Square-wave (SQW) ultraviolet-B (UV-B: 280–315 nm) radiation supplementation systems continue to be used in outdoor experimental locations due to the economically restrictive installation and maintenance costs, and technical expertise required to effectively operate more advanced modulated (MOD) delivery systems. However, continued yet contentious criticisms of SQW delivery systems risk creating prejudices as to the validity of plant responses measured in these with potentially negative repercussions on future UV-B experimentation. Consequently, we quantified the magnitude of UV-B supplementation inaccuracies in our typical outdoor step-wise SQW delivery system using 7-year records of computer-modeled and instrument-measured solar UV-B irradiances and synchronous measurements of total solar (300–3000 nm) radiation and daily sunshine duration. Both broad- and narrow-band instrument measurements confirmed that our step-wise SQW delivery system rendered larger total daily supplemental UV-B exposures (time-integrated UV-B irradiances) than a MOD delivery system on only substantially overcast days (25% or less daily sunshine duration). These larger supplemental UV-B exposures were augmented with increased magnitude of the added artificial UV-B supplement. However, their ranges did not exceed those in a MOD delivery system by more than 10% for added UV-B supplements of realistic magnitude (30% or less above background), except on virtually completely overcast days (5% or less daily sunshine duration). Also, our step-wise SQW delivery system rendered higher photon flux ratios of UV-B/total solar radiation than a MOD delivery system on only substantially overcast days, the ranges of which were also augmented with increased magnitude of the added artificial UV-B supplement. However, these features were restricted to high solar angles, since with reduced solar angle these higher photon flux ratios also included progressively less overcast days. Nevertheless, ranges of photon flux ratio increases were well below reported thresholds inhibiting to plant growth at all solar angles for the added artificial UV-B supplements of realistic magnitude, except on virtually completely overcast days. Results point to an under-estimation of clear-sky UV-B irradiance by the computer-encoded semi-empirical model commonly utilized to predict background and supplemental UV-B irradiances for SQW delivery systems. They confirm the superiority of MOD delivery systems in providing more realistic conditions of UV-B increases but likewise demonstrate little justification on balance for branding results derived from all field-based SQW delivery systems as exaggerated where sensible irradiation protocols and realistic UV-B supplements are applied.
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