Hormonal Aspects of Senescence in Detached Tobacco Leaves

The objective of the present work was to describe the simultaneous changes in endogenous levels of cytokinins, abscisic acid, indoleacetic acid and ethylene in detached, senescing tobacco (Nicotiana rustica L.) leaves. These measurements were related to changes in chlorophyll contents, ¹⁴CO₂ fixation and proline contents — three parameters which have been considered to reflect senescence. Effects of exogenous hormonal treatments on these parameters, as well as on endogenous hormonal levels, provided further evidence for the interrelationships between hormones and for their roles in senescence. Starting with actively growing attached leaves and ending with well-advanced senescence in detached leaves, our data indicate a chronological sequence of three hormonal states: (a) cytokinins — high activity, abscisic acid, auxin and ethylene — low contents (actively growing, attached leaves); (b) cytokinins — low activity, abscisic acid — high, auxin and ethylene — low contents (apparent induction of senescence in detached leaves); and (c) cytokinins and abscisic acid — low, auxin and ethylene — high contents (senescence proper in detached leaves).     

The Interaction of Genetic, Environmental, and Hormonal Factors in Stem Elongation and Floral Development of Cucumber Plants

Hypocotyl length was found to vary between cucumber plants carryingdifferent genes controlling sex expression. Among lines havingonly unisexual flowers (genotype M/M), the homozygous monoeciousplants (st⁺/st⁺) had significantly longer hypocotyls than theirgynoecious counterparts (st/st), heterozygous gynoecious plants(st+/st) being intermediate. Similarly, hypocotyls of plantsof an andromonoecious line (st⁺/st⁺ m/m) were significantlylonger than in their hermaphrodite counterparts (st/st m/m).Differences in intemode length were also significant and inthe same direction. Since stem and particularly hypocotyl elongationin cucumber is known to be very sensitive to applied gibberellin,these findings suggest the existence of differences in the effectivelevels of endogenous gibberellins in the different sex types,higher levels being correlated with stronger male tendency.This conclusion is in good agreement with the known effect ofapplied gibberellin on sex expression (enhancement of the maletendency) in cucumber. Application of gibberellin (GA₄+GA₇) and exposure to ‘summer’conditions (long days and relatively high temperature) inhibitedthe development of pistillate flowers while ‘winter’conditions (short days, lower temperature) had a similar effecton staminate flowers. The effect was in either case specific,that is, limited to flower development. It is concluded thatexogenous and endogenous gibberellins affect not only the initiation,but also the further development, of flowers in the cucumber.     

Physiology of Sex in Cucumis Sativus (L.) Leaf Age Patterns and Sexual Differentiation of Floral Buds

One of the factors which control the sexual differentiationof the floral primordium in the cucumber plant may be the hormonalbalance in its vicinity; a balance in favour of growth substance,like IAA, may enhance the female tendency while low concentrationsof these substances or higher concentration of their antagonistsmay cause an increase in the proportion of male flowers. In the present work it is shown that whenever the female tendencyis strong, the floral buds develop next to younger leaves; thereverse is true for plants of a stronger male tendency. Thisrule was found to hold true where environmental conditions—like length of day and night-temperature—affected thesex tendency, as well as in the case of the main branch of thecucumber plant, where an increasing female tendency is found.No difference in leaf age-flowerbud relations was detected whendifferent levels of nitrogen were fed to the plants, and thedifferent N leveb had no effect on the sex tendency of the plants. It is pointed out that a similar situation was found in earlierwork where the difference in sex tendency was due to geneticfactors. It is suggested, therefore, that both environmentaland genetic factors, which are known to affect sex tendencyin the cucumber plant, work through changing the leafage-flowerbudrelationship. Presumably this is related to the hormonal balancein the vicinity of the developing floral primordium.     

Relative Time of Anthesis in Pistillate and Staminate Cucumber Flowers

Staminate (male) floral buds of cucumber plants reach anthesisat consecutive nodes of the main shoot at regular time intervals;however, pistillate (female) buds reach anthesis earlier thanmale ones, thus causing a marked deviation from the linear chronologicalsequence of male anthesia along the main shoot. Measured indays, this relative precedence of the female is greatest inwinter planting time, when short days and low temperatures causea shift towards femaleness in the whole plant, and it is smallestin late summer planting when environmental conditions favourmale tendency. There are also varietal differences in this respect—inone variety (Beth Alpha) the female precedes the male by a longerperiod of time than in another variety (Yorkstate Pickling).The position of the female flower on the main shoot does notseem to play an important role in this respect, but, as a rule,the precedence is greater for the first female than for thesecond or third female developing on the main shoot. It is suggested that the relative enhancement of one type ofsex organs (carpels) is a continuous process which determinesfirst the final sex type of the flower and later causes it toreach anthesis earlier.