternoon in response to increasing atmospheric demand (vapour pressure deficit between the air and leaf tissues, VPD_leaf), increasing water loss and reductions in stem water potential. This generally stabilised transpiration or reduced further increases in transpiration beyond a VPD_leaf of around 3 kPa.

Leaf level photosynthetic water use efficiency is not always optimised in apple trees. Under well-watered and atmospherically milder conditions, carbon assimilation is prioritised to meet the high demand for assimilates in bearing trees. There were indications in EGVV that ‘Cripps’ Pink’ maintained higher gas exchange rates later in the season than ‘Golden Delicious’ in response to the high assimilate (sink) demands of the fruit crop which is only harvested in April. Across all orchards, stomatal conductance and transpiration rates remained higher in high canopy cover (full-bearing) trees with increasing VPD_leaf compared to medium and low canopy cover trees, suggesting that higher water use in these orchards  is not only due to the high total leaf area, but also due to the high sink demand of the large fruit crop. Furthermore, analysis of stomatal conductance and stem water potential revealed a likelihood that high canopy cover trees with many fruit have an internal water buffer which is used for the higher rate of transpiration during the day, allowing for higher conductances and thus higher photosynthetic rates. Low canopy cover non-bearing trees with a much lower demand for assimilates and a limited water buffer kept conductances lower even under mild evaporative conditions, to reduce transpired water losses and prevent stronger reductions in stem water potential. Lastly, it was found that under the more stressful prevailing atmospheric conditions (higher evaporative demand) in KBV compared to EGVV, similar stem water potentials were maintained, suggesting some form of acclimation of xylem hydraulic characteristics to prevent damage to the xylem.

There were no clear effects of the high crop load on most fruit quality attributes. Only the full- bearing ‘Golden Delicious’ orchards in both production regions had smaller fruit size which affected packout of export quality fruit. The water use efficiency varied with production region and with cultivar given the different microclimates and canopy management practices for the Cripps’ Pink and Golden Delicious cultivars. The key driver of the water use efficiency of the trees was the leaf area which determined transpiration.

Table II. Summary of the seasonal (Oct-Jun) water use rates of apple orchards from planting to full-bearing in the KBV and EGVV production regions from the 2014/15 to 2016/17 season. T – represents orchard level transpiration, and ET represents the orchard evapotranspiration. Transpiration data was derived from sap flow measurements while ET was simulated from the Shuttleworth and Wallace model. Water productivity is based on measured transpiration.

Lower transpiration from smaller canopies of ‘Cripps’ Pink’ was compensated for by higher evaporation from the orchard floor, compared to these values in ‘Golden Delicious’, so that water use efficiency based on modelled ET was similar between the cultivars. The length of

the growing season was not important since the canopies continued to be highly active until the autumn in both cultivars. Water use efficiency was greater under higher yields.

Lastly, the water productivity (Rand of gross income per cubic metre of water consumed) was higher for ‘Cripps’ Pink’ than for ‘Golden Delicious’ (Table II). The primary reason for  this is that export-quality ‘Cripps’ Pink’ (Pink Lady®) fetches a higher price than export- quality ‘Golden Delicious’ and this has a significant influence on orchard gross value. A secondary reason was that the two full-bearing ‘Golden Delicious’ orchards produced a high proportion of small fruit of lower value. The medium canopy cover ‘Golden Delicious Reinders’ orchard in KBV had a lower water productivity than expected due to losses ascribed to sunburn and bruising. Generally, water productivity was found to be higher under higher yields, but in ‘Golden Delicious’ it is likely that a maximum water productivity is reached between 75 and 100 t ha^-1, where after further increases are constrained by fruit size issues.

This study showed that high apple yields can be produced sustainably without using excessive amounts of water provided the canopy is managed optimally. Current ‘Cripps’ Pink’ canopy management practices promote the development of the red colour on the fruit and also have water saving benefits. High crop loads in this study did not necessarily have a negative effect on most fruit quality attributes in the high yielding orchards. In the high yielding ‘Golden Delicious’ orchards only fruit size was affected, and management of crop load is essential in this cultivar to produce export quality fruit and maximise water productivity. Length of the growing season of different cultivars appeared not to influence the seasonal total water use. Thus, our hypothesis relating to water use and season length must be rejected.