Influence of drought stress and low irradiance on plant water relations and structural constituents in needles of Pinus ponderosa seedlings

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Abstract
The influence of low light on tolerance to prolonged drought was tested on unshaded and shaded seedlings of ponderosa pine (Pinus ponderosa var. scopulorum Dougl. ex Laws.). Unshaded seedlings of P. ponderosa var. ponderosa were also drought stressed to compare varietal responses to drought. The maximum irradiance received by shaded seedlings was 10% of full light. Seedlings were progressively drought stressed until predawn water potentials (Ψx) were –5.0 MPa. Relative water content (RWC) and the reciprocal of Ψx were analyzed by means of an unusual application of the pressure–volume relationship for determination of RWC of the apoplast (RWCa), osmotic potential at full turgor (Ψoft), and ratio of fully turgid weight to dry weight. Major varietal differences in drought response were in RWCa and needle cellulose content. The shaded seedlings showed tissue damage at relative water contents < 60%, and were killed by water deficits from which unshaded seedlings recovered. Correspondingly, shaded plants had significantly higher cell volume/cell mass ratio, Ψoft, less cellulose in needle tissue, and lower RWCa than unshaded plants. These differences suggest that low irradiance restricts drought adaptation in ponderosa pine.

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