SPRAY APPLICATION FACTORS AND PLANT-GROWTH REGULATOR PERFORMANCE - I - BIOASSAYS AND BIOLOGICAL RESPONSE

Bioassays were adapted to investigate effects of droplet size and carr ier volume on performance of daminozide, gibberellic acid (GA,) and 2, 4-D using Phaseolus vulgaris L. as a model system. Response to plant g rowth regulators was indexed by inhibition (daminezide), promotion of internode elongation (GA(3)) or ethylene production (2,4-D). Elongatio n of first plus second internodes above primary leaves was evaluated 1 4 days after treatment of primary leaves, while ethylene production wa s determined from head-space samples of incubated leaves 24 h after tr eatment. Daminozide inhibition of internode elongation was related to decreased cell size and number in pith and epidermis (range 49-70% of the untreated control). GA(3) increased cell size and number in both t issues 2.3- to 4.8-fold. Responsiveness to daminozide and 2,4-D marked ly decreased as seedling age increased from 8 to 12 days, but responsi veness to GA(3) increased. Decreasing droplet size (10-0.5 mu l) and i ncreasing carrier volume (10-200 mu l per leaf) at constant dose of da minozide (100 mu g per leaf) and 2,4-D (100 mu g per leaf) significant ly increased performance, but had little effect on performance of GA, (2 mu g per leaf). Effects of application factors on performance were related to their effects on the interface area between droplets and le af surface. Significant positive linear relationships were obtained be tween plant response and the logarithm of the droplet/leaf interface a rea for all growth regulators. (C) 1998 Society of Chemical Industry