Apical Damage Research Articles

​Performance of Pigeon Pea [Cajanus cajan (L.) Millsp] Genotypes under Cold Stress

Background: In north India, pigeon pea experiences low temperature stress during winter months (December-January). Information’s regarding cold stress and its impact on morphological characters, physio-biochemical processes and yield attributes in pigeon pea crop is very limited. Keeping the above facts in to consideration, the experiment was formulated to screen suitable genotypes of pigeon pea tolerant to cold stress based on morpho-physiological traits and ability to set flower and pods and yield performance. Methods: Field experiments with 98 genotypes of pigeon pea were conducted in augmented design including checks with all recommended cultural practices in 2018-19 and 2019-20 at New Research Farm, ICAR-IIPR, Kanpur. Morphological, biochemical and yield parameters were recoded periodically. Result: Based on the reaction of pigeon pea genotypes against the exposer of cold stress in terms of flower/pod drop and flower/pod retention, cold injury at the apical meristems and post regeneration of apical portion, the genotypes like IPA 15F, Dhule-D, JBT46/27, IPACT-6, IPACT-14, IPAC-1-17, IPACT-68, IPACT-22 showed greater than 90% flower/pod drop and retained only 0-10% flowers/pods, greater than 90% apical damage and less than 10% recovery of apical portion and were considered highly sensitive to cold stress. Genotypes namely MA-3, IPA 9F, ICP 15-9-1, VKG 27/161, H-26, ICPL 7035, LRG-30, PBT/SSL 2/73, KPL 1034-31, IPA 15-15, KPL 34, ICP 2073, IPACT-5, IPAB 10-66, IPACT-10, IPACT-11, IPAHT-26, IPACT-15, IPAD 2-8, IPACT-16, IPAD-8, IPACT-17, IPAHT-43, IPACT-21 showed greater than 80% flower/pod drop and retained 10-20% flower/pods, greater than 80%apical damage and less than 20% recovery of apical portion and considered moderately susceptible to cold stress. Genotypes NDA-2, MAL-13, ICP-2275, IPACT-2 showed around 20-40% flower/pod drop and have retained 60-80% flowers/pods, less than 20% apical damage and greater than 80% recovery of apical portion were considered as the highly tolerant to cold stress. The nitrogen balance index, chlorophyll and flavonols showed higher values in tolerant group whereas lower in susceptible group. The average yield/plant of pigeon pea genotypes tolerant to cold stress is considerably higher than the susceptible group of pigeon pea genotypes.

Inconsistent annual compensation for floral herbivory by an iterocarpic thistle.

Plants experiencing steep reproductive losses from herbivores should favor strategies promoting tolerance or resistance to that herbivory. However, the degree to which such strategies succeed in improving plant fitness under natural conditions needs further evaluation, especially for iterocarpic species. We tested whether reproductive effort by the iterocarpic Cirsium undulatum Spreng. (Wavyleaf thistle) provided within-season tolerance for floral herbivory through response to apical damage. We imposed apical damage and manipulated floral herbivory on later-flowering, non-apical flowerheads for two seasons. We asked: (1) is there evidence of compensatory potential to tolerate apical flowerhead damage? If so, (2) does the amount of herbivore pressure on non-apical flowerheads influence the magnitude of any compensatory response; and (3) is the response to apical damage sufficient to increase plant seed production under ambient floral herbivory over the flowering season? Plants showed compensatory potential for apical head loss; apical damage increased seed contributions from later, lower positioned flowerheads. Further, the intensity of subsequent herbivore pressure influenced compensation outcomes. Equitable seed production under both levels of ambient herbivory occurred only in the year in which plants were larger and insect pressure was lower. Finally, the response to apical damage was sufficient to compensate for apical seed loss, but it did not consistently increase overall annual seed production under ambient floral herbivory. Although this iterocarpic species can compensate for apical damage, tolerance for floral herbivory varied between years.

Aboveground herbivory causes belowground changes in twelve oak Quercus species: a phylogenetic analysis of root biomass and non‐structural carbohydrate storage

Plant ecosystem structure is understood to be a result of complex multitrophic interactions. Most multitrophic studies focus on plant aboveground adaptations to aboveground herbivore pressures, neglecting belowground adaptations in response to aboveground damage. Differential investment in root structures may allow plants to compensate for tissue loss or damage due to herbivores. Furthermore, phylogeny may constrain a plant's ability to adapt belowground. We examined the belowground responses of 12 species of oak Quercus to varying locations and intensities of simulated herbivore damage. We first established that oak belowground traits responded to aboveground herbivory by measuring patterns of investment in coarse versus fine root structures and re‐allocation of non‐structural carbohydrates (NSC) to root storage. We then tested whether phylogeny could explain variations in investment patterns using phylogenetic independent contrasts. Plant adaptations to aboveground herbivory included allocating biomass and carbon reserves to root structures, depending on the location and intensity of herbivore damage. NSC re‐allocation to root storage was observed when oak species experienced any type of damage, but damage to lateral tissues caused a greater re‐allocation than apical damage or control treatments. We found that most belowground responses to aboveground herbivory are species‐specific and may be adapted for environmental conditions or type of herbivory. Some responses to herbivore damage, such as changes in fine‐root mass and root sugar concentrations, were phylogenetically constrained. Phylogenetic constraints generally occur when there is severe damage at the apical meristem. Plants may adapt to aboveground tissue loss due to varying herbivore pressures (i.e. varying location and intensity of damage) by differentially investing in root types and NSC re‐allocation to root storage. Understanding linkages between and phylogenetic constraints of plant belowground responses to aboveground herbivory will improve our understanding of the ecological processes involved in multitrophic interactions.

Effect of Seedling Provenance and Site Heterogeneity on Abies cephalonica Performance in a Post-Fire Environment

Reforestation constitutes a challenge in post-fire ecosystem restoration, although there are limitations such as species and genotype selection, planting and management design, and environmental conditions. In the present study, the basic issue is the longevity of Abies cephalonica Loudon—the Greek fir seedlings planted extensively in Parnitha National Park (Central Greece), located near the metropolitan city of Athens, following the large-scale wildfire of 2007. Seedling performance was assessed for a 3-year monitoring period (2013–2015) through the establishment of 8 permanent transects, including 400 seedlings at the burned, reforested sites. According to the long-term reforestation project, two seedling provenances were used: (a) from Mt. Mainalon (South Greece, Vytina provenance) and (b) the local one from Mt. Parnitha. Both provenances showed a relatively successful survival rate reaching, in average, 73.8%, with the first summer after planting being crucial for seedling survival. The overall mean seedling height was 39.2 ± 1.1 cm, with a mean crown diameter of 47.3 ± 1.4 cm in the last monitoring survey. Although Parnitha seedlings seem to perform better in terms of growth, seedling performance in both provenances was affected by reforestation site characteristics, mainly altitude and aspect. Approximately one third of seedlings exhibited damage in their crown architecture (29.8%), while apical bud damage was less extensive (12.2%) in the final field measurement. Data indicate that seedling performance has proved to be quite promising for post-fire restoration, although long-term monitoring data should be considered.

Neosilba Perezi (Romero & Ruppel, 1973) (Diptera: Lonchaeidae) Damage Simulation on the Production of Stem Cuttings and the Productive Aspects of Cassava

The loss caused by the shoot-fly (Neosilba perezi) was simulated through the elimination of the cassava`s terminal buds, with the objective to quantify this effect towards the roots productivity, flour production and propagative material production (stem cuts). The experimental design was random blocks, with three repetitions, in a factorial scheme with an additional treatment (10 × 4 + 1). Ten damage levels (10, 20, 30, 40, 50, 60, 70, 80, 90 and 100%) were studied, including a witness treatment (0%). The cassavas were treated during each of the four seasons of the year. The studied variables were: root productivity (t ha-1), flour production (t ha-1) and number of stem cuttings with a diameter greater than 20 and 25 mm. The induced damage in the cassava`s meristem did not significantly affect the root, nor the flour production. However, the stem cuttings production demonstrated a linear decrease in relation to the apical meristem damage level.

Step-by-step colpotomy in total laparoscopic hysterectomy: a technique to avoid apical support damage to the upper vagina

The purpose of this video article is to demonstrate our colpotomy technique that enables maximal protection of the cervical ring, helps to prevent the ureteral injury by distancing, and avoids shortening of the vagina at total laparoscopic hysterectomy. Step-by-step explanation of the colpotomy technique is presented using educational video setting in university-affiliated private hospital. After the uterine artery transection, a VECTEC surgical uterine manipulator (VECTEC, Hauterive, France) was inserted into the vagina in place of the sharp curette. The plastic rotating blade of uterine manipulator was strongly pushed forward into the anterior vaginal fornix. Colpotomy incision was started from the uppermost middle point of an anterior vagina, and extended to both sides with a monopolar L-hook electrocautery at 40 watts cutting mode. Then the manipulator’s blade was maneuvered into the right lateral fornix, and THUNDERBEAT platform (Olympus Medical Systems Corp, Tokyo, Japan) was chosen as the modality of energy for the transection of the rest of the vagina. At the posterior part of colpotomy, the vaginal wall was cut from the uppermost part of uterosacral ligaments, as well. Finally, the left lateral fornix was cut by the same principles, and colpotomy was completed circumferentially. In conclusion, maximal preservation of paracervical ligaments with this technique preserve the apical support of vagina, and avoids shortening of vaginal length. The technique also minimizes the ureteral injury by distancing.

Effects of apical damage on plant growth and male and female reproductive investments in Ambrosia artemisiifolia, a wind-pollinated plant

In wind-pollinated plants, apical damage may decrease male fitness by reducing height-dependent pollen dispersal distance, but may not affect female fitness because plant height is not always correlated with female fitness. We hypothesized that Ambrosia artemisiifolia responds to apical damage by (1) restoring plant height through compensatory growth from lateral buds, and/or (2) increasing the sex allocation to female function to compensate for the loss of male fitness. We tested these hypotheses by comparing a group of experimental removal of the apical meristem with three control groups and by field surveys on apically damaged plants. Experimental apical damage suppressed main stem growth, but promoted vertical secondary growth from lateral buds. These responses resulted in compensation of stem height in the apically damaged plants to the same height as one of three control groups. The numbers of male and female flowers and male racemes did not differ between damaged and undamaged plants, indicating that apically damaged plants did not change their sex allocation. Therefore, our results support our first hypothesis. The results of a field survey of naturalized populations also supported the first hypothesis in that plant height and the number of male racemes did not change in plants with apical damage. Consequently, our results suggest that A. artemisiifolia has a high ability of fitness compensation after apical damage by restoring height and male function. This ability may contribute to its invasiveness in disturbed habitats.

Growing competitive or tolerant? Significance of apical dominance in the overcompensating herb Gentianella campestris.

As a compensatory response to herbivory, plants may branch vigorously when the growth of dormant meristems is triggered by shoot damage. Undamaged plants, on the other hand, often restrain branching, and this limitation on growth can be considered a cost of tolerance to herbivory. Restrained branching is caused by apical dominance and may, alternatively, be associated with fitness benefits in competitive environments that favor fast vertical growth. To test these hypotheses regarding selection for restrained branching, we compared the performance of two subspecies of the biennial grassland herb Gentianella campestris; the tall, apically dominant ssp. campestris and the short, multi-stemmed ssp. islandica, which shows reduced apical dominance. For both subspecies, we manipulated the height of surrounding vegetation (competition) and damage intensity in grasslands of differing productivity (high, medium, low), and examined population growth rates using matrix population models combined with life table response experiments. In the absence of damage, ssp. campestris exhibited a higher population growth rate than ssp. islandica in the tallest vegetation, however with the growth rate still being below one. In the medium and low productivity environments where the vegetation was shorter, the population growth rate of ssp. islandica was considerably higher than that of ssp. campestris as long as no more than about 50% of the plants were damaged. When plants were damaged, the apically dominant ssp. campestris showed a positive population growth rate (λ>1) and often overcompensatory seed production in all productivity levels, while ssp. islandica showed no compensation and therefore the population was predicted to decline (λ<1). We conclude that restrained branching in Gentianella cannot be selected for by competition alone, but that episodes of apical damage are required to maintain the trait. Furthermore, because of the costs of restrained branching, apical dominance should be selected against in grasslands where competition and disturbance are low.

Cumulative herbivory outpaces compensation for early floral damage on a monocarpic perennial thistle.

Floral herbivory represents a major threat to plant reproductive success, driving the importance of plant tolerance mechanisms that minimize fitness costs. However, the cumulative insect herbivory plants experience under natural conditions complicates predictions about tolerance contributions to net fitness. Apical damage can lead to compensatory seed production from late season flowering that ameliorates early season fitness losses. Yet, the compensation realized depends on successful development and herbivore escape by later season flowers. Using monocarpic perennial Cirsium canescens, we quantified seed-reproductive fitness of plants with vs. without experimental damage to the early-developing large apical flower head, with and without a 30-40% herbivory reduction on subsequent flower heads, for two flowering cohorts. Plants with reduced herbivory clearly demonstrated the release of apical dominance and compensation, not overcompensation, for apical damage via greater seed maturation by later flower heads. In contrast, plants that experienced ambient herbivory levels on subsequent heads undercompensated for early apical damage. Individuals had lower total seed set when the apical head was damaged. Compensation was, therefore, possible through a small increase in total flower heads, caused by a higher rate of floral bud survival, and a higher seed maturation rate by subsequent heads, leading to more viable seeds per matured flower head. With ambient cumulative floral herbivory, compensation for apical damage was not sufficient to improve fitness. Variation in the intensity of biological interactions played a role in the success of plant tolerance as a mechanism to maximize individual fitness.

An assessment of the molecular mechanisms contributing to tolerance to apical damage in natural populations of Arabidopsis thaliana

Herbivory imposes substantial selection pressure on plants, with the ability to regrow and maintain reproductive success a challenging but often necessary response by the plant. Despite the commonness of herbivore-induced damage, vast variation in tolerance ability exists among plants. Recent studies have suggested the role of endoreduplication (increasing ploidy within an individual) and the pentose phosphate pathway (a metabolic pathway that supports both primary and secondary metabolism) in contributing to the variation in tolerance ability among genotypes of Arabidopsis thaliana. We measured natural variation in apical meristem damage frequency, endoreduplication, and the sequence of G6PD1, an important gene in the pentose phosphate pathway, and related them to variation in tolerance of natural populations of A. thaliana over a portion of its native European range. Variation among populations in tolerance was significantly positively related to damage frequency, suggesting the potential for directional selection for tolerance ability as a product of damage frequency. We also discovered likely loss-of-function G6PD1 alleles in two populations, both of which displayed among the lowest levels of tolerance of all populations assessed. In addition, populations with the greatest increase in endopolyploidy also had the greatest ability to tolerate damage while populations with the greatest reduction in endopolyploidy had the lowest ability to tolerate damage. This study provides an assessment of variation in tolerance, damage frequency, G6PD1 sequence, and endopolyploidy in natural populations of A. thaliana, and also contributes to the growing body of research on the contributions of these specific molecular mechanisms to the tolerance response.

Genetic variation in the reduction of attractive floral traits of an annual tarweed in response to drought and apical damage

Genetic variation in the reduction of attractive floral traits of an annual tarweed in response to drought and apical damage

The role of invertases in plant compensatory responses to simulated herbivory.

BackgroundThe ability of a plant to overcome animal-induced damage is referred to as compensation or tolerance and ranges from undercompensation (decreased fitness when damaged) to overcompensation (increased fitness when damaged). Although it is clear that genetic variation for compensation exists among plants, little is known about the specific genetic underpinnings leading to enhanced fitness. Our previous study identified the enzyme GLUCOSE-6-PHOSPHATE DEHYDROGENASE 1 (G6PD1) as a key regulator contributing to the phenomenon of overcompensation via its role in the oxidative pentose phosphate pathway (OPPP). Apart from G6PD1 we also identified an invertase gene which was up-regulated following damage and that potentially integrates with the OPPP. The invertase family of enzymes hydrolyze sucrose to glucose and fructose, whereby the glucose produced is shunted into the OPPP and presumably supports plant regrowth, development, and ultimately compensation. In the current study, we measured the relative expression of 12 invertase genes over the course of plant development in the Arabidopsis thaliana genotypes Columbia-4 and Landsberg erecta, which typically overcompensate and undercompensate, respectively, when damaged. We also compared the compensatory performances of a set of invertase knockout mutants to the Columbia-4 wild type.ResultsWe report that Columbia-4 significantly up-regulated 9 of 12 invertase genes when damaged relative to when undamaged, and ultimately overcompensated for fruit production. Landsberg erecta, in contrast, down-regulated two invertase genes following damage and suffered reduced fitness. Knockout mutants of two invertase genes both exhibited significant undercompensation for fruit production, exhibiting a complete reversal of the wild type Col-4’s overcompensation.ConclusionCollectively, these results confirm that invertases are essential for not only normal plant growth and development, but also plants’ abilities to regrow and ultimately compensate for fitness following apical damage.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0655-6) contains supplementary material, which is available to authorized users.

Regrowth patterns and rosette attributes contribute to the differential compensatory responses of Arabidopsis thaliana genotypes to apical damage.

A plant's compensatory performance refers to its ability to maintain or increase its reproductive output following damage. The ability of a plant to compensate depends on numerous factors including the type, severity, frequency and timing of damage, the environmental conditions and the plant's genotype. Upon apical damage, a cascade of hormonal and genetic responses often produces dramatic changes in a plant's growth, development, architecture and physiology. All else being equal, this response is largely dependent on a plant's genotype, with different regrowth patterns displayed by different genotypes of a given species. In this study, we compare the architectural and growth patterns of two Arabidopsis thaliana genotypes following apical damage. Specifically, we characterise regrowth patterns of the genotypes Columbia-4 and Landsberg erecta, which typically differ in their compensation to apical meristem removal. We report that Landsberg erecta suffered reductions in the number of stems produced, maximum elongation rate, a delay in reaching this rate, lower average rosette quality throughout the growing period, and ultimately, less aboveground dry biomass and seed production when damaged compared to undamaged control plants. Columbia-4 had no reductions in any of these measures and maintained larger rosette area when clipped relative to when unclipped. Based on the apparent influence of the rosette on these genotypes' compensatory performances, we performed a rosette removal experiment, which confirmed that the rosette contributes to compensatory performance. This study provides a novel characterisation of regrowth patterns following apical damage, with insights into those measures having the largest effect on plant performance.

Tolerance to apical and leaf damage of Raphanus raphanistrum in different competitive regimes.

Tolerance to herbivory is an adaptation that promotes regrowth and maintains fitness in plants after herbivore damage. Here, we hypothesized that the effect of competition on tolerance can be different for different genotypes within a species and we tested how tolerance is affected by competitive regime and damage type. We inflicted apical or leaf damage in siblings of 29 families of an annual plant Raphanus raphanistrum (Brassicaceae) grown at high or low competition. There was a negative correlation of family tolerance levels between competition treatments: plant families with high tolerance to apical damage in the low competition treatment had low tolerance to apical damage in the high competition treatment and vice versa. We found no costs of tolerance, in terms of a trade‐off between tolerance to apical and leaf damage or between tolerance and competitive ability, or an allocation cost in terms of reduced fitness of highly tolerant families in the undamaged state. High tolerance bound to a specific competitive regime may entail a cost in terms of low tolerance if competitive regime changes. This could act as a factor maintaining genetic variation for tolerance.

Manner of Apical Meristem Destruction Affects Growth, Reproduction, and Survival of Sea Oxeye Daisy

Although herbivory may result in plant death, the mode and timing of damage may produce variable wounding responses in the attacked plant. In this study, effects of different types of apical meristem damage on growth and performance of sea oxeye daisy Borrichia frutescens (L.) DC were compared. Damage involved either clipping or galling of the apical meristem by the gall midge Asphondylia borrichiae Rossi and Strong. Apical dominance was relatively weak before flowering and stronger in short ramets that were shaded by taller neighbors later in the season. Clipped ramets delayed sprouting new stems, and galled ramets sprouted new stems quickly compared to intact ramets, but final stem counts were similar across treatments. Clipping significantly delayed flowering, reduced seed head ripening time, and resulted in fewer seed heads and seeds. Galling did not significantly impact reproduction compared to intact ramets. Nitrogen supplementation significantly increased stem count and seed count and lengthened seed ripening time. Borrichia frutescens responds differently to clipping versus galling by A. borrichiae and better tolerates galling in terms of nonreproductive performance and survival. Galling from A. borrichiae likely prevents Borrichia frutescens from flowering, which suggests resource regulation of meristems by the midge.

Transcriptomics of plant responses to apical damage reveals no negative correlation between tolerance and defense

While one may expect the loss of plant tissue by animal herbivores to be universally detrimental to a plant’s fitness, a wide range of tolerance responses exists, including undercompensation (lower fitness when damaged), equal compensation, and even overcompensation (increased fitness when damaged). Theory predicts that these responses could be constrained by the investment into defensive chemicals and structures produced for improving resistance in damaged tissues, and thus tolerance and induced defense could be considered alternative strategies to the selection pressure imposed by herbivory. To determine which genetic pathways underlie differences in compensatory (i.e., growth and fitness) responses to damage by tissue loss, and to test their relation with pathways involved in defense, we performed a controlled greenhouse study to measure total gene expression via RNA-sequencing of undamaged and mechanically damaged plants of three Arabidopsis thaliana genotypes that differ in their compensatory performances: Columbia-4, Landsberg erecta, and a recombinant inbred line (RIL) from a Columbia-4 × Landsberg erecta cross. Among the many genetic pathways that responded to clipping, Columbia-4 significantly up-regulated genes involved in secondary defense chemistry and equally compensated for fitness while Landsberg erecta and the RIL both undercompensated and significantly down-regulated secondary defense pathways. The genotypes’ different compensatory performances are thus positively correlated with their differential investments into secondary metabolism following tissue loss. This study identifies differential post-damage gene regulation of growth, developmental signaling, and environmental response pathways in A. thaliana, and provides the first transcriptomic evidence counter to the presumed tradeoff between tolerance and defense in plant-herbivore interactions.

Effects of apical meristem mining on plant fitness, architecture, and flowering phenology in Cirsium altissimum (Asteraceae).

• Interactions that limit lifetime seed production have the potential to limit plant population sizes and drive adaptation through natural selection. Effects of insect herbivory to apical meristems (apical meristem mining) on lifetime seed production rarely have been quantified experimentally. We studied Cirsium altissimum (tall thistle), whose meristems are mined by Platyptilia carduidactyla (artichoke plume moth), to determine how apical damage affects plant maternal fitness and evaluate both direct and indirect mechanisms underlying these effects.• In restored prairie, apical mining was manipulated on tall thistles by applying insecticide, water, or no spray to apical meristems. We quantified effects on lifetime seed production, plant architecture, and flowering phenology. Seed germinability and seedling mass were evaluated in a greenhouse.• Apical meristem miners decreased lifetime seed production of C. altissimum, but not seed quality. Higher mortality rates of damaged plants contributed to reduced seed production. Apical damage reduced plant height and increased the proportion of blooming flower heads in axial positions on branches. Apical damage delayed flowering and shortened flowering duration.• Apical meristem mining reduced plant maternal fitness. The shift in the identity of blooming flower heads from terminal to axial positions contributed to this reduction because axial heads are less fecund. Shorter, meristem-mined plants may have been more susceptible to competition, and this susceptibility may explain their higher mortality rates. The kinds of changes in architecture and phenology that resulted from apical damage to C. altissimum have been shown to affect floral visitation in other plant species.

Plasticity in ploidy underlies plant fitness compensation to herbivore damage.

How plants mitigate damage by animal herbivores is a fundamental ecological and evolutionary question of plant-animal interactions. Some plants can increase their fitness when damaged in a phenomenon termed 'overcompensation'. Despite overcompensation being observed in a variety of plant species, its mechanistic basis remains elusive. Recent research has shown that the Arabidopsis thaliana genotype Columbia-4 employs endoreduplication, the replication of the genome without mitosis, following damage and that it overcompensates for seed yield. The related genotype Landsberg erecta, in contrast, does not increase its endoreduplication following damage and suffers reduced seed yield. While these results suggest that a plant's ability to plastically increase its ploidy during regrowth may promote its mitigation of damage, no studies have explicitly linked the endoreduplication genetic pathway to the regrowth and fitness of damaged plants. By comparing fitness and ploidy between undamaged and damaged plants of Columbia-4, Landsberg erecta and their offspring, we provide evidence that endoreduplication is directly involved in compensatory performance. We then overexpressed an endoreduplication regulator and compared this mutant's endoreduplication and compensation with its background genotype Columbia-0, an undercompensator. Enhancing Columbia-0's ability to endoreduplicate during regrowth led to the complete mitigation of the otherwise detrimental effects of damage on its fitness. These results suggest that the ability of these plants to increase their ploidy via endoreduplication directly impacts their abilities to compensate for damage, providing a novel mechanism by which some plants can mitigate or even benefit from apical damage with potential across the wide range of plant taxa that endoreduplicate.

Ontogenetic contingency of tolerance mechanisms in response to apical damage.

Plants are able to tolerate tissue loss through vigorous branching which is often triggered by release from apical dominance and activation of lateral meristems. However, damage-induced branching might not be a mere physiological outcome of released apical dominance, but an adaptive response to environmental signals, such as damage timing and intensity. Here, branching responses to both factors were examined in the annual plant Medicago truncatula. Branching patterns and allocation to reproductive traits were examined in response to variable clipping intensities and timings in M. truncatula plants from two populations that vary in the onset of reproduction. Phenotypic selection analysis was used to evaluate the strength and direction of selection on branching under the damage treatments. Plants of both populations exhibited an ontogenetic shift in tolerance mechanisms: while early damage induced greater meristem activation, late damage elicited investment in late-determined traits, including mean pod and seed biomass, and supported greater germination rates. Severe damage mostly elicited simultaneous development of multiple-order lateral branches, but this response was limited to early damage. Selection analyses revealed positive directional selection on branching in plants under early- compared with late- or no-damage treatments. The results demonstrate that damage-induced meristem activation is an adaptive response that could be modified according to the plant's developmental stage, severity of tissue loss and their interaction, stressing the importance of considering these effects when studying plastic responses to apical damage.

Plasticity in apical dominance and damage tolerance under variable resource availability in Medicago truncatula

The activation of dormant meristems following apical damage is an important mechanism for tolerance of herbivore damage, but its impact could vary with resource availability. Here, we examined central predictions of the limiting resource model (LRM), according to which high resource availability can support damage tolerance in plants with deterministic apical dominance, but will have limited or no effect in plants that are induced to increase branching by increased resource availability regardless of damage. We examined these predictions by studying the branching patterns of Medicago truncatula plants in response to both light and water availabilities and their effects on tolerance of apical damage. We used plants from environments that were predicted to select for different levels of apical dominance. Intact plants from the more productive and competitive population exhibited strong apical dominance and refrained from branching even under full light, whereas plants from the less productive and sparser population exhibited greater plasticity in apical dominance and readily branched under high water and light. In accordance with the LRM, these differences translated into differential responsiveness to apical damage: given abundant water, apical damage induced the activation of lateral meristems and increased pod and seed production in plants from the more productive environment, but not in plants from the less productive environment. These results suggest an adaptive association between deterministic inhibition of lateral meristems and compensatory ability, which supports the hypothesis that greater compensatory responsiveness to apical damage could be a derivative of adaptation to other environmental stresses, such as light competition.