Growth of White Clover, Dependent on N2 Fixation, in Elevated CO2 and Temperature

Clonal plants of white clover (Trifolium repens L ), wholly dependent on N2 fixation, were grown for 6 weeks in controlled environments providing either (C680 regime) 23/18 °C day/night temperatures and a CO2, concentration of 680 μmol mol−1, or (C340 regime) 20/15 °C day/night temperatures and a CO2, concentration of 340 μmol mol−1 During the first half of the experimental period the C680 plants grew faster than their C340 counterparts so that by week 3 they were twice the weight this 2 1 superiority in weight persisted until the end of the experiment The faster initial growth of the C680 plants was based on an approx 70 % increase in leaf numbers and an approx 30 % increase in their individual area Initially, specific leaf area (cm2 g−1 leaf) was lower in C680 than in C340 leaves but became similar in the latter half of the experiment Shoot organ weights, including petioles and stolons, reflected the C680 plant's better growth in terms of photosynthetic surface Throughout, C680 plants invested less of their weight in root than C340 plants and this disparity increased with time Acetylene reduction assays showed that nitrogenase activity per unit nodule weight was the same in both C680 and C340 plants Both groups of plants invested about the same fraction of total weight in nodules Nitrogen contents of plant tissues were similar irrespective of growth regime, but C680 expanded leaves contained slightly less nitrogen and their stolons slightly more nitrogen than their C340 counterparts However, C680 leaves contained more non-structural carbohydrate Young, unshaded C680 leaves possessed larger palisade cells, packed more tightly within the leaf, than equivalent C340 leaves The reason for the C680 regime's loss of superiority in relative growth rate during the second half of the experiment was not clear, but more accumulation of non-structural carbohydrate, constriction of root growth and increased self-shading appear to be the most likely causes