Seed production by skeleton weed (Chondrilla juncea L.) in Western Australia in relation to summer drought

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Abstract
Seed production in Forms A and C of C. juncea was measured at 11 wheatbelt sites in Western Australia and under glasshouse conditions. Most field-grown plants produced abundant viable seeds, even without substantial summer rainfall. Up to 27.6 x l03 seeds per plant were recorded, and their viability was frequently between 80 and 90%. Even in their first year, some plants produced more than 10000 seeds each. Through the summer, seed production often followed either a positively skewed, or a bimodal curve. Falls of rain in summer did not increase seed output or affect its quality in established plants. Exceptionally high air temperatures appeared to depress seed numbers and/or viability at several, but not all, sites. At the end of summer, when young plants were 9-12 months old, soil water extraction had occurred to 310 cm, indicating root penetration to this depth. Conservative water use by C. juncea during summer was indicated by low values of soil water depletion. In glasshouse-grown plants, simulated drought reduced seed numbers, viability, primary dormancy and seed weight, although the two forms responded differently. Given adequate winter rainfall to recharge soil water storage, C. juncea appears capable of producing large quantities of viable seeds throughout the Western Australian wheatbelt.