Photosynthesis in Climatic Races of Mimulus. I. Effect of Light Intensity and Temperature on Rate

Abstract

Turesson (14, 15, 16) demonstrated that species occurring in a wide range of climates are composed of genetically distinct races, and developed the concept that such races are products of natural selection. This concept is fundamental to an understanding oi mechanisms underlying natural selection and evolution of higher plants. Extensive later experiments bv other wvorkers on the response and survival of climatic races transplanted in contrasting environments have established Turesson's concept as a general principle. Another advance in the study of climatic races is the clarificatioin of major aspects of their comlplex genetic structure (3, 4, 5, 6). In addition to the infornmation being provid(led by field, transplant, ancI genetic investigations, further elucidlation of the nmechanisms of natural selection requires knowledge of the physiology of climatic races. Working principles for a physiological approaclh to the problemii were reviewved by Hiesey, MIilner, and(Nobs (8). Of the many species subjected to tralnsplanit and(l genetic study, members of the Minmulus cardinalis-lewisii species complex are exceptionally well suited for physiological study. These plants range in habitat froml Mlexico to southern Alaska and from near sea level to alpine conditions. They show a wi(le diversity in ability to survive in contrasting climates. Vickery (17) demonstrated that AM. cardinialis Dougl. and Al. lezeisii Pursh., classified as clistinct species, are in reality memnbers of a single genetically conm-patible species complex. Nobs anid Hiesey (12, 13) confirmiied by crossing and transplant experiniiients that this species conmplex contains a widle diversity of readlil) recognized climiatic races. The plants, easily propagated vegetatively as clones, have leaves well suite(d for physiological measurements. The present paper is linmited to the effect of light intensity and temperature on the photosynthetic rates of 6 climatic races of Al. cardinalis native to 6 different habitats. Our first objective is to discover whether the rates of the races differ over a wide range of external con(litions. The kinetic and(phy-siological aspects of the photosynthetic process in iNlimulus receive only incidental attentioii. Materials and Methods The climlatic races used in this wvork, and(for associatecl transplant and genetic experiments, were Received July 8, 1963. brought into culture fronm seed collected in each of the native habitats. Clones from seedlings are mlaintaieled in continuous culture by vegetatixe propa-g,ation in the greenhouse at Stanford. Six climatic races of .lL. cardinialis Dougl., represented by 3 or 5 clones each, Mwere chosen for measure-mlents of photosynthesis. The location and altitu(le of the native habitats of the races are shown in tahle I. The races are numbered in order of increasing elevatioln and will l)e denotedl below by these numbers. Thev rainge froml the lowest to nearly the highest altitude known for the natural (listribution of ,lI. cardinialis. Transplant experiments show themii to be distinct climiiatic races (12). Photosynthetic rates are mleasured with young actively growing plants before any flower buds appear. Plants of each clone are use(d at as nearly as possible the sanme stage of development. usually 10 to 15 cml tall with 5 or 6 pairs of leaves. The top 4 well developed leaves, still attache(d to the plant. are enclose(d in the leaf chamlber of the analytical apparatus. One pair is nearly full size, the other smaller. At the beginining (and end of a d(ay's mzeasuremiients the total leaf area is (letermiiined witlh a planimileter fromii tracings of the leaves. The area increases 2 to 5'il in a day and for any houir (an iiiter-polated value is used. Decker's (7) mletho(an(l apparatus for mleasurinig photosynthesis wvere miiodified to fit the needs of the present investigationl. The bottom of the plzant chamiiber has a removable half to facilitate placemlent of the plant stem in a hole provide(d for it. The