PHYSIOLOGICAL EFFECTS OF SALINITY ON FOUR CLONES OF JOJOBA, SIMMONDSIA CHINENSIS (LINK) SCHNEIDER

Abstract

Four clones of jojoba, Simmondsia chinensis (link, Schneider, were used to study the effects of salinity on the growth and physiological processes of jojoba. The vegetative growth as measured by shoot elongation, new branch development, node number, leaf expansion, new leaf production, and defoliation was studied using an iso-molar mixture of NaCl + CaCl₂ at -2, -4, -8, and -16 bars. Physiological processes including chlorophyll concentrations, CO₂ exchange rates, stomatal resistances, leaf water potentials, relative water content, leaf succulence, specific leaf weight, proline accumulation, and protein concentrations were measured at weekly intervals after the addition of salts. Twenty four days after exposure to salinity, plants were transferred to control solution in order to study the reversibility of the salt effects on jojoba. The inhibitory effects of salinity on shoot elongation was evident at -16 bars after 23 days of exposure to salinity. The number of new branches developed during the treatment period did not differ, however, the number of nodes formed on the new branches was less than that of the control. Leaf expansion was inhibited at -4, -8 and -16 bars. Leaf production was significantly reduced at -16 bars and it recovered during stress-release period. Leaf drop (defoliation) increased with increasing salinity. The inhibitory effects of salinity on jojoba's growth was reversible except for leaf expansion. Total chlorophyll concentrations were significantly reduced at 4 salinity levels and chlorophyll synthesis did not recover at -4, -8 and -16 bars during the stress-release period. CO₂ exchange processes including apparent photosynthesis, dark respiration, light respiration, and gross photosynthesis were not significantly affected up to -8 bars. However, at -16 bars, apparent photosynthesis and gross photosynthesis were reduced. Dark respiration was not affected significantly, however, light respiration was reduced at -4, -8, and -16 bars after 16 days of exposure to salinity. This reduction was due to the loss of the post-illumination burst of CO₂. It was concluded that the theory of growth suppression due to accelerated respiration is not true for jojoba. Therefore, the decreased rate of apparent photosynthesis could be attributed to the increased stomatal resistances which increased dramatically at -16 bars. Leaf water potentials were markedly reduced at -16 bars for 23 days which indicated a possible osmotic adjustment and jojoba's ability to tolerate higher salinity levels. Although the leaves produced during the treatment period did not show severe damage symptoms, however, frequent tip burn of the younger leaves at -16 bars indicated that -16 bars was near the salinity limit for jojoba. Leaf relative water content was reduced markedly at -8 and -16 bars which was correlated with leaf water potentials. Leaf succulence based on the ratio of fresh weight/dry weight and specific leaf weight were significantly increased at -16 bars. Increases in specific leaf weight which correlated with succulence, indicates that it was a function of increased leaf succulence and water accumulation. Free proline accumulated in response to salinity. However, there was no significant increase in proline levels until 23 days of exposure to -16 bars. Proline levels rapidly decreased to the control level during the stress-release period. It is believed that the accumulation of proline in jojoba does not function in osmotic regulation because of its low accumulation. Total protein concentrations were found to decrease due to high salinity levels (-16 bars). Although protein synthesis increased during the stress-release period, however, at -16 bars, no significant recovery was evident which indicated that the inhibition of protein synthesis rather than its breakdown was caused by high salt concentrations.