Assessment of Upland Rice Rooting Depth: Does the Herbicide Injection Technique Work?

Water stress is a major constraint to upland rice production. Plants with deep root systems are needed to increase water uptake. For varietal screening or for field diagnosis, breeders and agronomists need methods to monitor rice rooting depth. We assessed a simple, manual technique based on the injection of herbicide at a chosen soil depth using long needles mounted on a veterinary syringe. The time of appearance of herbicide phototoxicity symptoms in the leaves of four rice genotypes with contrasting rooting systems was monitored in the glasshouse and field ‐experiments. Three herbicides (atrazin, diuron and metribuzin) were tested. Metribuzin was selected as the reference herbicide as it consistently induced distinct leaf phototoxicity symptoms 4 days after its application, regardless of the genotype tested. Through the injection of metribuzin at different depths, significant differences in he number of plants affected by the herbicide were detected under field conditions between the extreme genotypes. The ranking of genotypes obtained in the glasshouse and field experiments was consistent with what is known about their respective rooting patterns. However, risks of herbicide leakage above the treated zone reduce the reliability of the technique and precautions were necessary‐ to avoid them. Following the precautions increased he time needed to treat one hill to about 5min, which limits the field of application of the manual technique. Moreover, metribuzin remained present at levels toxic to rice seedlings 7 months after being injected into the soil, in spite of its fast downward mobility and degradation rate. This long residual effect might cause environmental concern.