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Improve water use of citrus

By Johan Campher (Agronomist - Highveld / Lowveld Business Unit)

Agriculture is globally the largest consumer of water and 59% of fresh water in South Africa is used for agricultural purposes. Part of this fresh water is used to irrigate citrus and therefore the agricultural sector has a responsibility to use water more effectively.

Citrus Research International (CRI) launched a project to determine the actual water requirements of citrus for optimal production under different South African conditions. In short: how to use less water without negatively influencing fruit quality and yield. The results of the project can be used to develop a model for irrigation scheduling on citrus to improve water use efficiency (WUE). To do this, researchers needed the specific water use of different citrus varieties under different circumstances.

An important aspect of the project was the water that moves through the plant (transpiration process). Transpiration happens when plant stomata take up carbon dioxide (CO2) from the atmosphere and then release water (H2O) in gaseous form back into the atmosphere. For every CO2 molecule, 500 water molecules are released. Therefore 97% of absorbed water is lost. The amount of CO2 taken up by the plant plays an important role in the growth and yield of citrus. There is a strong correlation between the stomatal conductance and photosynthesis. The plant has strict control over transpiration through the stomata.

The project measured transpiration through the sap flow to determine how much water is transpired by a citrus plant during the year. Readings were taken on four different farms throughout the country. The climatic factors of every farm differed and different citrus varieties were used:

  • Groblersdal – Delta Valencia;
  • Groblersdal – Bahianinha Navel;
  • Citrusdal – Rustenburg Navel; and
  • Malelane – Midknight Valencia.

The transpiration rate, measured in mm per day, did not differ much. It did indeed vary to a degree as the plant requirements changed and according to different growth stages. However, a maximum transpiration rate was reached. The following readings were taken from the different farms that formed part of the project:

  Citrusdal Groblersdal Malelane
Summer 2.27 mm/day 1.86 mm/day 2.02 mm/day
Autumn 1.72 mm/day 1.41 mm/day 1.41 mm/day
Winter 1.15 mm/day 0.95 mm/day 0.95 mm/day
Sping 1.18 mm/day 1.85 mm/day 1.85 mm/day

It was concluded that the water use of citrus, subject to the fact that soil moisture is unlimited, is not controlled solely by pressure from environmental factors, but also by internal resistance of water movement in the plant. The pressure from environmental factors and the internal resistance of the water movement in the plant causes a limitation in the amount of water a citrus plant can transpire on a warm day. Water stress is caused by:

  1. the inability of the soil to supply sufficient moisture to the plant; or
  2. high evaporation rates.

As the change in transpiration in the plant does not vary by much, applying water on a warm, dry day will not necessarily increase transpiration, but can possibly even have the opposite reaction. Therefore it is important to schedule irrigation to maintain soil moisture status at field capacity.