Chemical ripening of sugarcane with trinexapac-ethyl (Moddus®) — Mode of action and comparative efficacy

Chemical ripeners cause growth suppression, thus increasing sucrose accumulation in sugarcane by lowering the growth sink demand for sucrose. Trinexapac-ethyl (Moddus®), the most recent chemical to be introduced as a ripener with a hormonal mechanism, inhibits production of the plant hormone gibberellic acid, which leads to restriction of internode elongation. This study reports novel findings on a dual mode of action, by which Moddus® affects growth processes in both the stalk and leaf canopy above a certain concentration, and how this influences ripening efficacy. In addition the efficacy of Moddus® is compared to Ethephon®, the other ripener with a hormonal mechanism. An irrigated field trial was planted to variety N32 at the South African Sugarcane Research Institute’s research station in Pongola, South Africa. The plant and subsequent three ratoons were used as experimental crops and harvested in April of each year. Replicated treatment plots consisted of 6 cane rows, 16m long and spaced 1.4m apart. Treatments comprised an unsprayed control, Ethephon® applied according to standard practise, and Moddus® applied at three dosages. Products were applied with CO2-pressurised equipment and a hand-held overhead spray boom. Measurements on the stalks (length, individual internode elongation and juice quality) and leaves (green and dead leaf mass per stalk) were conducted at intervals until harvest. At harvest the cane and sugar yield was determined for each treatment. A novel finding was that Moddus®, above a certain concentration, ripened the crop through a dual mode of action involving restriction of both internode elongation and leaf growth. Characterisation of effects on individual internode elongation and mass of the green leaf canopy, coupled to yield data, provided insights to be considered in future when attempting to explain varietal differences in response to Moddus®. The easy-to-measure plant processes identified in this study as sensitive indicators of ripening efficacy might enable initial pot-based screening of large numbers of varieties for responsiveness to Moddus®, before embarking on more time-consuming and expensive field-based testing. The data acquired in this study could also be used in attempts to simulate Moddus®-induced ripening with mechanistic sugarcane crop models.