THE EFFECT OF GIBBERELLIC ACID ON THE ABSORPTION AND TRANSLOCATION OF PHOSPHORUS-32 BY BEAN PLANTS

Linck , A. J., and Theodore W. Sudia . (U. Minnesota, St. Paul.) The effect of gibberellic acid on the absorption and translocation of phosphorus-32 by bean plants. Amer. Jour. Bot. 47(2) : 101—105. Illus. 1960.–Plants of Phaseolus vulgaris, variety ‘Black Valentine,‘ treated with 1 p.p.m. gibberellic acid supplied to the roots for 4 hr. were compared with nontreated. plants for phosphorus-32 uptake. Plants treated with gibberellic acid where transpiration was either rapid or restricted absorbed more phosphorus-32 than those not treated. More phosphorus-32 was recovered from plants free to transpire than from plants in high humidity. In plants free to transpire, significantly more phosphorus was present in the treated plants after 28 hr. and significantly more phosphorus-32 accumulated in treated plants in those parts actively growing, i.e., stem apex, second internode and first trifoliate leaf, and in the roots. For plants in high humidity atmosphere significantly more phosphorus-32 was absorbed by the treated plants at the end of 4 hr. than in the non-treated plants and this difference was maintained throughout all times of harvest. For plants in high humidity atmosphere, significantly more phosphorus-32 accumulated in the lower portions, i.e., roots, hypocotyl, first internode and primary leaves, of treated plants than of non-treated plants, while the differences for the second internode, the first trifoliate leaf and the stem apex were not significant between treated and non-treated plants.     

THE ABSORPTION AND TRANSLOCATION OF C14-LABELED PROTEINS IN YOUNG TOMATO PLANTS

The uptake of C¹⁴-labeled proteins (lysozyme, hemoglobin, lactoglobulin, and ovalbumin) from solution by tomato plants with sterile roots was studied. It was found that C¹⁴-compounds (proteins and/or protein-degradation products) were translocated to the foliage if the roots had undergone minor mechanical injury or if the plants were subjected to temporary wilting, i.e., physiological damage. C¹⁴-lysozyme was not transported to foliar tissue in healthy plants; C¹⁴-hemoglobin showed radioactivity in leaves of both healthy and injured plants, but there was evidence of a breakdown of the molecule; C¹⁴-ovalbumin gave a faint labeling of foliar tissues of some plants in which wilting or mechanical damage was below the threshold of detection. It is concluded, however, that translocation of proteins from roots in nutrient solution to tomato leaves does not occur in significant amounts in healthy plants in spite of the large uptake of proteins by root cortex, as found in earlier studies.     

LIGHT ABSORPTION BY PLANTS AND ITS IMPLICATIONS FOR PHOTOSYNTHESIS

The preceding account has attempted to examine the interactions between light absorption and photosynthesis, with reference to both unicellular and multicellular terrestrial and aquatic plants. There are, however, some notable plant groups to which no direct reference has been made, e.g. mosses, liverworts and lichens. Although many have similar optical properties to terrestrial vascular plants (Gates, 1980) and apparently similar photosynthetic responses (see Green & Snelgar, 1982; Kershaw, 1984) they may possess subtle, as yet unknown differences. For instance, the lichen thallus has a high surface reflectance although the transmittance is virtually zero (Gates, 1980; Osborne, unpublished results). It is envisaged, however, that differences in optical properties between species will reflect differences in degree not kind. Although not all variation in photosynthesis is due to differences in light absorption a number of accounts suggest that this is a contributing factor. Variations in leaf absorptance have been found to account for most of the variation in leaf photosynthesis at low J_is (see Ehleringer & Björkman, 1978a; Osborne & Garrett, 1983). There is, however, little direct experimental evidence on light absorption and photosynthesis in either microalgal species or aquatic macrophytes. We also do not know over what range of incident photon flux densities photosynthesis is determined largely by changes in light absorption. Plants growing under natural conditions also experience large diurnal and seasonal fluctuations in J_i, unlike species grown under laboratory conditions. The occurrence of transitory peaks in J_i tends to overshadow the fact that the average J_i is often lower than the J₁ required to saturate photosynthesis, i.e. 1500–2000 μmol m^-2 s^-1, depending on the growth treatment. Using the data of Monteith (1977) and I W m²= 5 μmol m^-2 s^-1, and with photosynthetically active radiation 50% of total solar radiation, the daily mean value for Britain is approximately 450 μmol m^-2 s^-1, with a maximum in June of 1000μmol m^-2 s^-1 and a minimum during the winter of 75 μmol m^-2 s^-1. Such values could be even lower on shaded understory leaves and considerably lower for aquatic species. Based on average values of net photosynthesis for a terrestrial plant leaf, light saturation would only be expected in June while for most of the year the average values would lie largely on the light-limited portion of the photosynthesis light response curve. Although the daily average values in tropical climates may be higher during the winter months, they are remarkably similar throughout the world for the respective summers in the northern and southern hemispheres, because the increased daylength at high latitudes compensates for the lower J_is. The expected lower dark respiration rates during the winter may also partially offset the effects of a lower light level. There is therefore a trade-off between high J_is for a short period of time against a lower J_i for a longer period of time. We might expect different photosynthetic responses to these two very different conditions. Importantly, a low J_i with a long daylength may enable a plant to photosynthesize at or near its maximum photon efficiency for most of the day. Although the response of the plant to fluctuations in J_i is complicated because it is affected by the previous environmental conditions, this may indicate that light absorption has a much greater significance under natural conditions, particularly for perennial species. The bias in many laboratories towards research on terrestrial vascular plants also tends to ignore the fact that a number of multicellular and unicellular aquatic species survive in very low light environments. Furthermore, the direct extrapolation of photosynthetic responses from measurements on single leaves to those of whole plants is clearly erroneous. Although this is obvious, many physiological ecologists have attributed all manner of things to the photosynthetic responses of ‘primary’ leaves. Most researchers have ignored problems associated with composite plant tissues and internal light gradients. Clearly caution is required in interpreting the photosynthesis light-response curve of multicellular tissues based on biochemical features alone. Also, the importance of cell structure on light absorption and photosynthesis has generally been ignored and attributed solely to the effects of structural features on CO₂ diffusion. In doing so the work of two or three generations of plant physiologists has been ignored. Haberlandt (1914) at the turn of the century probably first implicated the role of cell structure in leaf optics, and Heath (1970) stressed that in order to completely understand the role of light in photosynthesis we need to know the flux incident on the chloroplast itself. Even this suggestion may need modification because of the capacity of the internal chloroplast membranes for scattering light. It is worth emphasizing the importance of light gradients within tissues and their role in regulating photosynthesis, particularly at light saturation. Measurements of light gradients are fraught with problems because of experimental difficulties and the majority (few) are based on reflectance and transmittance measurements. Seyfried & Fukshansky (1983) have shown that light incident on the lower surface of a Cucurbita cotyledon produced a larger light gradient than light incident from above, indicating the importance of the spatial arrangement of the tissues with respect to the light source. Also, light incident on the lower surface of leaves of Picea sitchensis was less ‘effective’ in photosynthesis than light from above (Leverenz & Jarvis, 1979). Clearly, two tissues could have the same gross absorptance but different photosynthetic rates because of differences in the internal light environment. Fisher & Fisher (1983) have recently found asymmetries in the light distribution within leaves, which they related to asymmetries in photosynthetic products due to differences in solar elevation. Such modifications in light distribution could be important for a number of solar-tracking species. Changes in light absorption are brought about by a whole gamut of physiological, morphological and behavioural responses which serve to optimize the amount of light absorbed. Perhaps the simplest way of regulating the amount of light absorbed is by restricting growth either to particular times of the year or to conditions when the light climate is favourable. We are still largely ignorant of many details of these modifications. In particular, differences in tissue structure such as the size and number of vacuoles or the effects of organelles on the scattering component of the internal light environment of photosynthetic tissues are not understood. A better understanding of the interaction of light with plants in aquatic systems is also required. It is unfortunate that light-absorptance measurements are not routinely made in photosynthetic studies, and this is quite clearly a neglected area of study. That these measurements are not made is even more surprising, since techniques have been available for over sixty years (Ulbricht, 1920). Absorptance measurements are of particular importance in the photosynthetic adaptation of microalgae, where only a small proportion of the incident photon flux density is absorbed. For multicellular species more detailed information is required on internal light gradients and their variability. Light-absorptance measurements are also important in any study relating kinetic data on CO₂ fixation to in vivo photosynthesis, especially when there are large variations in the morphology and structure of the photosynthetic organ.     

AUTORADIOGRAPHIC STUDY OF SUGAR AND AMINO ACID ABSORPTION BY EVERTED SACS OF HAMSTER INTESTINE

Autoradiographs were prepared from frozen sections of everted sacs of hamster jejunum which had been incubated in vitro with C¹⁴- or H³-labeled sugars and amino acids. When such tissue was incubated in 1 mM solutions of L-valine or L-methionine, columnar absorptive cells at tips of villi accumulated these amino acids to concentrations ranging from 5 to 50 millimoles per liter of cells. Quantitative data were obtained by microdensitometry of C¹⁴ autoradiographs. Similar, though less striking, results were obtained with the sugars: galactose, 3-0-methylglucose, α-methylglucoside, and 6-deoxyglucose. In all cases the marked "step-up" in concentration occurred near the brush border of the cell, and a "step-down" in concentration occurred at the basal pole of the cell. Known inhibitors of intestinal absorption, e.g., phlorizin in the case of sugars, blocked the concentrative step at the luminal border of the absorptive cell. It is inferred from these data that active transport systems for sugars and amino acids reside in the brush border region of the cell. Additional evidence suggests that the basal membrane of the cell may be the site of both a diffusion barrier and a weak transport system directed into the cell.