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Betaine Synthesis and Accumulation in Barley During Field Water-Stress¹

Betaine accumulation by water and salt-stressed plants may have adaptive significance. Accordingly, the timing and extent of betaine accumulation by mature leaves of barley (Hordeum vulgare L.) were followed in irrigated (I) and non-irrigated (N-I) plots under rain-shelters. In the N-I crop, leaf water potential (leaf)began to fall at the five-leaf stage, continued to drop steadily until maturity, and reached a minimum of about –35 bars. Betaine accumulation started in the N-I crop about a week after the decline in leaf began and continued until about 10 days post-anthesis. The maximum betaine concentration attained by N-I leaves (100 µmol/g dry wt) was three times that in I leaves.

Betaine accumulation by upper leaves was due mainly to de novo synthesis in these leaves, because: (1) there was little ¹⁴C-import into upper leaves when [¹⁴C]betaine was applied to lower leaves, and (2) attached upper leaves of N-1 plants rapidly converted supplied [¹⁴C]ethanolamine to [¹⁴C]betaine during the peak period of betaine accumulation. Phosphatidylcholine (PC) behaved as an intermediate in the conversion of [¹⁴C]ethanolamine to betaine. The estimated peak metabolic cost of betaine biosynthesis via PC by stressed leaves (about 2 mg hexose/g dry wt/day) approached the cost of protein turnover in the same leaves (3 to 5 mg hexose/g dry wt/day) estimated from [³H] lyslnc incorporation. In N-I plants, cessation of betaine synthesis preceded the onset of senescence by several days, indicating that continuous betaine production is not mandatory for leaf function at lowered leaf.

These field results are consistent with an adaptive value for betaine accumulation in barley during prolonged water stress. A search for genetic variation in bataine-accumulating potential in barley is now warranted.

Key Words: Betaine degradation • Betaine translocation • Flag leaf • Hordeum vulgare • N,N,N,-trimethylglycine • Phosphatidycholine metabolism • Protein turnover

² Research associate, graduate assistant and assistant professor, Michigan State Univ.Dep. of Energy Plant Research Laboratory, Michigan State Univ., East Lansing, MI 48824.

Received for publication March 13, 1981.

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