GENETIC-VARIATION IN STOMATAL AND BIOCHEMICAL LIMITATIONS TO PHOTOSYNTHESIS IN THE ANNUAL PLANT, POLYGONUM-ARENASTRUM

Terrestrial plant photosynthesis may be limited both by stomatal behav ior and leaf biochemical capacity. While inferences have been made abo ut the importance of stomatal and biochemical limitations to photosynt hesis in a variety of species in a range of environments, genetic vari ation in these limitations has never been documented in wild plant pop ulations. Genetic variation provides the raw material for adaptive evo lution in rates of carbon assimilation. We examined genetic variation in gas exchange physiology and in stomatal and biochemical traits in 1 6 genetic lines of the annual plant, Polygonum arenastrum. The photosy nthesis against leaf internal CO2 (A-ci) response curve was measured o n three greenhouse-grown individuals per line. We measured the photosy nthetic rate (A) and stomatal conductance (g), and calculated the inte rnal CO2 concentration (ci) at ambient CO2 levels. In addition, the fo llowing stomatal and biochemical characteristics were obtained from th e A-ci curve on each individual: the degree of stomatal limitation to photosynthesis (L(s)), the maximum ribulose 1,5-biphosphate carboxylas e-oxygenase (Rubisco) activity (Vc(max)) and electron transport capaci ty (J(max)). All physiological traits were genetically variable, with broad sense heritabilities ranging from 0.66 for L(s) to 0.94 for J(ma x). Strong positive genetic correlations were found between Vc(max) an d J(max), between g and biochemical chemical capacity. Path analyses r evealed strong causal influences of stomatal conductance and leaf bioc hemistry on A and ci. Path analysis also indicated that L(s) confounds both stomatal and biochemical effects, and is an appropriate measure of stomatal influences on photosynthesis, only when biochemical variat ion is accounted for. In total, our results indicate that differences among lines in photosynthesis and ci result from simultaneous changes in biochemical and stomatal characteristics and are consistent with th eoretical predictions that there should be co-limitation of photosynth esis by ribulose-1,5-biphosphate (RuBP) utilization and regeneration, and by stomatal conductance and leaf biochemistry. Gas exchange charac teristics of genetic lines in the present study were generally consist ent with measurements of the same lines in a previous field study. Our new results indicate that the mechanisms underlying variation in gas exchange include variation in both stomatal conductance and biochemica l capacity. In addition, A, g, and ci in the present study tended also to be positively correlated with carbon isotope discrimination (Delta ), and negatively correlated with time to flowering, life span, and le af size based on earlier work. The pattern of correlation between phys iology and life span among genetic lines of P. arenastrum parallels in terspecific patterns of character correlations. We suggest that the ra nge of trait constellations among lines in P. arenastrum represents a continuum between stress avoidance (rapid development, high gas exchan ge metabolism) and stress tolerance (slow development, low gas exchang e metabolism), and that genetic variation in these character combinati ons may be maintained by environmental variation in stress levels in t he species' ruderal habitat.