Encuesta dirigida a los estudiantes

The physiological pri nciples g ove rning the reg rowth of pere n n ial forages are appl icable both u nder cutti n g and g raz i n g (Van Keure n & Matches, 1 988). Defo liatio n by cutting or g razing physically re moves all or part of the p hotosynthetic tissue. U ntil sufficie nt photosynthetic tissue is produced to support g rowt h , conti n ued g rowth must come from carbon co m pou nds previously accum ulated in the plant. This was p ri ncipall y i nferred from two studies. F i rstly, G raber et aI. , ( 1 927) i n a widely quoted publication, fou nd that when l ucerne plants were placed i n darkness after defoliatio n , top g rowth accounted fo r 1 7% of the o riginal dry weight. Secondly, the carbohydrate content of legu m e roots fl uctuates, with a decrease after defo liatio n and subsequent i ncrease thereafter (Grandfield, 1 93 5 ; Smith, 1 962) .

Evidence to support this view was revi ewed by Wei n man n ( 1 948, 1 96 1 , ) and Deregibus et aI . , ( 1 982). However, several other researchers i ncluding May ( 1 960 ) , questioned whether a cause-effect relationship e xisted betwee n depletion o f taproot carbohyd rate reserves and s hoot reg rowth. H e drew attentio n to the role of carbohydrates i n root respiration followi n g the rem oval of leaves. Davidson & Milthorpe ( 1 965, 1 966) poi nted out t h e possible use of other m etabolites as respi ratory substrate followin g severe defoliatio n i n g rasses, whereas Mitchell & D e n n e ( 1 967) drew attention to the possible l oss of absorbing surfaces by the roots. Recent experi me nts by Richard & Caldwell ( 1 985) i nvolvi n g etiolation of Agropyron

spp, supported the laboratory studies of Davidson _& Mi lthorpe ( 1 966) and the review of May ( 1 960). Culvenor et aI . , ( 1 989a) concluded i n the i r study of defoliated subterranean clover swards that partitio n i ng of g rowth to l amina and mobilization of carbohyd rates and nitrogen were i mportant for the recovery fro m defoliatio n . Fu rther, i n a concurrent study with t h e same sward, Culvernor e t aI . , ( 1 989) showed that root respiration comprised a large respi ratory cost of up to 75% of n et p h otosynthesis duri ng reg rowth. In additio n , othe r studies on the role of carbohyd rate in relation to stress tolerance i n l ucerne (Fankhauser et aI. , 1 989 ;

Fankhauser & Volenec, 1 989; H abbe n & Vole nec, 1 990 ; Volenec et aI. , 1 99 1 ; Boyce & Volen ec, 1 992), suggest that h i g h taproot starch concentratio n s may be i mportant for tolerance of specific enviro n me ntal stresses, but b road applicability of this concept to e ncom pass all stresses, i ncludi n g defoliation, we re not consistent i n their studies.

In New Zealand, the role of reserves i n the recovery of ryegrass/white clover swards after defoliation is not considered s i g nificant, but it is unequivocally acknowledged to be a recog nised factor i n the manage m e nt of lucerne pastures (Lang e r & Keog han, 1 970; Sheaffer et al . , 1 988 ; White & Lucas, 1 990).

Leaf area is a widely reco g nized dete rm i nant of reg rowth followin g defoliation in a rang e of p lant species (Watson , 1 947 ; Donald & Black, 1 958; Brown & Blaser, 1 968), and the basic concepts h ave bee n extended to pastu re g rasses and legumes ( B rougham, 1 956; Davidson & D o n ald, 1 958) . Howeve r, even in an erect g rowin g species like lucerne there have b e e n conflicting views o f the i m portance o f residual leaf area after defoliation. It is considered as of little value in som e situations ( B rown et aI. , 1 966 ; Leach , 1 969; Langer & Keoghan , 1 970), but in oth e rs it has i nflu en ced the rate of regrowth ( Langer & Stei nke , 1 965; S m ith & Nelso n , 1 967; Hodgki nson et aI. , 1 972) , possibly because photosynthetic rates of residual leaves rejuvenate when exposed to full daylight just after cutting ( H odgki nso n , 1 974).

Thus, a knowledge of the accumulation and use of reserve carbohydrate, and t h e role of residual leaf area followi n g defoliatio n , is fundamental to the u nderstandin g of management responses i n peren n ial legu mes. The e xtrapolation of the responses of one species may n ot always be used to explai n those of another species (Smith, 1 962). Although sulla is widely used in the Mediterranean cou nt ries such as southern Italy for hay and silage, and to a lesser extent in New Zealand, i nformatio n on its p h ysiological response to defoliation is lacki ng. This study i nvestigated the effects of plant g rowt h stage at fi rst defoliation and defoliat i o n i ntensity o n the p hYSiological response of s u l l a over the fi rst regrowth cycle, i n t h e absence of confou nding factors such as selective g razi ng , treading a n d excreti o n i mposed b y t h e g razing ani m al .

4.2 M ATERIAL AND M ETHODS