Leaf Toughness

Solvent leaf extracts of Terminalia catappa were evaluated for
their cytotoxic and their radical scavenging potential. Brine shrimp
assay was used for exploring cytotoxic activity, all the extracts
namely hexane, DCM, chloroform, acetone and methanol were
tested at the dose level of 25, 50, 100 and 200microgram /ml. A
dose dependent response was obtained in all the extracts.
Chloroform extract showed 100percent activity at the highest dose
of 200μg/ml. Antioxidant activity was evaluated by one
qualitative(TLC based) and two quantitative radical scavenging
assays namely DPPH (2, 2- Diphenyl-1- picryl hydrazyl) assay
and ferric reducing antioxidant power (FRAP) assay. In both
assays acetone extract exhibited significant reduction of free
radicals. The IC50 value of acetone extract was 36.9μg/ml.
Ascorbic acid was used in these assays as reference compound.
FRAP value of acetone extract was found to be 6.73 much higher
than all the other extracts.

Background and Aims In tropical lowland rain forest (TLRF) the leaves of most monocots differ from those of most dicots in two ways that may reduce attack by herbivores. Firstly, they are tougher. Secondly, the immature leaves are tightly folded or rolled until 50–100% of their final length. It was hypothesized that (a) losses of leaf area to herbivorous invertebrates are generally greatest during leaf expansion and smaller for monocots than for dicots, and (b) where losses after expansion are appreciable any difference between monocots and dicots then is smaller than that found during expansion.

Methods At six sites on four continents, estimates were made of lamina area loss from the four most recently mature leaves of focal monocots and of the nearest dicot shoot. Measurements of leaf mass per unit area, and the concentrations of water and nitrogen were made for many of the species. In Panama, the losses from monocots ( palms) and dicots were also measured after placing fully expanded palm leaflets and whole dicot leaves on trails of leaf-cutter ants.

Key Results At five of six sites monocots experienced significantly smaller leaf area loss than dicots. The results were not explicable in terms of leaf mass per unit area, or concentrations of water or nitrogen. At only one site was the increase in loss from first to fourth mature leaf significant (also large and the same in monocots and dicots), but the losses sustained during expansion were much smaller in the monocots. In the leaf-cutter ant experiment, losses were much smaller for palms than for dicots.

Conclusions The relationship between toughness and herbivory is complex; despite the negative findings of some recent authors for dicots we hypothesize that either greater toughness or late folding can protect monocot leaves against herbivorous insects in tropical lowland rain forest, and that the relative importance varies widely with species. The difficulties of establishing unequivocally the roles of leaf toughness and leaf folding or rolling in a given case are discussed.

Despite the potential for changes during transit or preservation, the physicochemical properties of leaves are typically measured in a laboratory setting. A suite of laboratory methods adapted for use in the field is described here. The equipment is portable and operable in remote environments. Each technique has been validated against laboratory standards and has been tested throughout the tropics in a variety of ecological contexts. The properties of canopy and understorey leaves from Central Panama are reported here. The results show clear differences between leaves growing in different light regimes. Canopy leaves are tougher and possess greater concentrations of protein, phenols, and tannins. The implication of these results to the resource availability hypothesis, which proposes trade-offs between physiology and defences against herbivory, is dis- cussed.

The two major mechanical defences of plants are toughness and hardness. These have different material causes and ecological functions. In any non-metal, high toughness is achieved by composite construction (i.e. by an organized mixture of components). The primary source of toughening in plants is the composite cell wall (cellulosic microfibrils set in a hemicellulose and, sometimes, lignin matrix), with a toughness of 3.45 kJ m-2, which is ten-times the probable toughness of its individual components if they could be isolated. The toughness of most plant tissues is roughly proportional to the volume fraction of tissue occupied by cell wall (Vc) and, compared to animal tissues and non-biological composites, is very low. High toughness in plant cells is not produced by the walls themselves, but by their plastic intracellular collapse. This is a truly cellular toughening mechanism, one of the most potent ever discovered by materials scientists, depending on an elongate cell shape with microfibrils directed uniformly at a small angle to the cellular axis. Only `woody' cells, tracheids and fibres, have this framework and only in the S2 layer of their secondary wall. Despite this non-optimum configuration, toughness is elevated by this mechanism ten-times above that due to cell wall resistance alone. The effectiveness of toughness in preventing herbivory is indisputable, but largely indirect due to confusion over a false equivalence between nutritional `fibre content' and toughness. In contrast, generalized hardness requires high density. If hardness is due to high Vc , this conflicts with `woody' toughness because there is then no lumen for cell walls to collapse into. Thus, dense seed shells may be brittle (i.e. low toughness) even if built from fibres. However, solid cell wall is not very hard. Instead, high hardness in plants is associated with amorphous silica and is always localized. The efficacy of hardness is more difficult to evaluate than toughness because some animals specialize in coping with it.