Water relations of the pine exine

Abstract

Background and aims: Water adhesion forces, water absorption capacity and permeability of the pine exine were investigated to consider a possible function of sporopollenin coatings in the control of water transport.

Methods: The experiments were carried out with sporopollenin capsules obtained from pine pollen consisting of an empty central capsule and two sacci. Changes in the concentration of excluded dextran molecules in the medium were analysed to quantify water absorption by purified exine fragments and the osmotic volume flow out of the intact central capsule.

Key results: The contact angle of sporopollenin to water is higher than the one to ethanol and lower than the one to n-heptane. The water-filled pore space in pine sporopollenin amounts to only 20.6 % of the matrix volume. A monosaccharide was excluded from 15 % and a trisaccharide from about 38 % of this space. Shrinkage of the central capsule induced by permeable osmotica was transient, whereas that induced by sodium polyacrylate (2100 g mol(-1)) was stable. Values obtained for the hydraulic conductance L(P) of the exine (0.39-0.48 microm s(-1) MPa(-1)) are comparable in size to those of biomembranes. Sodium sulfate solutions induced a significant osmotic flow through the exine (reflection coefficient at least 0.6). The exine around the central capsule can be ruptured by equilibration of its lumen with a concentrated electrolyte solution and subsequent transfer to water. The denatured protoplast along with the intact intine was ejected when pollen grains were subjected to this osmotic shock treatment.

Conclusions: The pine exine is easily wetted with water and does not represent a significant barrier to water exchange either liquid or gaseous. Through osmotic burst, it can be separated from the intine. The effect of salts and small solute molecules on water fluxes may be functionally significant for rehydration upon pollination.

Fig. 1.

Air-lift system for circulation of the dextran medium and polarimetric registration of the dextran concentration. 1, Thermostated and aerated incubation vessel containing the pine exines; 2, cylindrical particle filter covering a rifled drainage area and a ring channel in front of the filtrate outlet; 3, thermostated polarimeter cell (1 mL).

Fig. 2.

Partition of sporopollenin capsules within a water/n-butanol emulsion. Water-saturated capsules prepared from Pinus sylvestris pollen (5 mg of a filtered mass) were suspended in 0·5 mL of n-butanol and the suspension mixed with 1 mL of a suspension of Indian ink in water to stain the aqueous phase.

Fig. 3.

Sporopollenin capsules, prepared from Pinus sylvestris pollen, in a sodium polyacrylate solution. Concentration of polyacrylate 2100 = 50 g L⁻¹ (0·47 MPa). The sacci are brought together by the shrinkage of the central capsule.

Fig. 4.

Osmotically induced water efflux from the central capsules. (A) Course of the dextran concentration change after the addition of sodium polyacrylate. The polarimetric system contained 902 mg (f. wt) of packed water-saturated sporopollenin capsules (prepared from Pinus sylvestris). Dextran concentration, 30 g L⁻¹; volume outside of the central capsule, 9 mL; volume V_c of the central capsule, 322 μL. At time zero, a volume of 0·5 mL of a dextran/sodium polyacrylate solution was added (final polyacrylate concentration = 24 g L⁻¹, π = 0·225 MPa). The scale of recording was calibrated by addition of water (100 μL) as indicated. (B) Control experiment, osmoticum added to the dextran solution without suspended capsules. The transient change in the recording was caused by schlieren in the polarimeter cell.

Fig. 5.

Dependence of capsule shrinkage on the osmotic potential of sodium polyacrylate 2100 solutions. Final shrinkage was determined from records of the type shown in Fig. 4.

Fig. 6.

Course of the volume changes induced by sodium polyacrylate 2100 and sodium sulfate in isotonic concentration. Final osmotic pressure, 0·225 MPa; final concentrations: sodium sulfate, 0·035 M; sodium polyacrylate, 23·9 g L⁻¹. Sporopollenin capsules were prepared from Pinus sylvestris. Water efflux is given as percentage of the central capsule volume.

Fig. 7.

Integrity of central capsules before and after osmotic shock. (A) Control preparation. Only a small percentage of the central capsules is stained (due to fissures in the exine). (B) Preparation after loading with phosphoric acid (final concentration, 850 mL L⁻¹) and subsequent osmotic down-shock. Most of the central capsules do not exclude the dye. Sporopollenin capsules prepared from pollen of Pinus sylvestris. Concentration of Evans blue, 10 g L⁻¹.

Fig. 8.

Pine pollen grain (Pinus nigra) after osmotic shock. The denatured gametophyte has been catapulted out of the exine. Cellulose staining with Calcofluor White demonstrates that the ejected protoplast remained covered by the intine. The pollen grain has been slowly equilibrated with phosphoric acid (final concentration 850 mL L⁻¹) and subsequently transferred to water.