Germination responses of the invasive hedge cactus (Cereus uruguayanus) to environmental factors

Abstract

Hedge cactus (Cereus uruguayanus R. Kiesling; syn. Cereus hildmannianus K. Schum.) is a columnar cactus that was introduced to Australia as an ornamental plant and has since become invasive in subhumid regions of Queensland and New South Wales. Compared to its congener Cereus jamacaru DC. (which currently is invasive in both eastern and southern Africa), information on seed biology of C. uruguayanus is lacking. Experiments were conducted to study the effects of alternating day/night temperature, salt stress, water stress, and burial depth on germination and seedling emergence of four seed accessions of C. uruguayanus. Seeds were also subjected to a Controlled Aging Test (CAT) to obtain an estimate of potential persistence under field conditions. The optimum temperature regime for germination of all accessions was 30/20 C. Germination decreased with an increase in sodium chloride (NaCl) concentrations, but germination of all accessions (range 26–81%) occurred at 160 mM NaCl, indicating very high salt tolerance. Seed germination gradually decreased with an increase in water stress, but germination in all accessions (range 19–47%) occurred at -0.8 MPa. Seed viability and potential germinability were unaffected by exposure to salt level (320 mM NaCl) and water (-1.6 MPa) stress under which germination did not occur. Germination responses to all three factors were generally similar to those documented for C. jamacaru. The emergence of C. uruguayanus decreased with an increase in seed burial depth. The highest emergence (43%) was recorded for surface-sown seeds and emergence was reduced to 0 at a burial depth of 2 cm. CAT results for two seed accessions indicated that seeds of C. uruguayanus are likely to demonstrate extended (> 3 yr) persistence under field conditions, a prediction that is supported by evidence that germination of its small (2 mm) seeds is markedly reduced by burial.