Growth Regulator and Nitrogen Fertilization Effects on Performance and Nitrogen-Use Efficiency of Tall and Dwarf Varieties of Rice (Oryza sativa L.)

Little information is available on control of lodging, particularly when early-maturing, high-yielding, tall rice (Oryza sativa L.) varieties are heavily fertilized with nitrogen (N). Two plant growth regulators, PGRs-chlormequat (applied at 30 mg pot-1) and ancymidol (applied at 1.5 mg pot-1) were tested on two rice cultivars (early maturing, tall 0S-6 and late maturing, dwarf IR-5) after treatment to three N rates (0, 2.5 and 45 mg N kg-1) in an effort to evaluate PGRs and N effects on rice crop performance and N-use efficiency (NUE). Significant (p<0.5) responses to N-treatment were obtained with respect to plant height and tiller count. Number of fertile tillers increased, resulting in higher grain yields. At crop half (1/2) -life, N effect was generally insignificant on plant height but IR5 tillers increased while PGRs reduced height of both rice varieties but failed to dwarf them at maturity (full life). At full life, N application increased numbers of tillers, parnicles, spikelets and grains as well as weights of panicles, straw and grains; OS6 plants were taller than the control while IR5 plant heights were not affected just as panicle lengths remained the same. The NUE by OS6 plants increased with increase in N application rate but reduced in IR5 plants. Only OS6 had its grain yield increased by the PGRs. Increased plant heights caused more lodging in the early maturing variety (OS-6) despite the application of the PGRs, implying the inadequacy of PGRs rates sprayed on the plants while PGRs/N interactions were insignificant.