Plastids In Root Of Taeniophyllum

Photosynthetic roots deserve special mention in this discussion because they are featured by so many epiphytic orchids and aroids. Plastids responsible for the green color in an orchid root reside in cortical (Fig. 2.16) and less often stelar (Fig. 2.14) parenchyma, including the endodermis. Chloroplasts are scattered and relatively few per cell, much as they are in a CAM plant's succulent leaf and stem. If exposed on all sides, roots are uniformly pigmented. Those growing against solid objects remain achlorophyl-lous or pale green where contact blocks light. Embedded roots, of course, are incapable of autotrophy. Most orchid roots are oval to round, but those of shootless Campylocentrum pachyrrhizum and several Phalaenopsis have become almost planar. Taeniophyllum rhizophyllum roots are, as the name implies, even more leaflike. Structural features that might logically reflect trophic capacity do in fact seem to be related to photosynthetic vigor. One of these is root mantle development. Shootless species and relatives with noticeably reduced leaf surfaces never possess more than two to three layers of velamen. Campylocentrum pachyrrhizum sloughs its delicate rhizoder-mis on the exposed side, a process that promotes light penetration (Figs. 2.10, 2.16).

A second notable feature is the pneumathode (Fig. 4.27A). When roots are dry, the velamen offers little resistance to ventilation, but the exodermis is restrictive (Fig. 3.19G). When the velamen is wet, however, gases can still reach the exodermis via air-filled pneumathodes; here, certain U cells that

Velamen Radicans

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Figure 2.5. The diurnal course of CO, exchange by the shootless orchid Campylocentrum tyrridion. (gfw, grams fresh weight.) (From Winter et al. 1985.)

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Figure 2.5. The diurnal course of CO, exchange by the shootless orchid Campylocentrum tyrridion. (gfw, grams fresh weight.) (From Winter et al. 1985.)

promote aeration (Fig. 3.19A,B,D,E) have thinner boundaries, and inner tangential walls may be missing entirely. A third factor affecting trophic capacity is the volume of cortical airspace, which ranges from the high of a C3 plant's leaf to the low of a CAM organ (Benzing, Friedman, Peterson, and Renfrow 1983). Roots of shootless and transitional species exhibit the largest intercellular volume, doubtless a requirement for substantial C02 supply to deep-seated green cells. Cortical parenchyma cells in Kingidium taeniale are surprisingly similar to the irregularly shaped mesophyll cells in many C3 leaves (Fig. 3.19A).

One of the most important unknown aspects of green root biology is its impact on plant economy. Is gas exchange across the velamen-exodermis interface regulated or passive? Does the pneumathode-aeration cell combination function as a stoma or as a lenticel? Shootless Chiloschista usneoides apparently has no stomatal analog in its roots but avoids excessive carbon loss by regenerating C02 from malic acid just rapidly enough to refix it with available photosynthetically active radiation (PAR; Cockburn, Goh, and Avadhani 1985). Campylocentrum tyrridion was less effective in retaining C02, although the loss was less than 25% of the total fixed during the preceding dark period (Fig. 2.5; Winter 1985; Winter et al. 1985).

Separate analyses of individual species must be carried out if one is to establish how green aerial roots affect whole-plant carbon balance. Studies of gas exchange, 14C02 incorporation, enzyme activity, and acid fluctuation

Adaptations to specific exposures 61

show that roots of shootless orchids and such related semileafless species as Kingidium taeniale are indeed primary trophic organs; none, however, fixed C02 as vigorously as leaves from species with fully developed shoots (Benz-ing and Ott 1981). The shootless orchid's unusually high N and chlorophyll levels and the presence of chloroplasts in just about every root parenchyma cell (Fig. 2.14, 2.16) challenge the claim that shoot reduction has paralleled increased tapping of host substrates via root-inhabiting fungi (Johansson 1977). Although roots of leafy taxa (e.g., Epidendrum radicans) showed weaker activity than did those of shootless specimens (C02 exchange was continuously negative with efflux slowest during the day), this does not mean that they have no trophic significance to a leafy plant. On the contrary, roots sometimes comprise half or more of the vegetative body (Fig. 2.18) and are often exposed to light, especially during juvenile stages. At the very least, green roots reduce maintenance costs by recovering considerable dark-and light-respired carbon.

Adaptations to specific exposures

Light harvest

Photon capture and processing (transduction) by green plants is a complicated and only partially understood phenomenon. A quantum of energy (i.e., a photon) is first trapped by antennae containing various pigments. The photon's energy is then funneled via resonance transfer from one molecule to another and ultimately to a reaction center containing a special chlorophyll (Chi) a molecule. These pigment-protein assemblies for trapping and processing photons are called "quantasomes"; they occur in vast numbers in the lamellae of chloroplasts (thylakoids).

A different reaction center serves each of two distinct photosystems which together drive the overall photochemical reaction. Photosystem I reduces nicotinamide adenine dinucleotide phosphate (NADP) to NADPH, using hydrogen from water. Photosystem II synthesizes adenosine triphosphate (ATP), releasing molecular oxygen in the process. The two photosystems are connected by a series of molecules that act as electron carriers. The familiar "Z" scheme illustrated in plant physiology textbooks graphically depicts the photolysis of water, electron flow, and ultimately energy storage through the coupled syntheses of ATP and NADPH. Some of the captured energy may be used to process nitrate (NOf) to ammonium (NHJ") ions, but most is utilized to transform C02 into photosynthate.

Variation in quantasome size depends on the number of molecules pres ent. In exposed habitats, quantasomes will be smaller. In the shade, antenna size (thus quantasome size) increases, the better to harvest scarce photons; the Chi a/b ratio decreases. Investments in carboxylases, electron carriers, and other components of the energy-transducing apparatus are also cut; dark respiration lessens as metabolic rate is generally reduced. A lower specific leaf weight (dry weight per surface area) is an especially effective economy measure in low-energy sites. Other distinctions between sun and shade photosynthesis are based on more complex, poorly understood mechanisms (Chazdon and Pearcy 1986). Shade leaves quickly respond to sunflecks and can store enough acquired energy to continue fixation briefly after shade returns. Sun leaves are better able to dissipate excess energy and avoid photoinhibition of photosystem II.

Temperate-forest understory herbs sometimes alter the light responses of their foliage throughout the growing season; successive, or even the same, leaves switch from high- to low-light-adapted photosynthesis as overhanging branches leaf out in late spring. As yet no similar mechanisms have been identified in epiphytes, but they certainly seem probable. After all, many vining hemiepiphytes show dramatic shifts in leaf size, shape, and, more than likely, photosynthetic properties as well. Hemiepiphytic Monstera dubia produces only thin, small, sessile foliage appressed against the substratum during early stages of its climb. Once exposure reaches some threshold level, much larger petiolate organs, held well above the trunk surface, develop. Some Marcgraviaceae exhibit similar juvenile morphology on the main axis but, in the upper canopy, generate freely suspended, determinate, axillary branches equipped with robust leaves. Photomorphogenesis that allows vines such as those of Monstera gigantea to locate and ascend tree trunks (Strong and Ray 1975) is probably part of many more secondary hemiepiphytes' makeup.

A recent study (Winter, Osmond, and Hubick 1986) of the efficiency of photon harvest in Australian Pyrrosia and several other shade-adapted, nonfern, CAM, or CAM-cycling epiphytes produced unexpected results. Quantum yields (in this instance expressed as moles of quanta absorbed per mole of oxygen evolved) indicated that these species were about as efficient in dim light as were typical C3 plants. Thick foliage and CAM, usually considered to be a poor combination for life in shade, had no diminishing effect. Performance varied a great deal among species, however. In another investigation of C3 and CAM epiphytes (Luttge, Ball, Kluge, and Ong 1986), quantum yields (here expressed as moles of C02 absorbed per mole of quanta) ranged from 0.002 to 0.070 (Table 2.4). Although energy is captured with comparable efficiency in dim light, under stronger illumination C3 spe cies usually achieve higher photosynthetic yields than do CAM types (Osmond 1978; Osmond 1987).

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