Marta Oñate

Photo‐oxidative stress markers reveal absence of physiological deterioration with ageing in Borderea pyrenaica, an extraordinarily long‐lived herb

In animals, age-associated disorders are believed to be connected to shifts in the antioxidant/pro-oxidant balance in favour of oxidative stress. However, the contribution of oxidative stress to ageing in long-lived perennials has not been explored to date. Here, we tested age- and sex-related changes in several photo-oxidative stress markers in Borderea pyrenaica, a small dioecious geophyte relict of the Tertiary with one of the longest life spans ever recorded for a non-clonal herb (more than 300years). Given that survival increases with age in B.pyrenaica, we hypothesized that oxidative stress does not increase with ageing because the species develops improved anti-oxidant defence. In three field samplings performed during 2008, 2010 and 2011 in the Central Pyrenees (NE Spain), we examined the effects of ageing and sex on photosynthetic pigment levels, PSII integrity (Fv/Fm ratio), lipid peroxidation, and the extent of photo and anti-oxidant protection in chloroplasts. Furthermore, we explored whether age and sex affect plant response to severe natural desiccation. Both male and female plants maintained chlorophyll levels intact, as well as the Fv/Fm ratio and the levels of lipid peroxidation, irrespective of age. This finding suggests the absence of age-associated oxidative stress at the organismal level. Furthermore, photoprotection mechanisms were found to be similarly efficient in the oldest individuals as in juvenile plants, in terms of xanthophyll cycle de-epoxidation and accumulation of low-molecular-weight antioxidants (carotenoids and tocopherols). Indeed, females over 100years of age were the most resistant to severe desiccation, maintaining higher leaf hydration levels, less chlorophyll degradation and better PSII integrity under stress than females below 100years, males below or above 100years, and juveniles. Synthesis. Neither males nor females of the extraordinarily long-lived B.pyrenaica show age-dependent signs of oxidative stress. This observation suggests that age-induced oxidative stress is not a universal feature of ageing in perennial plants. Indeed, females older than 100years showed signs of negative senescence, in that they registered improved physiological performance with increasing age.

Influence of plant maturity, shoot reproduction and sex on vegetative growth in the dioecious plant Urtica dioica

BACKGROUND AND AIMS Stinging nettle (Urtica dioica) is a herbaceous, dioecious perennial that is widely distributed around the world, reproduces both sexually and asexually, and is characterized by rapid growth. This work was aimed at evaluating the effects of plant maturity, shoot reproduction and sex on the growth of leaves and shoots. METHODS Growth rates of apical shoots, together with foliar levels of phytohormones (cytokinins, auxins, absicisic acid, jasmonic acid and salicylic acid) and other indicators of leaf physiology (water contents, photosynthetic pigments, alpha-tocopherol and F(v)/F(m) ratios) were measured in juvenile and mature plants, with a distinction made between reproductive and non-reproductive shoots in both males and females. Vegetative growth rates were not only evaluated in field-grown plants, but also in cuttings obtained from these plants. All measurements were performed during an active vegetative growth phase in autumn, a few months after mature plants reproduced during spring and summer. KEY RESULTS Vegetative growth rates in mature plants were drastically reduced compared with juvenile ones (48 % and 78 % for number of leaves and leaf biomass produced per day, respectively), which was associated with a loss of photosynthetic pigments (up to 24 % and 48 % for chlorophylls and carotenoids, respectively) and increases of alpha-tocopherol (up to 2.7-fold), while endogenous levels of phytohormones did not differ between mature and juvenile plants. Reductions in vegetative growth were particularly evident in reproductive shoots of mature plants, and occurred similarly in both males and females. CONCLUSIONS It is concluded that (a) plant maturity reduces vegetative growth in U. dioica, (b) effects of plant maturity are evident both in reproductive and non-reproductive shoots, but particularly in the former, and (c) these changes occur similarly in both male and female plants.

Meristem aging is not responsible for age-related changes in growth and abscisic acid levels in the Mediterranean shrub, Cistus clusii.

To obtain new insights into the mechanisms underlying aging in perennials, we measured abscisic acid levels, growth and other stress indicators in leaves of Cistus clusii Dunal plants of different ages grown under Mediterranean field conditions. Recently emerged leaves from 9-year-old plants were compared to those of 1-year-old plants (obtained from cuttings from 9-year-old plants) to evaluate the effects of meristem aging on plant aging. Rooting and successful establishment of the cuttings allowed us to compare the physiology of plants with old meristems, but of different size. Plants obtained from cuttings were rejuvenated, with new leaves displaying a higher leaf area and chlorophyll content, but smaller leaf mass per unit area ratios and endogenous abscisic acid levels than those of 9-year-old plants. A comparative study in 1-, 4- and 9-year-old plants revealed that abscisic acid levels increase during the early stages of plant life (with increases of 90% between 1- and 4-year-old plants), but then remain constant at advanced developmental stages (between 4- and 9-year-old plants). Although leaf biomass was 53% smaller in 9-year-old compared to 4-year-old plants, the dry matter produced per shoot apical meristem was equivalent in both plant groups due to an increased number of leaves per shoot in the former. It is concluded that (i) C. clusii plants maintain the capacity to rejuvenate for several years; (ii) newly emerged leaves accumulate higher amounts of abscisic acid during early stages of plant life, but the levels of this phytohormone later remain constant; and (iii) although plant aging leads to the production of smaller leaves, the amount of biomass produced per shoot apical meristem remains constant at advanced developmental stages.

No signs of meristem senescence in old Scots pine

Ageing and senescence in plants remain poorly understood. Although meristem totipotency may allow woody perennials to be immortal, relative growth and photosynthetic rates typically decline with age. Trees of ages between 129 and 534years were selected in one of the oldest extant populations of Scots pine. Apical branches were propagated by grafting onto homogeneous juvenile rootstock to eliminate the effects of size and environmental variability and isolate those due to age. The hormonal profile of leaves and seeds along with markers of the physiological status of leaves and their pattern of DNA cytosine methylation were measured 15years after grafting. The percentage of total methylated loci in nuclear DNA increased with increasing meristematic age. However, only very few significant relationships were found between levels of phyto-hormones, pigments or physiological markers either in leaves or seeds and age of the meristem. In addition, shoots grafted from old trees grew as fast as those from younger trees and produced the same number of germinable seeds. Synthesis. We conclude that changes in DNA methylation can occur in old trees. The lack of apparent physiological deterioration in the grafted plants suggests that meristem senescence is not the main factor triggering whole-plant ageing in Scots pine.

Loss of flower bud vigour in the Mediterranean shrub, Cistus albidus L. at advanced developmental stages

To better understand aging in perennials, age-related changes in the physiology of leaves and flower buds of the Mediterranean shrub, Cistus albidus L. were evaluated. Two groups of different ages (5 and 10 years old), both at advanced developmental stages but of similar size, were compared. Total plant biomass, biomass produced per apical meristem and levels of cytokinins, abscisic acid and jasmonic acid in leaves and flower buds, as well as flower production, were measured. No differences in plant size, vegetative growth rates and levels of phytohormones in leaves were observed between 5- and 10-year-old plants. However, they showed significant differences in flower bud development; the older plants having reduced vigour, with 29.6% of flowers reaching anthesis compared to 52.5% in the younger plants. Furthermore, endogenous concentrations of zeatin and abscisic acid in flower buds at stage I (start of flower organ formation) were 61% and 41%, respectively, smaller in 10- than in 5-year-old plants. At stage II (with all flower organs formed), zeatin and abscisic acid concentrations decreased by ca. 90% and 80%, respectively, but differences between age groups were still evident (60% and 29% for zeatin and abscisic acid, respectively). Jasmonic acid levels in flower buds decreased by 80% from stage I to II, but did not differ between age groups. Despite reductions in flower bud vigour, total number of flowers per individual was not significantly different between age groups, so that an age-related loss in reproductive vigour at the organ level did not lead to a decrease in flower production at the whole plant level.

Influence of stress history on the response of the dioecious plant Urtica dioica L. to abiotic stress

Background: Reaching maturity is thought to affect plant responses to stress, but few studies have examined thus far how stress periods during the juvenile phase can alter the response of plants in a mature stage, particularly in dioecious plants. Aims: This study tested the hypothesis that sex and stress history can alter the response of Urtica dioica plants to abiotic stress. Methods: Stress indicators (water content, chlorophylls, the F v/F m ratio and the extent of lipid peroxidation) were measured in leaves of juvenile and mature plants exposed to a combination of water and nutrient deficit, with an emphasis on evaluating the effects of stress during the juvenile phase on the stress response of mature plants. Results: Stress treatment during the juvenile phase affected plant response to stress during the mature phase. Leaves of reproductive shoots were the most sensitive to stress history, as shown by increases in lipid peroxidation in leaves of reproductive shoots relative to non-reproductive ones. Leaves of both shoot types responded similarly in males and females, which showed no significant differences in any of the parameters measured. Conclusions: Stress history appears to determine the response of mature U. dioica plants to abiotic stress, plants with a stress history showing acclimation to subsequent stress, leaves of non-reproductive shoots of both males and females being more stress tolerant and allowing plant survival under severe stress.