SAINFOIN REGROWTH DECLINES AS METABOLIC-RATE INCREASES WITH TEMPERATURE

Sainfoin (Onobrychis viciifolia Scop.) often exhibits poor growth foll owing periods of high ambient temperatures. Our objective was to inves tigate the influence of air and soil temperature on regrowth and persi stence of sainfoin. At 99 d post planting, 'Renumex' sainfoin grown in 5- by 40-cm tubes were placed into growth chambers. Treatments were c hamber air temperatures of 15, 25, or 35 degrees C with soil temperatu res of 10, 20, or 30 degrees C. After a 21-d adjustment period, all pl ants were clipped to 2 cm (Harvest 1). Plants regrew for 35 d before t he final harvest (Harvest 2). Measurements were number of leaves, leaf area, shoot mass, taproot mass, fine root mass, crown mass, taproot c arbohydrates, starch degrading enzyme activity, whole plant metabolism , and plant survival. Leaves per plant decreased with increasing air t emperature and this caused a decrease in sainfoin leaf area per plant and shoot yield per plant. Increasing air temperature from 15 to 35 de grees C or soil temperature from 10 to 30 degrees C decreased crown, t aproot, and fine root mass by as much as 98%. Concentration of taproot carbohydrates showed a negative correlation (P < 0.05) with increasin g air temperature. Metabolic Fates of leaves and roots were two and fi ve times higher, respectively, when the air or soil temperature increa sed by 20 degrees C. Increasing air temperature showed a positive corr elation (P < 0.05) with the number of dead plants per treatment (R(2) = 0.90). Few plants grown at 35 degrees C survived. Severe defoliation during periods of high temperatures may cause plant death because hig h metabolic rates cannot be supported by existing leaf area or taproot carbohydrates.