Relationships between mineral nutrition, drought resistance and clone in Populus
Date
2018-07-20
Authors
Harvey, Helen Penelope Penny
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Abstract
Effects of mineral nutrition on drought and cavitation resistance of poplars were
examined in two sets of greenhouse-grown trees. First, two drought-sensitive and two
drought-resistant hybrid clones of black cottonwood (Populus trichocarpa Torr. & Gray)
and eastern cottonwood (P. deltoides Bartr.) were grown at three concentrations of
nitrogen (N) applied factorially with two concentrations of phosphorus (P) in a sub-irrigation
sand-culture system. The trees were subjected to 0, 4, 6, and 8 days of gradual
drought stress before measurements of cavitation, anatomical features affecting
cavitation, and nutrient mobilization during drought. High foliar concentrations of N
increased cavitation compared to barely adequate concentrations, whereas high
concentrations o f P decreased cavitation as measured by both hydraulic flow apparatus
and dye perfusion techniques. For one test, cavitation was 48% at high N and low P, but
only 28% at high N and high P. Vessel pit membrane mean pore diameters were 0.132
μm at low P and 0.074 μm at high P: smaller pores would decrease air-seeding cavitation.
No other significant effects of mineral nutrition on vessel dimensions were observed.
Scanning election microscopy showed less damage to pit membranes, suggesting greater
membrane strength in drought-resistant clones than in drought-sensitive clones.
In the second experiment, three drought-resistant and three drought-sensitive poplar
clones (including P. trichocarpa) were grown at two levels of N and three levels of
potassium (K) and either well-watered, cyclically droughted, or droughted once.
Cavitation, osmotic potential, gas exchange, and nutrient mobilization were measured at
each stage of drought and re watering, and fall nutrient retranslocation was monitored.
Cavitation was greater with adequate foliar N than at deficiency levels. Moderate
supplies of K increased cavitation, but luxury levels sometimes reduced cavitation by
decreasing foliar water loss and thus xylem tension. Preconditioning did not reduce
vulnerability to cavitation, but there was some evidence of cavitation reversal in a
drought resistant clone at high N supply. Vessel diameters were 36.6 μm at low N but
45.2 μm at high N, so within Populus, larger diameter vessels correlated with
susceptibility to cavitation.
High N supply increased water stress during the first drought, but also increased
instantaneous water use efficiency (WUE) before drought occurred, and osmotic
adjustment and hardening after drought. Increased K also increased WUE before drought
and decreased water stress (decreasing transpiration and wilting) at luxury levels, but did
not influence osmotic adjustment or hardening.
Mobilization of nutrients differed with speed and intensity of drought. Gradual
drought led to resorption of N and P. In the second experiment, drought was too rapid for
retranslocation. Nutrients became more concentrated; some (e.g., N) facilitated hardening
and osmotic adjustment, and some (e.g., K) moved out of the leaves on re watering. In
fall, N, P, Cu and K were resorbed, the latter more proficiently with greater N supply at
low levels of K.
Clones which were more productive on dry sites resisted severe, but not moderate
cavitation. Cavitation-resistant clones maintained high transpiration rates (and less
negative water potentials) in drought, especially after hardening, had more, but smaller,
stomata and decreased leaf loss in drought, but did not have increased WUE or osmotic
adjustment.
Nitrogen fertilization increased cavitation, greater P supply reduced this effect, and K
fertilization may make vessels more vulnerable to cavitation but decrease the tension on
the xylem that causes cavitation. Nitrogen fertilization levels should be tailored to site
water supplies, and appropriate P, and possibly K additions may increase drought
resistance.
Description
Keywords
Poplar, Growth, Drought resistance, Mineral nutrition