Oaks that occupy Mediterranean phytoclimates share common leaf features (evergreenness, high leaf dry mass per unit area, LMA). Due to this phytoclimatic, morphological, and phenological convergence it has been suggested that they might constitute a coherent functional group. To confirm this hypothesis, some physiological parameters were determined after calculating pressure-volume curves (P-V curves) using the free-transpiration method. Seventeen Quercus species from contrasting phytoclimates were studied: six Mediterranean evergreen species (Q. agrifolia, Q. chrysolepis, Q. coccifera, Q. ilex ssp. ballota, Q. ilex ssp. ilex, and Q. suber); seven nemoral deciduous species (Q. alba, Q. laurifolia, Q. nigra, Q. petraea, Q. robur, Q. rubra and Q. velutina), and four nemoro-Mediterranean deciduous species (Q. cerris, Q. faginea, Q. frainetto, and Q. pyrenaica). Two-year-old seedlings growing under homogeneous environmental conditions (no water restrictions, uniform light, and nutrient supply) were used. The statistical analyses (correlation, mean-value comparisons, principal component analysis) of leaf features and parameters derived from the P-V curves confirmed the functional homogeneity of the three phytoclimatic groups, which were characterized by their contrasting ecophysiological response to water stress. The Mediterranean oak species developed mechanisms to avoid an excessive loss of cell water (e.g. high cell-wall rigidity). Conversely, the nemoral oaks showed the opposite. The nemoro-Mediterranean oaks perform better than nemoral oaks under water-stress conditions, but they cannot perform as well as the Mediterranean oaks on relatively dry soils.

Most of the ice and snow-free land in the Antarctic summer is found along the Antarctic Peninsula and adjacent islands and coastal areas of the continent. This is the area where most of the Antarctic vegetation is found. Mean air temperature tends to be above zero during the summer in parts of the Maritime Antarctic. The most commonly found photosynthetic organisms in the Maritime Antarctic and continental edge are lichens (around 380 species) and bryophytes (130 species). Only two vascular plants, Deschampsia antarctica Desv. and Colobanthus quitensis (Kunth) Bartl., have been able to colonize some of the coastal areas. This low species diversity, compared with the Arctic, may be due to permanent low temperature and isolation from continental sources of propagules. The existence of these plants in such a permanent harsh environment makes them of particular interest for the study of adaptations to cold environments and mechanisms of cold resistance in plants. Among these adaptations are high freezing resistance, high resistance to light stress and high photosynthetic capacity at low temperature. In this paper, the ecophysiology of the two vascular plants is reviewed, including habitat characteristics, photosynthetic properties, cold resistance, and biochemical adaptations to cold.

Deschampsia antarctica Desv. (Poaceae) and Colobanthus quitensis (Kunth) Bartl. (Cariophyllaceae) are the only two vascular plants that have colonized the Maritime Antarctic. The primary purpose of the present work was to determine cold resistance mechanisms in these two Antarctic plants. This was achieved by comparing thermal properties of leaves and the lethal freezing temperature to 50% of the tissue (LT50). The grass D. antarctica was able to tolerate freezing to a lower temperature than C. quitensis. The main freezing resistance mechanism for C. quitensis is supercooling. Thus, the grass is mainly a freezing‐tolerant species, while C. quitensis avoids freezing. D. antarctica cold acclimated; thus, reducing its LT50. C. quitensis showed little cold‐acclimation capacity. Because day length is highly variable in the Antarctic, the effect of day length on freezing tolerance, growth, various soluble carbohydrates, starch, and proline contents in leaves of D. antarctica growing in the laboratory under cold‐acclimation conditions was studied. During the cold‐acclimation treatment, the LT50 was lowered more effectively under long day (21/3 h light/dark) and medium day (16/8) light periods than under a short day period (8/16). The longer the day length treatment, the faster the growth rate for both acclimated and non‐acclimated plants. Similarly, the longer the day treatment during cold acclimation, the higher the sucrose content (up to 7‐fold with respect to non‐acclimated control values). Oligo and polyfructans accumulated significantly during cold acclimation only with the medium day length treatment. Oligofructans accounted for more than 80% of total fructans. The degrees of polymerization were mostly between 3 and 10. C. quitensis under cold acclimation accumulated a similar amount of sucrose than D. antarctica, but no fructans were detected. The suggestion that survival of Antarctic plants in the Antarctic could be at least partially explained by accumulation of these substances is discussed.

Cyclic hydroxanc acids present in some species of Gramineae have been reported to be Important In resbtace of these plnts to fngi aNd insects. Since The amount of DIMBOA5 (Fig. 1) and its glucoside may total more than 1% of the dry wt of corn plants (14,16). In aqueous solutions near neutral pH, free DIMBOA has been reported to decompose quantitatively to MBOA and formic acid (28,29). This facile decomposition of DIMBOA to MBOA has been used for the measurement of DIMBOA content ofcorn by isotopic dilution (15), spectrofluorometry (3), and IR analysis (22). By each method, MBOA is the compound actually measured, and 100% conversion of DIMBOA to MBOA is assumed.

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