LEAF EXPANSION, PHOTOSYNTHESIS, AND WATER RELATIONS OF SUNFLOWER PLANTS GROWN ON COMPACTED SOIL

Leaf expansion and growth response of sunflower (Helianthus annuus, L. ) to soil compaction were investigated in relation to compaction effec ts on water relations, nitrogen nutrition, and photosynthesis. A serie s of field experiments were conducted with plants grown in 20 cm-diame ter cylinders with soil bulk densities ranging from 1.2 to 1.7 g cm-3 at the 0-20 cm depth (equivalent to 0.8 to 2.4 MPa soil strength measu red with a soil penetrometer). Relative leaf expansion rate (RLER) dec reased linearly with increasing soil strength. Smaller plant size in c ompacted treatments was due not only to slower expansion rates, but al so smaller maximum size of individual leaves. Sensitivity of leaf expa nsion to soil strength was best illustrated by a reduction in RLER and maximum size of the first leaf to emerge in a treatment with only the lower 10-20 cm of the profile compacted (bulk density of 1.7 g cm-3). Root growth was less affected than shoot growth by compaction and roo t:shoot ratios of compacted treatments were significantly higher than the control. Soil compaction had no significant effect on pre-dawn or midday leaf water potential, osmotic potential or leaf turgor. Specifi c leaf weight was usually higher in plants grown on compacted soil, an d leaf nitrogen and photosynthesis per unit leaf area were either unaf fected by treatment or significantly higher in compacted treatments. T he results suggest that early growth reduction of sunflower plants gro wn on compacted soil was more sink- than source-limited with regard to water, nitrogen, and carbon supply. Further evaluation of this hypoth esis will require verification that these whole-leaf measurements prov ided a sufficiently accurate approximation of treatment effects on the dynamic equilibria of expanding cells.