Abstract:
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, 16mlong and spaced 1.4mapart. 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.
