two weeks. In breaking dormancy in the field it produces green shoots about four Dormancy of Grasses days after suitable conditions prevail. Observations obtained over a five-year period indicated this species entered temperature, day-length, available complete dormancy-when all green coloration had disappeared-at approxi- moisture factors on rangelands mately the same time regardless of the date of the last spring rain. This indicated that high temperatures and long day- Horton M. Laude lengths might be related to the initiation of dormancy in this plant. On California rangelands, the peren- grass, California bromegrass, Harlan The breaking of dormancy followed nial pine bluegrass-Poa scabreh-may bromegrass, nodding stipa, purple stipa, the first fall rains by a few days suggest- enter complete dormancy in late spring and pine bluegrass-was used the fol- ing that moisture coupled with the cooler when surrounding annual vegetation is lowing summer to determine the effect temperatures then prevailing might be still green and vigorous. on growth resumption of watering after instrumental in this response. Efforts to Summer dormancy-reduced or halted several durations of dormancy. Nodding break summer dormancy of pine blue- growth-of pine bluegrass and other stipa, purple stipa and pine bluegrass grass in the field by watering failed until perennial grasses limits the production represented the group failing to continue temperatures moderated toward autumn. of green forage on California pastures summer growth when supplied with sup- To determine whether dormancy could and range lands during the arid summer plemental water in previous tests. By July be broken in midsummer with water, if period. 20 the nine species had ceased growth, cooler temperatures prevailed, plants in The factors involved in summer dor- but some green tissue was still visible in the field approaching their third summer mancy of perennial grasses in California all but perennial ryegrass and pine blue- were potted in late February. The pots were investigated in a study of 20 species grass. were sunk to the rim in soil among un- in field and greenhouse plantings at One replication was thoroughly wa- disturbed plants. Davis. tered on July 20, the second on August Growth of the potted plants was similar Relatively high summer temperatures 10, the third on September 7, and the to that of the undisturbed until early with meager rainfall for five to six months fourth on September 29. In each instance April. The potted plants, with greatly re- place most plants under moisture stress decided growth was obtained seven to 10 stricted root systems, entered dormancy by late spring or early summer. days after a watering in all species except four weeks earlier than those undis- The field test plantings-made on a the stipas and the pine bluegrass. The turbed. On August 4, 10 potted plants deep fine sandy loam in November of stipas made appreciable growth only were moved to a laboratory where maxi- mum temperatures averaged four years-were in replicated rows thfee after the September waterings, while pine 76’ F, while daily maximum field temperatures feet apart and were kept free of wekds. bluegrass did not break dormancy until cool weather prevailed coincident with averaged 87’ F. Plants in both the labora- Water was supplied during the summer the first fall rain. This suggests that those tory and field were watered to maintain to determine whether vegetative growth species in which summer growth may be available soil moisture. On the fourth day would continue. Commencing in the prolonged by watering will also break after the initial watering, plants in the spring following planting and before dormancy when watered after being dor- laboratory commenced to break dor- moisture deficiency was apparent, half mant a portion of the summer. mancy, and by the sixth day all were the plots received thorough weekly wa- Nodding stipa, smilo, and harding- growing. There was no growth resump- terings which continued until the first ef- grass made vigorous vegetative growth tion in the field. fective autumn rain. at Davis in September before autumn Cooler temperatures in the field with- Vegetative growth was continued by rains and after a period of dormancy or out rain or applied water were ineffective 13 of the 20 species when supplied with of greatly reduced growth during the in breaking the dormancy. water. They were desert wheatgrass, summer. Such fall growth prior to pre- In. view of the behavior of pine blue- California bromegrass, prairie brome- cipitation was recorded in a stand of two- grass in the field, studies to determine grass, Harlan bromegrass, orchardgrass, year-old spaced plants of hardinggrass in the factors involved in the induction of veldtgrass, blue wild-rye, tall fescue, per- 1949. The plots were mowed at 3” height dormancy were conducted during the ennial ryegrass, smilo, hardinggrass, big June 1 about two weeks after the last winter in a greenhouse where particular bluegrass, and Canada bluegrass. How- spring shower. By July 12 there was no attention could be given to the effects of ever, their growth differed considerably new growth and the plant bases were day-length and of temperature. A con- in vigor and in the time when the un- nearly devoid of green tissue. Growth trolled environment chamber was used to watered plants of the same species ceased evidenced by green leaf 6” to 8” tall was provide exposures to high temperature. growth. noted on all plants September 13, Responses due to deficient soil moisture Of the seven species which ceased although the first fall rain did not come were eliminated by daily watering of the growth, even though watered, four- until November 9. The source of moisture plants. California melicgrass, Nevada bluegrass, for this growth was obtained by roots It was found that plants grown under nodding stipa, and purple stipa-retained at considerable depth. short-day-photoperiod-at greenhouse green tissue, but three-bulbous blue- Pine bluegrass exhibited such preci- temperatur40’ F day and 65” F night grass, pine bluegrass, and Sandberg blue- sion in its summer dormancy behavior --continued vegetative growth, as did grass-retained no green tissue. Evi- that the species was selected for critical those plants under short-day subjected dently factors other than or in addition study of the factors involved. When dor- to the high temperature stresses in the to soil moisture are involved in this mant in the field, this plant is devoid of controlled environment chamber. These dormancy behavior. visible green tissue. The species when stresses consisted of three 4-hour ex- A fall planting of four replications of entering dormancy first shows a bronzing posures on alternate days to air at 130’ F nine species-perennial ryegrass, blue of the leaf tips and then loses all green and 50% relative humidity. With long wild-rye, orchardgrass, prairie brome- coloration in a period of approximately Concluded on page 14

10 CALIFORNIA AGRICULTURE, JULY, 1953 AVOCADO The observations obtained during these In the group dormant 20 months, only studies, to the effect that Guatemalan two plants-out of 18-failed to grow. Continued from page 3 stocks are far more susceptible to what- New shoot growth was rapid. The aver- heim, Dickinson, Itzamna, Nabal, Hass, ever soil condition causes chlorosis, are age length of eight shoots of the group Challenge, and Taft-and 45 on six Mexi- limited in scope. Perhaps, as the root sys- dormant 20 months measured 1/2” on the can varieties-Ganter, Topa Topa, Duke, tems expand, trees now considered recov- fourth day, l%’ron the fifth, 1%’’ on Northrop, Mexicola, and Blake. Two trees ered may again become chlorotic or the sixth, and 2,/rr on the seventh day are on Waldin, a West Indian variety hitherto normal trees on both types of after watering was resumed. Seven days from Florida. As in the first plot, all stocks may show the disease. after the first watering, vigorous plants Guatemalan seeds were of mixed origin, Whether the occurrence of chlorotic averaged 27 green shoots per plant. but seeds of the Mexican, except Ganter, and normal trees in some cases only about The greenhouse studies demonstrate came from single trees. The trees were 20’ apart and on the same rootstock va- that the initiation of summer dormancy propagated in the Subtropical Horticul- riety, is due to soil variation or genetic in pine bluegrass is associated with long ture Nursery at the University of Califor- differences in the rootstock seedlings is day-length and high temperatures. nia, Los Angeles. This nursery also fur- an open question. All that can be said Growth resumption, after extended nished trees for three additional plots. at present is that none of the 10 Guate- periods of dormancy, was obtained when In June, 1950, one year after planting, malan rootstock varieties used in the two the plants were subjected to relative cool- 7870-43 trees-of the trees on Guate- severely affected rootstock plots is im- ness and were watered. These conditions malan stocks showed chlorosis in varying mune. The number of trees on these stocks parallel those which normally prevail in degrees. Only one tree on Mexican stock varied from three to 18. This, and the the field when the plant enters and breaks was affected, but it soon recovered. The fact that in one plot 14 trees on a certain dormancy. two trees on West Indian stock remained variety showed 43% chlorosis and in the Horton M. Laude is Associate Professor of normal. As of September, 1951, a little other, eight trees on the same variety Agronomy, University of California, Davis. over a year after the disease appeared, showed loo%, stress the necessity for The above progress report is based on Re- 56%-24 trees-of the chlorotic trees more extensive information for valid search Project No. 1194A. were recovered or nearly so and 44%- comparison. 19 trees-were either dead or worthless. The loss on the basis of the 55 trees on F. F. Halma is Professor of Subtropical Horti- Guatemalan planted was 3576, as com- culture, University of California, Los Angeles, KARA pared to a 40% loss in the other plot. The late F. A. White was Farm Advisor, Santa As in the other plot, chlorotic trees are Barbara County, University of California, at Continued from page 4 the time the study reported here was made. scattered throughout the area and in some in bearing habit than most mandarins cases affected and normal trees on the The above progress report is based on Re- search Project No. 1458. and mandarin hybrids. In off years it same rootstock variety are side by side. usually produces at least light and often Five commercial trees-presumably on medium crops. The combination of heavy Mexican stock-which were planted ,in yield and large size on the Riverside trees place of severely chlorotic ones, so far DORMANCY in 1952 is resulting in light crops for have shown no symptoms of the disease. on most trees. Continued from page 10 1953 Also, of several hundred commercial In the Riverside planting the Kara has Fuerte trees on Mexican stock-planted day and greenhouse temperatures plants made vigorous trees on sweet orange, by the grower in 1948 and 1949 in the continued active growth, but those grown sour orange, grapefruit, and Cleopatra same orchard in which the experimental under long-day and subjected to the high mandarin rootstocks. On trifoliate and plot is located-only three show chloro- temperature stresses ceased growth and Rough lemon rootstocks the trees are sis. Neither the old nor the young orange within one week after the last high tem- smaller and on Cunningham citrange tree trees are chlorotic. perature exposure lost all or virtually all condition is poor. Fruit quality in 1952 Three additional plots were planted in visible green tissue. These plants were was best on the sweet orange, grapefruit, 1949 with trees from the same nursery and trifoliate stocks. which furnished the Fuerte trees for the dormant as they could be induced to re- sume growth under favorable conditions Some young plantings of Karas have plot in Orange County. Two of them are been established recently in the Riverside located in Ventura County and one in two to three months later. When pine bluegrass was permitted to area. One orchard of about 280 trees and Los Angeles County. Only three trees--- another of some 170 trees are just com- all in one plot in Ventura County-of grow in a warm greenhouse under natural day-length and without the high tempera- ing into bearing. A group of Karas top- about 200 on Guatemalan stocks planted worked on grapefruit in 1946 seem vigor- have shown chlorosis. None of a similar ture exposures, the plant was dormant by early June even though watered daily. ous, and averaged moderate crops in number on Mexican stock and three on 1952, with rather light crops set for 1953. West Indian are affected. This is similar to the field behavior. Two weeks after such dormancy had set in, Near Fullerton, in Orange County, a In the same orchard in which the three Kara topworked on a large grapefruit chlorotic Fuerte trees arelocated, 96 Hass watering was stopped on a series of these plants. The pots then remained dry on tree gave fruit of good size and color in and 54 Anaheim on Guatemalan as well 1952. In the Piru area of Ventura County as a similar number on Mexican and 11 greenhouse benches for 5, 7,11, 17, and 20 months. At the end ef each interval, a block of 86 three-year-old Karas is on West Indian stocks were planted in just beginning to bear. 1950. A year later 5% of the Hass and six pots were removed to a section of 10% of the Anaheim on Guatemalan greenhouse having daily maximum tem- James W. Cameron is Assistant Geneticist, stocks showed chlorosis. However, prac- peratures not exceeding 75’ F, and daily University of California, Riverside. tically all seem to be recovering. None watering was resumed. All plants of the Robert K. Soost is Assistant Geneticist, Uni- of the trees on Mexican or West Indian groups dormant 5, 7, 11, and 17 months versity of California, Riverside. H. B. Frost, Associate Plant Breeder, Emeri- are affected. Paradoxically, in the same resumed growth by the fifth day after tus, University of California, Riverside, intro- orchard eight of about 40 Mexican seed- the resumption of watering. The majority duced the Kara mandarin in 1935. lings planted by the grower showed the of these plants developed vigorous new The above progress report is based on Re- disorder a few months, then recovered. shoots 3/8“ to long on the fourth day. search Project No. 261.

14 CALIFORNIA AGRICULTURE, JULY, 1953