Onion growth and water use patterns in relation to drought stress

Abstract

Four distinct growth stages of storage onions were established using growth analysis techniques; stand establishment (0 - 4 leaves), leafing (4 - 8 leaves), bulbing (8 - 10 leaves), and sizing (less than 10 leaves. Visible developmental observations such as number of functional leaves were determined to be the best indicator of crop growth stage. Linear relationships between soil matric potential (SMP) and evaporative demand (ED) were developed for each of the latter three growth stages in order to determine proper irrigation timing. It was assumed that SMP i.e. soil moisture "need" is a function of stage of growth, EPA, soil texture and root zone recharge. It was determined that to limit SMP to no greater than -40 kilopascals (kPa), irrigation or a significant rainfall should occur at or before 789 ml of water are evaporated from a black Bellani plate atmometer during the leafing growth stage. In a like manner, soil recharge should occur at or before 340 ml are evaporated during the bulbing and sizing growth stages. Onions were stressed during the leafing, bulbing, and sizing growth stages by withholding irrigation until a soil matric potential of -70 kilopascals was reached. No significant differences were observed in either the growth pattern or final yield between each stress treatment and the unstressed control. Hydroponically grown onions were transplanted into containers located in a greenhouse so that roots were subjected to six different levels of polyethylene glycol (PEG) 3500 osmotic solutions for five weeks. Leaf, bulb, and root dry weights were found to be significantly reduced in the treatment of -274 kPa or more negative osmotic potential than treatments of -147 kPa or less negative potential. Root:shoot ratio was found to be significantly greater in only one instance. After three weeks of growth the highest stress level (-622 kPa) resulted in the largest root:shoot ratio. This observation was believed to result from aproportionately greater leaf tissue decline than root tissue decline in response to high osmotic stress.