PHYSIOLOGICAL AND MORPHOLOGICAL RESPONSE

Water defi cit stre ss, whi ch re sult s when the supply of water to the plant i s stoppe d or li mite d, cau se s dire ct change s in the phy si cal environment of the plant . The se change s may su bse quently affe ct phy siologi cal pro ce sse s. La ck of water limit s glo bal crop pro du ctivity more than any other stre ss fa ctor ( Fi scher an d Turner , 197 8 ; Boyer , 19 8 2 ). The mo st

2 1 u ni t ra cem e leng th did no t vary signi fican tly.

Th e na tur e of th e r espons e curv e in s eed yi eld to ni trog en f er tiliz er appli ca tion is usually an ini tially s teep in cr eas e in yi eld, follow ed by a diminishing ra te of r espons e as m aximum yi eld is approa ch ed. Furth er in cr eas es in f ertiliz er ra te can caus e a r edu ction in s eed yi eld ( Hump hr eys and Riv eros , 19 8 6) . How ev er , th e mos t ef fici en t and pro fitabl e r espons e

can o ccur a t high ra tes of nitrog en f ertiliz er wh en spli t appli ca tions ar e us ed wi th an irriga tion sys tem provid ed ( Harlan et aI. , 19 66), or i t is du e to th e low soil nitrog en l ev els u nd er whi ch i t is usually grown ( Cam eron and Mullaly , 19 69), al though this d ep ends on th e ini tial soil ni trog en s ta tus.

2.5.2.2 Species and cultivar

G en erally , i t is consid er ed tha t maximum s eed yi elds from es tablish ed s tands of tropi ca l grass es ar e ob tain ed by applying 100 kg N lh aJcrop as r eport ed by many work ers viz. wi th Chloris gayana cv . Mbarara ( Boo nman , 19 7 2a) , Eragrostis curvula ( Ahring , 1 9 70) ;

Paspalum plicatulum cv. Rodds ' Bay ( Chadhokar and Humphr eys , 19 7 3b ; Cam eron and Hump hr eys , 19 7 6; Humphr eys , 19 7 6) , and Setaria sphacelata var. sericea cv. Nandi and Na ndi II ( Boo nman , 19 7 2a ; S tillma n and Tapsa ll , 19 7 6).

Despi te this g en eraliza tion , th e ni trog en r equir emen t can also vary wi th diff er en t sp eci es and cul tivars. In th e Philippin es , Mendoza et al. ( 19 7 5) r epor ted tha t Dichanthium

aristatum r equir ed o nly 5 5 kg N lhalcrop to a chi ev e maximum s eed yi elds , bu t for Brachiaria mutica in north Qu eensla nd a ra te of 50-100 kg N lh aJcrop was r equir ed ( Grof , 19 69). Cam eron a nd Mullaly ( 19 69) also r epor ted a con tinuing r espons e by Cenchrus ciliaris cv. Molopo up to 3 3 6 kg N lh aJy ear . In terms of cul tivar diff er en ces , Ha ck er a nd Jon es ( 19 71) r epor ted tha t maximum s eed yi eld of Setaria sphacelata var . s eri cea ( C P I 3 2 9 3 0) show ed a quadra ti c r espons e to in cr easing ni trog en up to 8 4 kg N lha , bu t C P I 3 3 4 2 5 con tinu ed to r espons e lin early to 168 kgN lha .

2.5.2.3 Seed quality

In te rms of seed quality, both positive and negative effe cts of nitrogen have been reported. Fo r example a negative effe ct was reported fo r Setaria sphacelata var. se ri cea ( Boonma n, 19 7 2a), fo r Chloris gayana ( Boonma n, 19 7 2b), a nd fo r Paspalum plicatulum

( Came ron and H umphreys, 19 7 6) i.e. a redu ction in seed viabi lity a nd seed weight, but positive effe cts have a lso been reported by Bahnis ch ( 19 75) fo r Setaria anceps and Chadhokar a nd Hump hreys ( 19 7 3b) fo r Paspalum plicatulum, i.e. nitrogen app lied at flo ral initiation in creased seed viabi lity and seed weight . Howeve r, Javie r et al. ( 19 75) con cluded that seed qua lity is no t usua lly affe cted by nitrogen ferti lize r o r is not consistent ly related to nitrogen leve l

( Came ron and Mul la ly ( 19 69) and Ma cedo et al. ( 19 8 3) fo r Cenchrus ciliaris; Me jia et al.

( 19 7 8a) fo r Panicum maximum; Grof ( 19 69) fo r Brachiaria mutica; Sat jipa non et al. ( 19 89) fo r Brachiaria ruziziensis; and Ruiz et al. ( 199 6) fo r Brachiaria spp.

2.5.2.4 Crop maturity

The effe cts of nit rogen ferti lize r on flo we ring and crop matu rity have shown va ried res ults . Fo r example, nit rogen hastened flo ral initiation in Setaria sphaceiata va r. se ri cea ( Bahnis ch , 19 75; Bahnis ch and Humph reys, 19 7 7 a, 19 7 7b) and flo we ring a nd crop maturity in Chloris gayana (Boonman, 19 7 2b) , a nd in Paspalum plicatulum ( Chadhoka r a nd Humphreys, 19 7 3a, 19 7 3b). On the othe r hand, Stillman and Tapsall ( 19 7 6) reported a delay flowe ring in Setaria anceps whi le Boonma n ( 19 7 2a) found no effe ct on crop maturity in

Setaria sphacelata cv . Nandi.

2.6 EFFECT OF WATER STRESS ON SEED PRODUCTION

2.6.1 PHYSIOLOGICAL AND MORPHOLOGICAL RESPONSE

Wate r de ficit stress, whi ch resu lts when the supp ly of wate r to the plant is stopped o r limited , causes di re ct changes in the physi ca l envi ronment of the plant . These changes may subsequent ly affe ct physio logi ca l p ro cesses . La ck of wate r limits global crop p rodu ctivity mo re than any othe r st ress fa cto r (Fis che r a nd Turne r, 19 7 8; Boye r, 19 8 2). The most

24 ( 1 994), the caespitose Hyparrhenia rufa and Andropogon gayanus and the stoloniferous

Brachiaria mutica and Echinochloa polystachya were all subjected to moderate water stress. Net photosynthesis compensation point was reached at the lowest leaf water potential in A. gayanus which was considered as the most drought tolerant species.

Leaf e xtension is particularly sensitive to water stress. In grasses, the zone of leaf extension is limited to the lower region of the leaf enclosed by the leaf sheath of the preceding leaf, with cell division confined more to the basal region (Langer, 1 979). Generally, leaf elongation rate declines with decreasing water potential (Boyer, 1 968, 1 970; Meyer and Boyer, 1 972; Ludlow, 1 975; Ludlow and Ng, 1 976), but the leaf water potential at which leaf elongation ceases varies with the species concerned. For example, Ludlow and Ng's ( 1 976) experiment on leaf elongation of Panicum maximum var. trichoglume showed that leaf expansion ceased as leaf water potential reached - 1 1 bars which was lower than that for Zea mays (-9 bars) and for the average for a number of herbaceous C3 plants (-9 bars, range -3 to - 1 6) grown under controlled conditions (Ludlow, 1 975). Low leaf water potentials may also restrict leaf production through their effects on leaf initiation and the subsequent rate of cell division (Hsiao, 1 973; Slatyer, 1 973). Leaf area alteration can also be influenced by changes in the time of leaf appearance, in the rate and duration of leaf expansion, and in leaf senescence (Day, 1 98 1 ). As a consequence, such restriction in leaf area can reduce yield (Hsiao and Acevedo, 1 974; Paez et aI. , 1 995).

The total dry matter yield of a stressed crop differs from that of an unstressed crop as a result of the integrated effect of many changes in crop physiology, and can be considered as three processes (Day, 1 98 1 ). Firstly, light interception; the total green area index was decreased by water stress both by a shortening of the season and by a smaller maximum green area and as a result, a non-irrigated crop may intercept 40% less l ight than a fully irrigated crop. Secondly, by altering the efficiency of photosynthesis; although measurements did not distinguish any systematic differences in the internal photosynthetic performance of stressed and unstressed leaves in Day's experiment ( 1 98 1 ), reduction in net photosynthesis, resulting from closure of stomata, was nevertheless recorded with the tropical grass, Panicum maximum

var. trichoglume (Ludlow and Ng, 1 976). Finally, the fraction remaining after respiration; this represents the efficiency of conversion of assimilated carbon into stored dry matter and it is

probable that the increased temperature within the stressed crop leads to enhanced respiration rates (McCree, 1 970).

Tillering is one of the major growth components involved in pasture herbage production. In general, tiller production and the survival of tillers are inversely related to soil water deficit (Langer, 1 979). The variation in total tiller numbers is, according to Luxmoore and Millington ( 1 97 1 ), the major morphological parameter associated w ith variation in total plant dry weight and leaf area.

The differential effects of water deficits on different plant parts will influence not only dry matter production but also the quality of the herbage. With the tropical grass Panicum maximum var. trichoglume, Wilson and Ng ( 1 975) demonstrated that water stress reduced the herbage quality of specific plant tissue, compared on a physiological basis, with comparable tissue on a well-watered plant. Water stress may accelerate the death of older leaves, thereby rapidly decreasing their digestibility . Younger leaves may be retarded in development and the normal ontogenetical decline in digestibility temporarily halted. However, stress occurring during early vegetative growth may have a beneficial effect on the quality of grass forage, by retarding stem elongation and flowering and maintaining a higher nitrogen content than in well-watered plants which have flowered and matured rapidly. But if flowering stems are well developed when stress occurs, their quality may possibly be markedly reduced through accelerated maturation and through the effects of water stress in lowering cell wall digestibility.

2.6.2. REPRODUCTIVE STAGES AND THEIR SENSITIVITY TO WATER