What you need to know about orchard water use

Every fruit farmer knows irrigation is an essential part in growing the perfect crop. Horticulture experts Stephanie Midgley, Wiehann Steyn and Michael Schmeisser explain why this is, and how trees actually use water. By Esté Beerwinkel.

• Plants need to take up carbon dioxide for photosynthesis to take place.
• Carbon dioxide enters the leaf through very small openings on the leaf surface, called stomata (Huidmondjies in Afrikaans).
• The space within the stomata is saturated with water vapour while the air outside of the leaf is generally not saturated during the day.
• Due to the difference in vapour pressure in the air outside the leaf and the vapour pressure inside the leaf, water moves out of the leaf while the stomata are open. This process is called transpiration.
• There is a trade-off between plant productivity and water lost through transpiration. Stomata can close partially or fully to manage water loss through transpiration, but this impacts on the uptake of carbon dioxide, which may again impact on dry-matter production — fruit growth, sugar levels, vegetative growth, sugar and metabolite production, energy for various processes, etcetera.
• Factors that affect the water-vapour pressure in the air can affect the rate of transpiration.
  • Low relative humidity at high temperature equates to a low air water -vapour pressure and therefore a strong driving force for transpiration.
  • Wind removes the more saturated layer of air in immediate contact with the leaf and therefore also increases transpiration.
  • Nets can increase the water-vapour pressure of the orchard air and decrease wind, thereby decreasing the driving force for transpiration.
• Deciduous fruit trees will allow high levels of transpiration and therefore also high rates of photosynthesis until the water pressure in the plant becomes quite low (or the water tension within the plant becomes quite high).
• The water in the plant can come under considerable tension on days of high transpiration when supply to leaves through the roots and stem cannot keep up with the demand. The plant finds it more difficult to access soil water as the soil dries out. At a certain soil moisture level, the plant cannot take up any water. Due to limitations on the rate at which water can flow through the plant, water demand can outstrip supply even when the soil contains enough water. Partial to full closure of stomata at this point can restrict transpiration, but will limit photosynthesis due to lower uptake of carbon dioxide.
• The following factors affect water use at an orchard level:
  • The water use of deciduous fruit trees is linked to number of stomata, which relates to the leaf area per ground area. More stomata — in other words, more leaves — per square metre of orchard floor equals higher transpiration rates. Yield can also relate to the effective leaf area since the level of light interception affects the potential photosynthesis that can take place. We speak about effective leaf area, because leaves on vigorous shoots may use a lot of water, yet contribute little to tree productivity.
  • Cover crops also use water but provide biological functions to the orchard, such as serving as reservoirs for mite predators.
  • Evaporation from the bare orchard floor contributes to total orchard water use. The greater the area of orchard floor wetted during irrigation, the greater the potential evaporative water loss – this is why drip irrigation is more effective than micro-irrigation.
  • Drainage or run-off of excess irrigation water contributes to orchard water use but contributes nothing to productivity — it’s a dead loss.
  • Use of water for other purposes than to supply the transpiration needs of the plant, such as overhead or floor-level evaporation cooling, should be considered very carefully since it does not directly contribute to productivity — although it may affect fruit quality.
• Given the trade-off between transpiration and photosynthesis, fruit growers should aim to maintain soil water levels so to not limit photosynthesis by water restriction, while working hard to prevent or restrict any non-beneficial water use by the orchard.