Cuttings of Wollemi pine tolerate moderate photoinhibition and remain highly capable of root formation

Abstract

The Wollemi pine was discovered in 1994 at the base of a deep rainforest canyon in the Wollemi National Park, southeastern Australia. Only three populations have been found, comprising less than 100 adult trees. Ex situ conservation collections were quickly established, and also a commercial propagation program to produce plants for horticulture. We investigated vegetative propagation methods, focussing on rooting of cuttings with different amounts of foliar pruning, irradiance or fertiliser application, and we compared anatomy and rooting of cuttings at two different stages of stem development. We related root production to levels of photoinhibition experienced during the propagation phase, quantifying photoinhibition by measuring chlorophyll fluorescence. Maximum photochemical efficiency (FV/FM) of cuttings declined during daily stress periods, particularly after many weeks without roots in the propagation environment, but these declines did not inhibit root formation. Mean percentage of cuttings that formed roots was usually high (79.4–94.0%). Treatments that alleviated photoinhibition, such as low irradiance or foliar fertilisation, did not increase rooting percentages. Other treatments that did not alleviate photoinhibition, such as incorporation of fertiliser in the medium or no pruning, did accelerate formation of an extensive root system. Soft cuttings had a less-developed sclerenchyma ring in their cortex compared with firm cuttings, but both cutting types provided high rooting percentages. Our results showed that cuttings under mist irrigation can be prone to long periods of moderate photoinhibition and yet remain highly capable of root production.