Response of rice varieties to soil salinity and air

PlantandSoil 177: 11-19, 1995. (~) 1995KluwerAcademicPublishers. Printedin the Netherlands.

Response of rice varieties to soil salinity and air humidity: A possible involvement of root-borne ABA Folkard Asch 1'2, Karl D0rffling2 and Michael Dingkuhn t 1West Africa Rice Development Association (WARDA), B.P. 96, St. Louis, Senegal* and 2Universitdt Hamburg, Institut fiir Allgemeine Botanik und Botanischer Garten, Ohnhorststrafle 18, 22609 Hamburg, Germany' Received27 October 1994.Acceptedin revisedform 16 June 1995

Key words: abscisic acid, humidity effects, rice, root-to-shoot communication, soil salinity

Abstract

In a phytotron experiment four rice varieties (Pokkali, IR 28, IR 50, IR 31785-58-1-2-3-3) grown in individual pots were subjected to low (40/55% day/night) and high (75/90%) air humidity (RH), while soil salinity was gradually increased by injecting 0, 30, 60 or 120 mM NaCI solutions every two days. Bulk root and stem base water potential (SWP), abscisic acid (ABA) content of the xylem sap and stomatal resistance (rs) of the youngest fully expanded leaf were determined two days after each salt application. The SWP decreased and xylem ABA and rs increased throughout the 8 days of treatment. The effects were amplified by low RH. A chain of physiological events was hypothesized in which high soil electric conductivity (EC) reduces SWP, followed by release of root-borne ABA to the xylem and eventually resulting in stomatal closure. To explain varietal differences in stomatal reaction, supposed cause and effect variables were compared by linear regression. This revealed strong differences in physiological reactions to the RH and salt treatments among the test varieties. Under salt stress roots of IR 31785-58-1-2-3-3 produced much ABA under low RH, but no additional effect of low RH on rs could be found. By contrast, Pokkali produced little ABA, but rs was strongly affected by RH. RH did not affect the relationships EC vs. SWP and SWP vs. ABA in Pokkali, IR 28, and IR 50, but the relationship ABA vs. rs was strongly affected by RH. In IR 31785-58-1-2-3-3 RH strongly affected the relationship SWP vs. ABA, but had no effect on ABA vs. rs and EC vs. rs. The results are discussed regarding possible differences in varietal stomatal sensitivity to ABA and their implications for varietal salt tolerance.

Introduction

More and more cultivated land is threatened by or has already been lost to salinity, thus further salinization must be avoided and degraded soils must be desalinized and regenerated (Flowers et al., 1977). Although rice (Oryza sativa L.) is susceptible to salinity, irrigated rice culture is highly suited to saline soils because it tolerates standing water, which is necessary for reclamation (Neue et al., 1990). To date most rice research in the tropics has been conducted in Asia in humid environments with low evapotranspiration. Humidity mitigates the effects of water, salt, and temperature stresses (Capell and D6rffling, 1989; Lauter and Munns, 1987; Mizrahi * Fax no: + 221 626491

et al., 1971). Arid atmospheric conditions enhance such stresses, and therefore, crops appear to be less salt tolerant when grown under dry than under humid conditions (Fageria, 1985; Neue et al., 1990). Many stresses are associated with changes in the plant water status, for example chilling, heat and salt stresses (Dingkuhn and Miezan, 1991; Greenway and Munns, 1980). Low shoot water potential normally results in stomatal closure and thus, in reduction of transpiration. Although controversially discussed in some papers (Munns, 1992; Munns and King, 1988; Zhang and Davies, 1991) most authors agree on the plant hormone abscisic acid (ABA) being a major regulator of stomatal aperture (e.g. Henson et al., 1989; Tardieu and Davies, 1992). ABA was recently also found to act as a stress signal in root-shoot communication.

12

Table 1. Classificationof variety-salt tolerance (IRRI Standard Evaluation Systemfor Rice) Variety

WARDAa

IRRIb

IR 50

4.45

4.8

IR 28

4.1

6.0

IR 31785 Pokkali

9.0 4.34c

6.0 4.3

(range0-9; 0= not affected;9= almost all plants dead) aArid environment, conducted by authors, WARDA,BP 96, St. Louis, Senegal. bHumid environment, GEU data base, IRRI, P.O. Box 933, Manila, Philippines. COnlyscored in wet seasons.

Bano et al. (1993) found an increase of xylem ABA associated with a decrease of stomata/conductance in rice growing in drying soil even before the root/shoot water potential was negatively affected by the decreasing water content of the soil. They concluded, that an ABA signal allows the plant to keep its water status balanced for short drought periods. The present study investigates the effects of salt stress and relative humidity on the ABA concentration in the xylem sap and the stomatal resistance of four rice varieties differing in salt-tolerance. A causal chain of events that eventually leads to stomatal closure was used as underlying theory: we assumed that with increasing soil electric conductivity the root water potential decreases; this triggers a hormonal root-signal (ABA-synthesis and release to the xylem); ABA is transferred to leaves via the xylem and induces stomatal closure and thus, reduces transpiration. The validity and limitations of this theory are discussed on the basis of regression analyses among the parameters thought to be involved in rice root-shoot communication under salinity.

Varietal salt tolerance was classified using the IRRI Standard Evaluation System for Rice (IRRI, 1988) as given in Table 1. The varieties were subjected to different levels of soil salinity and air humidity in growth chambers at the Institut for Allgemeine Botanik, Universit~it Hamburg. Plants were sown in plastic containers containing 7 kg of sandy loam soil mixed 4:1 with dried animal manure. 21 days after sowing (DAS) single seedlings were transferred into 330 mL-polystyrol beakers containing ca. 400 g of puddled soil. Fertilizer was applied as 60mg K2SO4, 100mg (NH4)2SO4 and 50 mg "FLORY 5" (12% N+60% P) kg - l soil in a single dose at transplanting. The conditions in the growth chambers were: 30/25 °C day/night temperatures and 12 h photoperiod at 30 #tool m -z s -~ photosynthetically active radiation (PAR) using xenon lamps. Relative humidity was maintained from sowing onwards at 40%/55% RH day/night or 75%/95% RH day/night, depending on the treatment.

Salinity treatments and experimental design A three-factorial experimental design was used. The factors were relative humidity (2 levels), variety (4 levels) and NaCI concentration (4 levels). The experiment was replicated twice. At 42 DAS, 64 plants of each variety were divided into 4 groups of 16 and given 50 mL of one of four levels of NaCI concentration: 0 mM, 30 mM, 60 mM, 120 mM. Salt solutions were injected into the soil using glass tubes that had been installed before planting. Salt applications were replicated four times in intervals of 48h, resulting in gradually increasing NaC1 concentrations in the soil. Two days after each application, 4 plants were used for measurements and destructive sampling. Soil electric conductivity was determined with a conductivity meter (WTW LF 530, sensor LTA 01, values standardized at 25 °C) based on 20 g of dry soil extracted in 30 mL of distilled water.

Physiological measurements Material and methods

Plant materials and growth conditions Seeds of four rice varieties were obtained from WARDA, Senegal (IR 50, IR 31785-58-1-2-3-3) and from IRRI, Philippines (IR 28, Pokkali).

Measurements and samplings of plant material were conducted at mid-light period of the growth chambers. Each set of measurements comprised stomatal resistance, bulk water potential of roots and stems, and the ABA concentration in the xylem sap, for four plants per treatment, date and experiment.

13 Table 2. Effect of a constant level of RH on the stomatal resistance of four rice varieties under non-saline conditions

6 ¸

5

--0-- OmM --D-- 30mM - - m - - 60mM - - i - - 120rnM

l_~-ia

-~ 2-

/

~- ,

8_______0_____O~o

0

D-------'D

~ 2 3 4 5 6 7 8 Days after initial application

9

Fig. 1. Changes in soil electric conductivitydue to the application of salt solution throughoutthe treatments. Averagesof five measurements (3 of replication 1; 2 of replication 2) of randomlypickedpots regardless of RH and variety.

Stomatal resistance was measured on the abaxial surface of the youngest fully expanded leaf with a Li-Cor diffusion resistance meter (LiCor, Lincoln, Nebraska, model Li-60, sensor Li-20, HUM II). For measurements of bulk water potential and collection of xylem sap plants were detopped below the leaf blades, leaving 20-25 cm of the culm attached to the root system. The detopped potted plants with their entire root system were inserted into a Scholander bomb (Roth Ger~itebau, Baiersdorf, Germany). After determining the root/shoot water potential (SWP), sap was collected with a microliter syringe for 2 min from the cut surface while applying a constant pressure of 1.5 MPa. The sap of 4 plants was combined in ice-cold Eppendorf tubes and stored at - 2 0 °C for further analysis. Free A B A in the combined xylem sap samples was determined for each sampling and treatment without further purification by radioimmunoassay (RIA) following the method of Quarrie et al. (1988). Cross reaction tests (Weiler, 1980) showed no interference or non-specific activity from compounds other than ABA.

Results Treatment effects

Soil electric conductivity (EC) was determined as a measure of the impact of the applications of different NaC1 concentrations on the soil. EC increased roughly proportionally to the amount of NaCI applied (Fig. 1). Kinetics for the measured parameters are illustrated for

Variety

rs(high RH) (s.cm-~)

rs(lowRH) (s-cm-~)

Rel. effect (%)

IR 31785-58 IR28 IR50 Pokkali

0.76 4- 0.20 1.134-0.22 1.35±0.43 0.97 4- 0.22

1.83 4- 0.34 2.454-0.26 2.134-0.14 3.12 4- 0.26

241 217 158 322

Mean LSD(5%) 0.51

1.05 5=0.22

2.38 ± 0.48

235

Data presented as means 9= SE of all sampling dates of the 0 mM NaCl-treatment.

Pokkali and IR 31785-58-1-2-3-3 in Figure 2. The bulk root and stem base water potential SWP was between - 0 . 2 and - 0 . 3 MPa throughout the experiment if no NaC1 was applied, but decreased to about 4 times this value two days after the last application of 120 mM NaC1. Relative air humidity had no effect on SWP in the absence of salinity. With 120 mM applied, humidity effects were small and, in most cases, not significant (Fig. 2a). Xylem sap A B A content was 5 to 15 pmol mL-1 and not affected by RH in the control treatment. With the application of 120 mMNaCI, however, xylem A B A content was increased 2- to 4-fold and, in IR 3178558-1-2-3-3, was significantly higher in the low than in the high RH treatment. Xylem A B A content was not affected by RH in Pokkali, however. In contrast to SWP and xylem A B A content, rs was sensitive to RH even under non-saline conditions (Table 2). The low RH treatment generally resulted in higher rs, but varieties differed markedly: stomata of' Pokkali strongly responded to RH and salt applications, as well as to the combined treatments (Fig. 2c). Effects were also significant (p