Hydraulic Properties of Rice and the Response of Gas Exchange to Water Stress

Abstract

We investigated the role of xylem cavitation, plant hydraulic conductance, and root pressure in the response of rice (Oryza sativa) gas exchange to water stress. In the field (Philippines), the percentage loss of xylem conductivity (PLC) from cavitation exceeded 60% in leaves even in watered controls. The PLC versus leaf water potential relationship indicated diurnal refilling of cavitated xylem. The leaf water potential causing 50 PLC (P 50 ) was Ϫ1.6 MPa and did not differ between upland versus lowland rice varieties. Greenhouse-grown varieties (Utah) were more resistant to cavitation with a 50 PLC of Ϫ1.9 MPa but also showed no difference between varieties. Six-day droughts caused concomitant reductions in leaf-specific photosynthetic rate, leaf diffusive conductance, and soil-leaf hydraulic conductance that were associated with cavitationinducing water potentials and the disappearance of nightly root pressure. The return of root pressure after drought was associated with the complete recovery of leaf diffusive conductance, leaf-specific photosynthetic rate, and soil-leaf hydraulic conductance. Root pressure after the 6-d drought (61.2 Ϯ 8.8 kPa) was stimulated 7-fold compared with well-watered plants before drought (8.5 Ϯ 3.8 kPa). The results indicate: (a) that xylem cavitation plays a major role in the reduction of plant hydraulic conductance during drought, and (b) that rice can readily reverse cavitation, possibly aided by nocturnal root pressure.Upland (aerobically grown) rice (Oryza sativa) and rain-fed lowland (periodically flooded) rice, which combined account for about one-half of the world's rice, are subjected to unpredictable periods of drought (Chaudhary and Rao, 1982). Rice even suffers from water stress when grown in permanently flooded paddies (Ishihara and Saito, 1987;Jiang et al., 1988;Hirasawa et al., 1992 Hirasawa et al., , 1996. Much effort has been put into research that addresses critical yield and productivity constraints in relation to water stress (Wade et al., 1999;Zeigler, 1999). However, progress in the development of drought-resistant cultivars has been slow (Cooper et al., 1999;Fukai et al., 1999). One aspect that has been largely overlooked is the hydraulic conductance of rice and its droughtinduced reduction due to xylem cavitation. Work on woody plants has demonstrated an important hydraulic limitation on stomatal conductance and photosynthesis (Hubbard et al., 2001), a factor that could be equally important for herbaceous crop plants. Recently, Miyamoto et al. (2001) found the hydraulic conductivity of rice roots to be substantially lower than that of other herbaceous roots. This low root conductivity, combined with a relatively high transpiration rate (Tanguilig et al., 1987), could explain the steep drops in leaf water potential and leaf gas exchange found in flooded rice studies. Decreased xylem water potential carries the inherent danger of xylem cavitation, which can substantially reduce the hydraulic conductivity of plants (Tyree et al., 1986;Sperry et al., 1993). R...

Herb Hydraulics: Inter- and Intraspecific Variation in Three Ranunculus Species

“…With daily C min being 0.3 to 1.4 MPa lower than P k50 , at least two of our species would incur significant water transport reduction during the course of the day (Tables I and II; Stiller et al, 2003), limiting transpiration and, thus, according to Ohm's law, preventing a further decrease of C. However, the staining experiment suggests that embolism plays only a minor role in hydraulic decline at the observed in situ C (Fig. 2); therefore, water transport is limited mainly by reversible extraxylary effects (e.g.…”

Stomatal Responses in Rainfed Lowland Rice to Partial Soil Drying ; Evidence for Root Signals

“…4). This apparent increase in Ψ L may be partially explained by the findings of Stiller et al (2003), who showed that even at -2 MPa, rice roots were vulnerable to cavitation, with xylem conductivity in leaves dropping by as much as 90%. Nevertheless, the plants were able to refill cavitated xylem conduits and restore lost xylem hydraulic conductance.…”

“…They concluded that a plant with excised roots could supply the shoot with suffi cient water for it to survive by way of its remaining xylem vessels embolizing less frequently or refilling more quickly, so that the remaining root was more effi cient. Stiller et al (2003) also observed this ability in rice to refill cavitated xylem conduits and restore lost xylem hydraulic conductance. They also noted that reduction in hydraulic conductance could have come from cavitation in roots and stems and soil drying.…”

Plant Responses to Salt Stress: Adaptive Mechanisms

“…Whole plant hydraulic conductivity and individual conductance components are considered the main factors that control the movement of water through the soil-plant system and have been linked with plant transpiration and other physiological processes related with transpiration [126][127][128][129]. Differences in root hydraulic conductance may cause differences in water transport from soil to plant, influencing the leaf water status and therefore in the growth and other physiological responses of the plant [130].…”