Which of the following is the most likely reason for the difference in leaf growth?

Abstract

Herbaceous plants are plants that have no persistent woody stem above ground. They are classified following life-cycle classification as annuals, biennials or perennials. Native herbaceous species usually colonize on remediated soils with potential use in phytoremediation of heavy metals. In case of cadmium, many herbaceous species are suitable for phytoextraction and phytostabilization. Their mechanisms involving are immobilization, exclusion and compartimentalization. They can adapt and synthesize phytochelatins, metallothioneins, stress proteins and phenolic compounds to tolerate cadmium and other metals. In addition, endophytic microorganism also involves in the plants tolerant mechanism. Growing herbal plants to cover and remediate cadmium contaminated areas must be concerned. Especially shoot parts of hyperaccumulative plants, spreading of cadmium in top soil can be increased from falling leaves of their life cycles. The cadmium also enters the food chain via insects, birds, herbivores, etc. Moreover, application of herbal plants collected from contaminated area for folk medicines and raw materials of drug and food/feed should be prohibited or controlled base on reliable researches and knowledges. Finally, phytomanagement and harvesting processes for cadmium phytoremediation are depending on the tolerant mechanisms and life cycles of each herbal plant species.

Tree Growth

Herbaceous plants tend to be small and soft, whereas woody plants can grow into large trees as a result of their lignified secondary xylem. Trees grow by a mixture of both primary and secondary growth. Shoot tips grow by cell division and expansion from a primary apical meristem. If a single apical meristem on the main shoot is dominant, a tree with a single trunk and smaller branches will form. If the apical meristem is less dominant, then various more heavily branched forms result. As the shoot matures, a procambium is formed as cells undergo specialization to form vascular bundles containing primary xylem and phloem. After further maturation, cells between the xylem and phloem within each bundle undergo further specialization to form a fascicular cambium, which expands laterally to form a continuous secondary meristem around the full circumference of the stem forming secondary phloem to the outside and secondary xylem or wood to the inside. Trees therefore grow in height by cell division and expansion at the shoot tips (primary growth), and undergo circumferential growth by cell division and differentiation from the vascular cambium (secondary growth). Bark is formed by transient cork cambia, which form initially in the cortex of young stems and subsequently within the secondary phloem (Figures 1 and 2).

In bamboo and palms, which form woodlike stems, only primary growth occurs. Xylem is arranged in vascular bundles along with phloem and this tissue along with the parenchyma in which it is embedded becomes lignified and woodlike but does not grow in circumference beyond the initial expansion of tissue from the apical meristem (or intercalary meristems at the nodes in bamboo).

The structure and chemistry of wood formed by young trees differs from that formed by old trees so that the wood at the center of the stem at the base of the tree has different properties from the mature wood near the outside and is said to be juvenile. Typically, the first 10–15 growth rings will be juvenile wood, which generally has smaller cells, lower basic density, higher microfibril angle, higher spiral grain, and higher lignin content, and may contain a greater proportion of reaction wood. In terms of wood utilization, this juvenile wood has inferior quality including low stiffness and high longitudinal shrinkage. The terms corewood and outerwood are also often used to refer to wood from different locations in the stem. At the base of the tree, corewood corresponds to juvenile wood and outerwood to mature wood.

Water conducting tissues in sapwood undergo programmed cell death (apoptosis) before they can begin to conduct water and are essentially empty pipes. The non–water conducting tissues may remain alive for several years and in some cases may continue cell wall development for prolonged periods. As the tree ages, the wood in the center of the stem may undergo changes in moisture content and extractive content associated with heartwood formation. During heartwood formation, any remaining living cells undergo programmed cell death. The amount of sapwood versus heartwood varies considerably among species and individual trees. Some tree species may have many sapwood growth rings, whereas others may only have a single sapwood outer growth ring actively conducting water. The relative amount of sapwood to heartwood may also be related to climate and other environmental factors such as the presence of pathogens or physical damage to the stem (Figure 3).

Occurrence

Most herbaceous plants are colonized by VA mycorrhizae. However, VA mycorrhizae are not restricted to herbaceous plants and are found on a large number of temperate tree species and most tropical trees. The only family of tropical trees that are not typically VA mycorrhizal is the Dipterocarpaceae. Trees of savanna grasslands and semiarid bushlands are also dominated by VA mycorrhiza-forming species. Although, in temperate regions, VA mycorrhiza-forming species are less important in geographical area than ectomycorrhiza-forming species, the majority of temperate tree species form VA mycorrhizae. Whereas the Pinaceae (Abies, Larix, Picea, Pinus, and Pseudotsuga) are ectomycorrhizal, most other gymnosperms form VA mycorrhizae. This includes the Cupressaceae, Taxaceae, and Taxodiaceae. A number of families of angiosperms also commonly contain VA mycorrhizal-forming species, including the Rosaceae (Malus and Prunus), Leguminosae, Oleaceae, and Tiliaceae. VA mycorrhizae are reported to be the ecologically most important type in New Zealand forests. However, a large number of species also form both VA mycorrhizae and ectomycorrhizae. The formation of both VA mycorrhizae and ectomycorrhizae is particularly well described on Eucalyptus, Populus, and Salix. In most cases, VA mycorrhizae develop at earlier successional stages than ectomycorrhizae. VA mycorrhizae are more frequent on plants growing on mineral soils.

4.8.3 Coriander

The herbaceous plant coriander (Coriandrum sativum L.) serves culinary and medicinal purposes. Its leaves (cilantro) as well as fruits (coriander) serve as flavoring agents in various dishes. The seeds are also valued for their fatty acid content, in particular, petroselinic acid. Wang and Kumar (2004) developed transgenic coriander plants in an attempt to investigate the role of mutated ethylene receptor ERS1 from A. thaliana in tissue senescence of heterologous plants. Transgenic coriander was regenerated by cocultivating hypocotyl segments with A. tumefaciens harboring binary vector pCGN1547 that carried the ERS1 gene. The Arabidopsis ERS1 mutant effectively conferred ethylene-insensitive phenotype to coriander plants with a transformation efficiency of 6.6%.

Herbaceous Plants

Because herbaceous plants and shrubs have vascular systems and protective epidermal layers, they access most of their nitrogen through the soil and are not as sensitive as nonvascular species to high concentrations of nitrogenous compounds in the air. Once deposited, however, large impacts can occur because of their shallow root systems, short life spans, and rapid growth rates compared with forest trees. Some plants respond negatively to N deposition, declining in occurrence and/or abundance when N inputs are high, others show positive responses benefiting from the additional N through direct or indirect mechanisms. In many temperate grasslands, savannahs, and shrublands, grasses become more dominant whereas the cover and biodiversity of forbs and other species declines. Some species are particularly sensitive, such as slow growing long-lived species that historically dominated much of the US great plains, and/or acid-sensitive species from heathlands and acid grasslands of Europe. In forests, herbaceous species (e.g., the understory) have a disproportionate significance to the biome compared with their abundance (Gilliam, 2007). Indeed, although they only represent ∼0.2% of aboveground biomass, herbaceous understory species make up 90% of plant biodiversity and produce >15% of the litter biomass. Thus, their losses may have large effects on many species that depend on them for forage and habitat.

Guttation

In herbaceous plants the most common evidence of root pressure is the exudation of droplets of liquid from the margins and tips of leaves. The quantity of liquid exuded varies from a few drops to many milliliters, and the composition varies from almost pure water to a dilute solution of organic and inorganic substances. Guttation usually occurs through stomalike openings in the epidermis called hydathodes, which are located near the ends of veins. In tropical rain forests, guttation is common at night, but it is uncommon in woody plants of the Temperate Zone because the necessary combination of warm, moist soil and very humid air is less common than in the tropics. A few instances of guttation from the twigs of trees have been reported (Büsgen and Münch, 1931). Raber (1937) observed sap flow from leaf scars of deciduous trees in Louisiana after leaf fall, and Friesner (1940) reported exudation from stump sprouts of red maple in February in Indiana. Exudation of liquid from roots and root hairs of woody plants also has been reported (Head, 1964), and, since this probably is caused by root pressure, it may be termed root guttation. No guttation has ever been reported in conifers, as would be expected because of the absence of root pressure, but artificial guttation can be caused by subjecting the root system to pressure (Klepper and Kaufmann, 1966).

Guttation is of negligible importance to plants. Occasionally, injury to leaf margins is caused by deposits of minerals left by evaporation of guttated water and it is claimed that the guttated liquid provides a pathway for the entrance of pathogenic organisms. In general, however, guttation can be regarded as simply an incidental result of the development of hydrostatic pressure in the xylem of slowly transpiring plants.

Water in Plants

In herbaceous plants, water normally constitutes more than 90% of fresh weight, although in rare cases it can be less than 70%. In woody plants, over 50% of fresh weight consists of water. Of the total water content of plants, 60–90% is located within cells, the rest (10–40% of total water) is mainly in cell walls. The water in cell walls forms a continuum with specialized transport cells throughout the plant.

The total volume of water within a plant is very small in relation to the total volume of water transpired during its lifetime. Even on a daily basis, the volume of water within plants is insufficient to buffer appreciably daily transpiration requirements on a warm, sunny day. The very large amounts of water transpired by plants, in relation to that retained within plant tissue, can be viewed as the ‘cost’ that plants incur as a consequence of stomatal opening to allow CO2 absorption for photosynthesis.

A primary function of water contained in cells is the maintenance of cell and tissue turgor. Cell turgor is essential for cell enlargement and therefore for optimal plant growth. Other primary functions are the transport of solutes and participation in metabolic activities. Because of its high dielectric constant, water acts as a solvent for many mineral and organic solutes, enabling their transport within cells and throughout the plant. Also, water is directly involved in chemical reactions in cells such as CO2 reduction in photosynthesis. Cooling is another primary function of water in plants. Because of the high energy requirement for water vaporization (10.5 kJ mol− 1 at 25 °C), water evaporating from leaf surfaces (during transpiration) cools the leaf, thereby avoiding excessive daytime heating from incoming solar radiation.

The water status of plants is a primary determinant of plant growth and development, and therefore of crop productivity in agricultural systems, and of plant survival in natural systems. Almost every plant physiological process is directly or indirectly affected by plant water content. For example, cell enlargement is dependent on the level of cell turgor, photosynthesis is directly inhibited by insufficient water, and stomatal control of transpiration and CO2 absorption is dependent on the water status of stomatal guard cells.

The water status of plants is the sum of the interaction of various atmospheric, plant, and soil factors. The availability of soil water, the atmospheric demand (determined by radiation, humidity, temperature, wind), the capacities of the root system to absorb water and of the plant to transport absorbed water to transpiring leaves, and stomatal responses for regulating transpiration, can all appreciably influence plant water status.

Plant water status is commonly characterized by its water potential (Ψ). Water potential is a measure of the free energy status of water, which, because of its applicability to each component of the soil–plant–atmosphere system, enables water movement between these components to be considered. It theoretically represents the work involved in moving one mole of water from a selected point within the plant (or soil) to a reference point of pure water at the same temperature and at atmospheric pressure. Ψ varies from zero at the reference point to negative values within the plant and soil. It is normally measured in units of pressure, with megapascals (MPa) being most commonly used.

Leaves are the plant organs where most of the exchange of CO2 and H2O between the plant and the atmosphere occurs. The pathway for the inward diffusion of CO2 is much the same as that for the outward diffusion of H2O vapor. The outward diffusion of H2O vapor from the saturated surfaces within the plant to the drier atmosphere follows a gradient of the partial pressure of H2O vapor. To maximize CO2 fixation by photosynthesis, stomata must remain open for as long as possible during daylight periods. This also maximizes the period of water loss by transpiration.

The water status of leaves (considered as Ψ) is the balance between the water lost to the atmosphere by transpiration (T) and the water absorbed by the plant from soil, which is a function of soil water potential (ψsoil) and the combined resistance to water movement within the roots and shoots (r). These relationships are described by the equation:

[1]ψ=ψsoil−Tr

Even in saturated soils (where ψsoil = 0), Ψ is negative when transpiration occurs. During daylight periods, absorption of water lags behind transpiration owing mainly to the high resistance to water flow from soil into root xylem tissue. As atmospheric evaporative demand increases during the morning, transpiration increases, which lowers the water potential of cells from which water is evaporating. Within the plant, water then moves from nonevaporating parenchyma cells of leaves, which have a higher Ψ, toward the evaporating cells, establishing a Ψ gradient. This gradient is transmitted throughout the plant–soil system, enabling continuous water movement. In the afternoon, transpiration decreases on account of reduced atmospheric evaporative demand. However, water uptake by roots continues until parenchyma cells fully rehydrate, and their Ψ equals soil Ψ, which usually occurs during the night. At this stage, plant and soil water are in equilibrium, and absorption by roots ceases. In some species under certain climatic conditions (high nighttime vapor pressure deficit, wind), transpiration can occur at night; when it does, it is generally relatively small compared to daytime transpiration. However, it can be sufficient to prevent nighttime equilibration of plant and soil water potentials.

Morphology

Peanuts are herbaceous plant and need symbiotic nitrogen-fixing bacteria in their root nodule for growing. It grows up to 30–50 cm above the ground. Leaves of peanuts tree are opposite and pinnate with four leaflets opposite to each other and have size 1–3 cm. These leaves are nyctinastic in nature, i.e., they have sleep movement at night (closing at night). Flowers are yellowish to orange in color. They grow twisted on the stem and last for only one day. Peanut pods are developed underground. After fertilization, pedicel, i.e., ovary, elongates to form a thread-like structure known as peg, which grows in soil, and at the tip pods will grow and become mature. Each pod is 3–7 cm and contains two to four seeds. Various parts of peanuts are an outer shell, cotyledons, seed coat, radicle, and plumule10 (Fig. 32.10).

I. INTRODUCTION

Forage is herbaceous plants or herbaceous plant parts made available for animal consumption. Forage can be harvested directly by the grazing animal from the standing crop (pasturage) or mechanically harvested and then fed to herbivores (harvested or conserved forages). Forage crops are plant crops grown for feeding as forage to ungulate herbivores, but the term is sometimes used to exclude pasturage. Forage consists broadly of the total aboveground part of herbaceous plants, but only selected portions of the aerial parts of the plant may be included in harvested forages. The term “forage” may be extended to include browse (the edible leaf and stem portions of woody plants), but this enlarged usage is mostly associated with grazing mixed rangeland vegetation.

Herbage is similar to forage in pertaining to aboveground herbaceous vegetation, but differs in that it may include plant material not acceptable or physically available to herbivores, these differences being greatest in pasturage. Because roughage is described as edible but bulky, coarse plant materials high in fiber and low in digestible nutrients, it is synonymous with forage only in part. Forages do contain significant amounts of plant cell-wall materials, the nutritive value of which is generally significantly lower than that for the cell-contents materials. However, many forages may still be relatively high in digestible energy (70%) and in total protein (25%).

Harvested forages are produced almost exclusively for feeding to livestock, principally ruminants and horses. Harvested forages are commonly fed on the farm where they are produced. However, an alternative is to sell harvested forages, primarily hay, off the farm where produced for feeding elsewhere. Regardless of which utilization alternative—or combination of alternatives—is followed, the production of harvested forages should be considered an earning enterprise on the farm and planned and operated accordingly.

Although not covered in detail in this text, hay production is often locally important at restrictive sites such as mountain meadows, wetlands and flood plains, certain native prairie sites, and selected range seedings (the last on the better sites or in abundant rainfall years). Whereas most of the principles of harvested forage production covered in this book apply to these unique sites as well, additional information, adaptations, and suggestions may be desired.

Further adaptation of management techniques to hay production at these cites can be found in the following references: on native prairies, Hyde and Owensby (1975), Conrad (1954), Coon and Leistritz (1974), Klebesadel (1965), Burzlaff and Clanton (1971), Streeter et al. (1966), Keim et al. (1932), and Towne and Ohlenbusch (1992); on mountain meadows, Siemer and Delany (1984), Delaney and Borelli (1979), Hart et al. (1980), Rumberg (1975), Lewis (1960), Hunter (1963), Willhite et al. (1962), Eckert (1975), Seamands (1966), and Barmington (1964); on flood meadows, Gomm (1979), Cooper (1956), Rumburg (1963), Britton et al. (1980), and Raleigh et al. (1964); and on introduced wheatgrass grazing lands, Peake and Chester (1943).

4.1.2 Plant uptake and fate of elemental pollutants

Like in herbaceous plants, trees take up most elemental pollutants from the soil water phase, which moves freely in the capillary spaces between the outer layers of root cells (cortex). Besides factors such as soil composition and pH, uptake is substantially influenced by root exudates and the plant-associated microbiota, two factors tightly connected (Doty, 2008; Thijs et al., 2016). Regarding pH, metals tend to form insoluble carbonates and phosphates in alkaline soils. This affects their bioavailability, as recently shown for Salix trees growing on adjacent soils with very different pH, shale overburden and acidic clay (Mosseler & Major, 2017). Tree rhizospheres are more acidic than neighbouring non-vegetated soils and, moreover, contain natural chelators and other compounds that promote metal uptake by root cells (Ullah, Heng, Munis, Fahad, & Yang, 2015; Wenzel, 2009). In fact, inoculation with appropriate ectomycorrhizal fungal strains has been shown to increase metal absorption by tree roots (Huang, Han, & Li, 2018; Ma et al., 2014). Also interesting for phytoremediation are the effects of plant growth-promoting bacteria (e.g. Durand et al., 2018). The application of soil amendments is another common strategy to improve metal bioavailability and even root-to-shoot translocation (Shahid et al., 2014). Caution must be exerted, however, as some compounds have proven toxic for local plants and/or mycorrhizae; they can also induce pollutant leaching into the soil (Agnello, Huguenot, Van Hullebusch, & Esposito, 2014; Lu et al., 2017).

The cell wall polysaccharide matrix is known to bind the ionic forms of various metals (Chen, Liu, Wang, Zhang, & Owens, 2013), acting as a first barrier for root cell uptake. A more selective barrier is the plasma membrane that contains specific transporters for nutritional ions (Fe2 +, Cu2 +, Mg2 +, Ca2 +, Mn2 +, and others) but not for non-nutritional ones (Kramer, 2010; Pilon-Smits, 2005). The same is true for the tonoplast, the third barrier (Peng & Gong, 2014). Membrane-mediated transport is unavoidable at some point if the pollutants are to reach aboveground parts, because the Casparian band blocks xylem loading through the apoplast. This band confers therefore a pivotal role to the endoderm cell membranes for the success of phytoremediation. Once inside root cells, toxic metals can accumulate intracellularly or be translocated into adjacent cells via membrane transporters or plasmodesmata. The latter make up the cellular continuum known as symplast. The study of Cd movement in roots of hybrid poplars has provided a well-characterized example of symplastic translocation (He et al., 2015). Ricachenevsky, Araújo, Fett, and Sperotto (2018) have recently reviewed the role of the tonoplast as a checkpoint for this type of transport. Regardless of the specific mechanism, intercellular transport allows metals to reach the root xylem vessels and, from there, aboveground tissues. Whether for root cell uptake, vacuolar sequestration or xylem loading, toxic metals highjack membrane transporters for nutritional ions. In recent years, genome-wide analyses and other molecular tools have allowed the identification of numerous metal transporters in trees (e.g. Fan et al., 2018; Migeon et al., 2010; Yanitch et al., 2017; Yildirim, 2017). Functional equivalences have been established with well-characterized transporters from herbaceous species (e.g. Tan et al., 2017). Albeit sparse, these studies have shed light on the molecular mechanisms involved in metal tolerance and detoxification in trees. Major proteins involved in toxic metal transport include ATP-binding cassette (ABC) transporters, heavy metal(loid) ATPase (HMA) transporters, metal transporters of the NRAMP family, transporters of the zinc/iron regulated protein (ZIP) family, and cation diffusion facilitators (CDFs), also called metal tolerance proteins (MTPs). The unusual heterogeneity detected for these transporters in trees (e.g. Fan et al., 2018; Migeon et al., 2010) may be indicative that metal translocation is more complex than in herbaceous plants. In any case, the extraordinary capacity of some fast-growing trees to remediate metals from polluted substrates deserves further study. New species and varieties should be analysed, given the vast genetic variability found in trees for most traits analysed.

When the intracellular accumulation of metals reaches toxic levels, plants develop symptoms like chlorosis, leaf deformities, defoliation, reduced growth and general weakness towards biotic and abiotic stress. Major protective mechanisms include pollutant sequestration/deactivation and ROS-scavenging machinery (Jalmi et al., 2018; Kramer, 2010; Peng & Gong, 2014; Pilon-Smits, 2005). It is noteworthy that the antioxidant systems are typically upregulated in hyperaccumulator plants, even in the absence of metal pollution (Kramer, 2010). The rationale of metal sequestration is to protect metalloenzymes and other vulnerable cell machinery. Cell walls represent a major sequestering site in trees, which accumulate metals in wood and outer bark (Chen et al., 2013; Evangelou, Robinson, Günthardt-Goerg, & Schulin, 2013; Zárubová et al., 2015). This capacity is probably beneficial against the attack of insects as well as pathogenic and wood-degrading microorganisms, i.e., constitutes an adaptive advantage. The notion that metal accumulation has a defensive role is supported by several lines of evidence (Hörger, Fones, & Preston, 2013), including rhizosphere studies (Lakshmanan, Cottone, & Bais, 2016). Besides cell walls, sequestration in vacuoles is an important strategy to cope with metal toxicity (Peng & Gong, 2014; Sharma, Dietz, & Mimura, 2016). Putative tonoplast isoforms of ZIP, CDF (MTP) and HMA transporters have been identified in trees, mostly through genomic comparisons. Studies with herbaceous plants have shown that, prior to vacuolar storage, metallic ions are chelated by glutathione (GSH), phytochelatins, metallothioneins, and other metabolites (amino acids, other organic acids, phenolics) (Pilon-Smits, 2005; Sharma et al., 2016; Song et al., 2014). Whereas experimental evidence is still scarce for tree species, genomic comparisons and reverse genetics experiments suggest that both trees and herbaceous plants have similar mechanisms of vacuolar sequestration (Ariani et al., 2015; Di Baccio et al., 2011; He et al., 2013, 2015). The narrow correlation found in hybrid poplars between vacuolar sequestration and Cd tolerance is of particular interest (He et al., 2013). This work also reported that sulphur-containing chelators are probably involved. Evidence for metal sequestration in vacuoles has been obtained in field studies with different poplar varieties (e.g. Luo et al., 2016; Marmiroli, Imperiale, Maestri, & Marmiroli, 2013). When a yeast ABC transporter (YCF1) that sequesters cytosolic GSH-conjugated cadmium into the vacuole was overexpressed in poplar (Shim et al., 2013), the resulting plants performed better than wild type controls in mine tailing soil contaminated with Pb, Cd, and As. The transgenic lines also showed enhanced extraction capacities. Improved vacuolar sequestration is the most likely reason why transgenic hybrid poplars overexpressing a P. trichocarpa ABC metal transporter show improved Hg tolerance and accumulation (Sun et al., 2018).