Introduction
Auxins can modulate root architecture, fruiting and stress responses. In hydroponic and substrate soilless systems, exogenous root-zone applications at very low ppm sometimes boost yield or quality. Push the dose and you flip the response. Below I review peer-reviewed work on widely grown crops, focusing on species, timing, exact dosages converted to ppm, and toxic thresholds. Where possible I prioritize reviews to frame context, but yield data come from primary trials.
Evidence & discussion
Sweet pepper. Two lines of evidence exist. First, fertigation with a commercial IBA product at 0.4 percent active (4000 ppm in the stock) applied weekly from early fruit development at 0.5 L ha⁻¹ outperformed 1.0 L ha⁻¹, increasing marketable yield while improving root mass and water and nutrient uptake in perlite culture (1). Second, a separate trial compared root fertigation vs foliar using a formulation containing 6.75 g L⁻¹ NAA and 18 g L⁻¹ NAA-amide. The fertigation rate was 0.6 mL L⁻¹ of product in the solution, equal to ~4 ppm NAA plus ~10.8 ppm NAA-amide per application; foliar used 0.4 mL L⁻¹ or ~2.7 ppm NAA plus ~7.2 ppm NAA-amide. Early and total yield were higher with fertigation, while foliar favored some quality traits like firmness and soluble solids (5). Practical read: peppers respond to root-zone auxin in the single-digit ppm range, but more is not better.
Melon. The same IBA approach that helped pepper flopped in melon. In perlite greenhouse culture, 0.4 percent IBA applied weekly at 0.5 or 1.0 L ha⁻¹ did not improve yield or water or nutrient relations. Authors concluded it is not an effective tool for commercial melon in soilless culture (2). Species matter.
Strawberry. In long recirculating systems, autotoxic phenolics depress growth and fruiting. A one-time root or crown dip in NAA before transplant at 5.4 μM NAA, which is ~1 ppm, mitigated autotoxicity and restored flower and fruit numbers compared with untreated plants. A higher 54 μM dose, about 10 ppm, was less effective (3). Timing was everything.
Toxic thresholds from hydroponic seedlings. While not a yield trial, maize in nutrient solution shows the margins. IBA at 10⁻¹¹ M is ~0.000002 ppm and stimulated root growth, but 10⁻⁷ M is ~0.02 ppm and significantly stunted primary root elongation and biomass. The same hormone switches from helpful to harmful across four orders of magnitude (4). That narrow window explains why melon trials can miss and pepper trials can hit. For broader context on root-zone biostimulation via fertigation programs, see this review (6).
Tables
Table 1. Positive responses to exogenous auxin at the root zone in soilless crops
| Crop & system | Auxin and delivery | Dose in root zone (ppm) | Timing | Outcome |
|---|---|---|---|---|
| Sweet pepper, perlite | IBA 0.4 percent product via fertigation | Stock is 4000; applied 0.5 L ha⁻¹ weekly | From early fruit development | Higher marketable yield at 0.5 vs 1.0 L ha⁻¹; improved root mass and water and nutrient uptake (1) |
| Sweet pepper, soilless | NAA + NAA-amide via fertigation | ~4 NAA + ~10.8 NAA-amide per application | Weekly during production | Higher early and total yield vs foliar; foliar favored firmness and °Brix (5) |
| Strawberry, recirculating hydroponics | NAA root or crown dip | ~1 optimal; ~10 less effective | One time at transplant | Mitigated autotoxic yield loss; restored flower and fruit counts under closed reuse (3) |
Table 2. Null results and toxic thresholds
| Crop or context | Auxin & delivery | Threshold or tested dose (ppm) | Timing | Result |
|---|---|---|---|---|
| Melon, perlite greenhouse | IBA 0.4 percent via fertigation | Stock 4000; 0.5 or 1.0 L ha⁻¹ weekly | Season-long | No improvement in yield or water or nutrient relations (2) |
| Maize seedlings, hydroponic assay | IBA in solution | 0.000002 stimulatory vs 0.02 inhibitory | Continuous exposure | Root growth stimulation at ultra-low ppm but marked stunting by 0.02 ppm (4) |
Conclusion
Root-applied auxins are not a silver bullet. They can raise yield or preserve quality, but only when dose and timing line up with the crop’s physiology. Peppers respond to single-digit ppm root fertigation with higher early and total yields, while melons do not. Strawberries benefit from a ~1 ppm pre-plant dip that preempts autotoxicity, whereas ~10 ppm underperforms. Hydroponic seedling work reinforces the risk: ~0.02 ppm IBA already suppresses maize roots. The safe play is to trial low, crop-specific ppm near published values, apply at the stage that matters, and stop if marketable yield does not move. If you treat auxins like a nutrient and “turn them up,” they will punish you. If you treat them as a precise signal, they can pay off.
