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Vol. 2 No. 6
2011

Root Development Under Drought Stress

J. A. Franco

Departamento de Producción Vegetal, Unidad Asociada al CSIC de “Horticultura Sostenible en Zonas Áridas” (UPCT-CEBAS), Universidad Politécnica de Cartagena, Paseo Alfonso XIII, 48, 30203 Cartagena, Spain
(e-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it )

Additional keywords: root dynamic, plant stress, ornamentals, xerogardening, water deficit

INTRODUCTION

Serving as interfaces between plant and the soil, roots are much more exposed to drought stress than the upper plant parts. Therefore, the root system can be as affected, or even more affected, than the aerial parts of the plant for drought stress (Franco et al., 2011). Nevertheless, the influence of this stress on root activity and development has been much less studied. Undoubtedly, this is due to limitations on accessibility for root observations; being studies on root system dynamics especially difficult because they require successive, non-destructive measurements (Franco and Abrisqueta, 1997; Franco et al., 2002a).

ROOT DEVELOPMENT UNDER DROUGHT STRESS

Root development is strongly influenced by growing conditions such drought stress. However, root growth is usually less affected by drought stress than shoot growth. Thus, a decrease in shoot:root ratio is a common observation under drought-stress, which results either from an increase in root growth or from a relatively larger decrease in shoot growth than in root growth. This was so with Lonicera implexa (Navarro et al., 2008), Lotus creticus (Franco et al., 2001; Bañón et al., 2004), Myrtus communis (Bañón et al., 2002), Nerium oleander (Bañón et al., 2006), Rhamnus alaternus (Bañón et al., 2003), Rosmarinus officinalis (Sánchez-Blanco et al., 2004) and Silene vulgaris (Arreola et al., 2006; Franco et al., 2008). In addition, in the review article by Franco et al. (2006), it is well-documented a reduction in the shoot:root ratio as a result of pre-conditioning deficit-irrigation processes during nursery period.

Not only the shoot:root ratio but also other root characteristics (root length, fresh weight –FW–, dry weight –DW–, diameter and surface area, deep rooting and cortex thickness) and behaviours (root turnover, metacutisation, hardening and hydraulic conductivity) may be strongly affected by drought.

Deep rooting is a critical factor influencing the ability of the plant to absorb water from the deeper layers of the soil (Franco et al., 2006; 2011). Also, a greater percentage of fine roots, capable of penetrating smaller soil pores, presumably optimises the exploratory capabilities of the root system as a whole, and may have an important role for survival of plants to drought stress. Thinner roots compared with controls were reported for drought-stressed Silene vulgaris (Franco et al., 2008). However, deficit-irrigation increased the percentage of thick roots, and reduced the percentage of medium and fine roots in Myrtus communis and Nerium oleander plants (Bañón et al., 2002; 2006).

Branching of the roots and total root length of Silene vulgaris plants was increased under moderate drought-stress (Franco et al., 2008). Likewise, root surface area of Silene vulgaris plants was increased under moderate drought-stress (Franco et al., 2008). This can minimise localised water depletion around roots, thus minimising resistance to water transport to the root system (Franco et al., 2006). Those plants had higher water-use efficiency (WUE) than well-watered plants. Studies by Fernández et al. (2006) in Phillyrea angustifolia also reported that WUE improved with limited water availability in different species.

The combined effect of deficit-irrigation and low air humidity during the nursery phase reduced the mortality rate of Myrtus communis (Bañón et al., 2002) and Nerium oleander (Bañón et al., 2006) seedlings after transplantation under drought and heat conditions. Such behaviour was related to morphological changes observed in the roots (i.e., shorter, thicker, more dense and less ramified) and in the shoot:root ratio (i.e., reduced by approx. 60% in both species) of the pre-conditioned plants. This behaviour was related to the reordering of the assimilate gradient as the flow of solutes towards the roots intensified.

Hardening of roots, as revealed by an increased percentage of brown roots, is frequent in drought-stressed plants (Franco et al., 2006). The change in the root colour from white to brown (and, for some species, roots showing beads) is associated with suberisation of the exodermis, and may reflect a metacutisation process (i.e., suberisation of one or more root-cap cell layers that result in a resting root that is protected against adverse environmental conditions, which is capable of regrowth when conditions ameliorate), which was correlated with the capacity to grow under drought conditions of Limonium cossonianum (Franco et al., 2002b), Lotus creticus (Franco et al., 2001) and Silene vulgaris plants (Franco et al., 2008).

One indicator of plant capacity to absorb and transport water is the density of the xylematic vessels in the roots. Thus, a high vessel density would improve resistance to water-deficit situations. This was so with Lotus creticus in a study by Bañón et al. (2004), who reported that plants subjected to deficit-irrigation showed higher density of xylematic vessels in roots than well-watered plants.

Plants affected by drought can exhibited signs of tissue dehydration, evidenced by increasing their root DW:FW ratios (Franco et al., 2011). This was reported in Rosmarinus officinalis by Sánchez-Blanco et al. (2004) and in Nerium oleander by Bañón et al. (2006). Consequently, a greater cortex thickness:root radius (C:R) ratio for Silene vulgaris plants grown under drought-stress, noted by Franco et al. (2008) in two experiments, is significant to improve resistance to dehydration. Thereby, tissues internal to the endodermis are relatively well protected against dehydration at the expense of the cortex. Therefore, changes in the C:R ratio of Silene vulgaris seedlings could influence plant capacity to obtain water and survive adverse conditions.

LITERATURE CITED

ARREOLA, J., FRANCO, J. A., VICENTE, M. J. and MARTÍNEZ-SÁNCHEZ, J. J. (2006). Effect of nursery irrigation regimes on vegetative growth and root development of Silene vulgaris after transplantation into semi-arid conditions. Journal of Horticultural Science & Biotechnology, 81: 583-592.

BañÓn, S., Ochoa, J., Franco, J. A., Alarcón, J. J., Fernández, T.and Sánchez-Blanco, M. J. (2002). The influence of acclimation treatments on the morphology, water relations and survival of Myrtus communisL. plants. In: Sustainable Use and Management of Soils in Arid and Semiarid Regions. (Faz, A., Ortiz, R. and Mermut, A. R., Eds.). Quaderna Editorial, Murcia, Spain. 275-277.

BAÑÓN, S., OCHOA, J., FRANCO, J. A., SÁNCHEZ-BLANCO, M. J. and ALARCÓN, J. J. (2003). Influence of water deficit and low air humidity in the nursery on survival of Rhamnus alaternus seedlings following planting. Journal of Horticultural Science & Biotechnology, 78: 518-522.

Bañón, S., Fernández, J. A., Franco, J. A., Torrecillas A., Alarcón, J. J. and Sánchez-Blanco, M. J. (2004). Effects of water stress and night temperature pre-conditioning on water relations and morphological and anatomical changes of Lotus creticus plants. Scientia Horticulturae, 101: 333-342.

Bañón, S., Ochoa, J., Franco, J. A., Alarcón, J. J. and Sánchez-Blanco, M. J. (2006). Hardening of oleander seedlings by deficit irrigation and low air humidity. Environmental and Experimental Botany, 56: 36-43.

FERNÁNDEZ, J. A., BALENZATEGUI, L., BAÑÓN, S. and FRANCO, J. A. (2006). Induction of drought tolerance by paclobutrazol and irrigation deficit in Phillyrea angustifolia during the nursery period. Scientia Horticulturae, 107: 277-283.

FRANCO, J. A. and ABRISQUETA, J. M. (1997). A comparison between minirhizotron and soil coring methods of estimating root distribution in young almond tress under trickle irrigation. Journal of Horticultural Science, 72: 797-805.

Franco, J. A., Bañón, S., Fernández, J. A. and Leskovar, D. I. (2001). Effect of nursery regimes and establishment irrigation on root development of Lotus creticus seedlings following transplanting. Journal of Horticultural Science & Biotechnology, 76: 174-179.

Franco, J. A., Cros, V., Bañón, S., González, A. and Abrisqueta, J. M. (2002a). Effects of nursery irrigation on postplanting root dynamics of Lotus creticus in semiarid field conditions. HortScience, 37: 525-528

Franco, J. A., Cros, V., Bañón, S. and Martínez-Sánchez, J. J. (2002b). Nursery irrigation regimes and establishment irrigation affect the postplanting growth of Limonium cossonianum in semiarid conditions. Israel Journal of Plant Sciences, 50: 25-32.

FRANCO, J. A., MARTÍNEZ-SÁNCHEZ, J. J., FERNÁNDEZ, J. A. and BAÑÓN, S. (2006). Selection and nursery production of ornamental plants for landscaping and xerogardening in semi-arid environments. Journal of Horticultural Science & Biotechnology, 81: 3-17.

FRANCO, J. A., ARREOLA, J., VICENTE, M. J. and MARTÍNEZ-SÁNCHEZ, J. J. (2008). Nursery irrigation regimes affect the seedling characteristics of Silene vulgaris as they relate to potential performance following transplanting into semi-arid conditions. Journal of Horticultural Science & Biotechnology, 83: 15-22.

FRANCO, J.A., BAÑÓN, S., VICENTE, M.J., MIRALLES, J. and MARTÍNEZ-SÁNCHEZ, J.J. (2011). Root development in horticultural plants grown under abiotic stress conditions – a review. Journal of Horticultural Science & Biotechnology, 86: 543-556.

NAVARRO, A., VICENTE, M. J., MARTÍNEZ-SÁNCHEZ, J. J., FRANCO, J. A., FERNÁNDEZ, J. A. and BAÑÓN, S. (2008). Influence of deficit irrigation and paclobutrazol on plant growth and water status in Lonicera implexa seedlings. Acta Horticulturae, 782: 299-304.

Sánchez-Blanco, M. J., Ferrández, T., Navarro, A., Bañón, S. and Alarcón, J. J. (2004). Effects of irrigation and air humidity preconditioning on water relations, growth and survival of Rosmarinus officinalis plants during and after transplanting. Journal of Plant Physiology, 161: 1133-1142.