Apple Tree Water use under Shade Netting

Changes in microclimate, yield and fruit quality of “Rosy Glow” were measured under a fixed white net.

A new four-year research project was launched in April 2018, to compare the water use of high producing open and netted (fixed and draped) full bearing apple orchards, under optimal management and optimal irrigation. The aim was to determine water savings per hectare and per ton (see Midgley et al., 2019). The project is jointly funded by the Water Research Commission of SA (WRC) and Hortgro Pome. Measurements for the first year have now been completed by the research team at the “Rosy Glow” Orchard of the Future, at Paardekloof, Witzenberg Valley. This orchard was planted in 2010 on MM109 rootstock, with an M9 interstem, on sandy soil, at a spacing of 3.5 m x 1.25 m. The flat white 20% net was installed over a portion of the orchard in December 2014, with irrigation of the netted and open areas being managed separately. This brief update serves to inform the apple industry of the early results. We focus on the changes in microclimate under the net, compared to the adjacent open block, preliminary results on whole tree transpiration through the season, differences in stomatal conductance and stem water potential, and the yield and fruit quality achieved under the net and in the open.

Microclimate changes under a fixed open-sided white net

Most studies of protective netting over fruit orchards have report-ed reductions in direct solar radiation, air temperature and wind speed, and sometimes increases in air relative humidity, so that the microclimate under shade netting is often milder (Smit et al., 2009; Mupambi et al., 2018). In the current study, midday (12:00 to 14:00) peak solar radiation was on average 15% lower under the nets than in the open, and the daily total solar radiation was 12% lower, at most, under the nets than in the open treatment. These figures are very similar to what is known about the percentage reduction in radiation under flat 20% white nets. Mean and maximum daily air temperature under the net followed those in the open very closely, as measured at a similar height above the orchard floor. In published studies elsewhere, temperature of the air, canopy and soil under netting compared to the open can vary widely, and can be both lower and higher, or the same (Middleton and Mc-Waters, 2002; Kalcsits et al., 2017; Mupambi et al., 2018). This is because temperature is the outcome of a complex interacting set of factors, including solar radiation and the shading factor of the net, position of the sensor in the canopy or soil, changes in air circulation, and the local climate. Nevertheless, air and leaf temperatures are frequently reduced (Middleton and McWaters, 2002; Iglesias and Alegre, 2006; Smit et al., 2009; Solomakhin and Blanke, 2010). Differences in air temperature between covered and open orchards are, however, less pronounced on windy days (Solomakhin and Blanke, 2010) and air temperature may even increase where wind is significantly reduced (Iglesias and Alegre, 2006). In our study, there was a clear and significant reduction in the wind speed, by up to 56%, under the net compared to the open. We found no difference in the air relative humidity (RH) measured inside and outside the nets. The results for air temperature and RH imply that the vapour pressure deficit (VPD) of the air was also similar in the two treatments. Calculation of the reference evapotranspiration (ETo) showed that the net reduced ETo by around 14%, al-though the figure may be as high as 20% under some conditions. The data suggests that the reduction in atmospheric evaporative demand is primarily driven by the reduced radiation levels and wind speed under nets. Our preliminary results also show a 12% reduction in whole tree water use under the nets, partly attributed to the lower ETo. Transpiration is, however, not only influenced by climatic factors, but also by the regulation of the opening and closing of the stomata, and thus the flux of water vapour through these pores in the leaves. When protective netting reduces atmospheric stress relative to an open orchard, it can lead to higher stomatal conductance and leaf-level transpiration rate (Nicolás et al., 2005; Smit et al., 2009). In our study, stomatal conductance was increased slightly under the net, only on one measurement date towards the end of May 2019. In mid-January 2019, stomatal conductance was slightly reduced under the net, but no differences were found between the treatments at other times in the growth season. The 2018/19 season was characterised by consistently mild climatic conditions, with no days that were very hot. The measurement of pre-dawn leaf water potential gives an accurate reflection of any water stress a tree may be experiencing. No differences were found from December to June in this parameter between trees, under nets and in the open. This indicates that the irrigation regime of both orchards did not give rise to any water deficits.

Figure 1. “Rosy Glow” Orchard of the Future at Paardekloof, Witzenberg Valley, under shade netting, shortly before harvest.

Figure 2.  Samples taken during the first pick on 17 April 2019, showing (left) apples grown in the open, and (right) apples grown under white net.

Yield and fruit quality under the net and in the open

One of the main objectives of the project is to quantify the physical water productivity and the economic water productivity of apple orchards under net, compared to the open. Water Use Efficiency is defined as CO2 gained per unit H2O lost. Physical water productivity is defined as the kilograms of fruit produced per cubic meter of water used by the tree, while the economic water productivity represents the gross income achieved per cubic meter of water consumed.
The potential differences in fruit production and fruit quality are, therefore, critical parameters and a comprehensive assessment of yield and quality was performed at harvest in April 2019. At a whole orchard level, yield under the net was 139 t ha-1 compared to 132 t ha-1 in the open.
In the open orchard, average fruit mass was 171 g. At first pick, the netted fruit had a slightly lower percentage red colour than the control fruit, a common response in red apples (Mupambi et al., 2018). But overall, the red colour was good and a high proportion of fruit from both treatments met the colour requirements for marketing as Pink Lady®. The mild season meant that sunburn was low (5%) even in the open orchard, and almost absent (<1%) under the net. Hail damage in the open was 4%, and 1% under the net. Final pack-out and price data will be used to calculate the water productivity of apple trees in the two treatments. This trial will be repeated in the next season (2019/20) and an additional trial will be started on the water balance of a ‘Golden Delicious Reinders’ orchard under draped netting.

Acknowledgement

We acknowledge funding from the Water Research Commission of SA (Project no. WRC
K5/2815//4) and the Hortgro Pome. We also thank the farm management of Paardekloof and Dutoit Agri for their cooperation and support.

STEPHANIE MIDGLEY; ELMI LÖTZE; EDWARD LULANE: Department of Horticultural Science, Stellenbosch University
SEBINASI DZIKITI: Council for Scientific and Industrial Research, Natural Resources and Environment
THERESA VOLSCHENK: Soil and Water Science Programme, ARC Infruitec-Nietvoorbij, Stellenbosch

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