Jojoba: Temperature Adaptation as Expressed in Growth and Leaf Function

Abstract

The adaptation of jojoba [Simmondsia chinensis (Link) Schneider] to temperature was studied under controlled conditions. Shoot extension and leaf area development reflected the very low rate of growth of this species, even under favourable conditions, and were stable with an increase in temperature from 20 to 30°C. However growth was markedly reduced at temperatures below 20°C and at 6°C there was no net gain in dry weight over a 42 day period. Root: shoot ratios were near unity and showed a small drop in response to increasing temperature. Leaves adapted to low temperature by an increase in thickness, specific leaf weight and starch content. Chlorophyll formation was retarded in young leaves developing at 15/10°C, but there was no sign of photodestruction of previously formed chlorophyll in mature leaves. Young leaves developing at 30/25°C had a very high chlorophyll a/b ratio of 9.5, but otherwise leaf chlorophyll was apparently normal (2.3-3.4) over a wide range of temperatures.

Light saturation of net CO₂ exchange (NCE) occurred at about 1000 µE m^-2 s^-1 for leaves grown over a wide range of temperatures and the maximum NCE of approximately 16 mg CO₂ dm^-2 h^-1 (0.45 mg m^-2 s^-1) occurred between 19 and 25°C. Pulse labelling with ¹⁴CO₂ indicated that low temperature (18°C) reduced the rate of transfer of ₁₄C from the primary products of fixation to sucrose. The rate of movement of ₁₄C-labelled photosynthate out of the leaf was negligible at 18°C, and reached only about 3% h^-1 at 30°C. In the stems, shortly after ¹⁴CO₂ uptake by the leaf, 86% of the ¹⁴C activity was in sucrose, indicating that this was the preferred form of translocate in the vascular system. However glucose was more abundant in the leaves than sucrose, particularly at low temperatures. Starch accumulated in the leaves at low temperatures, reaching nearly 30% of the dry weight at 18/13°C.

Photosynthetic stability rather than active adaptation appears to form the basis of resistance to temperature stress in jojoba. With low rates even under optimal conditions this is essentially one of adaptation for survival rather than adaptation for production.