Paclobutrazol application to increase the productivity of different rice ecotypes under rainfed lowland conditions

Aside from rainfed rice varieties, other ecotypes are also cultivated under rainfed lowland ecosystem. Paclobutrazol (PBZ), one of the most well-known plant growth regulators is generally considered as growth retardant but has the ability to improve yield of tillering crops by increasing the number of tillers or panicles. This study aimed to determine the effect of PBZ on the growth and yield response of different rice ecotypes under rainfed condition. The experiment was laid out in split-plot design with PBZ concentrations (0, 250, 500 ppm) as main-plot applied at 14 days after transplanting (DAT) while rice ecotypes, i.e. rainfed lowland (PSB Rc14), irrigated lowland (NSIC Rc222), upland-special quality (Dinorado) and lowland-special quality (NSIC Rc216) as sub-plot , arranged in RCBD with three replications. Rainfall is the only source of water from 14 DAT until harvesting. Water level below soil surface was 44-66 cm with fluctuating soil moisture content of 18-76% from DAT until harvesting. Cracks were observed from early stage until harvesting indicating soil drying. PBZ temporarily suppressed plant height at 28 DAT of different rice ecotypes as plants were able to recover at 35 DAT resulting to even taller plants at maturity. PBZ also increased tiller number as early as one week after application contributing to higher number of panicle at maturity. Other agronomic parameters, i.e. panicle length, numbers of spikelet and filled spikelet including percent filled grains per panicle, and biomass also improved with PBZ application. Most importantly, grain yield increased PBZ application at different concentrations for each rice ecotype. A lower concentration (250 ppm) of PBZ had the highest grain yield for lowland rice ecotype, i.e. PSB Rc14 (Rainfed) and NSIC Rc222 (irrigated); while higher concentration (500 ppm) for special-quality rice, i.e. Dinorado (traditional upland) and NSIC Rc216 (irrigated lowland).