Water Transport in Crop Plants with Special Reference to Rice: Key to Crop Production Under Global Water Crisis

Abstract

The knowledge of water flow in agricultural plant species is vital for successful crop production and its management; therefore, investigations were carried out on water transport in rice, one of the most important cereal crops of the world. In these studies, five distinct pieces of evidence of the existence of, at whole-plant level, a previously unappreciated mechanism of water transport similar to that of a "water forced upward-like device," have been presented. The first evidence relates to instant rolling of rice leaves, a symptom of turgor loss, within 60 seconds of plant excision, i.e., separation from roots under water in situ. The second evidence came from the fact that the previously dehydrated intact plant took only 12-15 minutes, upon rewatering, to raise the water potential of top portion of leafless stem, i.e., leaves removed prior to watering, from -2.0 MPa to -0.3 MPa. However, dehydrated and excised plants placed upright with their cut ends under water took, in the absence of roots, 40 minutes, thrice the time taken by similar intact plants to raise the potentials only to the level of -1.0 to -1.4 MPa.The third evidence emerged from complete unrolling (score 1) of rolled leaves (score 5) with a concurrent increase in their water potential and relative water content (92%) within 30 minutes following application of pneumatic pressure of +0.2 MPa to force entry and transport of water through the cut ends of the excised shoots up to the leaves. However, no such changes either in the leaf form or its water status were noticed without pressure application. The fourth evidence is from the use of metabolic inhibitors like HgCl 2 and KCN, which retarded the water intake and transport to the leaves as revealed by delayed leaf unrolling (score 3 or 4) and slow rise of water potential (from -1.6 to -1.4 MPa) and relative water content (from 56 to 65%). The fifth evidence, identified as the most convincing and unequivocal, came from the forcing of water, contained in a specially designed cup-like structure sealed and stuck to previously dehydrated intact plant, by pneumatic pressure through incisions in the stem. A pneumatic pressure of +0.025 MPa raised the leaf water potential by 1.7 MPa (elevated from -2.5 MPa to -0.8 MPa) and the relative leaf water content to 85%. No such positive changes in either water potential or relative water content (55%) occurred in the absence of external pneumatic pressure, though water was abundantly and freely available for intake and upward transport around incisions made in the stem. We concluded that at the whole-plant level, roots possessed enormous capacity to absorb through the involvement of some bio-regulator(s), aquaporins, and to propel water up to the shoot. The potential role of the proposed mechanism of water transport that enables plants to survive during harsh climatic conditions and is supplementary to the cohesion-tension mechanism is discussed for agronomic success of crops under stressful situations.

Living with Limited Water: I. Shoot Hydraulic Resistance as Water-Saving Strategy and its Significance in Varietal Improvement of Rice

“…Therefore, this trait may be advantageous for rice cultivars, enabling them to complete their growth cycle, vegetative and reproductive, satisfactorily under limited-water supply conditions. In fact, plant species and ecotypes differ widely in their responses to environmental parameters and these differences are genetically controlled (Richards 2006;Singh, Singh, and Chauhan 2009c). The present results are supported by the fact that C4 1 plants, highly efficient in water use, seem to have a reduced transport capacity in relation to C3 2 plants (Kocacinar and Sage 2004;McClenahan, Macinnis-Ng, and Eamus 2004).…”

“…In all probability, the hydraulic rope, i.e., transpiration stream being broken, cavitated and embolized in stem, sheath, and lamina by water deficit following withdrawal of water supply, appears to result in hindered water flux leading to the slow and the nonrecovery of leaf water status while incubated with their cut ends in water (Linton, Sperry, and Williams 1998;Linton and Nobel 1999). This proves the relevance and requirement of water forced upward-like mechanism recently proposed by Singh, Singh, and Chauhan (2009b) for pushing the water up in the leaves from the soil to overcome the hydraulic resistance present in the shoot, particularly during water replenishment following irrigation or rainfall. However, the leaf-level homeostasis in water status is maintained through ontogeny, despite varied and changing soil and atmosphere conditions, which is largely because of whole plant changes in the physical resistance to liquid water flow caused by morphological and anatomical adjustments.…”

“…It is suggested that by adopting this new stop-flow technique, which is simple, easy, rapid, cost-effective, and eco-friendly, a large amount of germplasm and breeding materials can be screened and developed for shoot hydraulic resistance. This technique being rapid is very likely to cut down the duration of time required for field investigations enormously, thereby enabling the selection of desirable genotypes; for example, setting a time limit of 55 min for leaf unrolling in this case and discarding all those, i.e., 'NDRH-2', 'NDR-359', 'Jal-priya' and 'Jal-lahari' exhibiting time period less than this cut-off point, much faster than those obtained by field trials often ill-defined for yielding capacity under drought (Paleg and Aspinall 1986;Singh, Singh, and Chauhan 2009a). The duration of time taken for leaf unrolling can further be proportionately shortened by increasing the magnitude of external pressure proposed to be applied.…”

“…Moreover, we agree that, although such changes include adjustments in plant physiology and anatomy, the main control at the time scale of ontogeny is exerted by transformations in whole-plant morphology without which the capacity to acquire water below-ground could not possibly match water demand from above-ground. Though lately the plant water literature is often found to refer to the role of plant growth regulators (Paleg and Aspinall 1986;Voisin et al 2006;Thompson et al 2007) and other metabolic clues to water scarcity-induced effects (Singh, Singh, and Chauhan 2009c); it is recognized that the hydraulic and chemical signals can interact for better adaptation to water constraints (Drake and Franks 2003;Katul, Leuning, and Oren 2003).…”

“…In addition to these macro-and micro-water conservation and agronomic avenues for soil-water saving, the plant strategies, though mostly invisible, that may help save water in the soil and enhance water-use efficiency include, among others, plant hydraulic resistance, an agriculturally important plant trait (Niklas 1992;Zotz et al 1998;Salleo et al 2004;Sangsing et al 2004;McCulloh and Sperry 2005). In spite of its significance, however, this trait has attracted little attention of plant biologists, agronomists and plant breeders for crop improvement owing mainly to lack of a simple, easy, quick and reliable technique of measuring plant hydraulic resistance (Williams, Bond, and Ryan 2001;Singh, Singh, and Chauhan 2009a).…”