In glasshouse experiments, high temperatures increased vegetative growth in the tropical rambutan (Nephelium lappaceum L. seedlings of cv. Rapiah). In absolute terms, there was generally a greater response to night temperature (day/night temperatures of 32 degrees/14 degrees C vs. 32 degrees/28 degrees C) than to day temperature (22 degrees/14 degrees C vs. 32 degrees/14 degrees C), and better growth at 32 degrees/28 degrees C than at 32 degrees/22 degrees C. The effects of temperature on nutrient concentrations were small, whereas average leaf water potential (Psi(L)) during the day was -1.5 MPa at 32 degrees/28 degrees C, 0.1 to 0.4 MPa higher with nights of 14 degrees or 22 degrees C, and 0.7 MPa higher at 22 degrees/14 degrees C. Average net CO2 assimilation rate (A) was 5.3 mu mol m(-2)s(-1) at 32 degrees/22 degrees and 32 degrees/28 degrees C, 50% lower at 32 degrees/14 degrees C and 80% lower at 22 degrees/14 degrees C. When seedlings of cv. Rapiah were grown in water baths, the two critical root temperatures at which 90% of maximum plant weight occurred were similar for leaves and stems (about 24 degrees to 30 degrees C), but lower for roots (19 degrees to 25 degrees C). There was only a small effect of temperature on the concentration of most nutrients,and on average day time Psi(L), whereas average A was 4.5 mu mol m(-2)s(-1) at 28 degrees C, 30% lower at 20 degrees C, and 80% lower at 150 and 38 degrees C. These experiments show that rambutan growth was strongly reduced when night or root temperatures fell to 14 degrees or 15 degrees C. The main effect was through lower CO2 assimilation, whereas the changes in water and nutrient status were relatively small. Areas where night or root temperatures remain at or below 14 degrees C for prolonged periods should be avoided for rambutan production.
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