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 -1.6 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 -1.9 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 cavitation-inducing 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 +/- 8.8 kPa) was stimulated 7-fold compared with well-watered plants before drought (8.5 +/- 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.

Figure 1.

A, Vulnerability curve showing PLC in leaf xylem versus Ψ_leaf in field-grown rice. Even watered plants (solid symbols, n = 33) experienced up to 90 PLC. The drought treatment (white symbols, n = 41) was mild, perhaps because of accidental flooding; even so, nearly the complete vulnerability curve was obtained. Symbols represent different varieties: Bala (circles, n = 21), Moroberekan (squares, n = 17), Azucena (down triangles, n = 11), IR62266 (up triangles, n = 10), and Lemont (hexagons, n = 15). Weibull curves fit to the data of each variety were not different; therefore, data were pooled and fit to a single Weibull function (solid line). Insert, Same data grouped by measurement time (white symbol, 6–8; gray symbol, 9–11; and black symbol, 13–16). Letters above symbols indicate significant differences. Means and se, n = 22–29, P < 0.05, lsd test. B, Vulnerability curves of leaf (solid symbols) and stem (white symbol) xylem in greenhouse-grown rice of variety Azucena (circles) and IR64 (triangles). Means and se, n = 4–8 (leaves), n = 4 (stems). Lines represent a Weibull fit to the leaf data (solid and dashed lines) and to the stem data (dotted line). Vulnerability curves of the leaves were not different between the two varieties.

Figure 2.

Leaf-specific assimilation rate (A; A) and soil-leaf hydraulic conductance (k_L; B) relative to d 1 during a soil drought/rewatering cycle. Well-watered rice plants (n = 6, varieties Azucena and IR62266) were subjected to 6 d of increasing soil drought. The vertical dotted line indicates the time of rewatering after the 6th d. After rewatering, lights were either turned off overnight (black bar, n = 3) or kept on (white bar, n = 3). Arrows in B, Cession of guttation (downward arrow) and its reappearance (upward arrow). Asterisked means are different from d 1 values (P < 0.05, lsd test). Absolute A values on d 1 for Azucena and IR62266 were 27.6 ± 3.3 μmol s^–1 m^–2 and 14.4 ± 0.8 μmol s^–1 m^–2, respectively. Absolute k_L values were 10.2 ± 3.4 and 4.7 ± 1.3 mmol s^–1 s^–2 MPa^–1, respectively.

Figure 3.

A, g_L and k_L during a soil drought/rewatering cycle for IR62266 (circles) and Azucena (triangles). Data points from the same experiment as in Figure 2. Within each variety, g_L saturated at a similar value with increasing k_L. An exponential rise to a maximum function [y = a(1 –b^x)] was fitted to the pooled data (a = 495.8, b = 0.80, R² = 0.85, P < 0.001). B, Relationship between A and k_L for the same varieties and experiment. Within each variety, A saturated with increasing k_L. However, maximum A and k_L were greater for Azucena than IR62266. An exponential rise to a maximum function [y = a(1 –b^x)] was fitted to the data of each variety. Azucena, a = 28.73, b = 0.68, R² = 0.86, P < 0.001; IR62266, a = 14.17, b = 0.38, R² = 0.73, P < 0.001.

Figure 4.

Root pressure of a representative rice plant (variety Bala) during 1 week of increasing soil drought. The plant was rewatered in the evening of d 8. Numbers on the x axis indicate midnight.

Figure 5.

Root pressures of well-watered rice plants before and after a 7-d drought period. Means and se, n = 11 plants, P < 0.001.