Mark A. Else

Stomatal Closure in Flooded Tomato Plants Involves Abscisic Acid and a Chemically Unidentified Anti-Transpirant in Xylem Sap

We address the question of how soil flooding closes stomata of tomato (Lycopersicon esculentum Mill. cv Ailsa Craig) plants within a few hours in the absence of leaf water deficits. Three hypotheses to explain this were tested, namely that (a) flooding increases abscisic acid (ABA) export in xylem sap from roots, (b) flooding increases ABA synthesis and export from older to younger leaves, and (c) flooding promotes accumulation of ABA within foliage because of reduced export. Hypothesis a was rejected because delivery of ABA from flooded roots in xylem sap decreased. Hypothesis b was rejected because older leaves neither supplied younger leaves with ABA nor influenced their stomata. Limited support was obtained for hypothesis c. Heat girdling of petioles inhibited phloem export and mimicked flooding by decreasing export of [14C]sucrose, increasing bulk ABA, and closing stomata without leaf water deficits. However, in flooded plants bulk leaf ABA did not increase until after stomata began to close. Later, ABA declined, even though stomata remained closed. Commelina communis L. epidermal strip bioassays showed that xylem sap from roots of flooded tomato plants contained an unknown factor that promoted stomatal closure, but it was not ABA. This may be a root-sourced positive message that closes stomata in flooded tomato plants.

Export of Abscisic Acid, 1-Aminocyclopropane-1-Carboxylic Acid, Phosphate, and Nitrate from Roots to Shoots of Flooded Tomato Plants (Accounting for Effects of Xylem Sap Flow Rate on Concentration and Delivery)

We determined whether root stress alters the output of physiologically active messages passing from roots to shoots in the transpiration stream. Concentrations were not good measures of output. This was because changes in volume flow of xylem sap caused either by sampling procedures or by effects of root stress on rates of whole-plant transpiration modified concentrations simply by dilution. Thus, delivery rate (concentration x sap flow rate) was preferred to concentration as a measure of solute output from roots. To demonstrate these points, 1-aminocyclopropane-1-carboxylic acid (ACC), abscisic acid, phosphate, nitrate, and pH were measured in xylem sap of flooded and well-drained tomato (Lycopersicon esculentum Mill., cv Ailsa Craig) plants expressed at various rates from pressurized detopped roots. Concentrations decreased as sap flow rates were increased. However, dilution of solutes was often less than proportional to flow, especially in flooded plants. Thus, sap flowing through detopped roots at whole-plant transpiration rates was used to estimate solute delivery rates in intact plants. On this basis, delivery of ACC from roots to shoots was 3.1-fold greater in plants flooded for 24 h than in well-drained plants, and delivery of phosphate was 2.3-fold greater. Delivery rates of abscisic acid and nitrate in flooded plants were only 11 and 7%, respectively, of those in well-drained plants.

Contributions of roots and rootstocks to sustainable, intensified crop production

Sustainable intensification is seen as the main route for meeting the worlds increasing demands for food and fibre. As demands mount for greater efficiency in the use of resources to achieve this goal, so the focus on roots and rootstocks and their role in acquiring water and nutrients, and overcoming pests and pathogens, is increasing. The purpose of this review is to explore some of the ways in which understanding root systems and their interactions with soils could contribute to the development of more sustainable systems of intensive production. Physical interactions with soil particles limit root growth if soils are dense, but root-soil contact is essential for optimal growth and uptake of water and nutrients. X-ray microtomography demonstrated that maize roots elongated more rapidly with increasing root-soil contact, as long as mechanical impedance was not limiting root elongation, while lupin was less sensitive to changes in root-soil contact. In addition to selecting for root architecture and rhizosphere properties, the growth of many plants in cultivated systems is profoundly affected by selection of an appropriate rootstock. Several mechanisms for scion control by rootstocks have been suggested, but the causal signals are still uncertain and may differ between crop species. Linkage map locations for quantitative trait loci for disease resistance and other traits of interest in rootstock breeding are becoming available. Designing root systems and rootstocks for specific environments is becoming a feasible target.

Ionic and pH signalling from roots to shoots of flooded tomato plants in relation to stomatal closure

Soil flooding damages shoot systems by inhibiting root functioning. An example is the inhibition of water uptake brought about by decreased root hydraulic conductance. The extent of any resulting foliar dehydration this causes is limited by partial stomatal closure that begins within 4 h and is maintained for several days. Root to shoot signals that promote closure in flooded tomato plants have remained elusive but may include changes in solute delivery to the shoot by transpiration. Accordingly, we examined total osmolites and selected mineral ions in samples of xylem sap flowing at rates approximating whole plant transpiration. After 2.5 h flooding,delivery of total osmolites and of PO43-SO42-Ca2+K+NO3− and H+strongly decreased while Na+ remained excluded. Several hours later, deliveries of osmolites, PO43-, SO42-, Ca2+, and Na+ rose above control values, suggesting that, after approximately 10 h, root integrity became degraded and solute uptake de-regulated. Deliveries of NO3− remained below control values. Reducing or eliminating the supply of K+ to detached leaves to test the potential of decreased K+ delivery to close stomata proved negative. Decrease in H+ delivery was associated with sap alkalisation. However, raising the pH of buffer from 6.0 or 6.5 to 7.0 did not close stomata when tested in the presence of abscisic acid (ABA) at a concentration (10 μmol m−3) typical of the transpiration stream of flooded plants. It is concluded that despite their rapidity and scale, negative messages in the form of increased pH and decreased solute delivery from roots to shoots are, themselves, unlikely initiators of stomatal closure in flooded tomato plants.

Expression of Fragaria vesca PIP aquaporins in response to drought stress: PIP down-regulation correlates with the decline in substrate moisture content.

PIP aquaporin responses to drought stress can vary considerably depending on the isoform, tissue, species or level of stress; however, a general down-regulation of these genes is thought to help reduce water loss and prevent backflow of water to the drying soil. It has been suggested therefore, that it may be necessary for the plant to limit aquaporin production during drought stress, but it is unknown whether aquaporin down-regulation is gradual or triggered by a particular intensity of the stress. In this study, ten Fragaria PIP genes were identified from the woodland strawberry (Fragaria vesca L.) genome sequence and characterised at the sequence level. The water relations of F. vesca were investigated and the effect of different intensities of drought stress on the expression of four PIP genes, as well as how drought stress influences their diurnal transcription was determined. PIP down-regulation in the root corresponded to the level of drought stress. Moreover, transcript abundance of two genes highly expressed in the root (FvPIP1;1 and FvPIP2;1) was strongly correlated to the decline in substrate moisture content. The amplitude of diurnal aquaporin expression in the leaves was down-regulated by drought without altering the pattern, but showing an intensity-dependent effect. The results show that transcription of PIP aquaporins can be fine-tuned with the environment in response to declining water availability.

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.

Determination of 1-aminocyclopropane-1-carboxylic acid (ACC) in leaf tissue and xylem sap using capillary column gas chromatography and a nitrogen/phosphorus detector

The Lizada and Yang method, commonly used for analyzing 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of the plant hormone ethylene, is subject to interference and lacks internal standards. The use of combined gas chromatography-mass spectrometry (GC-MS) overcomes these shortcomings but the method is expensive and unavailable to many laboratories. We describe an alternative physico-chemical method using a capillary column gas chromatograph fitted with a standard nitrogen/phosphorus detector. After forming the N-benzoyl n-propyl derivative, measurements of ACC concentrations in extracts of leaves and in xylem sap of tomato plants using the nitrogen/phosphorus detector were within 10% of those obtained by GC-MS. Concentrations in plants grown in well-drained soil were approximately 0.16 nmol g−1 fresh weight (leaves) and 0.04–0.01 mmol m−3 (sap). Flooding the soil for 48–72 h increased these values approximately 9-fold.

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.

Hormones and root-shoot relationships in flooded plants: an analysis of methods and results

Two aspects of root to shoot communication in flooded plants are discussed (i) the formation of porous aerenchyma that enhances the passage of oxygen, and other gases, from shoots to roots and (ii) the movement of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) from roots to shoots in the transpiration stream, and the effect of this on ethylene production and epinastic curvature in the shoots. For aerenchyma studies a highly sensitive photoacoustic laser detector for ethylene was used to avoid interference associated with other methods of ethylene measurement that require tissue excision. ACC concentrations in xylem sap were measured by physico-chemical means to ensure correct identification and account for processing losses. Solute concentrations, e.g., abscisic acid (ABA), in xylem sap are shown to be distorted by temporary contamination caused by the method used to collect sap. Concentrations of solutes in xylem sap (e.g., ACC) are also altered by changes in sap flow brought about by conventional methods of sap collection or by experimental treatments such as flooding the soil. Ways of for overcoming these problems are described together with a summary of preliminary results.

Regulation of sweet cherry fruit abscission: the role of photo-assimilation, sugars and abscisic acid

Summary Premature late-season fruit abscission is a major concern for UK sweet cherry growers, but its causes are not understood. Competition for resources such as photo-assimilates and plant hormones may trigger fruit abscission. We therefore examined the roles of photo-assimilation, sugars and abscisic acid (ABA) in the regulation of late-season fruit abscission. The experimental material consisted of phloem-girdled short shoots (‘spurs’) with two distinct ranges of leaf area to fruit number (LA:FN) ratios. These ratios were achieved by selecting spurs with similar leaf areas, but different fruit numbers. Our previous research showed that fruit rapidly abscise from spurs with a low LA:FN ratio (within 14 – 21 d after girdling; DAG), while fruit on spurs with a high ratio are typically retained. Photo-assimilation of leaves was measured over 15 DAG. Fruits were collected for the determination of sorbitol, ABA and 1-aminocyclopropane-1-carboxylic acid concentrations 0, 1, 3, 5, 7 and 9 DAG. After girdling, photo-assimilation of leaves from spurs with a low LA:FN ratio was greater than that of leaves from spurs with a high LA:FN ratio. However, fruit on spurs with a low LA:FN ratio potentially received three-fold less photo-assimilate per fruit compared to fruit on spurs with a high LA:FN ratio. Before the symptoms of abscission appeared, sorbitol concentrations in fruit were 30% lower on spurs with low LA:FN compared to fruit on spurs with a high ratio. As the symptoms of abscission developed for fruit on spurs with a low LA:FN ratio, fruit ABA concentrations increased as fruit sorbitol concentrations declined. Experiments to determine if the exogenous application of ABA induced fruit abscission showed a rapid induction of fruit loss. These results support the hypothesis that source limitation, through the supply of photo-assimilates, can cause late-season fruit abscission in sweet cherry. However, the delayed rise detected in fruit ABA concentration appears more consequential than abscission-related.