Impact of root and shoot temperature on bud dormancy and floral induction in lychee (Litchi chinensis Sonn.)

Abstract

Potted lychee trees (cv. Tai so) with mature vegetative flushes were grown under three day/night temperature regimes known to induce floral (18/13 °C), intermediate (23/18 °C) and vegetative (28/23 °C) shoot structures. Heating roots respective to shoots accelerated bud-break and shoot emergence, but reduced the level of floral initiation in emergent shoots. At 18/13 °C, root temperatures of 20 and 25 °C decreased the period of shoot dormancy from 9 weeks to 5 and 3 weeks, respectively. A root temperature of 20 °C also increased the proportion of both leafy and stunted panicles to normal leafless panicles, and reduced the number of axillary panicles accompanying each terminal panicle. A root temperature of 25 °C produced only vegetative shoots. At 23/18 °C, heating roots increased the proportion of vegetative shoots and partially emerged buds to leafy and stunted panicles as well as accelerating bud-break. Cooling of roots in relation to the shoot resulted in non-emergence of buds at both 28/23 and 23/18 °C. Bud-break did not occur until root cooling was terminated and root temperature returned to that of the shoot. At 23/18 °C, subsequent emergent shoots had a greater proportion of leafy panicles relative to control trees. At 28/23 °C, all emergent shoots remained vegetative. Lychee floral initiation is influenced by both root and shoot temperature. Root temperature has a direct effect on the length of the shoot dormancy period, with high temperatures reducing this period and the subsequent level of floral initiation. However, an extended period of dormancy in itself is not sufficient for floral initiation, with low shoot temperatures also a necessary prerequisite.