Dynamics of leaf water relations components in co-occurring iso- and anisohydric conifer species

Background/Question/Methods

There is considerable interest in determining if iso- versus anisohydric species are more likely to die during severe droughts. Two coniferous species, the anisohydric Juniperus monosperma and isohydric Pinus edulis, co-occur over large areas of the semiarid southwestern USA. In J. monosperma leaf tissue produced under relatively mesic conditions must cope with large seasonal variations in water potential (Ψ) after it is fully formed and mature. In contrast, more stringent stomatal regulation in P. edulis strongly constrains seasonal variation in plant Ψ. Based on their contrasting modes of regulation of plant Ψ, we hypothesized that leaf water relations components would exhibit greater seasonal and shorter term plasticity in the anisohydric J. monosperma than in the co-occurring isohydric P. edulis. To address this hypothesis, we conducted pressure-volume analyses on non-rehydrated and rehydrated shoots of both species collected over a 16-month period during which shoot Ψ of J. monosperma varied from ~ -1 to -6 MPa and that of P. edulis from ~ -1 to -3 MPa. Although many studies have applied the pressure-volume technique to characterize leaf water relations components of species representing a range of drought tolerance, few have explicitly compared species representing different points along the isohydry/anisohydry continuum.

Results/Conclusions

Seasonal courses of midday shoot Ψ were consistent with anisohydric behavior in J. monosperma and isohydric behavior in P. edulis. Midday shoot Ψ varied seasonally by about 3.6 MPa in J. monosperma and only 0.9 MPa in P. edulis. Consistent with our hypothesis, the shoot water potential at turgor loss (Ψ_TLP) in J. monosperma ranged from a mean of -3.4 MPa in artificially rehydrated shoots to -6.6 MPa in shoots with an initial Ψ of -5.5 MPa, whereas in P. edulis mean Ψ_TLP remained at ~ -3.0 MPa over a range of initial Ψ from -0.1 to -2.3 MPa. The shoot osmotic potential at full turgor and the bulk modulus of elasticity also declined sharply with shoot Ψ in J. monosperma, but not in P. edulis. The contrasting behavior of shoot water relations components in J. monosperma and P. edulis may be associated with their contrasting modes of regulation of plant water status and may be representative of anisohydric and isohydric species in general. Plasticity of shoot water relations components in J. monosperma may contribute to its greater capacity to withstand severe drought.