Interactions Between Rhinanthus minor and Its Hosts:A Review of Water, Mineral Nutrient and Hormone Flows and Exchanges in the Hemiparasitic Association

At the ecological level, the effects of the facultative root hemiparasite Rhinanthus minor on the structure and functioning of its host communities are relative well described; yet until recently, the mechanistic basis for parasitic plant-driven community change and the physiological basis for the host-parasite interaction were poorly understood. Empirical incremental flow models, based on the increase in water, mineral nutrients, carbon assimilates or phytohormones between two defined time points, have been successfully employed to investigate the physiology of resource acquisition by- and distribution within host-parasitic plant associations. In this study we review the application of these empirical flow models to Rhinanthus-host associations showing the extent of and physiological basis of resource abstraction from the host and how this is profoundly influenced by soil nutrient status. We show that Rhinanthus primarily abstracts water and mineral nutrients via the apoplastic pathway through direct lumen-lumen connections with little resource acquisition via symplastic pathways. Nutrient status of the soil is shown to significantly influence the resource acquisition. We also investigate the hormonal regulation of resource acquisition by Rhinanthus showing pivotal roles for the key for the phytohormones abscisic acid (ABA) and cytokinins

Effects of external NaCI on the growth of Atriplex amnicola and the ion relations and carbohydrate status of the leaves

Atriplex amnicola, was grown in nutrient solution cultures with concentrations of NaCl up to 750 mol m−3. The growth optimum was at 25–50 mol m−3 NaCl and growth was 10–15% of that value at 750 mol m−3 NaCl. Sodium chloride at 200 mol m−3 and higher reduced the rate of leaf extension and increased the time taken for a leaf to reach its maximal length. Concentrations of Na+, K+ and Mg2+ in leaves of different ages were investigated for plants grown at 25, 200 and 400 mol m−3 NaCl. Although leaves of plants grown at 200 and 400 mol m−3 NaCl had high Na+ concentrations at young developmental stages, much of this Na+ was located in the salt bladders. Leaves excluding bladders had low Na+concentrations when young, but very high in Na+ when old. In contrast to Na+, K+ concentrations were similar in bladders and leaves excluding bladders. Concentrations of K+ were higher in the rapidly expanding than in the old leaves. At 400 mol m−3 NaCl, the K+:Na+ ratios of the leaves excluding bladders were 0.4–0.6 and 0.1 for rapidly expanding and oldest leaves, respectively. The Na+ content in moles per leaf, excluding bladders, increased linearly with the age of the leaves; concurrent increases in succulence were closely correlated with the Na + concentration in the leaves excluding the bladders. Soluble sugars and starch in leaves, stems and buds were determined at dusk and dawn. There was a pronounced diurnal fluctation in concentrations of carbohydrates. During the night, most plant parts showed large decreases in starch and sugar. Concentrations of carbohydrates in most plant organs were similar for plants grown at 25 and 400 mol m−3 NaCl. One notable exception was buds at dusk, where sugar and starch concentrations were 30–35% less in plants grown at 400 mol m−3 NaCl than in plants grown at 25 mol m−3 NaCl. The data indicate that the growth of A. amnicola at 400 mol m−3 NaCl is not limited by the availability of photosynthate in the plant as a whole. However, there could have been a growth limitation due to inadequate organic solutes for osmotic regulation