Research paperPhosphate starvation during the transition phase increases the sex ratio and 12-oxo-phytodienoic acid contents in females of Urtica dioica
Introduction
Plants are exposed to varying degrees of stress factors during their lifetime. Stress can be biotic, imposed by other organisms, or abiotic, arising from an excess or deficit of a given stress factor (e.g. water, nutrients, light, temperature, etc) in the physical environment (Levitt, 1986). Many factors can affect plant response to abiotic stress, including the duration of the stress, its severity, previous exposure to stress, the part of the plant being examined, and the phase of plant growth or development when it is occurring (Bray, 1997). How a stress factor can influence sex lability in dimorphic species has been studied previously (reviewed by Korpelainen, 1998, Sakai and Weller, 1999, Barrett, 2002), but it is still poorly understood. In general, environmental stresses caused by less-than-optimal light, nutrition, weather or water conditions often favour maleness (Korpelainen, 1998, Munné-Bosch, 2015). However, this topic is still insufficiently explored considering the high diversity of dimorphic species and stress factors. For instance, to our knowledge the influence of Pi starvation during the transition phase on sex determination in dioecious perennial plants has not been explored thus far.
It is generally assumed that plant maturity, which is marked by the onset of flowering and seed production, has drastic effects on the capacity of plants to respond to stress (Bond, 2000). For example, it has been generally accepted that females often show higher proportional investments in reproduction than males in dioecious plant species (Hancock and Bringhurst, 1980). However, the theory that females grow less than males as a consequence of higher investment in reproduction appears to be true for some woody perennials, but it is not so clear or not generally applicable in herbaceous perennials (Obeso, 2002, Barrett and Hough, 2013, Juvany and Munné-Bosch, 2015). Indeed, recent in-depth analyses at the physiological level suggest that sex-related differences observed in dioecious species are highly species-specific (Juvany and Munné-Bosch, 2015) and may well respond to long-term compensatory mechanisms (Obeso, 2002). Therefore, it is essential to get further insight into our current knowledge on plant response to abiotic stress in dimorphic species, considering not only the differences caused by sex, but also integrating a better knowledge of what are the causes of biased sex ratios found in nature, an aspect that requires special attention in the frame of climate change (Munné-Bosch, 2015).
Phosphorus (P) is one the most abundant macronutrients in plant tissues, but the low availability of this element in the soil is often the limiting factor for plant growth and development. P is essential for the storage of genetic information (is necessary for forming nucleic acids and other essential cellular components), it plays a crucial role in cell membranes (such as the formation of membrane phospholipids), as well as in practically all energy transactions through ATP, and it is both a substrate and a regulatory factor in photosynthesis and oxidative metabolism, among others. The chemical form of P taken up by plants is inorganic phosphate (Pi), mostly in the form of orthophosphate (H2PO4−) under physiological pH (Ullrich-Eberius et al., 1984, Furihata et al., 1992). Due to its chemical properties, Pi is easily immobilized in the soil by forming insoluble complexes or precipitates with metals (Al or Fe in acidic soils and Mg or Ca in calcareous soils, Manning, 2008). Thus, Pi is one of the most important mineral nutrients limiting plant growth and development (Marschner, 2012).
Urtica dioica, often called common nettle or stinging nettle, is a dioecious, herbaceous and perennial plant distributed widely in many parts of the world (Taylor, 2009). Nettles have a strong association with human activity. In some cases, human and animal waste may be responsible for elevated levels of P and N in the soil, providing an ideal environment for nettles (Bolòs and Vigo, 1984). Here, we examined the effects of Pi starvation on sex expression and the response of males and females in terms of nutrient accumulation, C and N isotopic composition, photo-oxidative stress markers and hormone profiling. All measurements were performed in a growth chamber, which allowed comparing the stress response of male and female plants under identical environmental conditions.
Section snippets
Plant material, treatments and sampling
Urtica dioica L. a dioecious, herbaceous and perennial plant that requires high levels of P and other nutrients, was used for experiments. Seeds of U. dioica, which were purchased in Spicegarden (www.spicegarden.eu, Germany), were sown in 0.1-L pots containing a mixture of peat:perlite:vermiculite (1:1:1, v/v/v) and plants were grown in a constant environment chamber with 16-h photoperiod, 120 μmol quanta m−2 s−1, and air temperature 21–23 °C. After 15 days of growth, plants were transplanted
Maturity and growth
Pi deficiency for 12 d during the juvenile phase caused a small (1,2 d), but significant delay in maturation in males, but not in females (Fig. 1). In other words, females matured more slowly than males under control conditions, but not under Pi deficiency. Furthermore, Pi starvation during the juvenile phase increased the sex ratio from 1.6 in controls to 2.0, thus indicating female over male production in juveniles under stress conditions was favoured. The height and number of leaves per
Discussion
Sexual dimorphism can lead to profound differences in secondary sexual characters, such as vegetative growth and activation of chemical defences under stress, but the extent to which the trade-off between reproductive effort, vegetative growth and stress resilience is still poorly understood in dimorphic plant species (Obeso, 2002, Barrett and Hough, 2013, Juvany and Munné-Bosch, 2015, Morales and Munné-Bosch, 2016). U. dioica plants represent a particular model of interest for the study of
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