Of three computer‐generated classifications of Pyrus species, produced respectively from 29 chemical characters, 22 botanical characters and all 51 characters taken together, the classification based upon both chemical and botanical characters agrees best with the known geographical distributions of the species. Where either chemical or botanical characters are separately employed, the classifications contain seriously misplaced species. The numerical classification based upon all 51 characters lends some support to the division of the genus into four principal groups of species: the East Asian pea pears, the larger‐fruited East Asian pears, the North African pears and the European and West Asian pears. P. longipes, P. betulifolia and P. cordata are considered as being connecting links between the East Asian pea pears, the larger‐fruited East Asian pears and the European and West Asian pears. Some speculations are made on the phylogenetic relationships among Pyrus species.
1984. Effect of root anaerobiosis on the water relations of several Pyrus species. -Physiol. Plant. 62; 245-252.Solution culture experiments were designed to investigate the plant water relations of 3 Pyrus species subjected to root anaerobiosis. Root anaerobiosis induced partial stomatal closure prior to alterations in leaf water potential (WLW) or root osmotic potential ("PRn). In contrast, stomatal closure was accompanied by a decline in root hydraulic conductivity (Lp). Anoxia markedly reduced WLW for Pyrus communis L. and eventually led to wilting and defoliation. Pyrus betulaefolia Bunge and Pyrus calleryana Deene, however, were less affected by root anaerobiosis. To delineate if the increased root resistance was in the radial or longitudinal direction, 10^ M cistrans abscisic acid (ABA) was added to detopped root systems of P. communis in solution culture after steady-state rates of Lp were established. A consistent 25 to 30% promotion of Lp was observed 1.5 h after the addition of ABA for aerobically treated plants. ABA did not influence Lp when applied to roots previously deprived of O; for 4 days. Additional evidence against the limiting resistance being in the radial direction was obtained when water fluxes were compared through intact P. communis roots, roots with all feeder roots detached, and stems without root systems. Severing feeder roots from anaerobically treated plants did not increase water flux to rates observed for aerobically treated plants. Resistance progressed basipetally to eventually encompass the stem itself. These results can only be explained by occlusion of the xylem vessels.
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