Relationships between available soil water and indicators of plant water status of sweet sorghum to be applied in irrigation scheduling

Optimal crop production depends greatly on available soil water, and it is therefore important to know when and how much to irrigate in order to attain agronomic potential. In this work, plant indicators are used to assess water stress. These are compared with available soil water to find the critical point for irrigation scheduling of sweet sorghum. The experimental trial was carried out in Bečej, in the Vojvodina region, on a sweet sorghum crop, grown in moderate climatic conditions on a well-drained, deep chernozemolic meadow soil. Plant indicators tested were predawn and midday leaf water potential and crop and air temperature difference. All the methods were sensitive to water deficit in plants, but not all can be used for irrigation scheduling. The predawn leaf water potential was the most reliable parameter among those tested due to its relative independence from weather conditions and a valid indicator of plant water status. This was not the case with canopy-air temperature difference. The predawn leaf water potential corelation with available soil water indicates that the threshold value for irrigation scheduling of sweet sorghum is when the former has decreased to –0.45 MPa. This corresponds to a soil water depletion to about 10% of available water in the active root zone. The canopy-air temperature difference was sufficiently sensitive to indicate the onset of mild plant water stress, although it showed a certain threshold value when water shortage appeared, which occurred when the air and canopy temperature were the same.