REGULAR ARTICLE Water uptake efficiency and above- and belowground biomass development of sweet sorghum and maize under different water regimes Walter Zegada-Lizarazu & Alessandro Zatta & Andrea Monti Received: 14 March 2011 /Accepted: 18 July 2011 # Springer Science+Business Media B.V . 2011 Abstract Background and aims Lately sweet sorghum (S) has attracted great interest as an alternative feedstock for biofuel production due to its high yielding potential and better adaptation to drought than maize (M). However, little is known about the response of newly developed sweet sorghum genotypes to water deficits, especially at the root level and its water uptake patterns. The objective of this study was to compare the water uptake capacity, growth and developmental characteristics at the root and canopy levels of a sweet sorghum hybrid (Sorghum bicolor cv. Sucro 506) with those of maize (Zea mays cv. PR32F73) at two water regimes. Methods The trial was setup in a total of 20 rhizotrons (1 m3), where calibrated soil moisture probes were installed for monitoring and adjusting the soil moisture content to 25% (well-watered, W) and 12% (drought stress, D). Results DS was able to sustain its physiological activity close to that of WS plants, while maize was not. The biomass production potential of DS was reduced about 38%, while in maize the reduction was 47%. The water use efficiency (WUE), however, was increased by 20% in sweet sorghum and reduced in 5% in maize. Moreover, in contrast to maize the root length density and water uptake capacity of DS was enhanced. Root water uptake efficiency in DM was sustained close to its potential, but not in sweet sorghum. Conclusions In summary, the better adaptation to drought of sweet sorghum is explained by increased WUE, sustained physiological activity and enlarged root system. It is also associated with a reduced water uptake efficiency compared to its control but maintained compared to maize. Keywords Bioethanol . Energy crops . Drought . Roots . Water uptake efficiency. Water use Introduction Currently, maize is the predominant starch source used for biofuel production. In USA, for example, 32.4% of maize agriculture is devoted to ethanol production (USDA 2010). The evergrowing maize- ethanol based production, however, would result in severe competition between food, feed, and energy feedstock supplies (Yuan et al. 2008). Moreover, in semiarid areas or in climates with major summer droughts the production of maize require high inputs Plant Soil DOI 10.1007/s11104-011-0928-2 Responsible Editor: Hans Lambers. W. Zegada-Lizarazu : A. Zatta : A. Monti (*) Department of Agroenvironmental Science and Technology, University of Bologna, Viale G. Fanin 44, 40127 Bologna, Italy e-mail: a.monti@unibo.it W. Zegada-Lizarazu e-mail: walter.zegadalizarazu@unibo.it

in terms of water and fertilization. Without supple- mental irrigation maize-ethanol based production would not be viable in such regions. An alternative to maize is sweet sorghum a fast growing C4 plant native to tropical zones, highly productive and that can be grown without supplemental irrigation and in less fertile or marginal soil (Guiying et al. 2000 Prasad et al. 2007 Miller and Ottman 2010) thus limiting the negative effects brought about by inten- sive cultivation practices and land competition issues. Moreover, sweet sorghum has a high content of fermentable sugars (sucrose, glucose and fructose) in the stem juice that can be more easily converted into ethanol than maize starch and cellulose rich-stover residues. Then, compared with maize, sweet sorghum can yield more ethanol per hectare and with fewer inputs (e.g. water, nutrient Miller and Ottman 2010). It is for these reasons that sweet sorghum is attracting great interest as a new energy crop throughout the Mediterranean area. But, its development as an energy crop is still far behind the development of maize (Smith et al. 1987 Rooney et al. 2007). It is, therefore, imperative to focus on studies that evaluate the whole plant performance, integrating roots and canopy responses to water deficit such as those prevailing in drought prone environments. An important characteristic of a drought resistant species is an extensive and widely spreading branching root system. However, root proliferation and functioning in response to water stress is not clear, especially for newly developed sweet sorghum genotypes. Indepen- dent of the modified rooting patterns and water uptake capacity and efficiency, it is reported that grain sorghum roots are still active and extract water even when soil water potential goes below ���1.5 MPa (Blum and Arkin 1984 Hundal and De Datta 1984) while in maize embolism occurs at around ���1.6 MPa (Cochard 2002). In general, under drought conditions grain sorghum develops a deeper and more evenly distributed root system than maize. Garrity et al. (1982) and Stone et al. (2001), however, found that rooting characteristics of various improved sorghum lines differed significantly, then their functioning, such as water uptake and root water uptake efficiency, may also vary widely. Compared with a drought susceptible cultivar, for example, a drought tolerant sorghum cultivar showed higher water uptake efficiency, fewer late metaxylem vessel per nodal root, smaller leaf area, and well developed sclerenchyma (Salih et al. 1999). Then the magnitude of the impact of water stress may vary with the sorghum cultivars according to the nature of their physiological and morphological traits. In the case of sweet sorghum the variability may be even greater as Massacci et al. (1996) indicated that the physiology of sweet sorghum is different from that of grain and fibre sorghum due to the accumulation of large amounts of sugars in the stems which may play a fundamental role in its drought adaptation mechanisms. However, the relationship between structural (increased root length density RLD) and functional (water uptake capacity and root water uptake efficiency) changes at root level caused by water stress in sweet sorghum genotypes is not well documented. Under drought stress, water use efficiency (WUE aboveground biomass produced per unit of water evapotranspired) is the main concern rather than the absolute production. Several studies have demonstrated that the WUE of sweet sorghum is higher than that of maize, grain sorghum, and other C4 crops and that it changes in function of the timing and intensity of the drought stress (Gosse 1996 Mastrorilli et al. 1995 Mastrorilli et al. 1999 Dercas and Liakatas 2007). Mastrorilli et al. (1999), for example, indicated that WUE was either maintained or slightly increased when drought stress occurred at later growth stages while with an early stress WUE decreased by about 20%. Since the ability to take up water and WUE seems to operate independently on drought tolerance (Passioura, 1983), new sweet sorghum species may have either one or both of these traits but not necessarily both of them, however, information on that regard is not available. Although the phenological, morphological, and physiological mechanisms of grain sorghum and maize adaptation to drought are generally well understood, little is known about the response of newly developed sweet sorghum genotypes to water deficits, especially at the root level and their water uptake patterns. Moreover, no studies that relate sweet sorghum root development, water uptake capacity and interactions with the aerial parts of the plant are available. A deeper understanding of these traits at the whole plant level would be beneficial for further improving the drought resistance and productivity of potential new energy crops under resource-limited environments. The objective of this study was to compare the water uptake capacity and growth and developmental characteristics of the roots and canopy Plant Soil