Moisture Stress

If you are irrigating your crop at the recommended intervals for your region and making frequent assessments of the water status of the crop using the wilting score and using that score to confirm when you should irrigate, moisture stress should not become a problem. Irrigation frequency and amount will differ depending on many factors, but there are four stages additional to sowing when water should not be limiting. These are crown root initiation when tillering is starting, the jointing stage, anthesis and the grain milk stage. Of the four stages, tillering and anthesis are most sensitive to water stress.

Is moisture stress a problem?

A stressed crop rapidly loses potential yield. When a young crop has too little water its first response is to conserve what water there is by closing its stomata. These are small pores all over green surfaces that let water out and carbon dioxide in. Without carbon dioxide photosynthesis stops and so the sugars it normally makes from the carbon are no longer available for growth. So growth stops. The first growth response to water shortage is that leaves stop expanding. The tiller buds that are ready to grow into tillers remain as dormant buds. Usually the main shoot struggles on with its development. If moisture remains limiting, the crop eventually runs out of time to produce leaves, tillers and spikes and the consequence is a thin canopy with few small spikes, few grains and low yield.

If moisture stress occurs after anthesis the grains are affected, as they are now the growing part of the crop. Again the stomata close, the leaves roll and die from oldest to youngest and the plant rushes to move as much material from storage into the grains to fill them as full as possible before everything dies. The consequence of post-anthesis stress (milk stage) is that grains are pinched, shrivelled and small.

Click here to estimate how much soil moisture is available to the crop.

Is the crop short of water?

  • Is the wilting score more than 1? A score of 1 means that yield potential will not decline if water is applied today. Higher scores mean that potential yield is already declining due to water shortage
  • Count the number of tillers on average plants and the number of main shoot leaves as described in the field sheets. The drawings of the wheat growth stages will help you to identify the tillers and main shoot leaves. Are there sufficient tillers for the number of main shoot leaves? Typically a normal healthy crop has one fewer tillers than leaves on the main stem. Too few tillers indicates that irrigations have been too far apart.
  • After heading, are there at least 2.5 green leaves? A stressed crop loses leaves very quickly during grain filling.
  • Are leaves warm to touch or hold on a sunny day? Leaves that are not stressed transpire rapidly when the sun is shining and on hot days cool themselves below air temperature. Stressed leaves close their stomata, stop transpiring, and heat up.

Causes of water shortage

  • Irrigations were too far apart for the water holding capacity of the soil and the demands of the crop.
  • Rain may have fallen that was overestimated in its useful quantity because of incorrect measurement or run off.
  • The roots of the crop may be restricted to a part of the soil profile by a plough pan, or shallow or compacted soil.
  • The roots may be diseased with rots limiting water uptake.
  • The soil or the irrigation water may be saline resulting in the crop limiting its water uptake (see saline soils).
  • The soil may be poorly aerated/waterlogged (see saline soils) preventing water uptake.
  • Irrigation water was not available when it was needed.

Solutions to crop water shortage

  • Moisture required: Follow recommended irrigation scheduling for the region. Use the wilting score to time irrigations. Calculate when water should be applied by using the step method. Measure rainfall yourself with a rain gauge and measure pan evaporation with a homemade pan evaporimeter.
  • Plant observation: Check your calculations for when you should irrigate by regularly measuring whether the stomata are closing. Stomatal closure is a very early sign of water stress. It can be measured quickly and simply by a viscous flow porometer.
  • Soil depth: Check for plough pans and soil depth. Cultivate accordingly.
  • Roots: Look for root rots and apply the appropriate treatment.
  • Salinity: If the soil is saline, apply water in greater quantity than previously each time you irrigate to ensure that the salt is leached below the rooting zone. Also ensure that drainage is adequate so that salt is removed from the system. This applies particularly if the irrigation water is saline.
  • Drainage: Improve drainage if waterlogging is a problem.
  • Irrigation capacity: If too little water is available either increase on-farm storage or plant less area to crops that require irrigation.
  • Mulch: Conserve moisture by spreading a mulch on the soil.