Interactions between Mild NaCl Stress and Red Light during Lettuce ( Lactuca sativa L. cv Grand Rapids) Seed Germination

Abstract

The sensitivity of lettuce (Lactuca sativa L. cv Grand Rapids) seeds to red light was reduced by NaCQ concentrations which had no effect upon the germination of continuously illuminated seeds. The germination capacity of the seeds was fully restored by increased red light exposures. Indirect evidence indicates that NaCI does not affect the photoconversion of red-absorbing form of phytochrome to the far-red absorbing form of phytochrome. Instead, the increased red light requirements are attributable to increases in the threshold levels of the far-red absorbing form of phytochrome necessary to induce germination and to changes in the slopes of the fluence-response curves. Results also show that the sensitivity of the seeds to NaCI decreased as the time between red light irradiation and the imposition of NaCl stress increased. Reductions in the germination percentage and delays in the onset of germination are well-documented effects of salinity stress. However, salinity stress seldom acts alone, and its effects on seed germination may be modified by interactions with other environmental parameters, such as temperature (14) and light (8). As the effects of NaCl on germination control mechanisms remain largely undefined, the present study investigates phyto-chrome-dependent germination in NaCl-stressed seeds. In particular , this study attempts to establish the level at which NaCl stress affects the phytochrome system in lettuce (Lactuca sativa L.) seeds and describes some of the effects of NaCl upon phyto-chrome-initiated pregermination events. MATERIALS AND METHODS Lettuce cv Grand Rapids seeds (correctly termed achenes) were obtained in 1981 from Ferry-Morse Seed Company (Moun-tain View, CA; batch No. 32210-18639) and stored in the dark at 4°C until required. Seeds were sown in 9-cm Petri dishes lined with two 9-cm filter paper discs moistened with 4 ml of the appropriate germination medium. Five replicates, each containing 25 seeds, were used for each treatment. All experiments were conducted at 25°C. Germination, adjudged by radicle emergence, was determined 48 and 72 h after R2 irradiation for seeds germinated in water and NaCl, respectively. For studies involving continuous illumination, the Petri dishes 'Recipient of a Postdoctoral Research Fellowship financed by the Karnsberg Foundation. Present address: Centro de Investigaci6n Cienti-fica de Yucatan, A. C., Apartado Postal 87, 97310 CORDEMEX, Yu-catAn, Mexico. 2Abbreviations: R, red light; FR, far red light. were wrapped in two transparent polyethylene bags and placed in a temperature-controlled incubator. Illumination was provided by two 20-w cool white fluorescent lamps located 50 cm above the Petri dishes. For dark germination experiments, the Petri dishes were enclosed in two opaque, black polyethylene bags and the seeds incubated at 25°C in a temperature-controlled darkroom. All dark manipulations, including seed sowing, were carried out under a dim green safelight. Seeds germinated in the dark were routinely irradiated with 5 min FR light 60 to 75 min after the start of imbibition. Unless stated otherwise, the seeds were R-irradiated 6 h after the start of imbibition. The R was provided by a tungsten light source projected through an interference filter with peak transmission at 650 nm (Schott Optics, Mainz). The light beam was directed through a neutral density screen to yield a final fluence of 0.35 w/m2 at seed level. Light from a bank of incandescent lamps filtered through two layers of red and one of blue cellophane provided the FR source with a fluence rate at seed level of 30 w/m2. Fluence rates were measured with a YSI-Kettering model 65 radiometer with a black body detector. Unless otherwise stated, datum points represent the mean of three separate experiments, each containing five replicates. Error bars indicate the limits of ± 1 SD. RESULTS Figure 1 shows the effects of a range of NaCl concentrations on the germination of continuously illuminated lettuce seeds. In water, germination was essentially complete within 24 h. At a concentration of50 mm, NaCl had little effect upon germination, and, after 24 h, the percentage germination was similar to that of the water controls. Higher NaCl concentrations increased the time required for germination, and, in 100 mm NaCl, maximum germination levels were attained 48 h after the start ofimbibition. Above 100 mM NaCl, the delay in the onset of germination was accompanied by reductions in the final germination percentage which decreased as the NaCl concentration increased. At NaCl concentrations of 200 mM and above, no germination was observed within 72 h of the start of imbibition. Although the seeds become sensitive to saturating light irradiations shortly after the start of imbibition, maximal sensitivity to R develops more slowly (7, 1 1). Figure 2 shows the germination response of seeds exposed to a suboptimal R dose at varying times after the start of imbibition; the seeds were imbibed and germinated in water and 100 mM NaCl. In water, the germination percentage in response to 40-s R progressively increased as the length of the preirradiation period increased; reaching a maximum about 6 h after the start of imbibition. In 100 mM NaCl, maximal sensitivity to 85-s R was attained about 4 h after the start of imbibition. Longer preirradiation periods, up to at least 8 h, had little further effect upon the R sensitivity of the seeds. NaCl greatly reduced the germination response of the seeds to 149