The influence of temperature on the oil content and composition of sunflower was studied on plants grown under field conditions and in a range of controlled environments. Traces of oil were detectable in cypsela (seed) almost immediately after pollination. Much of this appeared to be present in the hull (pericarp), which is well developed at this stage. Significant production of oil commenced with the development of the embryo about 150 day-degrees after pollination, and the oil content reached a maximum value just prior to physiological maturity of the seed. Linoleic acid constituted the major component of the oil at all stages of seed development, and under favourable temperature conditions increased from c. 50% soon after pollination to over 70% at physiological maturity. High temperature during the development of the seed was associated with a reduction in total oil yield. However, under field conditions this effect was variable owing to confounding with other environmental factors such as moisture stress, which also influence the yield of oil through their effects on growth and development of seed. Elevated temperatures, and in particularly high night temperatures, caused a marked reduction in the percentage of linoleic acid, apparently due to the effect of temperature on the activity of the desaturase enzymes which are responsible for the conversion of oleic to linoleic acid.These results support the hypothesis that reduced yields and altered composition of sunflower oil from crops matured under high temperature conditions in midsummer are due to the effects of heat stress on the biosynthesis of fatty acids.

The effects of light and water stress upon chilling injury of chloroplasts have been assessed by electron tnicroscopy in seedlings of three species known to differ in their chilling susceptibility. Chilling injury to chloroplasts was first manifested by distortion and swelling of thylakoids, reduction in starch granule size, and the fortnation of stnall vesicles of the envelope, called the peripheral reticulum. More prolonged treatment produced accumulations of lipid droplets, increased staining of the stroma, disintegration of the envelope, and mixing with cytoplastnic contents. Cotton, a notably chilling-sensitive plant, and bush bean, a somewhat less sensitive plant, showed damage within 6 h when exposed to both light and water sttess at chilling temperatures (5 C). Even collard, a chilling-resistant species, exhibited signs of chilling itijury to chloroplasts after 6 h when exposed to both light and water stress but the plastids retnained intact throughout the 48 h ol^ treattnent. Cotnparable chilling injury does not occur in cotton until around 72 h if the plants are exposed to water stress or light separately. Bush bean was affected less by separate treatments of light and water stress. The least chilling injury occurred in all three species when they were kept in the dark at a high hutnidity.

This review examines the impact of drought and salinity on agricultural production in both the semiarid and humid regions of the world. The frequency and nature of drought events and some recent approaches to drought prediction are discussed along with the distribution of salinity and the various factors leading to its occurrence under both irrigated and dryland conditions. Strategies to minimize the impact of drought and salinity on the rural community and on the stability of land resources have been developed largely in high-income countries. However, because of the high cost, little has been done in the developing world where extensive areas of valuable land are deteriorating, representing a serious threat to food security.

Cotton (Gossypium hirsutum) and bean (Phaseolus vulgaris) seedlings wilted when entire plants or roots alone were chilled in the light. The water relations and gas exchange characteristics of these chilling-sensitive species have been compared with a chilling-resistant species, collard (Brassica oleracea), following exposure to a chilling temperature at 5°C. Chilling either whole seedlings or roots alone had little or no effect on leaf water potentials or gas exchange of collard seedlings but induced a rapid response in cotton and bean. Leaf water potentials were reduced below - 1.5 MPa within 1-2 h; however, this response was reduced when seedlings were chilled in the dark or when leaves or roots of bean were pretreated with abscisic acid. Chilling also caused a rapid reduction in photosynthesis but a more gradual decline in transpiration over the first 2-4 h due to the slow closure of stomata. These results suggest that water deficits in chilling-sensitive seedlings when exposed to low temperature in the light are caused by a reduction in the hydraulic conductivity of the root membranes and by the loss of stomatal control in the leaves. The reduced flow of water into the plant through the roots and continued transpiration, because of the slow closure of stomata, bring about a rapid decline in leaf water potential and wilting of primary leaves.

An important factor in the survival of Phalaris tuberosa, a typical Mediterranean perennial grass, is the ability of its deep root system to supply water during the summer to the dormant culms at the soil surface. This behaviour contrasts with that of the related annual P. minor, which is unable to exploit subsoil moisture, and dies as soon as the surface moisture is exhausted. The volume of water supplied by the perennial roots is sufficient to offset transpiration losses and maintain a favourable water balance in the dormant culms during the summer stress. The importance of this water source for survival is indicated by the death of plants whenever this supply is interrupted by severing deep roots. In field soils under drought conditions roots of the perennial have been followed to a depth of 7 ft in subsoil containing available moisture. The large metaxylem vessels and heavily suberized endodermis which are a feature of these roots suggest that they are well adapted to transport water up through the dry surface soil to the base of the dormant culms. The culms also show typical xerophytic characteristics which help to minimize water loss during the summer, and maintain favourable conditions for the survival of the dormant buds which develop at the basal nodes. These conclusions concerning the survival of the perennial have been drawn from plants growing under natural conditions, and also from more detailed studies under controlled environments. They appear to be of general significance for the perennial grasses adapted to the drier Mediterranean environments and form the basis of the perennial response found in this group.