Comparative assessment of the overall development of young rubber tree grown in association with cassava, corn and groundnut

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Strategies deployed by plants to access essential soil resources

The strategy of root proliferation as a means to increase competitive advantage is seen in the soil exploration patterns (root foraging strategies) of many species. For example, Mordelet et al. (1996) measured the distribution of roots and N in the patchy savanna of Cote d’Ivoire dominated by the palm tree Borassus aethiopum (Mart.). Root mass and total N concentration were significantly greater under clumps of trees (and termite mounds) than outside the same clumps and mounds. Palm trees extended their roots as far as 20 m towards the nutrient-rich patches where they proliferated. This foraging strategy of root proliferation under tree clumps or termite mounds results in both a large area explored and efficient resource exploitation, because high root lengths only occur in nutrient-rich patches.
For a given mineral nutrient, feedback regulating signals on the nutritional status of the shoot to the roots may lead to contrasted responses of the uptake system in different plant species as discussed below for iron. Depending on their response to iron deficiency, plants can be classified into two categories or strategies (Strategy I and Strategy II). In both strategies the responses are confined to the apical zones of growing roots and are fully repressed within about one day after re-supply of iron. Strategy I is typically for dicots and non-graminaceous monocots, and characterized by at least two distinct components of iron deficiency responses: increased reducing capacity and enhanced net excretion of protons. In many instances also the release is enhanced of reducing and/or chelating compounds, mainly phenolics (Olsen et al., 1981; Marschner et al., 1986). These root responses are often related to changes in root morphology and anatomy, particularly in the formation of transfer cell-like structures in rhizodermal cells. In leaves of all plant species the major symptom of iron deficiency is inhibition of chloroplast development. For roots, however, both morphological and physiological changes brought about by the deficiency and responses to this lack of iron depend upon plant species. In both dicots and monocots, with the exception of the grasses (graminaceous species), iron deficiency is associated with inhibition of root elongation, increase in the diameter of apical root zones, and abundant root hair formation (Romheld and Marschner, 1981; Chaney et al., 1992). These morphological changes are often associated with the formation of cells with a distinct wall labyrinth typical of transfer cells. These transfer cells may be induced either in the rhizodermis or in the hypodermis (Landsberg, 1989). The iron deficiency-induced formation of rhizodermal transfer cells (Kramer et al., 1980) is part of a regulatory mechanism for enhancing iron uptake.
Drew et al. (1973), Drew and Saker (1975, 1978) and Drew (1975) demonstrated that barley responded to a localized supply of nitrate, ammonium or phosphate (but not potassium) by increasing the number of primary lateral roots per unit length of axis. Those laterals became longer and, in turn, carried more secondary laterals compared with plants receiving a uniform supply of nutrients.

Root system architectures / rooting profiles

Root architecture, the spatial configuration of a root system in the soil, is used to describe distinct aspects of the shape of root systems. Lynch (1995) stated that studies of root architecture do not usually include fine details such as root hairs, but are primarily concerned with the general arrangement of roots within the entire root system of an individual plant. From the architecture, both the topology (a description of how individual roots are connected through branching) and the distribution (the presence of roots in a spatial framework) can be derived, whereas neither topology nor distribution can be used to derive architecture. Root architecture is quite complex and varies between and within plant species. Drawings of excavated root systems of crops and other species show the differences in shape between monocotyledons and dicotyledons and allow some broad generalizations to be made about the depth of rooting and the relative distribution of roots (Kutschera, 1960). Nearly all such drawings show that, with the exception of the tap root which grows almost vertically throughout, most other root axes grow initially at some angle relative to the vertical but gradually become more vertically orientated. Gravitropic responses combined with responses to light, water and soil mechanical impedance, together with the predominance of vertical cracks in deeper soil layers, produce these patterns.
Root architecture’s importance lies in the fact that many of the resources that plants need from soil are heterogeneously distributed and/or are subject to local depletion (Robinson, 1994), In such circumstances, the development and growth of root systems may become highly asymmetric, and the spatial arrangement of the root system will substantially determine the ability of a plant to secure those resources (Lynch, 1995), Such ideas have been investigated in a series of experiments and models using common bean (Bonser et al., 1996; Ge et al., 2000). While root trajectories are essentially under genetic control, phosphorus deficiency was found to decrease the gravitropic sensitivity of both the tap root and the basal roots, resulting in a shallower root system. It was hypothesized that the shallower root system was a positive adaptive response to low soil P availability by: first, concentrating roots in the surface soil layers where soil P availability was highest; and second, reducing spatial competition for P among roots of the same plant. This hypothesis was tested by modeling root growth and P acquisition by bean plants with nine contrasting root systems in which basal root angle was varied but not root length or degree of branching. Shallower root systems acquired more P per unit carbon cost than deeper root systems and in soils with higher P availability in the surface layers, shallower root systems acquired more P than deeper root systems because of less inter-root competition as well as increased root exploration of the upper soil (Ge et al., 2000). In practice, the plant may have multiple resource constraints to contend with (e.g. heterogeneously distributed P and soil water) and will try to optimize its investment in roots. Ho et al., (2004) investigated this optimization with respect to beans grown under different combinations of water and P availability. They postulated that an ideally optimized plant would grow roots deeper into the profile until the marginal benefit of extra deeper roots exactly equaled the marginal cost of constructing those roots; through modeling, they found (Ho et al., 2004) that the basal root angle would be shallower for localized shallow P, and deeper for localized deep water compared to the case of uniformly distributed water and P. When P was concentrated in the surface and water was located deep, the optimal basal root angle depended on the relative rates of change with depth in the values ascribed to the available resources. While useful in indicating general principles, it should be remembered that not all of the responses of roots to a heterogeneous environment (e.g. changes in branching frequency and root hair growth) are yet captured in such models; this remains a substantial challenge.

Functions of root system architectures

The root system serves several functions simultaneously (Gregory, 2006). It provides a stable platform for the shoot so that the photosynthetic organs can intercept sunlight, and forms a network that can exploit the water and nutrient resources of the soil. The availability and mobility of soil resources varies depending on the particular resource being considered, so that in contrast to the shoot which is essentially harvesting only two resources, light and carbon dioxide, the roots and root system have evolved to cope with a more challenging environment.

Root anchorage

Although anchorage is a major function of the root system, it has not received as much attention as other functions such as water and nutrient absorption. While it has long been assumed that anchorage is merely a byproduct of roots ‘main’ function as an absorbing organ, it is now realized that the need for anchorage influences the overall root system size and shape (Ennos, 2000).

Water uptake

Water is essential to the life of terrestrial plants and to biota that live in the soil. It carries nutrients in the soil to the roots, is the solvent for, and medium of, most biochemical reactions within plants, and its loss from plants is of the driver of CO2 exchange with the atmosphere. For most plants, soil is a major source of water, so that the modalities through which soil water is acquired by roots has been, and continues to be, a major topic of soil/plant research (Gregory, 2006).

Nutrient uptake

Unlike water, there is no potential external demand for nutrients that can be readily calculated. Demand for nutrients is driven by the metabolic demands of the plant, and the plant exerts considerable, but not always perfect, control over the quantities of nutrients and other ions that are allowed to enter it. In general, all higher plants have similar requirements for nutrients, although there are some minor variations. An element is essential to a plant if: (1) a deficiency makes it impossible for the plant to complete its life cycle; (2) such deficiency is specific to a particular element and can be prevented or corrected by supplying this element; and (3) the element is directly involved in the physiological or biochemical functions of the plant (Marschner, 1997)

Root systems and competition for resources within the root system

The issue of the size of root system necessary to take up resources in sufficient amounts has been examined in detail in the crop production literature. However, there is no single answer to this question as it is influenced by many factors including the size, architecture and activity of the roots as well as the behavior of the particular resource under consideration in the soil. In general, a large, more intensely branched root system can extract the plant’s requirements from a soil more efficiently than a smaller root system, but the optimal size for a particular resource varies so that there can appear to be an element of redundancy or overprovision in many systems if a mobile resource is used as the basis for comparison (Gregory, 2006). For example, van Noordwijk (1983) calculated that a root length density of 0.1-1.0 cm cm-3 throughout the upper 0.2 m soil layer would be sufficient to supply the N requirements of most crop plants, whereas a root length density of 1-10 cm cm-3 would be required for less mobile nutrients such as P. For water, the required a root length density is similar to that needed for nitrate, assuming that roots are in intimate contact with the soil but rises to 1-5 cm cm-3 if there is an appreciable soil/root contact resistance (Veen et al., 1992). Under usual evaporative demand conditions and assuming that all roots are equally and uniformly active within a soil volume for which spatially uniform supply conditions prevail, root length density values ranging from 0.5 to 10 cm cm-3 are sufficient to cover plant needs. These values cover the range of root length density commonly measured for a range of crops in the cultivated layer of many soils.

Effect of soil patchiness

However, this does not hold under conditions of soil patchiness, i.e. spatially heterogeneous distribution of soil resources (Fitter, 1994), which have been reported to trigger “root races” in which vast amounts of assimilates are used to produce profuse roots (Passioura and Wetselaar, 1972), in an effort to secure benefit from resource enriched spots (Hodge et al., 1999). Farley and Fitter (1999) examined proliferation responses of roots of seven plant species chosen because they coexisted at a single site and would therefore encounter a similar suite of patch characteristics. The plants were offered patches of soil or a soil/sand mixture set in a background of sand. The patches varied in size (40, 70 and 160 cm3), but the probability of encounter was the same for all patches. Only five of the seven species proliferated roots in patches. The two that did not (Oxalis acetosella and Viola riviniana) had the smallest root systems and thickest roots of the group. There was also evidence that their nutrient uptake depended on mycorrhizal associations to a greater extent than that of the other species. All other species showed a proliferation response, but each did so in a unique fashion. One species (Glechoma hederacea) was sensitive to patch size and two other species (Siliene dioica and Veronica montana) responded to patch quality. Two species changed specific root length (length of root per unit weight of root) on encountering the patches, with finer roots being grown in patches. Four showed a change in branching pattern, becoming less herringbone-shaped in architecture in the patch, as predicted by theoretical models (Fitter et al., 1991).
Such idiosyncrasy of response means that it will be exceptionally difficult to predict the effect of complex variation in patch attributes, such as occurs naturally in soils, on species mixtures. At the same time, this opens up obvious opportunities for species’ coexistence and niche differentiation. These would arise from differential responses to a range of spatial and temporal patchiness. Species that respond weakly to nutrient-rich patches by proliferation may do so more strongly by physiological changes (Fitter et al., 2000).

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Root plasticity

Roots probably evolved plastic responses to their environment as they differentiated as specialized tissues throughout geological times (Raven and Edwards, 2001), optimized to explore and utilize resources in heterogeneous soils (Leyser and Fitter, 1998). Root plasticity is also a response to intra- and inter-specific competition. Robinson (2001), for example, showed that plastic root responses are triggered by intra-specific competition in a wheat monoculture but do not necessarily lead to greater uptake rates. Nutrient availability is known to influence many facets of root system morphology (Ford and Lorenzo, 2001): root branching, root growth (with growth of main axes generally less affected by nutritional effects than higher order axes), root diameter, root angle (e.g., low P availability decreases the angle of emission of basal roots in bean [Phaseolus vulgaris L.], soybean [Glycine max (L.) Merr.], and pea [Liao et al., 2001]), root hair length and density, and production of specific root types (cluster roots [Skene, 2000] or drought-induced roots [Vartanian, 1996]). The response of plants to variations in the location of nutrients has been well studied (see review by Robinson, 1994) compared to the influence of temporal variations in nutrient concentrations on root plasticity. Experimental observations of root responses to variations in the spatiotemporal availability of nutrients have generally been made under conditions wherein access to nutrients was artificially reduced. For example, a classic experimental design consists of providing nutrients to a small portion of the root system only, while the rest of it grows in nutrient-poor or sterile soil (Drew and Saker, 1975). Roots respond to such a heterogeneous system in two ways (Robinson, 1996): (i) the nutrient inflow rate increases but then returns to normal within hours, or (ii) roots proliferate toward and within the nutrient rich patch over a period of several days, while root growth in the rest of the root system is inhibited. These trends vary depending on the plant species, with the induced increases in root growth and nutrient uptake varying over one order of magnitude or with a total lack of response in some species (Robinson, 1996). The stimulation in uptake rate seems to be sensitive to the nutrient considered and the duration of the starvation period. Root proliferation appears less dependent on the nutrient considered (except for K in some species). Localized responses are generally assumed to be caused by direct nutritional benefits to the roots directly exposed to nutrient patches, but there is some evidence that they can also involve indirect, sophisticated mechanisms (Pierret et al., 2007a).

Common rubber tree intercrops

Cassava (Manihot esculenta Crantz, of the Euphorbiaceae family) is one of the important root crops of the tropics, grown mainly for its starchy edible roots. The original home of cassava is considered to be North East Brazil. In Asia, its cultivation is limited to a few countries such as, Indonesia, India and Thailand. It is used as a major source of carbohydrate in many African, Asian and American countries. Cassava is a perennial shrub, 1 to 5 m in height, with the stem branching or non-branching. It is harvested after a period of 9-12 months in hot areas and 16-24 months in cooler or dryer areas. The edible roots are adventitious roots, swelled by secondary thickening and deposition of starch and are conventionally referred to as cassava tubers. Usually, 5-10 tubers are produced per plant. These tubers are cylindrical or tapering, 15-100 cm long and 3-15 cm across each and occasionally branched.
Corn or maize (Zea mays, Poaceae or Gramineae Family) is the most valuable cereal crop of global importance. Corn is used for human consumption and for animal feeding. Besides, it is used in the manufacture of starch, syrup, sugar and industrial spirit. The products of milling include corn grits, meal, flour, germ and germ oil (Palaniappan and Sreenivasan, 1993).

The effect of inter-cropped species on rubber tree growth and yields

Laosuwan et al. (1988) tested different combinations of inter-cropping treatments between 1981 and 1986. They found differences in girth increments during certain periods of rubber growth. Legume cover and pineapple were more conducive to the growth of rubber than any other crops. Both crops covered the ground from the beginning of rubber planting and provided improved soil moisture conditions. Other crops, including legumes, cereals and banana gave similar growth rate of rubber and none of these crops, as compared with control, adversely affected the growth of rubber.
Wibawa et al. (2006) tested the planting of rubber with Paraserianthes falcataria in double row spacing (4 x 3 x 16 m): up to 18 months, growth was comparable to that in a system with normal spacing 6.7 x 3 m. The gap of rubber girth between those two treatments increased afterward and started to be significant after 24 months. The presence of P. falcataria at different densities reduced rubber growth significantly since 24 months. At 51 months, rubber girth at inter-cropped plots was 30% and 15% less than that at monoculture with normal and double row spacing, respectively. The monthly girth increments in inter-cropped, mono- and double-spacing plots were 0.6, 0.8 and 0.9 cm respectively. The slowest increment was observed during dry season during which the inter-crop reduced the girth increment by as much as 70% compared to the control and 50% compared to the monoculture with double row spacing.
Rubber tree canopy in double row spacing plots started to shade the soil after 30 months. About 60% of the incident light penetrated in the rubber tree monoculture with double row spacing, about 70% in the monoculture with normal spacing and between 36 and 52% in inter-crop treatments. After 54 months, in all double row plots, light intensity was less than 35%, however in normal density the light intensity was 50%. These data indicated that the intra-plant competition for light may start earlier in plots with inter-crop and in plot without intercrop with double row spacing, compared to normal spacing plots.
In line with the above-mentioned data, the light conditions in between rubber rows and in inter-crop rows, varied depending on treatments: P. falcataria with a density of 750 plants/ha reduced light intensity after 18 months, and a lower reduction in light intensity was associated with lower P. falcataria density. P. falcutaria shaded the soil more than 50% after 18 months.
Wibawa et al. (2006) showed that rubber growth in double row spacing plots was comparable to that in normal spacing (control) plots. These results were better than that mentioned above. This may be related to the wider rubber row spacing and the rubber clone used, RRIC 100, a fast growing clone. This trial also indicated that planting perennial inter-crops after rubber (almost 2 years) is a good strategy to minimize high competition with rubber. Eucalyptus sp. planted under rubber did not significantly reduce rubber growth. However, Acacia mangium a fast growing timber tree planted at the same time as rubber, competed with rubber and affected its growth very significantly two years after establishment. Rubber reached a tappable size between 56 to 63 months after planting. Up to 62 months after planting, no significant difference was observed between rubber tree girth in double row with or without inter-crop and monoculture with normal. The findings from this experiment may be very useful as a recommendation basis for planting rubber in double row spacing (4 x 3 x 16 m), i.e. as an alternative to normal, single row spacing (6 x 3 m or 7 x 3 m). The land in the wider space in between double rows (14 m) can be used by farmers to grow food crops over a longer period (more than three years) and for perennial tree crops (timber or fruit trees). The light intensity is expected to remain at levels higher than 70% up to 54 months after planting. The longer the inter-cropping period allowed by this design makes the plantation safer from the pressure of external factors (fire, pests and market fluctuations).

Table of contents :

CHAPTER I – INTRODUCTION: General Background
1. Role of Plant Roots
1.1 Main processes involved in water and nutrient uptake
1.2 Water transport in the soil-plant-atmosphere continuum
2. Strategies deployed by plants to access essential soil resources
3. Root system architectures / rooting profiles
3.1 Common types of architectures
3.2 Functions of root system architectures
3.3 Interactions
4. Concluding remarks and aim of the work
CHAPTER II – INTRODUCTION: General context of the research on belowground interactions in young rubber tree plantations of northeast Thailand 
1. The rubber tree industry
2. Inter-cropping
2.1 Common rubber tree intercrops
3. The effect of inter-cropped species on rubber tree growth and yields
3.1 Effects of inter-crops on rubber tree growth
3.2 Effects of inter-crops on rubber tree yields
4. Competition problems potentially associated with inter-cropping
4.1 Resources use by inter-cropped species
4.2 Below-ground interactions in inter-cropping and agroforestry systems
5. Context of the research and working hypotheses
6. Main question addressed by this research
7. Expected outputs and outcomes of the research project
7.1 Scientific outputs and outcomes
7.2 Applied outcomes for farmers
CHAPTER III: METHODOLOGY
1. Introduction
1.1 Tools used to assess rooting patterns
1.2 Analysis of root growth potential based on apical diameter measurements
1.3 Choice of root parameters measured in this work
2. Experimental Materials and methods
2.1 Greenhouse Experiment
2.2 Field Experiment
CHAPTER IV: RESULTS
1. Results of the rhizobox experiments
1.1 Introduction
1.2 Overall root system development (rubber tree -corn association)
1.3 Root growth rate analysis at the individual root scale
1.4 Analysis of the above-ground development of rubber trees and intercrops.
1.5 Comparative assessment of the overall development of young rubber tree grown in association with cassava, corn and groundnut
1.6 Discussion and conclusions
2. Results of the field experiments
2.1 2006 Field experiment
2.2 2007 Field experiment
2.3 2008 Field experiment
2.4 Discussion of the 2007-2008 field experiments
2.5 Results of the 2007 and 2008 field experiments on cassava inter-cropping
CHAPTER V – GENERAL DISCUSSION AND CONCLUSIONS
1. Rhizobox experiment – discussion
2. Field experiments – discussion
3. Perspectives for future research
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