Polyribosome Content in Young and Aged Wheat Leaves Subjected to Drought

The effect of a draughting treatment on the polyribosome content of wheat leaves was markedly different on leaves of different ages. Growing leaves showed a large decrease in their polyribosome population, while fully expanded leaves showed no loss of polyribosomes compared with controls. These results suggest that the large reductions in polyribosome population observed in growing tissues are due to reductions in growth, rather than a direct effect of water stress. INTRODUCTION Rapid decreases in the proportion of ribosomes organized on polyribosomes occur in young expanding tissue of many higher plants subjected to a sudden but moderate water stress (Hsiao, 1970; Morilla, Boyer, and Hageman, 1973; Rhodes and Matsuda, 1976) suggesting that polyribosome populations may be extremely sensitive to water stress. Decreases in polyribosome contents from 20 to 80% have been reported in these and in other investigations (Brandie, Schnare, Hickley, and Brown, 1973; Barlow, Munns, Scott, and Reisner, 1977) which varied in the extent and duration of the stress, the plant species, the plant organ, and the physiological age of the plant organ sampled. These variations have led to speculation as to whether polyribosomes in some species (Rhodes and Matsuda, 1976; or Brandie et al., 1973) and some plant organs (Barlow et al., 1977) are more resistant to drought. The influence of physiological age of a tissue on the response of its poly ribosome population to water stress has not been investigation. An alternative interpretation of the results quoted above is that the decrease in polyribosome content during water stress is a result of reduced growth (Hsiao, 1970) and is, therefore, only indirectly related to decreasing water potential. As no parallel studies on growing versus non-growing tissues have been reported previously, we tested Hsiao's hypothesis using both elongating and fully expanded leaves from the same plant during a water stress period. We conclude there is an interaction between the physiological age and position of a leaf and its response to water stress. 1 Plant Physiology Unit, CSIRO Division of Food Research, Macquarie University, North Ryde, N.S. W. , 2113. 2 CSIRO Division of Horticultural Research, Glen Osmond, S.A. , 5064. 3 School of Biological Sciences, Macquarie University, North Ryde, N.S. W. , 2113. 4 Department of Agronomy, The University of Western Australia, Nedlands, W.A. , 6009. This content downloaded from 157.55. 39.208 on Wed, 28 Sep 2016 04:51:29 UTC All use subject to http://about.jstor. org/terms 906 Scott, Munns, and Barlow—Leaf Polyribosomes and Drought METHODS Seedlings of Triticum aestivum L. cv. Heron were grown in potting mix in a growth chamber as described in a previous paper (Barlow et al., 1977). The pots were watered daily with half strength Hoagland nutrient solution and plants were droughted by withholding water from randomized pots 24 d after germination (day 0 of the experimental period. At this stage, leaf 4 was fully expanded and mature; Leaf 5 was approximately one-half its final size; leaf 6, although elongating, had not emerged from the sheath of leaf 5 and was about 60 mm long. The apex was about 1 mm long, with a large proportion of the cells dividing. The water potential of exposed leaves was measured with a pressure bomb calibrated against a thermocouple psychrometer (Wescor J-52 sample chamber). The water potential of enclosed leaves and apices was measured with the same psychrometer. All tissues for polyribosome determinations were frozen in liquid nitrogen within 30 s of harvest, the extraction and fractionation of polyribosomes and the measurement of the proportion of chloro plast and cytoplasmic ribosomes in leaves has been described (Brady and Scott, 1977). The extraction and measurement of polyribosomes from wheat apices has been described in a previous paper (Barlow et al., 1977).