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 (−0.4 MPa) 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 HgCl2 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 (−2.5 MPa) 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.