Salinity Effects on Photosynthesis and Growth in Alternanthera philoxeroides (Mart.) Griseb.

Abstract

Alternantheraphiloxeroides, alliator weed, was grown at five different NaCI concentrations to determine the effect of salinity on factors related to the net rate of CO2 uptake (Pa). Over the range of 0 to 400 millimolar NaCI, P. declined 51%. Stomatal conductance declined in parallel with P. and as a result there was no reduction in intercellular CO2 concentration and therefore no reduction in the amount of CO2 available for photosynthesis. The CO2 compensation point did not change with salt stress. Increases in leaf thickness tended to compensate slightly for the negative effects of salinity on leaf cell metabolism, at least in relation to P.. On a mesophyll cell area basis, soluble protein was relatively constant in leaves developed at 100 to 400 millimolar NaCI while total chlorophyll decreased at all salinities. Dry weight production and P. were closely correlated in alligator weed grown at different salinities. Plants produced less leaf area per unit dry weight as salinity increased, which may aid in water conservation. The PM3 of many plant species declines with increasing rhizo-sphere salinity (6, 10, 13-16, 23). The decline in Pn has been attributed to salinity effects on both stomatal (6, 10, 14, 16, 23) and nonstomatal (6, 10, 15, 18, 23) controls of P,,. Although there seems to be almost general agreement that salinity effects on nonstomatal factors (i.e. metabolic parameters; 9) reduce PF, the importance to P, of observed reductions in stomatal con-ductance with salinity is not as clear. Declining stomatal con-ductance could reduce P,, by lowering the CO2 concentration in the leaf, as hypothesized for spinach (19). In contrast, reductions in stomatal conductance coupled to lowered potential PF, could act to maintain a relatively constant intercellular CO2 concentration (8, 18, 26), fitting the response of Avicennia marina developed at 50 to 500 mm NaCl and a vapor pressure deficit of 0.6 KPa (2). Further examination of stomatal effects on the amount of CO2 available for photosynthesis is necessary before generalizations can be made about the relationship of stomatal conduct-ance to P, in salt-stressed plants. 95616. 3 Abbreviations: P,,, net rate of CO2 uptake; g,,, water vapor conduct-ance; ci2, average CO2 concentration in intercellular air spaces of leaf; A ?e/A, ratio of mesophyll cell surface area to leaf surface area. 24) and that certain of these changes in anatomy may have consequences for Pn (15) complicates interpretation of salinity effects on stomatal and nonstomatal controls of Pn. The effect of changes in leaf structure on Pn in salt-stressed plants can be analyzed by measuring the relative amount of mesophyll cell surface to leaf surface area (15). The present study was designed to examine the effect of salt stress on Pn in terms of stomatal, nonstomatal, and structural changes in a rapidly growing species that naturally experiences a range of salinities. Alternanthera philoxeroides (Mart.) Griseb. (a C3 member of the Amaranthaceae), alligator weed, can apparently survive changing concentrations of salinity (11) which may be more stressful than constant salinity (5). In a previous study, water potential in alligator weed declined in response to increases in rhizosphere salinity of 50 mm NaCl/d up to a final concentration of400 mm (3). These changes in water potential were ofsufficient magnitude to maintain a water potential gradient from rhizo-sphere to roots and were the result of decreases in minimum tissue osmotic potential. Salinity also had a dramatic effect on cell structural properties of alligator weed as measured by the bulk elastic modulus (3). Having established that alligator weed tolerates increases in rhizosphere salinity by osmotic adjustment, we asked how this adjustment was related to carbon gain. Specifically, how does salinity affect P,, in alligator weed and what is the relationship of the P,, response to changes in leaf structure and plant growth? To answer these questions, we measured maximum P,, at different salinities, and compared control by stomatal and nonsto-matal properties ofleaves. We also evaluated the effect ofsalinity-induced differences in leaf anatomy on P,,. Finally, to place the other measurements in a context of growth response, we measured increases in dry weight and leaf area for plants growing at different salinities. MATERIALS AND METHODS Culture. Propagules of Alternanthera philoxeroides (Mart.) Griseb., alligator weed, consisting of two node sections of stem, were collected from Campus Lake, at Louisiana State University, Baton Rouge, and rooted in a modified Hoagland solution for 7 to 10 d at 24°C (7). Rooted propagules were placed in holes in pieces of styrofoam floating on nutrient solution in 10-L plastic trays. Trays were placed in a growth chamber with a 12-h photoperiod (average photosynthetic photon flux density, 250 to 300 ,mol m-2 s-', measured with a Licor LI-185A quantum sensor) and 28°C days and 22°C nights. Solutions were maintained at pH 6.0 to 6.5 with 0.5 M KOH, replenished daily with distilled H20 and solutions were completely replaced every week. Propagules were grown until they had a well developed root system and a stem size of four to five nodes (approximately 5-6 weeks). NaCl was added to culture solutions in daily, 50 mM increments to produce five different salinity treatments (0, 100, 200, 300, and 400 mM). Salt additions were scheduled so that all 1044