PHOTOSYNTHETIC CHARACTERISTICS OF LEAVES OF MALE-STERILE AND HERMAPHRODITE SEX TYPES OF PLANTAGO-LANCEOLATA GROWN UNDER CONDITIONS OF CONTRASTING NITROGEN AND LIGHT AVAILABILITIES

Plantago lanceolata is it gynodioecious species: In natural population s male steriles (MS) coexist with hermaphrodites (H). Since male steri les have a reproductive disadvantage, without any compensation for the ir loss in male function by an increase in female function, they are e xpected to disappear from the population. In this study we investigate d the possibility that differences in ecologically important photosynt hetic characteristics, between MS and H lines of P. lanceolata, play a role in maintaining gynodioecy. One MS line and two H lines were grow n under conditions of high N and light availability, as well as under either N limitation or light limitation, to investigate whether the se x types respond differently to environmental constraints. Photosynthet ic light-response and CO2-response cut-yes were made, together with le af organic N and chlorophyll determinations. There were only few small differences between the lines and since the MS line did not differ in any of the determined photosynthetic characteristics from either H li ne, it is unlikely that these differences are involved in maintaining male sterility in populations of P, lanceolata. The low-light-grown pl ants showed a high degree of acclimation its shown by a two-fold highe r leaf area to leaf weight ratio (SLA), a two-fold higher investment o f N in light harvesting, and higher net photosynthetic rates under low -light conditions, as compared to the high-light-grown plants. The low -N-grown plants used their organic N more efficiently in photosynthesi s compared to plants grown at an optimal N supply. This was mainly due to the N-limited plants having leaves with a lower organic N content and thus lower photosynthetic capacities. To a lesser extent it was du e to the higher value for the curvature factor of the light-response c urves of the N-limited plants, to their decreased rates of photorespir ation and possibly to their relatively higher allocation of organic N to photosynthetic functions.