Thesis (MScAgric)--Stellenbosch University, 2008. / ENGLISH ABSTRACT: The climate around Prieska differs from other pistachio growing regions in the world in that it receives fewer winter chilling units, has higher maximum temperatures during winter and spring and receives summer rainfall. This possibly results in the observed delayed foliation, flower bud and inflorescence abortion, low fruit set and other flowering disorders, which lower yield potential. In order to increase yields, winter pruning, evaporative cooling and chemical rest breaking were investigated on ‘Ariyeh’, ‘Shufra’ and ‘Sirora’ pistachio trees. Tip-pruning (to remove <2.5cm) and severe heading cuts (to remove 35-45%) of one-year old wood were compared and 4% hydrogen cyanimide (Dormex®), 4% mineral oil (Budbreak®) as well as the combination (0.5% Dormex® + 4% Budbreak®) used as rest breaking agents. Bud break, reproductive bud differentiation, die-back, flower bud retention during winter and early summer as well as yield were evaluated. The results emphasised the interaction of rest breaking and pruning effects, with genetic chill requirements and environmental influences - specifically winter chill build-up. Severe pruning was detrimental to flower bud formation as well as yield. The bud break data suggests that the ability of some rest breaking chemicals to promote lateral development may be explained by their potential to impede the development of apical dominance, rather than a direct effect on the lateral buds. The inability of the chemical treatments to increase yield consistently might indicate other factors involved or that the average winter chill of Prieska is below the minimum amount necessary for adequate rest breaking effects on yield. Evaporative cooling was used to counteract potential negative effects of high maximum day temperatures during autumn and spring on flower bud retention, fruit set and yield. Cooling during autumn (May + June, Southern hemisphere), spring (August + September, Southern hemisphere) and the combination of autumn + spring were investigated during two seasons. Flower bud retention during winter and early summer, flowering patterns, as well as yield were evaluated. The significant effects obtained with evaporative cooling - specifically in autumn + spring, indicated the important role climatic conditions play during both stages of entering and exiting dormancy of pistachio trees. Although all differences are not yet clearly understood, the fact that evaporative cooling resulted in substantially higher yields in the case of ‘Ariyeh’ and ‘Shufra’ in an area with sub-optimal pre-blossom temperatures and less than 40% of the required winter chill of pistachio, emphasises its potential in horticultural management. / AFRIKAANSE OPSOMMING: Prieska se klimaat verskil van ander pistachio-produksie areas in die wêreld deurdat minder winterkoue-eenhede opgebou word, dit hoër maksimum temperature het gedurende die winter en lente en ’n somer-reënvalgebied is. Dit dra waarskynlik by tot die waargenome vertraagde bot, blomknop- en bloeiwyse abortering, lae vrugset en ander blom-afwykings. Aangesien hierdie faktore opbrengspotensiaal verlaag, is wintersnoei, verdampingsverkoeling en chemiese rusbreking ondersoek as moontlike bestuursoplossings. Tip- (om <2.5cm te verwyder) en topsnitte (om 35-45% te verwyder) van eenjarige lote is met mekaar vergelyk en 4% waterstofsianied (Dormex®), 4% minerale olie (Budbreak®) en hul kombinasie is as rusbrekers aangewend. Bot, blomknop-differensiasie, terug-sterwing, blomknopretensie gedurende winter en vroeë somer sowel as opbrengs is geëvalueer. Die resultate benadruk die onderlinge interaksie van rusbreking- en snoei-effekte met genetiese koue-behoeftes en omgewingseffekte - spesifiek die opbou van winterkoue. Topsnitte was nadelig vir blomknopvorming, sowel as opbrengs. Die bot-data doen aan die hand dat sommige chemiese rusbrekers se potensiaal om laterale breke te bevorder, verduidelik kan word deur hul vermoë om die ontwikkeling van apikale dominansie te onderdruk, eerder as ‘n direkte effek op die laterale knoppe. Die chemiese behandelings se onvermoë om opbrengs deurggaans te verbeter, mag daarop dui dat die gemiddelde winterkoue van Prieska laer is as die minimum hoeveelheid benodig alvorens chemiese rusbreker effekte op opbrengs verwag kan word. Potensiële negatiewe effekte van hoë maksimum dagtemperature gedurende die herfs en lente op blomknopretensie, vrugset en opbrengs is teengewerk deur middel van verdampingsverkoeling. Verkoeling gedurende herfs (Mei + Junie, Suidelike halfrond), lente (Augustus + September, Suidelike halfrond) en die kombinasie van herfs + lente is gedurende twee seisoene ondersoek. Blomknopretensie gedurende winter en vroeë somer, blompatrone, sowel as opbrengs is geëvalueer. Die betekenisvolle verskille verkry met verdampingsverkoeling, dui die belangrike rol aan wat klimaatstoestande gedurende beide stadiums van in-, sowel as uitgang uit dormansie speel in pistachiobome. Hoewel alle verskille nog nie verklaar kan word nie, dien die feit dat verdampingsverkoeling tot substansiële opbrengste in die geval van ‘Ariyeh’ en ‘Shufra’ kon lei in ‘n area met suboptimale voor-bot temperature en gemiddeld minder as 40% bevrediging van die kouebehoefte van pistachios, as beklemtoning van die belang daarvan as hortologiese bestuursmiddel.