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Abstract
It has been hypothesized that slow-growing plants are more likely to maximize above-ground biomass and fitness when defoliated by herbivores than those with an already high relative growth rate (RGR). Some populations of the annual herb Datura stramonium L. can tolerate foliar damage better than others. The physiological basis of this difference is examined here in a comparative study of two ecotypes that differ in tolerance and maximum growth rate, using a growth analytical approach. One hundred and fifty-four plants of each ecotype grown under controlled conditions were suddenly defoliated (35 % of total leaf area removed) and a similar sample size of plants remained undefoliated (control). Ontogenetic plastic changes in RGR and its growth components [net assimilation rate (NAR), specific leaf area and leaf weight ratio (LWR)] after defoliation were measured to determine whether these plastic changes maximize plant growth and fitness. Different ontogenetic phases of the response were discerned and increased RGR of defoliated plants was detected at the end of the experimental period, but brought about by a different growth component (NAR or LWR) in each ecotype. These changes in RGR are putatively related to increases in fitness in defoliated environments. At the intra-specific scale, data showed a trade-off between the ability to grow under benign environmental conditions and the ability to tolerate resource limitation due to defoliation.
Highlights
A different trend was observed in the Santo Domingo (SD) ecotype: the maximum relative growth rate (RGR) decreased by 29 % in relation to Patria Nueva (PN); it was 16.6 mg g21 h21 at the beginning of the experiment and decreased with a complex growth trajectory to zero at the end of the experiment, two sudden increases were observed between 1494 – 1536 and 1586 – 1638 h of age (Fig. 2)
The RGR of defoliated plants remained marginally lower than the control plants up to hour 1586, when defoliated plants reached the RGR values of control plants, producing an increase with respect to the values observed in the control plants at the end of the experiment (Fig. 2)
Regarding the general question on what growth determinants are involved in the behavioural changes of RGR in response to defoliation, we have found that, throughout their ontogeny, defoliated plants of D. stramonium are capable of restoring, equalling or even overcompensating RGR compared with undefoliated plants
Summary
Camargo et al — Trade-off between RGRmax and tolerance to defoliation response implies a series of plastic phenotypic changes in different traits that can determine the pattern of change in relative growth rate (RGR, the rate at which a given amount of existing biomass can produce new biomass per unit time; Shipley 2000) after defoliation (Oesterheld and McNaughton 1988; Trumble et al 1993; Strauss and Agrawal 1999; Stowe et al 2000; Tiffin 2000) These plastic responses to defoliation may include an increasing photosynthetic rate (Caldwell et al 1981; Wallace et al 1984), changing the allocation pattern to increase the production of new leaf area (McNaughton and Chapin 1985), or nutrient uptake (Ruess et al 1983; McNaughton and Chapin 1985) and improving plant water status (Toft et al 1987).
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