Water management in irrigated rice - Rice Knowledge Bank ...

ACO 2H 2 OB[CO 2 ] airBoundary layerBoundary layerresistanceEpidermis cellStomatalresistanceChloroplast[CO 2 ] Stomatal cavityStomatal cavityIntercellular spaceMesophyllresistanceMesophyll cell[CO 2 ] ChloroplastFig. 2.1. Schematic cross section of a leaf stomata (A) indicating components of resistance (B). Source: Lövenstein et al(1992).(with less resistance) than in narrow bundles (withlarge resistance). From the bundles, the water flowsthrough the cells or spaces between the cells ofthe leaves to the “stomata”: small cavities in theleaves that connect to the outside world (Fig. 2.1).The stomata are the last barrier (resistance) to waterflowing out of the plant. The process of waterrelease through the stomata is called (stomatal)transpiration (there is also a cuticular transpirationdirectly through cells of the leaves, but this is muchlower than stomatal transpiration). Figure 2.2 givesan example of water potentials in the soil-plant-atmospheresystem as water moves gradually fromthe soil through the plant into the atmosphere.The flow of water through the plant servesseveral purposes: it transports nutrients (in itsstream) from the soil to the plant organs where theyare needed, it provides the plant with water in itscells so it will stay erect (this is called “turgor”),and transpiration cools the plant so it doesn’t getoverheated. Plants can actively regulate the rate ofwater flow (transpiration) by regulating the size ofthe opening of the stomata. If there is not enoughwater in the soil to satisfy the demand from theatmosphere (that is, to give in to the pulling forceof the atmosphere), the plant can close its stomataand reduce or even completely stop transpiration.Besides the reduction in transpiration, severalgrowth processes of the plant become affectedwhen there is not enough water. We usually callthese “drought effects,” and they are summarizedin Chapter 2.2. Some typical tension levels of soilwater that are important for upland crops are givenin Table 2.2.Nonrice soils usually have a mixture of water,air, and solid soil particles, and the water potential isnegative (positive tension). However, under floodedconditions, as in the muddy layer above the plowpan of rice fields, the soil is saturated with waterand the potential is positive. Negative potentials(positive tensions) occur in this layer only when itdries out. Generally, rice plants experience the shiftfrom flooded conditions (negative tensions) to nonfloodedconditions (positive tensions) as “droughtstress.” A flooded soil that is saturated with water isalso called an “anaerobic soil,” whereas a soil thatis not saturated but has a mixture of air and waterin the pores is called an “aerobic soil.” The waterstatus of a soil has major implications for watersupply to the crop and for pH (acidity), nutrient12