UC Davis

Iodine’s impact on plant growth and development remains incompletely understood due to its volatility and its ubiquity in the environment that complicates experimental control. These attributes of iodine experimentation have led to a wide diversity of results that are often circumstantial to species, experimental growing conditions, and the rate, form, and timing of iodine application. Iodine has been demonstrated to improve the overall productivity of some plants in certain conditions when applied as a fertilizer, to influence bloom timing and fruiting, and to mitigate negative effects of salt and heavy metals in stress treatments. This study investigated iodine's potential as a biostimulant and beneficial nutrient in almond, pistachio, and avocado orchards. Field trials were conducted using a commercial potassium nitrate, iodine containing product (UltrasolINE, SQM) treatment applied via drip irrigation (0.154 kg I ha⁻¹) and as a foliar spray (3.5 g I ha⁻¹). Control treatments received identical rates of potassium nitrate without iodine (Ultrasol, SQM). Results showed no significant effects on plant tissue iodine concentration, leaf nutrient content, vegetation indices, or yield across all species. Iodine applied at pink bud in almond did not influence bloom timing at the individual spur or at the whole-tree level or have any yield impacts. Naturally occurring root iodine concentrations in almond and avocado were found to be higher than in leaves, consistent with observations in other species. This study contributes to the understanding of iodine's role in plant phenology and physiology, concluding that low-rate iodine application did not demonstrate agricultural benefits in the studied tree crops.