Element contents and guard cell physiology of the root hemiparasite Melampyrum arvense L. before and after attachment to the host plant

Before attachment to the host (Capsella bursa pastoris L. Med), the root hemiparasite Melampyrum arvense L. can live during a few weeks in an autotrophic-like way (the preparasitic stage) but its growth is very poor. The attachment of the hemiparasite to the host plant induced a series of significant changes, chiefly connected with solute content, osmolality, and stomatal physiology, which were quantitatively determined. As compared with the attached hemiparasite, a very small content of phosphorus (P) was found in the bulk leaf extract of the unattached one, a decreased level of chloride (Cl) and total nitrogen (NTOT) being also assessed. Moreover, the content of sodium (Na) was 3.2 and 2.9 times greater in the unattached and attached hemiparasite, respectively, than in the host. Like the host plant, stomatal aperture in the preparasitic stage depended upon light intensity. In the case of the hemiparasite attached to the host, this dependence disappeared and differences occurring in stomatal aperture did not exceed 10% between the light and dark phase. The opening of stomata was associated with an increase in pH of the guard cell (GC) compartments. The difference in pH of vacuole saps of GC and epidermal cells (ECs) was 1.1-1.2 units, and in the case of apoplasts, 0.2-0.4 units. With closed stomata, higher pH values occurred in the ECs. The difference in the pH of vacuolar saps of the ECs and GC was 0.4-0.6 units and between the apoplasts, 1.2-1.4 units. No significant differences were ascertained in the pH gradient between the GC and ECs in the attached hemiparasite plants. In the light phase, the content of potassium (K) in the GC of the attached hemiparasite was more than twice that in the preparasitic stage, while the level of Na was greatly decreased and had no significant effect on the osmoregulation process. Cyclic changes in inorganic solutes were correlated with the content of organic acids, mainly malic acid. In the preparasitic stage, its content in the vacuolar sap of GC was reduced to about 1/5 during the closure of the stomata, while in the ECs like in the GC of the attached hemiparasite, these changes were slight. The osmolality gradient between the host and the attached hemiparasite was determined. In the bulk leaf extract, the osmolality was 432, 421, and 779 mosmol/kg for the host, the unattached, and the attached hemiparasite, respectively. In the case of the GCs, the osmolality rates were higher, being 748, 573, and 928 mosmol/kg for the host, the unattached, and the attached hemiparasite, respectively, during the opening of stomata.