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Thesis (PhD(Agric) (Horticulture))--Stellenbosch University, 2008. / It is a common practise in the Western Cape to use micro sprinklers as the standard irrigation system for apple trees. Over the past forty years much effort has been put into the optimisation of the tree canopy. Less attention has been given to root proliferation, and the question as to whether root stimulation and proliferation, through intensive water and nutrient management, can contribute towards improved tree efficiency and more efficient water use. This is addressed in this study. ‘Brookfield Gala’ apple trees were studied in the Genadendal area near Greyton, in the Western Cape Province, South Africa. The trees were planted in Dundee soil (well aerated sandy loam soil) during winter 2003. Both horticultural aspects (tree growth, shoot growth, fruit yield and quality, trunk circumference and root growth) and gas exchange were studied from 2004/5 until 2007/8 under three different water application strategies, namely micro sprinkler irrigation, daily drip and pulsing drip irrigation and using two different rootstocks: M793 and M7. Irrigation under micro sprinkler irrigation was applied once to three times weekly, daily drip irrigation once daily/twice daily, and pulsing drip irrigation one to six times daily. Water use for bearing apple trees was calculated using long-term evaporation data (for Villiersdorp and Caledon) and existing crop factors for apples. Annual nutrient requirements were adapted from literature and divided percentage-wise into the requirements for five different phenological stages. Soil sensors were used to keep plant available soil water between 100% and 50%. A computer software program was used to incorporate all the above mentioned information and calculate the exact amounts of water and nutrients, and the application times. In general, drip irrigation systems used ±26% less water than micro sprinkler irrigation system. Significantly higher fruit yields were obtained with trees under daily or pulsing drip irrigation than those under micro irrigation during 2005/6 and 2007/8. During 2006/7 the crop load was low due to unfavourable weather conditions during flowering, resulting in poor fruit set and no differences in yield. There was a significantly higher number of thin plus medium roots (3mm and less in diameter) in the 0─400mm rooting zone and total root mass at 0─800mm rooting zone under drip irrigation systems than under micro sprinkler irrigation. Brookfield Gala’ apple trees grown under daily drip irrigation and pulsing drip irrigation performed better compared to those grown under micro sprinkler irrigation with respect to CO2 assimilation rate (A), stomatal conductance (gs), water use efficiency (WUE) and leaf water potential. None of the three irrigation systems affected the biochemical efficiency of the leaf significantly, except on a few occasions during the pre-harvest period. This implied that the changes in leaf biochemical efficiency were as a result of both stomatal and non-stomatal effects (temperature and vapour pressure deficit). The removal of fruit at harvest had a great influence on leaf photosynthetic capacity under micro irrigation but less so under drip irrigation systems. Higher chlorophyll a and chlorophyll b concentrations were observed under drip irrigation systems than under micro sprinkler irrigation, implying efficient biochemical efficiency under these systems compared to micro sprinkler irrigation during the post-harvest period. Use of daily drip irrigation and pulsing drip irrigation delayed the process of leaf ageing. This study demonstrated the benefits of more intensive water and nutrient application for apple trees. Improved root proliferation, increased fruit yield and photosynthetic efficiency have been found under drip irrigation system than under micro sprinkler irrigation.

Intact dark- and light-grown mung bean (Vigna mungo L.), black bean (Phaseolus vulgaris L.), pea (Pisum sativum L.) cowpea (Vigna unguicul a ta (L.) Wa lp.) and squash (Cucurbita pepo L.) seedlings on hydroponic medium were osmotically stressed by exposing their roots to PEG 8000 of various concentrations (-0.2 to -0.6 MPa) to determine stress effects on growth and tissue water status. Growth of dark-grown mung bean hypocotyls ceases within 40 sec upon exposure to any level of stress, and resumes within 10 to 45 min. Growth of all other seedlings were measured usually after 3 to 24 h stress, and in 3 h, elongation is inhibited in dark-grown and is stopped in light-grown tissues. In dark-grown mung bean, black bean and squash hypocotyls and pea epicotyls, growth rates after 24 h stress were found to be proportional to the Ψ of the medium. In mung bean hypocotyls, growth stopped before any change in Ψ or Ψ(π) occurred in the growing region. In this tissue and also in dark-grown squash hypocotyls, pea epicotyls, and in lightgrown cowpea hypocotyls, equivalent reductions in Ψ and Ψ(π) were evident in the growing region after 3 h, so turgor remained constant. In other species, osmotic adjustment with turgor maintenance was evident after 24 h in both the growing and expanded regions. The results with mung bean hypocotyls provided the first demonstration that stress causes an almost instantaneous stress-caused cessation of elongation in dicots. Since data from all plants showed that stress causes growth rate inhibition or cessation without a concomittant decrease in Ψ(p), it is concluded that turgor is not the factor regulating growth. More likely, stress-caused growth and water status changes are responses to an earlier signal, such as a stress-caused reduction in the apoplastic Ψ.

The response of cassava (Manihot esculenta Crantz) to water stress was studied in two distinct sites (Tucson, Arizona and Santander de Quilichao, Colombia). A third experiment was conducted in Palmira, Colombia to evaluate the relationship between photosynthesis, relative humidity and yield of cassava cultivars HCol 1684. Plants stressed after 2 months of growth in Tucson reduced growth by reducing leaf formation, expansion and leaf area. Reductions in LAI due to stress imposed to MCol 1684 in Quilichao were attained by reducing leaf expansion rates and leaf formation in plants stressed after 3 months of age. Plants stressed later increased leaf fall and did not reduce leaf formation. Stress reduced dry matter production in all cultivars, especially when given to young plants. Stress also altered the patterns of dry matter partitioning in 3-month-old plants of MCol 1684, but not in those stressed at 6 months. Transpiration and diffusive conductances of MCol 1684 were reduced after 40 days of stress. These parameters were correlated to photosynthesis and leaf temperatures for non-stressed plants, and additionally with relative humidity in the plants stressed after 3 months of growth. Interestingly, leaf temperatures were lower in stressed plants of MCol 1684, which suggests that stressed cassava plants can avoid excessive heating caused by stomatal closure simply changing leaf orientation and increasing reflectance. Leaf water potential was slightly reduced by stress in Tucson except for MVen 218. Stressed plants of MCol 1684 in Quilichao showed lower values of (L) than those of non-stressed plants after 30-40 days of treatment, suggesting an adaptation of stressed plants acquired during the stress period. The effects of air humidity on stomatal functioning of MCol 1684 seems to be strong as suggested by the dependence of transpiration, conductances and photosynthesis on relative humidity.

Two-year studies (1979-1980, 1980-1981) designed to measure the relation of plant development to yield of spring wheats were undertaken at the University of Arizona Mesa Experiment Station and at the Casa Grande Overpass Farm, Tucson. Low density seeding rates (15 to 20 kg/ha) were used in both studies. The soil at Mesa and in Tucson were clay and sandy barns, respectively. All plants grown under limiting water conditions received 150 kg/m² pre-plant irrigation. Supplemental water as rainfall in the 1979-1980, 1980-1981 seasons were 153 and 79 kg/m² atMesa respectively, and 134 and 116 kg/m2 in Tucson for the respective seasons. Experiments were also conducted in Tucson during the two seasons to evaluate yield performance of wheats under well-watered conditions (525 kg 1m2 and 400 kg/m² irrigation water in the 1979-1980, 1980-1981 seasons, respectively) and on plants which received a single added irrigation approximately 2 to 3 weeks prior to the estimated time of anthesis. Yields in Mesa ranged from 2400 to 3700 kg/ha in 1980 and from 1800 to 3600 kg/ha in 1981. When grown under limiting water conditions in Tucson yields ranged from 1100 to 3000 kg/ha in 1980 and from 2000 to 4100 kg/ha in 1981. Productivities of plants grown with a single added irrigation ranged from 2000 to 4200 kg/ha. Under minimal water conditions yield was correlated with head weight of the main culm, average head weight, grain weight per spike, number of grains per unit land area, Leaf area and Flag leaf area indices at later stages of development, total plant dry weights, root depths and dry weights. Differences were found in tissue water status and leaf growth of selected entries. Tissue water potential was higher in a higher yielding than in a low yielding entry. Additionally, tissues at the basal region tended to be more "sensitive" to changes in soil moisture than those at the mid-section of the expanding leaf. Difficulty in sample selection and variability with plants militated against using leaf growth as an index for determining the degree of stress in field grown wheat cultivars. Neutron probe analysis of soil profile water depletion of a high yielding, long rooted cultivar was greater at the lower depths (60 to 120 cm) after anthesis than for an intermediate or low yielder.

The effect of water stress on the growth, landscape performance, and plant-water relations of four ornamental ground cover species (gray santolina, Santolina chamaecyparissus; dwarf rosemary, Rosemarinus officinalis; Chihuahuan Desert lantana, Lantana velutina; and prostrate germander, Teucrium chamaedrys) was investigated for a twoyear period. Plants were established in a drip irrigated field and subjected to four soil moisture regimes (-1, -5, -10, and -15 bars), monitored by neutron probe. While water stress reduced vegetative covers, shoot growths, heights, and fresh and dry weights of the plants, species response to water deficit varied greatly. Minimum amounts of water required for growing each species were determined. Germander was found to be the most drought tolerant, followed by santolina, rosemary, and lantana in decreasing order. In earlier treatment period, adequately watered plants showed better aesthetic appeal and landscape performance than the plants grown under a high soil moisture tension. However, the effect of water stress on plants' landscape performance was less evident as plants became more established. In all species tested, with an exception of germander, leaf water, osmotic, and turgor potentials of nonstressed plants were higher than those of severely stressed plants. Using the pressure-volume technique, apoplastic water was found to contribute a significant proportion of the total tissue water content. The dilution of symplastic water by apoplastic water during osmotic potential determinations was found to be the major cause of the frequently observed negative turgidity. Methods for correcting for these apoplastic dilution effects were suggested. Osmotic potentials at full and zero turgor, symplastic water contents, changes in water, osmotic, and turgor potentials relative to changes in cell water content (Blifler diagrams), and cell wall elasticity varied significantly with species and treatments. Water stress caused a reduction in total chlorophyll and carotenoids concentrations and in the chlorophyll/ carotenoids ratio, without changing chlorophyll a/chlorophyll b ratio in plant tissues. Leaf reflectances to incident light as measured at 400 to 700 nm were found to differ substantially by species but not by treatments. No consistent correlations were found to exist between leaf pigment content and each of dominant wavelength, brightness, and the purity of leaf color.

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