Reproductive diversity and survival of the potential annual Diplotaxis harra (Forssk.) Boiss (Brassicaceae) in Egypt
Diplotaxis harra is a common annual species in the desert of Egypt. The investigated population was located in Wadi Hof near Helwan city. Additional water in relatively wet years cause these ephemeral plants to live longer and become perennials. The species has ephemeral, modular and coppiced life-cycles and may shift from a r- to a K-selected strategy. The germinable seed bank represented 15.6% of the total seed rain, out of which only 0.03% established as adult plants. About 72.7% of the adult plants were ephemeral, 27.0% modular and 0.3% coppiced plants. The highest contribution of the seed rain came from modular plants and the lowest contribution from coppiced plants. The survivorship curve of the coppiced plants exhibited a combination of Deevey type I and III curves. The ephemeral and modular plants showed steep curves. In contrast to ephemeral plants, the coppiced plants attained the highest reproductive value and the lowest reproductive effort, while the modular plants demonstrated intermediate values. The active phenological cycle started earlier in coppiced plants than in the modular and ephemeral plants. The period from the start of vegetative growth to shoot-die back extends from mid-December to early June in coppiced plants, from February to May in modular plants, and from late February to early May in ephemeral plants. The living stumps of the coppiced plants remained dormant during summer and autumn months. Seeds collected from coppiced plants attained high germinabiltiy and viability, while those from ephemeral and modular plants attained low values. Seed longevity was higher in the modular plants (>10 yr) than in seeds of ephemeral and coppiced plants (7–8 yr). The high energy content of the seeds produced from different cohorts over ten years of storage enabled the population to maximize the germinability, viability and longevity of seeds despite the unpredictable environment. The potential annual behaviour of D. harra is mainly attained by the persistence of viable seed bank and root-shoot stock. The ability to shift from a r- to a K-selected life-cycle, phenological variations among the different cohorts of the population, variations in seed longevity and energy content strengthen intra-population interactions.
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Document Type: Research Article
Publication date: August 1, 2001