The effect of plant growth regulators on the growth and yield of barley: a thesis submitted in partial fulfilment of the requirements for the degree of Master of Agriculture [i.e. Agricultural] Science in the University of Canterbury [Lincoln College]

(1.) Recent investigations have shown that applying plant growth regulators (PGRs) to barley can prevent yield losses which would otherwise be caused by lodging and, in some conditions, enhance yield even in the absence of lodging. The purpose of the work described here was to establish the physiological and morphological basis of such responses and to determine whether they are likely to occur under field conditions in New Zealand. (2.) Four experiments were conducted in 1982-83 to investigate the influence of the growth promoter (GA₃), and the growth retardants CCC, Terpal, barleyquat and Cerone (applied at the recommended stage) on the stem extension, tillering, grain and straw yield, and yield components of several cultivars of spring barley (Hordeum vulgare L.). Two of the experiments were done in the field, one of which was also done in a growth cabinet and glasshouse. There were no PGR X cultivar interactions with respect to the major variables measured. (3.) CCC applied before tillering (ZGS 13) in the controlled environment experiment slowed stem growth initially but had no influence on plant height at maturity. Terpal applied at ZGS 32-37 severely slowed stem extension and decreased plant height at maturity in all environments. An early application (ZGS 13) of GA₃ increased plant height initially in all environments but the effect disappeared within 3-4 weeks of application. barleyquat and Cerone had no effect on plant height at maturity. (4.) CCC stimulated tillering shortly after application but had no effect on ear production. Terpal not only stimulated tillering but also increased the number of ears plant⁻¹ in all environments except the growth cabinet where Ilumber of ears plant⁻¹ was decreased. These increases in tillering were associated with an initial restriction of stem extension caused by Terpal. GA₃ had no effect on tillering 1 or ears plant⁻¹ (5.) Terpal decreased grain yield only in the growth cabinet and the glasshouse and this was associated with very poor grain set. In the field Terpal decreased grain ea⁻¹ and 1000-grain weight. Other PGRs had no effect on yield or yield components. (6.) Terpal delayed ear emergence and maturity by 2 days in the field; the other PGRs were without effect. (7.) The six-row cultivar, Kakapo, yielded well in the growth cabinet where the two-row cultivars were handicapped by their failure to set many grains. (8.) Among the PGRs investigated, Terpal alone constantly decreased plant height. This suggests that Terpal was potential for preventing yield losses due to lodging in cultivars commonly grown in New Zealand. The likelihood of yield enhancement in the absence of lodging appears small.