C. J. Atkinson

Climate change impacts on UK top and soft fruit production.

The impacts of climate change for UK fruit growing regions requiring winter chill dormancy are discussed and insights are drawn from subtropical regions where chilling requirements for these perennial crops have been overcome. A further challenge facing UK soft fruit growers is water availability for irrigation. To maintain future productivity, more sustainable production systems are needed. The authors discuss recent advances in irrigation scheduling and deficit irrigation techniques, along with their potential to reduce water inputs while maintaining yields of high-quality, healthy berries.

INTERACTIONS OF CALCIUM WITH ABSCISIC ACID IN THE CONTROL OF STOMATAL APERTURE

Summary It has been recognised for many years that stomatal aperture is influenced by calcium ions. We have recently discovered that the action of abscisic acid on guard cells is dependent upon calcium. In this paper we discuss the interactions of abscisic acid with calcium ions, and in particular assess the contribution which extracellular and intracellular calcium may make to the control of stomatal aperture.

The effects of phloem girdling on the abscission of Prunus avium L. fruits

Summary Excessive premature abscission of developing fruitlets in UK cherry orchards often results in low fruit yields. An improvement in our understanding of the underlying causes of embryo abortion and fruitlet abscission will help rationalize effective remedies to this problem. The aim of this study was to determine the effects of limiting the availability of leaf-derived assimilates, during critical stages of fruitlet development, on the severity of fruitlet abscission. Experimentally, this was achieved by isolating individual “spur units” (short shoots (<10 cm) with leaves and fruit) from the tree by girdling (severing the phloem connections) branches on either side of the unit. In this way, the developing fruitlets within the associated spur would be able to derive their assimilates only from the associated spur leaves. Spur units with different total leaf areas and variable numbers of developing fruitlets were chosen to achieve a wide range of potential source and sink strengths. The spurs analysed varied in leaf number from 4–9 leaves as spur leaf area increased. The largest variability in the spur leaf area number relationship occurred in spurs with 6–7 leaves. When initially determining the total leaf area per spur in May, there was no obvious relationship with fruit number per spur. Subsequent analysis of the relationship between spur leaf area and fruit number per cluster showed that fruit had been lost from spurs with the smallest leaf areas. Spurs girdled later in the season in June also showed no obvious relationship between spur leaf area and fruit number. As with spurs girdled in May, those manipulated in June lost fruit from spurs with small leaf areas. By July, there was a positive curvilinear relationship between spur leaf area and fruit number for girdled spurs. Neither total nor average fruit fresh weight per spur, at harvest, could be related to spur leaf area. The average individual fresh weight of fruit in a spur was, however, limited by the number of fruit within that spur. When spurs were girdled, fruit loss was shown to take place preferentially where the spur leaf area per fruit was low. From this analysis, it was possible to predict which girdled spurs would lose fruit, using the calculated ratio of spur leaf area per fruit. It is concluded that fruit retention, not size, appears to be limited by the availability of leaf-derived assimilates.

OZONE, SULPHUR DIOXIDE AND NITROGEN OXIDES: SOME EFFECTS ON THE WATER RELATIONS OF HERBACEOUS PLANTS AND TREES

O3, SO2 and NO2 have all been shown to exert effects on plants which are likely to influence water relations. In this paper a review of evidence (often confusing) from the literature is presented, followed by a description of two phenomena which we have recently studied in detail. It has been known for some time that plants exposed to SO2, or to mixtures of SO2 and NO2, often display differences in stomatal behaviour from those in clean air. We have grown spring barley in SO2 and NO2 (24–35 ppb of each gas) and examined the ability of leaves to regulate water loss. The stomata were found to be much less responsive to ABA after exposure. In other respects the stomata of polluted plants appeared to function normally (e.g., they showed similar responses to CO2). Accumulation of ABA in water-stressed leaves appeared to be unaffected by the pollution treatment. We have concluded that the impairment of stomatal sensitivity to ABA may adversely affect the water relations of polluted plants. In other experiments, young trees of Fagus sylvatica (beech) were exposed to realistic doses of ozone in summer. In well-watered trees ozone inhibited stomatal opening, but in trees growing in dry soil, and thus experiencing water stress, stomatal closure was significantly inhibited by ozone. We suggest that ozone may have two deleterious effects on beech: inhibition of CO2 acquisition when water supplies are adequate, and interference with the protective role of stomatal closure during periods of water stress.

Interactions of SO2 with other environmental stresses in influencing leaf gas exchange

SummaryLeaves of two field growing co-occuring perennial shrubs (drought-deciduous Diplacus aurantiacus and the evergreen Heteromeles arbutifolia) from the Californian chaparral were exposed to small doses of SO2. During this exposure the leaf environment was manipulated to determine how the presence of SO2 alters the response of gas exchange to other environmental stresses. The data show that no direct changes in stomatal conductance (g) or net assimilation rate (A) could be attributed to short-term (7 h) SO2 (4.2 μmol m-3, 0.1 μl l-1) exposure. D. aurantiacus leaves possessed features which demonstrate that they were sensitive to changes in environment e.g. light flux and atmospheric relative humidity. The interspecific differences in stomatal sensitivity to water vapour were extremely important, as relative humidity is a major factor influencing carbon fixation and the rate of pollutant absorption. Conditions of high relative humidity and high xylem water potentials are suggested to pre-dispose leaves of D. aurantiacus to greater pollutant doses than the more stomatally-conservative evergreen, H. arbutifolia. In the presence of SO2 there was some indication of increased g for both D. aurantiacus and H. arbutifolia as ΔW became smaller. This SO2-effect was only obvious as increasing atmospheric humidity induced further stomatal opening. The important consequences of an SO2 enhanced g, were a reduction in WUE, which may cause earlier leaf abscission and a concomitant decline in productivity.