Morphological and Physiological Mechanisms of Temperature Compensation in Phosphate Absorption along a Latitudinal Gradient

The effects of temperature upon phosphate absorption by marsh plants were examined along latitudinal gradients of temperature and thermal stability in order to (1) document the adaptive differentiation of a physiological system in response to environmental severity and fluctuation, and (2) elucidate the mechanisms and evolutionary implications of compensation for temperature changes occurring over different time scales. Among the species and ecotypes studied, those that had evolved in colder climates differed from their warm—adapted counterparts in having (1) lower temperature optima for root initiation, elongation, and production; (2) larger surface—to—volume ratios of roots; (3) proportionately more nutrient—absorbing tissue; (4) higher phosphate absorption capacities at given measurement and acclimation temperatures; (5) lower potential of the phosphate absorption system to acclimate in response to temperature change; and (6) less temperature sensitivity of the phosphate absorption system. When plants were acclimated to a new temperature over a period of weeks, cold—acclimated individuals had higher rates of phosphate absorption at a given measurement temperature than did warm—acclimated individuals, and temperature sensitivity of phosphate absorption was lowest in those individuals of a population acclimated to a root temperature that most closely approximated the soil temperature at the site of origin of that population. When temperatures were changed over a period of minutes, roots exposed to low temperature had a higher affinity for phosphate than did roots exposed to warm temperature. Consequently, phosphate absorption was less temperature—sensitive at ecological than at saturating concentrations of phosphate, and the change in affinity of roots for phosphate occasioned by a change in temperature compensated for that temperature change.