Oral Session #4: Plant Ecology: Gas Exchange. Presiding: J. Pereira.
Monday, August 6, 2001. 8:00 AM to 12:00 PM. Hall of Ideas P&Q.

Physiological and developmental responses to drought in Cryptantha flava are size specific.

Forseth, Irwin1, Wait, D.2, Casper, Brenda3, 1 2 3

ABSTRACT- Large individuals of the herbaceous desert perennial, Cryptantha flava, respond to drought by shrinking in size, while small plants of the same age continue to grow. This rearranges size hierarchies and reproductive contributions within a cohort. These results led us to set up a water manipulation experiment to test the hypothesis that large plants are less water use efficient, enabling them to grow more quickly in wet years but making them less tolerant of drought. In 1998 and 1999, we erected rainout shelters in a natural population and examined demographic and physiological responses simultaneously. Under drought, small plants maintained higher stomatal conductances than large plants (F1,462=8.05; p<0.01). Photosynthetic response did not vary with plant size, thus larger plants had a HIGHER water use efficiency, indicated by both instantaneous gas exchange (F1,462=9.19; p<0.01) and 13C/12C isotope ratios (repeated measures ANOVA; F1,5=522.6, p<0.01). Large plants droughted in 1998 continued to show depressed conductance in 1999, although photosynthesis recovered to pre-drought levels. Large plants also committed a greater proportion of rosette meristems to flower production the year after exposure, a response not seen in small plants. Thus, both water use efficiency and reproductive effort were increased in large plants by exposure to drought in previous growing seasons. Based on these data, we believe larger plants have greater difficulty meeting their water requirements under drought, perhaps due to constraints imposed by hydraulic architecture.

KEY WORDS: Cryptantha flava, drought, water use efficiency, demography