© 1994 Heron Publishing—Victoria, Canada
Carbon allocation, gas exchange, and needle morphology of Pinus ponderosa genotypes known to differ in growth and survival under imposed drought
B. M. Cregg
USDA Forest Service, Center for Semiarid Agroforestry, Rocky Mountain Forest and Range Experiment Station, East Campus-UNL,
Lincoln, Nebraska 68583-0822, USA / Received July 14, 1993
Summary
Seedlings from 27 open-pollinated families of ponderosa pine representing nine geographically diverse origins were screened
for drought tolerance based on survival and growth under imposed drought. Seedlings that had been preconditioned to drought
survived 14 days longer than seedlings that had been well watered before being subjected to drought. Seed sources varied in
their ability to survive drought and this variation was accentuated by drought preconditioning. Seedlings from a South Dakota
source and a Nebraska source generally survived the longest under drought. Seedlings from a Montana source and a New Mexico
source succumbed the fastest after water was withheld. Significant family within source variation in drought survival was
observed for some sources. In general, drought survival was poorly correlated to climate indices of the seed sources. Allocation
of biomass to roots, stems, and needles varied significantly among the seed sources with the most drought-sensitive sources
(Montana and New Mexico) showing the most divergent allocation patterns. The relation between drought survival and shoot/root
ratio suggested that there is an optimum pattern of allocation for drought survival. A comparison of the most and least drought-tolerant
sources indicated that needle gas exchange (net photosynthesis and needle conductance to water vapor) and predawn needle water
potential were similar among the sources regardless of their relative ability to survive drought. Needle morphology traits
often associated with variation in drought tolerance, such as stomatal density and specific leaf area, did not differ among
the seed sources. However, seedlings from the drought-tolerant sources had shorter needles, less surface area per needle,
and fewer stomata per needle than seedlings from the drought-sensitive sources. The results suggest that drought tolerance
of ponderosa pine may be improved through seed source selection and, within certain sources, family selection. Allocation
patterns and needle morphology appear to play a larger role than needle gas exchange patterns in determining drought tolerance
in this species.
Keywords:
biomass partitioning, genetic diversity, genotypic selection, needle morphology, preconditioning, shoot/root ratio, stomatal
density.
